CBSE Class 10 Science Magnetic Effect Of Current Notes

Solenoid : A Coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called solenoid. 

The real picture of a solenoid
Magnetic field due to a current in a solenoid :

• Using R.H. Thumb Rule, we can draw the pattern of magnetic field lines around a current carrying ‘Solenoid’.
• One end of the solenoid behaves as a magnetic north pole, while the other end behave as the South Pole.
• The filed lines inside the solenoid are in form of parallel straight lines, that implies that magnetic field inside the solenoid is same at all points i.e. Field is uniform.
• The magnetic field inside the solenoid is more but less outside it. Magnetic field inside solenoid is uniform i.e. same everywhere.

The strength of the magnetic field produced depends upon
(a) the number of turns
(b) Strength of current in the solenoid used in making solenoid. Electromagnet : Strong magnetic field inside the solenoid can be used to magnetise a magnetic material for example soft iron, when it is placed inside the coil. The magnet so formed is called electromagnet. It is a temporary magnet. OR It is a magnet formed by putting a soft iron core inside a current carrying solenoid. It is a temporary magnet as the magnetic effect is lost when there is no current flowing into it.  

The polarity of electromagnet can also be changed on changing the direction of current 
Properties of Magnetic Field:
• The magnitude; of magnetic field increases with increase in electric current and decreases with decrease in electric current.
• The magnitude of magnetic field; produced by electric current; decreases with increase in distance and vice-versa. The size of concentric circles of magnetic field lines increases with distance from the conductor, which shows that magnetic field decreases with distance.
• Magnetic field lines are always parallel to each other.
• No two field lines cross each other. .Force on a current carrying conductor in a magnetic field. Andre Marie Ampere (1775–1836) suggested that the magnet also exert an equal and opposite force on the current carrying conductor.

We will observe that the rod will displace i.e. the rod will experience a force, when it is placed in magnetic field, in a perpendicular direction to its length.
• The direction of the exerted force will be reversed if the direction of current through the conductor is reversed.
• If we change the direction of field by inter changing the two poles of the magnet, again the direction of exert force will change.
• Therefore the direction of exerted force depends on
(a) direction of current
(b) direction of magnetic field lines.

Fleming’s left-hand rule
Fleming’s left hand rule states that the direction of force applied to a current carrying wire is perpendicular to both, the direction of current as well as the magnetic field.

• According to this rule, stretch thumb, forefinger, and middle finger of your left hand such that they are mutually perpendicular to each other. If fore finger represent direction of magnetic field & middle finger represent direction of current, then thumb will point in the direction of motion or force acting on the conductor.

Electric motor
Electric Motor is a device which converts electrical energy into mechanical energy.

Principle- When a rectangular coil carrying current is placed in a magnetic field, it experiences a force that rotates it. (Rectangular coil)
OR
When a rectangular coil is placed in a magnetic field and a current is passed through it, force acts on the coil, which rotates it continuously. With the rotation of the coil, the shaft attached to it also rotates

Function: To make contact with split rings. Split Rings Commutator formed by splitting of copper ring. Its function is to reverse the direction of the current.

Parts of a Electric Motor
• Insulated Copper wire: A rectangular coil of wire ABCD
• Magnet Poles: A magnet as placed above ie North Pole and South Pole. This creates a magnetic field as shown above. We can use u shape magnet or 2 bar magnets with the end of north and south as shown in figure.
• Split Rings: Two disjoint C-shaped rings P and Q. It acts as a commutator (which can reverse the direction of current)
• Axle: The split rings are placed on the axle which can rotate freely.
• Brushes: The outside of the split rings are connected to conducting brushes X and Y.
• Source Battery: To source current. Working of Electric Motor Imagine a coil ABCD is in a horizontal plane such that magnetic field is parallel to the plane of the coil. When electric current is passed through coil

A and B
▪ In segment AB current flows from A to B. Applying Fleming's left hand rule, we come to know that it experiences force in upward direction C and D. ▪ In segment CD, current flows from C to D and force acts downwards (By Fleming’s left hand rule).
▪ The two forces being equal and opposite form a couple and rotate the coil in clockwise direction.

When the coil turns an angle of 90o, commutator loses contact with brushes and thus, no force acts on the coil. But due to the momentum gained, the coil continues rotating till it covers an angle of 180o. After 180o, S1 connects with brush B2 and S2 connects with B1. This alters the direction of current in coil and new current flows in the direction BADC. This is how electric motor’s work.

OR IN SIMPLE WAYS :-
• Current enters arm AB through brush X and current flows through brush Y from C to D. Using Fleming’s LHR we find that the force pushes AB downwards and pushes CD upwards.
• In electric Motor the split rings PQ act as a commutator that reverses the direction of the current. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil.

Applications
• Electric Fans
• Refrigerators
• Mixers
• Washing machines

The term magnetic effect of electric current means that an electric current flowing in a wire produces a magnetic field around it.
A current flowing in a wire always gives rise to a magnetic field around it. The magnetic effect of current is also called electromagnetism which means electricity produces magnetism. In figure, the deflection of compass needle by the current carrying wire in the below experiment show that an electric current produces a magnetic field around it.

MAGNET

A magnet is an object, which attracts pieces of iron, steel, nickel and cobalt. It has two poles at ends – South and North Pole.

ØLike magnetic poles repel each other.

ØUnlike magnetic poles attract each other.

MAGNETIC FIELD

The space surrounding a magnet in which the force of attraction and repulsion is exerted is called a magnetic field.

MAGNETIC FIELD LINES

The magnetic field lines are the lines drawn in a magnetic field along which a north magnetic pole would move. These are also known as magnetic lines of forces.

PROPERTIES OF MAGNETIC FIELD LINES

1. A magnetic field lines originate from north pole and end at its south pole.

2. A magnetic field line is a closed and continuous curve.

3. The magnetic field lines are closer near the poles of a magnet where the magnetic field is strong and farther apart where the magnetic field is weak.

4. The magnetic field lines never intersect each other.

5. A uniform magnetic field is represented by parallel and equidistant field lines.

INTEXT QUESTIONS 

1. Why does a compass needle get deflected when brought near a bar magnet?

Ans. A compass gets deflected due to the forces acting on its poles due to the magnetic field of the bar magnet.

MAGNETIC FIELD DUE TO A CURRENT THROUGH A STRAIGHT CONDUCTOR

The magnetic field lines around a straight conductor carrying current are concentric circles whose centres lies on the wire.
The magnitude of magnetic field produced by a straight current carrying wire at a point- directly proportional to current passing in the wire. inversely proportional to the distance of that point from the wire.

RIGHT-HAND THUMB RULE

When a current-carrying straight conductor is holding in right hand such that the thumb points towards the direction of current. Then fingers will wrap around the conductor in the direction of the field lines of the magnetic field, as shown in below figure. This is known as the right- hand thumb rule

Thumb-points in the direction of current then direction of fingers encircle the wire give the direction of magnetic field around the wire.

1. Draw magnetic field lines around a bar magnet.

Ans.

2. List the properties of magnetic lines of force.

Ans. Refer in page no. 1

3. Why don’t two magnetic lines of force intersect each other?

Ans. If two magnetic lines of force intersect then there would be two directions of magnetic field at that point, which is absurd. That is why they never intersect.

MAGNETIC FIELD DUE TO A CURRENT THROUGH A CIRCULAR LOOP

The magnetic field lines are circular near the current carrying loop. As we move away, the concentric circles becomes bigger and bigger. At the centre, the lines are straight.

At the centre, all the magnetic field lines are in the same direction due to which the strength of magnetic field increase.

The magnetic of magnetic field produced by a current carrying circular loop at its centre is

* directly proportional to the current passing

* inversely proportional to the radius of the circular loop

The strength of magnetic field produced by a circular coil carrying current is directly proportional to both number of turns(n) and current(I) but inversely proportional to its radius(r).

MAGNETIC FIELD DUE TO A CURRENT IN A SOLENOID

The insulated copper wire wound on a cylindrical tube such that its length is greater than its diameter is called a solenoid. The solenoid is from greek word for channel.

* The solenoid is a long coil containing a large number of close turns of insulated copper wire.

* The magnetic field produced by a current carrying solenoid is similar to the magnetic field produced by a bar magnet.

* The current in each turn of a current carrying solenoid flows in the same direction due to which the magnetic field produced by each turn of the solenoid ads up, giving a strong magnetic field inside the solenoid.

The strong magnetic field produced inside a current-carrying solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the solenoid. The magnet thus formed is called an electromagnet.

So, a solenoid is used for making electromagnets.

The strength of magnetic field produced by a carrying current solenoid depends on

* number of turns(n)

* strength of current(I)

* nature of core material used in solenoid – use of soft iron as core in a solenoid produces the strongest magnetism.

ELECTROMAGNETS AND PERMANENT MAGNETS

An electromagnet is a temporary strong magnet and is just a solenoid with its winding on soft iron core.

A permanent magnet is made from steel. As steel has more retentivity than iron, it does not lose its magnetism easily.

Difference between Electromagnet and permanent magnet 

Q. Why soft iron is used for making the core of an electromagnet?

Soft iron is used for making the core of an electromagnet because soft iron loses all of its magnetism when current in the coil is switched off.

Q. Why steel is not used for making the core of an electromagnet?

Steel is not used for making the core of an electromagnet because steel does not loses all of its magnetism when current in the coil is switched off.

INTEXT QUESTIONS PAGE NO. 229 and 230

1. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.

For downward direction of current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop. Similarly, for upward direction of current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop, as shown in the given figure.

2. The magnetic field in a given region is uniform. Draw a diagram to represent it.

3. Choose the correct option: The magnetic field inside a long straight solenoid-carrying current (a) is zero. (b) decreases as we move towards its end. (c) increases as we move towards its end. (d) is the same at all points.

The magnetic field for a point inside a long straight solenoid carrying current is double than for a point situated at one of its ends. Thus, the correct option is (b).

FORCE ON A CURRENT-CARRYING CONDUCTOR IN A MAGNETIC FIELD

When a current carrying conductor is placed in a magnetic field it experiences a force, except when it is placed parallel to the magnetic field.

The force acting on a current carrying conductor in a magnetic field is due to interaction between:

1. Magnetic force due to current-carrying conductor and

2. External magnetic field in which the conductor is placed.

In the above figure, a current-carrying rod, AB, experiences a force perpendicular to its length and the magnetic field.
The displacement of the rod in the above activity suggests that a force is exerted on the currentcarrying aluminium rod when it is placed in a magnetic field. It also suggests that the direction of force is also reversed when the direction of current through the conductor is reversed. Now change the direction of field to vertically downwards by interchanging the two poles of the magnet. It is once again observed that the direction of force acting on the current-carrying rod gets reversed. It shows that the direction of the force on the conductor depends upon the direction of current and the direction of the magnetic field. We considered the direction of the current and that of the magnetic field perpendicular to each other and found that the force is perpendicular to both of them.

FLEMING’S LEFT HAND RULE

Fleming's left hand rule (for electric motors) shows the direction of the thrust on a conductor carrying a current in a magnetic field. The left hand is held with the thumb, index finger and middle finger mutually at right angles.
The First finger represents the direction of the magnetic Field. (north to south) The Second finger represents the direction of the Current (the direction of the current is the direction of conventional current; from positive to negative).
The Thumb represents the direction of the Thrust or resultant Motion. 

FLEMING’S RIGHT HAND RULE

Fleming's right hand rule (for generators) shows the direction of induced current when a conductor moves in a magnetic field.

The right hand is held with the thumb, first finger and second finger mutually perpendicular to each other {at right angles}, as shown in the diagram .

The Thumb represents the direction of Motion of the conductor.
The First finger represents the direction of the Field. (north to south)
The Second finger represents the direction of the induced or generated Current (the direction of the induced current will be the direction of conventional current; from positive to negative). 

INTEXT QUESTIONS PAGE NO. 231 AND 232

1. Which of the following property of a proton can change while it moves freely in a magnetic field?
(There may be more than one correct answer.) (a) mass (b) speed (c) velocity (d) momentum Whenever a charged proton moves in a magnetic field, its velocity changes and as a result of this its momentum change. Thus (c) and (d) are the properties which change when a proton moves freely in a magnetic field.

2. In Activity 13.7, how do we think the displacement of rod AB will be affected if (i) current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased?
(i) If the current in the rod AB is increased, force also increases.
(ii) When a stronger horse-shoe magnet is used, magnetic field increases as a result force also increases.
(iii)If the length of the rod AB is increased, force also increased.

3. A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is (a) towards south (b) towards east (c) downward (d) upward The direction of the motion of proton is the direction of current. The direction of force o the proton is towards north. Applying Fleming’s left hand rule, the direction of magnetic field is upward. The correct option is (d).

ELECTRIC MOTOR

An electric motor is a rotating device that converts electrical energy to mechanical energy.
Electric motor is used as an important component in electric fans, refrigerators, mixers, washing machines, computers, MP3 players etc.
Principle: When a coil carrying current is placed in a magnetic field, it experiences a torque.
As a result of this torque, the coil begins to rotate.
Construction:
It consists of a rectangular coil ABCD of insulated copper wire. The coil is placed between the two poles of a magnetic field such that the arm AB and CD are perpendicular to the direction of the magnetic field. The ends of the coil are connected to the two halves P and Q of a split ring. The inner sides of these halves are insulated and attached to an axle. The external conducting edges of P and Q touch two conducting stationary brushes X and Y, respectively, as shown in the below figure 

Working:
Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through brush Y.

Notice that the current in arm AB of the coil flows from A to B. In arm CD it flows from C to D, that is, opposite to the direction of current through arm AB.

On applying Fleming’s left hand rule for the direction of force on a current-carrying conductor in a magnetic field.. We find that the force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards. Thus the coil and the axle O, mounted free to turn about an axis, rotate anti-clockwise.

At half rotation, Q makes contact with the brush X and P with brush Y. Therefore the current in the coil gets reversed and flows along the path DCBA. A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator.

The reversal of current also reverses the direction of force acting on the two arms AB and CD.
Thus the arm AB of the coil that was earlier pushed down is now pushed up and the arm CD previously pushed up is now pushed down. Therefore the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.

Uses of electric motor:

The commercial motors use (i) an electromagnet in place of permanent magnet; (ii) large number of turns of the conducting wire in the current-carrying coil; and (iii) a soft iron core on which the coil is wound. The soft iron core, on which the coil is wound, plus the coils, is called an armature. This enhances the power of the motor.

INTEXT QUESTIONS PAGE NO. 231 AND 232

Question. State Fleming’s left-hand rule.
Ans. Fleming’s left hand rule states that if we arrange the thumb, the centre finger, and the forefinger of the left hand at right angles to each other, then the thumb points towards the direction of the magnetic force, the centre finger gives the direction of current, and the forefinger points in the direction of magnetic field. 

Question. What is the principle of an electric motor?
Answer: The working principle of an electric motor is based on the magnetic effect of current.
A current-carrying loop experiences a force and rotates when placed in a magnetic field.
The direction of rotation of the loop is given by the Fleming’s left-hand rule.

Question. What is the role of the split ring in an electric motor?
Answer: The split ring in the electric motor acts as a commutator. The commutator reverses the direction of current flowing through the coil after each half rotation of the coil. Due to this reversal of the current, the coil continues to rotate in the same direction.

ELECTROMAGNETIC INDUCTION

The production of electricity from magnetism is called Electromagnetic induction. When a straight wire is moved up and down rapidly between the poles of magnet, then an electric
current is produced in the wire. This is an example of electromagnetic induction The process of electromagnetic induction has led to the construction of generators for producing electricity at power stations The current produced by moving a straight wire in a magnetic field is called an induced current. In the below figure, moving a magnet towards a coil sets up a current in the coil circuit, as indicated by deflection in the galvanometer needle.

If the bar magnet moved towards south pole of the magnet towards the end B of the coil, the deflections in the galvanometer would just be opposite to the previous case. When the coil and the magnet are both stationary, there is no deflection in the galvanometer. It is, thus, clear from this activity that motion of a magnet with respect to the coil produces an induced potential difference, which sets up an induced electric current in the circuit.

ELECTRIC GENERATOR

In an electric generator, mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.

Principle: Whenever in a closed circuit, the magnetic field lines change, an induced current is produced.

Construction:
♦ An electric generator, as shown in the below figure, consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet.
♦ The two ends of this coil are connected to the two rings R1 and R2.
♦ The inner side of these rings are made insulated.
♦ The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively.
♦ The two rings R1 and R2 are internally attached to an axle.
♦ The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field.
♦ Outer ends of the two brushes are connected to the galvanometer to show the flow of current in the given external circuit.

Working:
When the axle attached to the two rings is rotated such that the arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet. Let us say the coil ABCD is rotated clockwise in the arrangement shown in the above figure.
By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions AB and CD. Thus an induced current flows in the direction ABCD. If there are larger numbers of turns in the coil, the current generated in each turn adds up to give a large current through the coil. This means that the current in the external circuit flows from B2 to B1.

After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current in the direction DCBA. The current in the external circuit now flows from B1 to B2.
Thus after every half rotation the polarity of the current in the respective arms changes. Such a current, which changes direction after equal intervals of time, is called an alternating current (abbreviated as AC). This device is called an AC generator.

To get a direct current (DC, which does not change its direction with time), a split-ring type commutator must be used. With this arrangement, one brush is at all times in contact with the arm moving up in the field, while the other is in contact with the arm moving down. We have seen the working of a split ring commutator in the case of an electric motor Thus a unidirectional current is produced. The generator is thus called a DC generator. The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically.

INTEXT QUESTIONS PAGE NO. 236

1. Explain different ways to induce current in a coil.
The different ways to induce current in a coil are as follows:
(a) If a coil is moved rapidly between the two poles of a horse-shoe magnet, then an electric current is induced in the coil.
(b) If a magnet is moved relative to a coil, then an electric current is induced in the coil 

INTEXT QUESTIONS PAGE NO. 237

Question. State the principle of an electric generator.
Answer: An electric generator works on the principle of electromagnetic induction. It generates electricity by rotating a coil in a magnetic field.

Question. Name some sources of direct current.
Answer: Some sources of direct current are cell, DC generator, etc

Question. Which sources produce alternating current?
Answer: AC generators, power plants, etc., produce alternating current

Question. Choose the correct option: A rectangular coil of copper wires is rotated in a magnetic field.
Answer: The direction of the induced current changes once in each (a) two revolutions (b) one revolution (c) half revolution (d) one-fourth revolution
(c) When a rectangular coil of copper is rotated in a magnetic field, the direction of the induced current in the coil changes once in each half revolution. As a result, the direction of current in the coil remains the same

Assertion-Reason Type Questions

For question numbers 1 and 2 two statements are given-one labeled as Assertion (A) and the other labeled
Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:
(a) Both ‘A’ and ‘R’ are true and ‘R’ is correct explanation of the Assertion.
(b) Both ‘A’ and ‘R’ are true but ‘R’ is not correct explanation of the Assertion.
(c) ‘A’ is true but ‘R’ is false.
(d) ‘A’ is false but ‘R’ is true.

Question. Assertion: Compass is a small magnet and gives direction of magnetic field lines.
Reason: It gets deflected when brought near a bar magnet.
Answer : B

Question. Assertion: A current carrying solenoid behaves like a bar magnet.
Reason: When soft iron is placed inside the solenoid it can also be magnetised.
Answer : B

DOMESTIC ELECTRIC CIRCUITS

Question. When does an electric short circuit occur?
Answer: If the resistance of an electric circuit becomes very low, then the current flowing through the circuit becomes very high. This is caused by connecting too many appliances to a single socket or connecting high power rating appliances to the light circuits. This results in a short circuit.
When the insulation of live and neutral wires undergoes wear and tear and then touches each other, the current flowing in the circuit increases abruptly. Hence, a short circuit occurs.

Question. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Answer: The metallic body of electric appliances is connected to the earth by means of earth wire so that any leakage of electric current is transferred to the ground. This prevents any electric shock to the user. That is why earthing of the electrical appliances is necessary.

Question. What is Electric fuse? What is the important of electric fuse?
Answer: Electric Fuse consists of a piece of wire made of a metal or an alloy of appropriate melting point, for example aluminium, copper, iron, lead etc. If a current larger than the specified value flows through the circuit, the temperature of the fuse wire increases. This melts the fuse wire and breaks the circuit. Fuse is the most important safety device, used for protecting the circuits due to short-circuiting or overloading of the circuits. The use of an electric fuse prevents the electric circuit and the appliance from a possible damage by stopping the flow of unduly high electric current. The fuses used for domestic purposes are rated as 1 A, 2 A, 3 A, 5 A, 10 A, etc.

INTEXT QUESTIONS PAGE NO. 238

Question. Name two safety measures commonly used in electric circuits and appliances.
Answer: Two safety measures commonly used in electric circuits and appliances are as follows:
(i) Each circuit must be connected with an electric fuse. This prevents the flow of excessive current through the circuit. When the current passing through the wire exceeds the maximum limit of the fuse element, the fuse melts to stop the flow of current through that circuit, hence protecting the appliances connected to the circuit.
(ii) Earthing is a must to prevent electric shocks. Any leakage of current in an electric appliance is transferred to the ground and people using the appliance do not get the shock.

Question. An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.
Answer: Current drawn by the electric oven can be obtained by the expression,
P=VI
I = P/V
Where, current = I. Power of the oven, P = 2 kW = 2000W
Voltage supplied, V = 220V
I = 2000/220 = 9.09A
Hence, the current drawn by the electric oven is 9.09 A, which exceeds the safe limit of the circuit. Fuse element of the electric fuse will melt and break the circuit.

Question. What precaution should be taken to avoid the overloading of domestic electric circuits?
Answer: The precautions that should be taken to avoid the overloading of domestic circuits are as follows:
(a) Too many appliances should not be connected to a single socket.
(b) Too many appliances should not be used at the same time.
(c) Faulty appliances should not be connected in the circuit
(d) Fuse should be connected in the circuit.

MAGNETISM IN MEDICINE

An electric current always produces a magnetic field. Even weak ion currents that travel along the nerve cells in our body produce magnetic fields. When we touch something, our nerves carry an electric impulse to the muscles we need to use. This impulse produces a temporary magnetic field. These fields are very weak and are about one-billionth of the earth’s magnetic field. Two main organs in the human body where the magnetic field produced is significant, are the heart and the brain. The magnetic field inside the body forms the basis of obtaining the images of different body parts. This is done using a technique called Magnetic Resonance Imaging (MRI). Analysis of these images helps in medical diagnosis. Magnetism has, thus, got important uses in medicine.

EXERCISE QUESTIONS PAGE NO. 240

Question. Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire
(b) The field consists of straight lines parallel to the wire
(c) The field consists of radial lines originating from the wire
(d) The field consists of concentric circles centred on the wire
Answer: (d) The magnetic field lines, produced around a straight current-carrying conductor, are concentric circles. Their centres lie on the wire.

Question. The phenomenon of electromagnetic induction is
(a) the process of charging a body
(b) the process of generating magnetic field due to a current passing through a coil
(c) producing induced current in a coil due to relative motion between a magnet and the coil
(d) the process of rotating a coil of an electric motor
Answer: (c) When a straight coil and a magnet are moved relative to each other, a current is induced in the coil. This phenomenon is known as electromagnetic induction.

Question. The device used for producing electric current is called a
(a) generator.
(b) galvanometer.
(c) ammeter.
(d) motor.
Answer: (a) An electric generator produces electric current. It converts mechanical energy into electricity.

Question. The essential difference between an AC generator and a DC generator is that
(a) AC generator has an electromagnet while a DC generator has permanent magnet.
(b) DC generator will generate a higher voltage.
(c) AC generator will generate a higher voltage.
(d) AC generator has slip rings while the DC generator has a commutator.
Answer: (d) An AC generator has two rings called slip rings. A DC generator has two half rings called commutator. This is the main difference between both the types of generators.

Question. At the time of short circuit, the current in the circuit
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.
Answer: (c) When two naked wires of an electric circuit touch each other, the amount of current that is flowing in the circuit increases abruptly. This causes short-circuit.

Question. State whether the following statements are true or false.
(a) An electric motor converts mechanical energy into electrical energy.
(b) An electric generator works on the principle of electromagnetic induction.
(c) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(d) A wire with a green insulation is usually the live wire of an electric supply.
Answer: (a) False
An electric motor converts electrical energy into mechanical energy.
(b) True
A generator is an electric device that generates electricity by rotating a coil in a magnetic field. It works on the principle of electromagnetic induction.
(c) True
A long circular coil is a long solenoid. The magnetic field lines inside the solenoid are parallel lines.
(d) False
Live wire has red insulation cover, whereas earth wire has green insulation colour in the domestic circuits.

Question. List three sources of magnetic fields.
Answer: Three sources of magnetic fields are as follows:
(a) Current-carrying conductors
(b) Permanent magnets
(c) Electromagnets

Question. How does a solenoid behave like a magnet? Can you determine the north and south poles of a current–carrying solenoid with the help of a bar magnet? Explain.
Answer: A solenoid is a long coil of circular loops of insulated copper wire. Magnetic field lines are produced around the solenoid when a current is allowed to flow through it. The magnetic field produced by it is similar to the magnetic field of a bar magnet. The field lines produced in a current-carrying solenoid is shown in the following figure.

In the above figure, when the north pole of a bar magnet is brought near the end connected to the negative terminal of the battery, the solenoid repels the bar magnet. Since like poles repel each other, the end connected to the negative terminal of the battery behaves as the north pole of the solenoid and the other end behaves as a south pole. Hence, one end of the solenoid behaves as a north pole and the other end behaves as a south pole.

Question. When is the force experienced by a current–carrying conductor placed in a magnetic field largest?
Answer: The force experienced by a current-currying conductor is the maximum when the direction of current is perpendicular to the direction of the magnetic field.

Question. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Answer: The direction of magnetic field is given by Fleming’s left hand rule. Magnetic field inside the chamber will be perpendicular to the direction of current (opposite to the direction of electron) and direction of deflection/force i.e., either upward or downward. The direction of current is from the front wall to the back wall because negatively charged electrons are moving from back wall to the front wall. The direction of magnetic force is rightward. Hence, using Fleming’s left hand rule, it can be concluded that the direction of magnetic field inside the chamber is downward.

Question. Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?
Answer: An electric motor is a rotating device that converts electrical energy to mechanical energy. Electric motor is used as an important component in electric fans, refrigerators,
mixers, washing machines, computers, MP3 players etc.
Principle: When a coil carrying current is placed in a magnetic field, it experiences a torque. As a result of this torque, the coil begins to rotate.

Construction:

It consists of a rectangular coil ABCD of insulated copper wire. The coil is placed between the two poles of a magnetic field such that the arm AB and CD are perpendicular to the direction of the magnetic field. The ends of the coil are connected to the two halves P and Q of a split ring. The inner sides of these halves are insulated and attached to an axle. The external conducting edges of P and Q touch two conducting stationary brushes X and Y, respectively, as shown in the below figure 

Working:
Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through brush Y.

Notice that the current in arm AB of the coil flows from A to B. In arm CD it flows from C to D, that is, opposite to the direction of current through arm AB.

On applying Fleming’s left hand rule for the direction of force on a current-carrying conductor in a magnetic field.. We find that the force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards. Thus the coil and the axle O, mounted free to turn about an axis, rotate anti-clockwise.

At half rotation, Q makes contact with the brush X and P with brush Y. Therefore the current in the coil gets reversed and flows along the path DCBA. A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator.

The reversal of current also reverses the direction of force acting on the two arms AB and CD. Thus the arm AB of the coil that was earlier pushed down is now pushed up and the arm CD previously pushed up is now pushed down. Therefore the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.

Question. Name some devices in which electric motors are used.
Answer: Some devices in which electric motors are used are as follows:
(a) Water pumps (b) Electric fans (c) Electric mixers (d) Washing machines

Question. A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary inside the coil?
Answer: A current induces in a solenoid if a bar magnet is moved relative to it. This is the principle of electromagnetic induction.
(i) When a bar magnet is pushed into a coil of insulated copper wire, a current is induced momentarily in the coil. As a result, the needle of the galvanometer deflects momentarily in a particular direction.
(ii) When the bar magnet is withdrawn from inside the coil of the insulated copper wire, a current is again induced momentarily in the coil in the opposite direction. As a result, the needle of the galvanometer deflects momentarily in the opposite direction.
(iii) When a bar magnet is held stationary inside the coil, no current will be induced in the coil. Hence, galvanometer will show no deflection.

Question. Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.
Answer: Two circular coils A and B are placed closed to each other. When the current in coil A is changed, the magnetic field associated with it also changes. As a result, the magnetic field around coil B also changes. This change in magnetic field lines around coil B induces an electric current in it. This is called electromagnetic induction.

Question. State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.
Answer: (i) Maxwell’s right hand thumb rule
(ii) Fleming’s left hand rule
(iii) Fleming’s right hand rule

Question. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?
Answer: In an electric generator, mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.
Principle: Whenever in a closed circuit, the magnetic field lines change, an induced current is produced.
Construction:
♦ An electric generator, as shown in the below figure, consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet.
♦ The two ends of this coil are connected to the two rings R1 and R2.
♦ The inner side of these rings are made insulated.
♦ The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively.
♦ The two rings R1 and R2 are internally attached to an axle.
♦ The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field.
♦ Outer ends of the two brushes are connected to the galvanometer to show the flow of current in the given external circuit.

Working:

When the axle attached to the two rings is rotated such that the arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet. Let us say the coil ABCD is rotated clockwise in the arrangement shown in the above figure.

By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions AB and CD. Thus an induced current flows in the direction ABCD. If there are larger numbers of turns in the coil, the current generated in each turn adds up to give a large current through the coil. This means that the current in the external circuit flows from B2 to B1. 

After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current in the direction DCBA. The current in the external circuit now flows from B1 to B2. Thus after every half rotation the polarity of the current in the respective arms changes.
Such a current, which changes direction after equal intervals of time, is called an alternating current (abbreviated as AC). This device is called an AC generator.

To get a direct current (DC, which does not change its direction with time), a split-ring type commutator must be used. With this arrangement, one brush is at all times in contact with the arm moving up in the field, while the other is in contact with the arm moving down. We have seen the working of a split ring commutator in the case of an electric motor Thus a unidirectional current is produced. The generator is thus called a DC generator. The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically. 

Question. When does an electric short circuit occur?
Answer: If the resistance of an electric circuit becomes very low, then the current flowing through the circuit becomes very high. This is caused by connecting too many appliances to a single socket or connecting high power rating appliances to the light circuits. This results in a short circuit.
When the insulation of live and neutral wires undergoes wear and tear and then touches each other, the current flowing in the circuit increases abruptly. Hence, a short circuit occurs.

Question. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Answer: The metallic body of electric appliances is connected to the earth by means of earth wire so that any leakage of electric current is transferred to the ground. This prevents any electric shock to the user. That is why earthing of the electrical appliances is necessary.

Key Learnings:

1. A compass needle behaves as a small magnet. Its one end pointing towards north is called a north pole, and the other end pointing towards south, is called a south pole.

2. The space around a magnet in which the force of attraction and repulsion due to the magnet can be detected is called the magnetic field.

3. A field line is path along which a hypothetical free north pole would tend to move. The direction of the magnetic filed at a point is given by the direction that a north pole placed at that point would take. Field lines are shown closer together where the magnetic filed is greater.

4. The magnetic field lines around a straight conductor carrying current are concentric circles.

5. The direction of magnetic field is given by Right Hand Thumb Rule.

6. The magnetic field inside a solenoid is similar to that of a bar magnet.

7. A current-carrying conductor when placed in a magnetic field experiences a force.

8. Fleming’s left-hand rule gives the direction of magnetic force acting on a conductor.

9. An electromagnet consists of a core of soft iron wrapped around with a coil of insulated copper wire.

10. An electric motor is a device that converts electric energy into mechanical energy and it works on the principle of force experienced by a current carrying conductor in a magnetic field.

11. The phenomenon in which an electric current is induced in a circuit because of a changing magnetic field is called electromagnetic induction.

12. The magnetic field may change due to a relative motion between the coil and a magnet placed near to the coil. If the coil is placed near to a current carrying conductor, the magnetic field may change either due to a change in the current through the conductor or due to the relative motion between the coil and conductor.

13. Fleming’s right hand rule is used to find the direction of induced current.

14. Electric generators are based on the principle of electromagnetic induction and converts mechanical energy into lectrical energy.

15. In our houses we receive AC electric power of 220 V with a frequency of 50 Hz.

16. One of the wires in the electricity wiring of houses is with red insulation, called live wire. The other one is of black insulation, which is a neutral wire. The third is the earth wire that has green insulation and this is connected to a metallic body deep inside earth.

17. The potential difference between live wire and neutral wire is 220 V.

18. Third wire in the wiring is used as a safety measure to ensure that any leakage of current to a metallic body does not give any server shock to a user.

19. Fuse is the most important safety device used for protecting the circuits due to short circuiting or overloading of the circuits.

Snow rule: According to this rule, when the current flows from the south to the north, the needle deflects towards the west.
Magnetic field produced by straight conductor 

Characteristics:
(a) Magnetic lines are concentric circles.
(b) The direction of magnetic lines reverses as we reverse the direction of the current. When current flows upward, the direction of magnetic lines is anticlockwise. When current flows downward, the direction of magnetic lines is clockwise.
(c) The Magnetic field produced is directly proportional to the current & inversely proportional to the distance from the conductor.
(d) To know the direction of magnetic field around a straight conductor, we have different sets of rules as given below:

Right-hand thumb rule
If a straight conductor is held in the right hand in such a way that the thumb points along the direction of the current, then the tips of the fingers or the curl of the fingers show the direction of magnetic field around it.

Magnetic field due to current through a circular loop
The right-hand thumb rule can be used for a circular conducting wire as well as it comprises of small straight segments. Every point on the wire carrying current gives rise to a magnetic field that appears as straight lines at the centre.

The Magnetic field produced around a circular loop is also circular. As we move away from the loop, the concentric circle becomes bigger. At the centre, the magnetic lines are parallel.

Characteristics
1. The magnetic lines are circular at the points from where the current enters or leaves the coil.
2. Within the space enclosed by the coil, the field lines are in the same direction.
3. Near the centre of the coil, the magnetic lines are almost parallel to each other.
4. At the centre of the coil, the plane of magnetic field lines is at right angles to the plane of circular coil.
5. Magnetic field produced is directly proportional to the current and inversely proportional to the distance from the conductor.
6. The part from where the magnetic lines enter the coil facing us is considered as south pole and the other is north pole.
Current flowing clockwise = south
Current flowing anti clockwise = north

MAGNETIC EFFECTS OF ELECTRIC CURRENT

1. Hans Christian Oersted (1777-1851) Oersted showed that electricity and magnetism are related to each other. His research later used in radio, television etc.

The unit of magnetic field strength is name Oersted in his honour.

2. Oersted Experiment

On passing the current through the copper wire XY in the circuit, the compass needle which is placed near the conductor gets deflected. If we reverse the direction of current, the compass needle deflect in reverse direction. If we stop the flow of current, the needle comes at rest. Hence, it conclude that electricity and magnetism are linked to each other. It shows that whenever the current will flow through the conductor, then magnetic field around. it will developer

3. Magnetic Field –

It is the region surrounding a magnet, in which force of magnet can be detected. It is a vector quantity, having both direction & magnitude.

4. Compass needle–

It is a small bar magnet, whose north end is pointing towards north pole and south end is pointing towards south pole of earth.

5. Magnetic field lines–

When a bar magnet is placed on a card board and iron fillings are sprinkled, they will arrange themselves in a pattern as shown below.

The lines along which the iron filling align themselves represent magnetic field lines. Hence, magnetic field line is a path along which a hypothetical free north pole tend to move towards south pole.

The lines along which the iron filling align themselves represent magnetic field lines.
Hence, magnetic field line is a path along which a hypothetical free north pole tend to move towards south pole.

6. Characteristics of Magnetic field lines :
(1) The direction of magnetic field lines outside the magnet is always from north pole to south pole of bar magnet and are indicated by an arrow. Inside the magnetic, the direction of field lines is from its south pole to north pole
Thus magnetic field lines are closed curve
(2) The strength of magnetic field is expressed by the closeness of magnetic field lines. Closer the lines, more will be the strength and farther the lines, less will be the magnetic field strength.
(3) No two field lines will intersect each other.
If they intersects, then at point of intersection the compass needle will show two direction of magnetic field which is not possible.

The above electric circuit in which a copper is placed paralled to a compass needle, shows the deflection in needle gets reversed, when the direction of current reversed. Hence electricity and magnetism are related to each other.

8. Right Hand Thumb Rule :–
It is a convenient way of finding the direction of magnetic field associated with current carrying conductor.
Hold the straight were carrying current in your right hand such that thumb points towards the direction of current, then your folded fingers around the conductor will show the direction of magnetic field.

Electromagnetic Induction and Electric Generators Faraday’s experiment
• Faraday discovered that a magnetic field interacts with an electric circuit by inducing a voltage known as EMF (electromotive force) by electromagnetic induction.
• Moving a magnet towards a coil sets up a current in the coil circuit, as indicated by deflection in the galvanometer needle.

Electromagnetic induction
The phenomenon of electromagnetic induction is the production of induced EMF and thereby current in a coil, due to the varying magnetic
field with time. If a coil is placed near to a current-carrying conductor, the magnetic field changes due to a change in I or due to the relative motion between the coil and conductor. The direction of the induced current is given by Fleming’s right-hand rule.

Fleming’s right-hand rule
According to Fleming’s right-hand rule, the thumb, forefinger and middle finger of the right hand are stretched to be perpendicular to each other as indicated below, and if the thumb indicates the direction of the movement of conductor, fore-finger indicating direction of the magnetic field, then the middle finger indicates direction of the induced current.

Rule can be defined as :
Stretch, thumb, forefinger, and middle finger of right hand, so that they are perpendicular to each other. The forefinger indicates direction of magnetic field, thumb shows the direction of motion of conductor, then the middle finger will shows the direction of induced current.

Electric generator
• The device that converts mechanical energy into electrical energy.
• Operates on the principle of electromagnetic induction.
• AC Generation: The axle attached to the two rings is rotated so that the arms AB and CD move up and down respectively in the produced magnetic field. Thus the induced current flows through ABCD.

• After half rotation the direction of current in both arms changes. Again by applying Fleming’s right hand rule, the induced currents are established in these arms along directions DC and BA, therefore the induced I flows through DCBA.
• DC Generation: They work just like AC, instead use half rings to produce current in one direction only without variations in magnitude.

Domestic Electric Circuits Fuse
• Fuse is a protective device in an electrical circuit in times of overloading.
• Overloading is caused when the neutral and live wire come in contact due to damage to the insulation or a fault in the line.
• In times of overloading the current in circuit increases (short circuit) and becomes hazardous. Joule’s heating(resistive or ohmic heating on the passage of current) in the fuse device melts the circuit and breaks the flow of current in the circuit.

Domestic electric circuits
• Livewire has a voltage of 220 V and is covered with red insulation.
• Earth wire has a voltage of 0 V (same as Earth) and is covered with green insulation.
• The neutral wire has black insulation.
• In our houses, we receive AC electric power of 220 V with a frequency of 50 Hz.

Power loss in transmission
Power losses in transmission lines over long distances occur due to Joule’s heating. This heat (H)∝l2R losses where R is the line resistance

Advantages of Alternate Current (AC) over Direct Current (DC)

Electric power can be transmitted to longer distances without much loss of energy. Therefore cost of transmission is low.

In India the frequency of AC is 50Hz. It means after every 1/100 second it changes its direction.

11. Solenoid– A Coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called solenoid.

12. Magnetic field due to a current in a solenoid– 

– Using R.H. Thumb Rule, we can draw the pattern of magnetic field lives around a current carrying solenod.
– One end of the solenoid behaves as a magnetic north pole, white the other end behave as the South Pole.
– The filed lines inside the solenoid are in form of parallel straigh lines, that implies that magnetic field inside the solenoid is same at all points i.e. Field is uniform.

13. Electromagnet– Strong magnetic field inside the solenoid can be used to magnetise a magnetic material for example soft iron, when it is placed inside the coil. The magnet so formed is called electromagnet.

14. Force on a current carrying conductor in a magnetic field.
Andre Marie Ampere (1775-1836) suggested that the magnet also exert an equal and opposite force on the current carrying conductor. 

We will observe that the rod will displace i.e. the rod will experience a force, when it is placed in magnetic field, in a perpendicular direction to its length.
– The direction of the exert force will be reversed if the direction of current through the conductor is reversed.
– If we change the direction of field by inter changing the two poles of the magnet, again the direction of exert force will change.
– Therefore the direction of exerted force depends on
(1) direction of current
(2) direction of magnetic field lines.

15. Left Hand Fleming Rule 

16. Michael Faraday– Gave the law of Electro magnetic Induction

17. Galvanometer→ It is an instrument that can detect the presence of a current in a circuit. If pointer is at zero (the centre of scale) the there will be no flow of current.
If the pointer deflect on either side right or left, this will show the direction of current. Represented by

18. Electro Magnetic Induction – Can be explained by two experiments

(a) FIRST EXPERIMENT → “SELF INDUCTION” 

In this experiment, when the north pole of bar magnet is brought closes to the coil or away from the coil, we see momentary deflection in the needle of galvanometer on either side of null point. First right and then left.
Similarly, if we keep the magnet stationary and coil is made to move towards or away from the north pole of magnet. Again we will observe deflection in the needle of galvanometer.
If both bar magnet and coil kept stationary, there will be no deflection in galvanometer.
This experiment can also be done with the south pole of magnet, we will observe the deflection in galvanometer, but it would be in opposite direction to the previous case.
ÞIt concludes that motion of magnet with respect to coil or vice-versa, changes the magnetic field. Due to this change in magnetic field lines, potential difference is induced in the same coil, which set up an induced current in the circuit.

(b) SECOND EXPERIMENT – Mutual Induction 

In this experiment plug in the key that is connect coil with battery and observe the deflection in galvanometer. Now plug out the key that is disconnect the coil-1 from battery and observe the deflection in galvanometer, which will be in reverse direction.
Hence, we conclude that potential difference is induced in secondary coil (coil-2), whenever there is a change in current, in primary coil (coil-1) (by on and off of key).

This is because, whenever there is change in current in primary coil 
                                                        ↓
Magnetic field associated with it also changes
                                                        ↓
Now, magnetic field lines around the secondary coil (coil-2) will change and induces the electric current in it (observed by the deflectionof needle of Galvanometer in secondary circuit)

This process, by which changing of strength of current in primary coil, induces a current in secondary coil is called Electromagnetic Induction” The induced current is found to be highest when the direction of motion of coil is at right angles to the magnetic field.

19. Fleming’s Right Hand Rule 

21. Advantages of Alternate Current (AC) over Direct Current (DC)

Electric power can be transmitted to longer distances without much loss of energy. Therefore cost of transmission is low.
In India the frequency of AC is 50Hz. It means after every 1/100 second it changes its direction.

22. Domestic Electric Circuits :–

In our homes, the electric power supplied is of potential difference V = 220V and frequency 50Hz.
It consist of three wires :–
(1) Wire with red insulation cover – LIVE WIRE (POSITIVE)
Live wire is at high potential of 220V
(2) Wire with black insulation cover – NEUTRAL WIRE (NEGATIVE)
Neutral wire is at zero potential
Therefore, the potential difference between the two is 220V.
(3) Wire with Green insulation cover – EARTH WIRE
it is connected to a copper plate deep in the earth near house.
The metallic body of the appliances is connected with the earth wire as a safety measure.

Function–
Earth wire provide a low resistance to the current hence any leakage of current to the metallic body of the appliances, keep its potential equal to that of earth. That means zero potential and the user is saved from severe electric shock. 

Point to be noted in domestic circuit

(1) Each appliance has a seperate switch of ON/OFF
(2) In order to provide equal potential difference to each appliance, they should be connected parallel to each other. So that they can be operated at any time. 

24. Short Circuiting –

Due to fault in the appliances or damage in the insulation of two wires, the circuit will offer zero or negligible resistance to the flow of current. Due to low resistance, large amount of current will flow.
2 According to Joule’s law of heating effect (HaI²) heat is produced in live wire and produces spark, damaging the device and wiring.

25. Overloading–

Overloading can be caused by (1) Connecting too many appliances to a single socket or (2) accidental rise in supply voltage if the total current drawn by the appliances at a particular time exceeds the bearing capacity of that wire, it will get heated up. This is known as overloading.
Fuse a safety device can prevent the circuit from overloading and short circuiting.

Chapter Notes

Key Learnings:

1. A compass needle behaves as a small magnet. Its one end pointing towards north is called a north pole, and the other end pointing towards south, is called a south pole.

2. The space around a magnet in which the force of attraction and repulsion due to the magnet can be detected is called the magnetic field.

3. A field line is path along which a hypothetical free north pole would tend to move. The direction of the magnetic filed at a point is given by the direction that a north pole placed at that point would take. Field lines are shown closer together where the magnetic filed is greater.

4. The magnetic field lines around a straight conductor carrying current are concentric circles.

5. The direction of magnetic field is given by Right Hand Thumb Rule.

6. The magnetic field inside a solenoid is similar to that of a bar magnet.

7. A current-carrying conductor when placed in a magnetic field experiences a force.

8. Fleming’s left-hand rule gives the direction of magnetic force acting on a conductor.

9. An electromagnet consists of a core of soft iron wrapped around with a coil of insulated copper wire. 

10. An electric motor is a device that converts electric energy into mechanical energy and it works on the principle of force experienced by a current carrying conductor in a magnetic field.

11. The phenomenon in which an electric current is induced in a circuit because of a changing magnetic field is called electromagnetic induction.

12. The magnetic field may change due to a relative motion between the coil and a magnet placed near to the coil. If the coil is placed near to a current carrying conductor, the magnetic field may change either due to a change in the current through the conductor or due to the relative motion between the coil and conductor.

13. Fleming’s right hand rule is used to find the direction of induced current.

14. Electric generators are based on the principle of electromagnetic induction and converts mechanical energy into electrical energy.

15. In our houses we receive AC electric power of 220 V with a frequency of 50 Hz.

16. One of the wires in the electricity wiring of houses is with red insulation, called live wire. The other one is of black insulation, which is a neutral wire. The third is the earth wire that has green insulation and this is connected to a metallic body deep inside earth.

17. The potential difference between live wire and neutral wire is 220 V.

18. Third wire in the wiring is used as a safety measure to ensure that any leakage of current to a metallic body does not give any server shock to a user.

19. Fuse is the most important safety device used for protecting the circuits due to short circuiting or overloading of the circuits.

KEY CONCEPTS & GIST OF THE LESSON

• Magnet: (i) is an object that attracts objects made of iron, cobalt & nickel.

(ii) Comes to rest in North-South direction, when suspended freely.

• Magnets are used: (i) In radio & stereo speakers, (ii) In refrigerator doors, (iii) on audio & video cassettes players, (iv) On hard discs & floppies of computers & (v) in children‘s toys.

• Magnetic field: The area around a magnet where a magnetic force is experienced is called a magnetic field. It is a quantity that has both direction & magnitude.

• Magnetic field lines: Magnetic field is represented by field lines. They are lines drawn in a Magnetic field along which a North magnetic pole moves. Magnetic field lines are called as Magnetic lines of force.

Refer to figure 13.3 & 13.4 page no. 225 of N.C.E.R.T Text book)

• Properties of Magnetic field lines:

(i) They do not intersect each other.

(ii) It is taken by convention that magnetic field lines emerge from North pole and merge at the South pole. Inside the magnet, their direction is from South pole to North pole. Therefore magnetic field lines are closed curves.

• Magnetic field lines due to a current through a straight conductor (wire)- consist of series of concentric circles whose direction is given by the Right hand thumb rule.

• Right hand thumb rule: If a current carrying straight conductor is held in your right hand such that the thumb points towards the direction of current, then the wrapped fingers show the direction of magnetic field lines. (Refer to figure 13.7, page no. 228 of N.C.E.R.T Text book)

• Magnetic field lines due to a current through a circular loop (Refer to figure 13.8, page no. 228 of N.C.E.R.T Text book)

• The strength of the magnetic field at he centre of the loop(coil)depends on:

(i) The radius of the coil- The strength of the magnetic field is inversely proportional to the radius of the coil. If the radius increases, the magnetic strength at the centre decreases.

(ii) The number of turns in the coil: As the number of turns in the coil increase, the magnetic strength at the centre increases, because the current in each circular turn is having the same direction, thus the  field due to each turn adds up.

(iii) The strength of the current flowing in the coil: as the strength of the current increases, the strength of thee magnetic fields also increases.

• Solenoid: (Refer to figure 13.10, page no. 229 of N.C.E.R.T Text book)

• (i) A coil of many turns of insulated copper wire wrapped in the shape of a cylinder is called a Solenoid.

(ii) Magnetic field produced by a Solenoid is similar to a bar magnet.

(iii) The strength of magnetic field is proportional to the number of turns & magnitude of current.

• Electromagnet: An electromagnet consists of a long coil of insulated copper wire wrapped on a soft iron core. (Refer to figure 13.11, page no. 229 of N.C.E.R.T Text book)

• Fleming‘s Left hand rule: Stretch the thumb, forefinger and middle finger of left hand such that they are mutually perpendicular. Forefinger points in the direction of magnetic field and

centre finger in the direction of current, then the thumb gives the direction of force acting on the conductor. (Refer to figure13.13, page no. 231 13.13 of N.C.E.R.T Text book)

• Electric motor: A device that converts electric energy to mechanical energy. (Refer to figure 13.15, page no. 232 of N.C.E.R.T Text book)

• Principle of Electric motor: When a rectangular coil is placed in a magnetic field and a current is passed through it, force acts on the coil, which rotates it continuously. With the rotation of the coil, the shaft attached to it also rotates. 

• Electromagnetic induction: Electricity production as a result of magnetism (induced current) is called Electromagnetic induction.

• Fleming‘s Right hand rule: gives the direction of induced current. Stretch the thumb, forefinger and middle finger of right hand such that they are mutually perpendicular. Forefinger points in the direction of magnetic field and centre finger in the direction of induced current, then the thumb gives the direction of motion of the conductor.

• Electric generator: A devise that converts mechanical energy to electric energy. (Refer to figure 13.19, page no. 236 of N.C.E.R.T Text book) Electric generator is of two types- (i) A.C generator (ii) D. C generator

• Principle of Electric generator: Electromagnetic induction

• Domestic electric circuits: (Refer to figure 13.20, page 238 of N.C.E.R.T Text book)

• We receive electric supply through mains supported through the poles or cables. In our houses we receive AC electric power of 220V with a frequency of 50Hz.

The 3 wires are as follows- (i) Live wire- (Red insulated, Positive)

(ii) Neutral wire- (Black insulated, Negative)

(iii) Earth wire- (Green insulated) for safety measure to ensure that any leakage of current to a metallic body does not give any serious shock to a user.

• Short circuit: is caused by touching of live wires and neutral wire

• Fuse: is a protective device used for protecting the circuits from short circuiting and over loading

• Important diagrams-

1. Magnetic field lines around a bar magnet.

2. Right hand thumb rule

3. Magnetic field lines through and around a current carrying solenoid.

4. An electromagnet.

5. A simple electric motor

6. Electric generator 

• Important activities-

1. Magnetic field lines around a bar magnet

2. Direction of electric current in a simple electric circuit.

3. Direction of Magnetic field lines depends on the direction of electric current.