NCERT Exemplar Solutions Class 10 Science Magnetic Effects of Electric Current

Multiple Choice Questions...................

Question 1:  Choose the incorrect statement from the following regarding magnetic lines of the field

(a) The direction of the magnetic field at a point is taken to be the direction in which the north pole of a magnetic compass needle points

(b) Magnetic field lines are closed curves

(c) If magnetic field lines are parallel and equidistant, they represent zero-field strength

(d) The relative strength of the magnetic field is shown by the degree of closeness of the field lines 

Solution 1: (c) If magnetic field lines are parallel and equidistant, they represent zero-field strength 

Option c) is inappropriate since uniform magnetic field is represented by parallel lines of magnetic field.

Question 2:  If the key in the arrangement is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane ABCD, the lines are

(a) concentric circles

(b) elliptical in shape

(c) straight lines parallel to each other

(d) concentric circles near the point O but of elliptical shapes as we go away from it 

Solution 2:  (a) concentric circles 

As open magnetic field lines are drawn over the horizontal plane ABCD, the magnetic field lines form concentric circles, with the centre at the conductor's axis. This is because there will be no current flow, and therefore no magnetic field produced by the conductor. As a result, only the Earth's magnetic field would be present at point O, causing the magnetic field lines to form concentric circles.

Question 3: A circular loop placed in a plane perpendicular to the plane of paper carries a current when the key is ON. The current as seen from points A and B (in the plane of the paper and on the axis of the coil) is anti-clockwise and clockwise respectively. The magnetic field lines point from B to A. The N-pole of the resultant magnet is on the face close to

(a) A

(b) B

(c) A if the current is small, and B if the current is large

(d) B if the current is small and A if the current is large

Solution 3:  (a) A 

The magnetic field would be directed from the south-pole to the north-pole. Since field lines are pointing from B to A, point A represents the north-pole.

Question 4:  For a current in a long straight solenoid N- and S-poles are created at the two ends. Among the following statements, the incorrect statement is

(a) The field lines inside the solenoid are in the form of straight lines which indicates that the magnetic field is the same at all points inside the solenoid

(b) The strong magnetic field produced inside the solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the coil

(c) The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet

(d) The N- and S-poles exchange position when the direction of current through the solenoid is reversed 

Solution 4: (c) The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet 

Since the solenoid behaves like a bar magnet, the magnetic field associated with it has the same pattern as the magnetic bar around a bar magnet.

Question 5:  A uniform magnetic field exists in the plane of paper pointing from left to right as shown in given figure. In the field an electron and a proton move as shown. The electron and the proton experience

(a) forces both pointing into the plane of paper

(b) forces both pointing out of the plane of paper

(c) forces pointing into the plane of paper and out of the plane of paper, respectively

(d) force pointing opposite and along the direction of the uniform magnetic field respectively

Solution 5: (a) forces both pointing into the plane of the paper 

The direction of electron movement is the inverse of the direction of electric current. This would accelerate the present upward trend. If the index finger represents the magnetic field, the ring finger represents the current and the orientation of the thumb into the paper.

Question 6: Commercial electric motors do not use

(a) an electromagnet to rotate the armature

(b) effectively large number of turns of conducting wire in the current-carrying coil

(c) a permanent magnet to rotate the armature

(d) a soft iron core on which the coil is wound 

Solution 6:  (c) a permanent magnet to rotate the armature

Electromagnets are used instead of permanent magnets in electric motors.

Question 7:  In the arrangement shown in Figure 13.4, there are two coils wound on a non-conducting cylindrical rod. Initially, the key is not inserted. Then the key is inserted and later removed. Then

(a) the deflection in the galvanometer remains zero throughout

(b) there is a momentary deflection in the galvanometer but it dies out shortly and there is no effect when the key is removed

(c) there are momentary galvanometer deflections that die out shortly; the deflections are in the same direction

(d) there are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions

Solution 7:  (d) there are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions 

When the key is inserted, the galvanometer displays a deflection in one direction, which reverses when the key is removed.

Question 8:  Choose the incorrect statement

(a) Fleming’s right-hand rule is a simple rule to know the direction of induced current

(b) The right-hand thumb rule is used to find the direction of magnetic fields due to current-carrying conductors

(c) 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

(d) In India, the AC changes direction after every 1/50 second  

Solution 8:  (d) In India, the AC changes direction after every  1/50 second 

In India, the AC frequency is 50 Hz. In each loop, the path shifts twice, resulting in a change of direction every  1/50 second.

Solution 9:  A constant current flows in a horizontal wire in the plane of the paper from east to west as shown in figure below. The direction of the magnetic field at a point will be North to South

(a) directly above the wire

(b) directly below the wire

(c) at a point located in the plane of the paper, on the north side of the wire

(d) at a point located in the plane of the paper, on the south side of the wire

Solution 9:  (b) directly below the wire

We may determine the position of the magnetic field underneath the wire by using the right-hand thumb law.

Question 10: The strength of the magnetic field inside a long current carrying straight solenoid is

(a) more at the ends than at the centre

(b) minimum in the middle

(c) same at all points

(d) found to increase from one end to the other 

Solution 10: (c) same at all points

Magnetic field lines within the solenoid are straight. This shows the presence of a solid magnetic field. As a result, the magnetic field within the solenoid is consistent.

Question 11:  To convert an AC generator into DC generator

(a) split-ring type commutator must be used

(b) slip rings and brushes must be used

(c) a stronger magnetic field has to be used

(d) a rectangular wire loop has to be used 

Solution 11: (a) split-ring type commutator must be used

After each half-turn of the armature, a split-ring style commutator reverses the current direction. This keeps a DC current flowing.

Question 12:  The most important safety method used for protecting home appliances from short-circuiting or overloading is

(a) earthing

(b) use of fuse

(c) use of stabilizers

(d) use of electric meter 

Solution 12:  (b) use of fuse 

Fuse has a thin, short-length wires made of tin and lead in a 75:25 percent ratio. When the current reaches the set limit, the fuse melts and the circuits are broken, protecting the home appliances.

Short Answer Questions....................

Question 13:  A magnetic compass needle is placed in the plane of paper near point A as shown in Figure 13.6. In which plane should a straight current-carrying conductor be placed so that it passes through A and there is no change in the deflection of the compass? Under what condition is the deflection maximum and why? 

Solution 13:   Inside the plane of the paper. The compass's axis is vertical, and the conductor's area is also vertical. It could cause a compass needle to dip, which is not likely in this situation (dips result only if the axis of the compass is horizontal).

When the conductor through A is perpendicular to the plane of the paper, the deflection is greatest, and the field due to it is greatest in the plane of the paper.

Question 14:  Under what conditions permanent electromagnet is obtained if a current-carrying solenoid is used? Support your answer with the help of a labelled circuit diagram. 

Solution 14:   The following conditions must be met in order to produce a permanent electromagnet from a current-carrying solenoid.

(a) Closed-circuit

(b) Core of soft iron

Question 15:  AB is a current-carrying conductor in the plane of the paper as shown in figure. What are the directions of magnetic fields produced by it at points P and Q? Given r1 > r2, where will the strength of the magnetic field be larger?

Solution 15:  At P, I'm in the plane of the paper, and at Q, I'm out of it. The magnetic field strength is stronger at the closer stage, i.e. at Q.

Question 16:  A magnetic compass shows a deflection when placed near a current-carrying wire. How will the deflection of the compass get affected if the current in the wire is increased? Support your answer with a reason. 

Solution 16:   When the angle of deflection increases, so does the amount of deflection. The magnitude of current flowing through the straight conductor will determine the intensity of the magnetic field.

Question 17:  It is established that an electric current through a metallic conductor produces a magnetic field around it. Is there a similar magnetic field produced around a thin beam of moving

(i) alpha particles, (ii) neutrons? Justify your answer. 

Solution 17:

i) Since alpha particles are positively charged, they form the current in the direction of motion.

ii) Since neutrons have no energy, they do not conduct electricity.

Question 18:  What does the direction of thumb indicate in the right-hand thumb rule? In what way this rule is different from Fleming’s left-hand rule? 

Solution 18:  In the right-hand law, the force encountered by a current-carrying conductor placed in an external magnetic field is indicated by the thumb held by curled fingers, while in the left-hand rule, the force experienced by a current-carrying conductor placed in an external magnetic field is indicated by the thumb held by curled fingers.

Question 19:  Meena draws magnetic field lines of the field close to the axis of a current-carrying circular loop. As she moves away from the centre of the circular loop she observes that the lines keep on diverging. How will you explain her observation? 

Solution 19:  The magnetic field's intensity decreases as the distance between them decreases. It is indicated by the decrease in the degree of closeness of the lines of ground.

Question 20:  What does the divergence of magnetic field lines near the ends of a current-carrying straight solenoid indicate?  

Solution 20:  Divergence of magnetic field lines at the ends of a current-carrying straight solenoid suggests a decrease in magnetic field strength near and beyond the solenoid's ends.

Question 21:  Name four appliances wherein an electric motor, a rotating device that converts electrical energy to mechanical energy, is used as an important component. In what respect motors are different from generators? 

Solution 21:  An electric motor, a spinning device that transforms electrical energy to mechanical energy, is used as an important component in electric fans, mixers, washing machines, and computer drives, to name a few.

Electrical energy is converted into mechanical energy by motors, while mechanical energy is converted into electrical energy by generators.

Question 22:  What is the role of the two conducting stationary brushes in a simple electric motor? 

Solution 22:  As they meet the outer sides of the two halves of the split rings, the two conducting stationary brushes draw current from the battery and supply it to the armature of the motor. The inner sides of the split rings are insulated and connected to the motor's axle.

Question 23:  What is the difference between a direct current and an alternating current? How many times does AC used in India change direction in one second? 

Solution 23:   The direction of current in an AC current will change continuously, while the direction of current in a DC current will not change. In India, the AC changes direction 100 times a second.

Question 24:  What is the role of fuse, used in series with any electrical appliance? Why should a fuse with defined rating not be replaced by one with a larger rating? 

Solution 24:  Fuse has a thin, short-length wire made of tin and lead in a 75:25 ratio. When the current reaches the set limit, the fuse melts and the circuits are broken, protecting the home appliances. When a fuse is replaced with one with a higher rating, the appliances can be harmed while the protective fuse remains lit. This method of using a fuse with an incorrect rating should be avoided at all costs. 

Long Answer Questions.........................

Question 25:  Why does a magnetic compass needle pointing North and South in the absence of a nearby magnet get deflected when a bar magnet or a current-carrying loop is brought near it. Describe some salient features of magnetic lines of field concept. 

Solution 25:   Current carrying loops behave like bar magnets with corresponding field lines. This causes a deflection of the current earth's magnetic field. The magnetic field has both a magnitude and a direction. Magnetic field lines travel southward from the north pole. The degree of closeness of the field lines represents the magnetic field intensity diagrammatically. Since field lines cannot overlap, two net field values cannot exist as a single point. There can only be one value, a unique net value. If the lines of field in a given region are parallel and evenly spaced, the field is said to be uniform.

Question 26:  With the help of a labelled circuit diagram illustrating the pattern of field lines of the magnetic field around a current-carrying straight long conducting wire. How is the right-hand thumb rule useful to find the direction of the magnetic field associated with a current-carrying conductor? 

Solution 26:  According to the right-hand thumb law, if a current-carrying straight conductor is placed in the right hand with the thumb pointing in the direction of current, the fingers will wrap around the conductor in the direction of the magnetic field lines.

Question 27:  Explain with the help of a labelled diagram the distribution of magnetic field due to a current through a circular loop. Why is it that if a current-carrying coil has n turns the field produced at any point is n times as large as that produced by a single turn? 

Solution 27:   Even in a current-carrying loop, the right hand thumb rule is followed. Magnetic field lines can be seen all the way around the conducting wire in this picture. However, since the conductor is circular, field lines at various points of the loop tend to form a ring around the loop's circumference. This appears to be a circular ring looping around the outside of a larger ring.

When the number of coils is increased, the magnetic field increases. As a result, the magnetic field strength increases as the number of turns in a coil increases.

Question 28:  Describe the activity that shows that a current-carrying conductor experiences a force perpendicular to its length and the external magnetic field. How does Fleming’s left-hand rule help us to find the direction of the force acting on the current-carrying conductor? 

Solution 28:   Take a small aluminium rod and write AB on it (of about 5 cm). Suspend it horizontally from a stand using two connecting wires, as seen in the diagram below.

Place a solid horseshoe magnet between the two poles with the magnetic field pointing upwards. Place the north-pole of the magnet vertically below the aluminium rod and the south-pole vertically above it for this.

Connect the aluminium rod to a battery, a key, and a rheostat in series.

Now, from end B to end A, run a current through the aluminium rod.

The rod seems to be shifted to the left. The rod will be displaced, as you will remember.

Reverse the direction of the current flowing through the rod and note the displacement in the opposite direction. It's now moving to the right.

Stretch the left hand's thumb, forefinger, and central finger perpendicular to each other, according to Fleming's left-hand law. The thumb would point in the direction of motion or force acting on the conductor if the forefinger points in the direction of magnetic field and the central in the direction of current.

Question 29:  Draw a labelled circuit diagram of a simple electric motor and explain its working. In what way these simple electric motors are different from commercial motors? 

Solution 29:   Between the two poles of a permanent magnet is a coil known as a BCD. The coil is positioned such that the direction of the current is perpendicular to the magnetic field's direction.

• The coil's arms are connected to a split loop, which is divided into two parts, P and Q. Arms AB and CD are connected to the half P and half Q, respectively.

• The split rings are connected to two static brushes, X and Y, from which they receive power. From the inside, the broken rings are insulated. The split rings are being passed around by an axle.

• The coil is powered by an electric current supplied by a battery. Present flows from point A to point B, and from point C to point D.

An electric motor is made up of a rectangular coil ABCD of insulated copper wire, as shown in Figure. The coil is positioned between the two poles of a magnetic field, with the arms AB and CD perpendicular to the magnetic field's path. The coil's ends are bound to the split ring's two halves, P and Q. These halves' inner sides are insulated and connected to an axle. P and Q's external conducting edges make contact with two stationary conducting brushes, X and Y, respectively. Current flows from the source battery to the coil ABCD by conducting brush X and back to the source battery via brush Y. It's worth noting that the current in the coil's arm AB flows from A to B. It flows from C to D in arm CD, which is the polar opposite of current flow through arm AB. 

Fleming's Left-Hand Rule is being followed. As current flows from point A to point B, the coil's arm AB goes down. As current passes from C to D, on the other hand, the arm CD goes up. 

When the coil has completed a half-turn. The split ring's P brushes brush Y, while the split ring's Q does the reverse. As a consequence, the current's course is reversed. As a result, current is now flowing from D to C and from B to A. CD is pushed down and AB is pushed up as a result of this. As a result, the coil continues to spin. 

In simple electric motor permanent magnet is being used while in commercial electric motor electromagnet is used.

In simple electric motor iron core is not used while soft iron core is used in commercial electric motor.

Turn number in coil is less in simple motor whereas Turn number in coil is large in commercial electric motor.

Question 30:  Explain the phenomenon of electromagnetic induction. Describe an experiment to show that a current is set up in a closed loop when an external magnetic field passing through the loop increases or decreases. 

Solution 30:  Electromagnetic induction is the phenomenon of altering the magnetic field to generate an electric current in a closed circuit. Induced current is the electric current caused by this phenomenon, and induced emf is the potential difference set in the circuit as a result of induced current.

Experiment:

• Take two copper wire coils with a large number of turns each (say 50 and 100 turns respectively). Over a non-conducting cylindrical roll, position them. (For this, you should use a thick paper roll.)
• Attach the coil-1, which has a greater number of turns, to a battery and a plug key in sequence. Also, as shown, attach the other coil-2 to a galvanometer.
• Insert the key. Keep an eye on the galvanometer. Is the needle deflected in some way? You'll note that the galvanometer's needle hops to one side and then quickly returns to zero, showing a momentary current in coil-2.
• Unplug coil-1 from the power source. The needle will pass for a brief moment, but to the opposite hand. It implies that the current in coil-2 is now flowing in the opposite direction.2.

Enable a magnetic field to be applied to the conductor coil ABCD. Magnetic flux can flow from the N pole to the S pole. The coil is connected to slip rings, and the load is connected to the slip rings through brushes that rest on them.

Consider Case 1 in the diagram above. Since the coil is spinning clockwise, the direction of induced current can be determined using Fleming's right-hand law, which is A-B-C-D.

The location of the coil would be as in the second case of the above figure after half of the time span, as the coil rotates clockwise. The path of the induced current will be along D-C-B-A in this case, according to Fleming's right-hand law. It shows that after half of the time span, the current's path shifts, indicating that we have an alternating current.

Question 31:  Describe the working of an AC generator with the help of a labelled circuit diagram. What changes must be made in the arrangement to convert it to a DC generator? 

Solution 31:   When a conductor travels in a magnetic field, Faraday's law electromagnetic induction states that an EMF is induced through the conductor. Induced emf allows current to flow in the circuit if a near path is given to the conductor. 

Enable a magnetic field to be applied to the conductor coil ABCD. Magnetic flux can flow from the N pole to the S pole. The coil is connected to slip rings, and the load is connected to the slip rings through brushes that rest on them.

Consider Case 1 in the diagram above. Since the coil is spinning clockwise, the direction of induced current can be determined using Fleming's right-hand law, which is A-B-C-D.

The location of the coil would be as in the second case of the above figure after half of the time span, as the coil rotates clockwise. The path of the induced current will be along D-C-B-A in this case, according to Fleming's right-hand law. It shows that after half of the time span, the current's path shifts, indicating that we have an alternating current. 

Question 32:  Draw an appropriate schematic diagram showing common domestic circuits and discuss the importance of fuse. Why is it that a burnt-out fuse should be replaced by another fuse of identical rating? 

Solution 32:

Importance of Fuse:

• A fuse is a protective device used in electrical appliances and household wiring.

• A fuse protects the circuit and the appliance from damage caused by an overload.

• A fuse protects against an unintentional fire caused by a short circuit. 

A fuse with a specific rating is used with a specific appliance. Because of the constant need to change the fuse, a fuse with a lower rating would trigger a lot of inconvenience. Even if the current reaches the device's cap, a fuse with a higher rating does not serve the function because it would not melt. As a result, a burnt-out fuse should be replaced with a fuse of the same grade.