CBSE Class 12 Physics VBQs Effect Of Current and Magnetism

Question 1. Mr Narasimhan a 65 year old person often complained of neck pain. One day his grandson Avinash, suggested that magnetic therapy is very effective in reducing such pains. He said that the permanent magnet/electromagnet , used in the device will help to produce Joule’s heating effects in the blood stream, which helps the blood flow better.He immediately contacted his friend in Chennai, who was running Magnetic Therapy Clinic. Mr Narasimhan who felt better.

(i) What two values did Avinash exhibit towards his grandfather? Mention any two

(ii) What is the SI unit of magnetic induction and define it?

Answer:

(i) Ans. Responsible behaviour, concern and awareness

(ii) Tesla

Question 2. Ms Udaya joined a PG course in Nanotechnology lab in IIT Chennai. The first day, when she went to the lab, she met Mr. Antonio, the lab assistant.He greeted her and advised her not to touch the wires which were suspended from the roof at every part of the lab as they were from high voltage lines. He also told her not to bring any of the two wires closer to each other during any experimental applications. He helped her in understanding about the precautions that has to be taken in the lab.

1. What value did Mr. Antonio exhibit towards Ms. Udaya? Mention any two

2.Why two high voltage power transmission lines should not be close to each other?

3.Give an expression for the magnetic force that acts between the wires?

Answer: 

Responsible behaviour, sensitivity, concern for others and alerting the people)

Question 3. In the birthday party of Bharat, a class 7student, his parents gave big slinkys to all his friends as return gifts. The next day, during the physics class Mr Mohan, the teacher explained them about the production of magnetic fields using current carrying coil and also said that they can make permanent magnets, using such coils by passing high currents through them. That night Sumanth, a friend of Bharat, asked his father about the coils, and their shape. His father asked him to bring the slinky, that his friend gave and explained the uses of toroid and solenoid.

(i) What value did Sumanth’s father exhibit towards his son?

(ii) What is the difference in the fields produced by the solenoid and Toroid ?

Answer: 

(i) Responsibility, makes his child to understand the concepts and to generate interest in the subjects

(ii) The magnetic field lines in a toroid is concentric circles whereas in solenoid it is straight within the turns

Question 4. Ms Nita chander found that her son could not hear properly. The specialist prescribed hearing aid for her son. Hearing aids consist of electromagnets in the loudspeakers used in the device.

(i) What two values does Ms Nita exhibit towards her son and students? Mention any two

(ii)What is an electromagnet? In what way its hysteresis curve is differen from that used for permanent magnets?

Answer:

(i) caring attitude, sensitive towards society, concern for others

(ii) Electromagnet- temporary magnet.Hysteresis curve has small are,small coercivity, small retentivity.

Question 5. Sartaj’s mother had put lot of clothes for washing in the washing machine, but the machine did not start and an indicator was showing that the lid did not close. Sartaj seeing his mother troubled tried closing the lid by force but realized that the mechanism was different. It was a magnetic system. In his effort he damaged the lid of the machine.

(i) What was the value developed by Sartaj?

(ii) What values did his mother impart to Sartaj?

(iii) A magnetic needle in a uniform magnetic field experiences a torque but not a force, why?

Answer:

(i) Sympathy, responsibility, helping nature, self reliance

(ii) Appreciation, thankfulness

(iii) The forces acting on both the poles are equal and opposite, so net force is zero but their lines of action are different so torque is not zero.

Question 6. Shubham once watching a TV channel saw that certain organisms have the ability to sense the field lines of earth’s magnetic field. By following these lines, they can travel from one place to another. Shubham in his quest to know more about earth’s magnetic field, brought a magnetic compass. By knowing the direction in which the compass comes to rest, he found the magnetic meridian. Then he fixed the compass on a card board and placed it vertically along the magnetic meridian. By measuring the inclination of the compass with the horizontal, he found the angle of dip at the given place.

(i) What value did Shubham have?

(ii) In which direction would a compass free to move in the vertical plane point to, if located right on the geomagnetic north or south pole?

(iii) Which device is used to find angle of dip?

Answer: 

(i) Nature of appreciation, Curiosity, Diligence, Self-reliance, Creative skill

(ii) At magnetic poles, earth’s magnetic field is vertical. So the compass points in any direction.

(iii) Dip circle

Question 7. From ancient times to present, human have sought healing power of magnetism (magneto therapy) claims that it can heal bunions eliminate tennis elbow and cure cancer have been made but none have been substantiated.
(i) How can magnetism help increase in effectiveness and reduce the side effect of drus?

(ii) What is hyperthermic technique?

(iii) How does magnetic field help to measure the speed of the blood?

(iv) Blood contains ions. Magnetic field can deflect positive and negative ions oppositely and the potential difference so created can measure the speed of blood

Answer: (i) Certain bacteria create nano permanent magnets. These magnets could be attached to drug molecules.

(ii) It is magnetically induced heating therapy.

Question 8. Geeta’s uncle was advised by his doctor to undergo an MRI scan test of his chest and gave him an estimate of the cost. Not knowing much about the significance of this test and finding it to be too expensive he first hesitated. When Geeta learnt about this, she decided to take help of her family, friends and neighbours and arranged for the cost. She convinced her uncle to undergo this test so as to enable the doctor to diagnose the disease. He got the test done and the resulting information greatly helped the doctor to give him proper treatment.

(i) What according to you, are the values displayed by Geeta, her family, friends and neighbours?

(ii) Assuming that the MRI scan test involved a magnetic field of 0.1 T, find the maximum and minimum values of the force that this field could exert on a proton moving with a speed of 104 m/s. State the condition under which the force can be minimum?

Answer: (i) prompt decision making, empathy, helping attitude, caring nature

(ii) maximum force F= qvB sin 900
=1.6 ×10^-16 N
minimum force F=0

Question 9. Alka and her sister were watching a movie in which the phenomenon of aurora borealis was shown. Alka could not believe her eyes that such a colorful display like the one during commonwealth games could be created by nature. She went to the library, but could not find the right book. So she consulted her teacher who guided her. Hence, Alka understood that during a solar flare, a large number of electrons and protons are ejected from the sun. Some of these get trapped in the earth’s magnetic field and move in a helical path along the field lines. As the density of the field lines increases near the poles ,these particles collide with atoms and molecules of the atmosphere emitting green and pink light. Alka shared this knowledge with her class when they studied the chapter of moving charges in magnetic field.

(i) What values did Alka have?

(ii) What is the radius of the path of an electron moving at a speed of 3 x 107 m / sec in a magnetic field of 6 Gauss perpendicular to it?

Answer: (i) Nature of appreciation, diligence, curiosity, research mindedness, communicative skills,

(ii) Using the formula r = mv/qB we get r= 0.29 m

Question 10. Anil was using a galvanometer in the practical class. Unfortunately it fell from his hand and broke. He was upset, some of his friends advised him not to tell the teacher but Anil decided to tell his teacher. Teacher listened to him patiently and on knowing that the act was not intentional, but just an accident, did not scold him and used the opportunity to show the internal structure of galvanometer to the whole class.

(i) What are the values displayed by Anil.?

(ii) Explain the principle of moving coil galvanometer.

Answer: (i) Courage to tell the truth, Gratitude to his teacher for her patience and tolerance

(ii) The current carrying coil kept in a magnetic field always experiences a torque.

Value Based questions 

Question 1. Geeta’s uncle was advised by his doctor to undergo an MRI scan test of his chest and gave him and estimate of the cost. Not knowing much about the significance of this test and finding it to be too expensive he first hesitated. When Geeta learnt about this, she decided to take help of her family, friends and neighbours and arranged for the cost. She convinced her uncle to undergo this test so as to enable the doctor to diagnose the disease. He got the test done and the resulting information greatly helped the doctor to give him proper treatment.

(i) What according to you, are the values displayed by Geeta, her family, friends and neighbor’s?

Answer:(i) prompt decision making, empathy, helping attitude, caring nature

(ii) Assuming that the MRI scan test involved a magnetic field of 0.1 T, find the maximum and minimum values of the force that this field could exert on a proton moving with a speed of 10⁴ m/s. State the condition under which the force can be minimum?

(ii) maximum force F= qvB sin 90 ̊ F =0.1X 10⁴X 1.6 X 10^‐19 N minimum force F= 0

Question 2. Alka and her sister were watching a movie in which the phenomenon of aurora borealis was shown. Alka could not believe her eyes that such a colorful display like the one during commonwealth games could be created by nature. She went to the library, but could not find the right book. So she consulted her teacher who guided her. Hence, Alka understood that during a solar flare, a large number of electrons and protons are ejected from the sun. Some of these get trapped in the earth’s magnetic field and move in a helical path along the field lines. As the density of the field lines increases near the poles ,these particles collide with atoms and molecules of the atmosphere emitting green and pink light. Alka shared this knowledge with her class when they studied the chapter of moving charges in magnetic field.

(i) What values did Alka have?

Answer: Nature of appreciation, diligence, curiosity, research mindedness, communicative skills,

(ii) What is the radius of the path of an electron moving at a speed of 3 x 10⁷ m / sec in a magnetic field of 6 Gauss perpendicular to it?

Answer: Using the formula r = mv/qB we get r= 0.29 m

Question 3. In the birthday party of Bharat, a class 7student, and his parents gave big Slinky’s to all his friends as return gifts. The next day, during the physics class Mr. Mohan, the teacher explained them about the production of magnetic fields using current carrying coil and also said that they can make permanent magnets, using such coils by passing high currents through them. That night Sumanth, a friend of Bharat, asked his father about the coils, and their shape. His father asked him to bring the slinky, that his friend gave and explained the uses of toroid and solenoid.

1. What value did Sumanth’s father exhibit towards his son?

Answer: (Responsibility, makes his child to understand the concepts and to generate interest in the subjects)

2.What is the difference in the fields produced by the solenoid and Toroid?

Answer: The magnetic field lines in a toroid is concentric circles whereas in solenoid it is straight within the turns

Question 4. MsSumathy wife of MrVaradan complained about the non availability of gas cylinders and explained to him to look out for alternate methods for cooking. MrVaradan bought an induction stove to overcome the fuel problem. The next day Sumathy used her copper bottom cooker and kept it on the induction stove. But even after using it for half an hour she found that the cooker was not hot and food not cooked. As she was not aware of the method to use the induction stove, she asked her elder daughter Dhanya, studying first year engineering about it. She told her, that some vessels can not be used on this stove. She took the instruction manual and explained to her mother, that the stove works on magnetic induction, and copper being adia magnetic material, will not respond to it.

1. What values did Mrvaradan and Dhanya exhibit towards MsSumathy? Mention any two

Answer: Awareness, concern for conservation of energy and fossil fuels, sharing the knowledge)

2.Give few examples of diamagnetic materials and explain how their suceptiblity varies with temperature?

Answer: Susceptibility is independent of temperature as they have no permanent dipoles e.g mercury, water, alcohol and all inert gases.

TWO MARKS QUESTIONS

Question. The horizontal component of the earth’s magnetic field at a place equals to its vertical component there. Find the value of the angle of dip at that place. what is the ratio of the horizontal component to the total magnetic field of the earth at that place?
Answer : BH / B = 1/√2
And cos δ = BH /B
Cos δ = 1/√2
i.e. Angle of dip ,δ = 45°

Question. Draw magnetic field lines when a (1) diamagnetic, (2) paramagnetic substance is placed in an external magnetic field. Which magnetic property distinguished this behavior of the field lines due to the two substances?
Answer : (i) Behaviour of magnetic field; lines when diamagnetic substance is placed in an external field. (Image 23)
(i) Behaviour of magnetic field; lines when diamagnetic substance is placed in an external field.

Question. State briefly an efficiency way of making a paramagnet magnet. Write two properties to select suitable materials for making permanent magnets.
Answer : Permanent magnet can be made by putting a steel rod inside the solenoid and a strong current is allowed to pass through solenoid. The strong magnetic field inside the solenoid magnetise the rod.
For making permanent magnets, the material used should have (I) high residual magnetism, and (II) high coercitivity.

Question. The following figure shows the variation of intensity of magnetisation versus the applied magnetic field intensity H for two magnetic materials A and B,
(1) Identify the material A and B.
(2) Why does the material B have a large susceptibility than A for a given field at constant temperature?
Answer : Paramagnetic Substance : Aluminium , Sodium
Diamagnetic Substance : Bismuth , Copper , the susceptibility of the diamagnetic materials is small and negative , i.e. -1 <ᵡm < 0, whereas for paramagnetic substance the susceptibility is small and positive, i.e. 0 < χm < a, where ‘ a’ is a small number.

Question. The following fig shows the variation of intensity of magnetisation versus the applied magnetic field intensity H for two magnetic materials A and B.
(1) Identify the material A and B.
(2) Draw the variation of susceptibility χm with temperature for B.
Answer : Material A : Paramagnetic substance (χm is small and positive )
Material B : Diamagnetic substance (χm is small but negative)
The susceptibility of a diamagnetic substance is independent of temperature and megnetising field.
This is the graph for variation of χm with temperature ( T ) of diamagnetic substance for material B.

Question. The following Fig shows the variation of intensity of magnetisation versus the applied magnetic field intensity H for two magnetic materials A and B .
Identify the material A and B.
For the material A, plot the variation of intensity of magnetisation.
Answer : (1) Material A is paramagnetic and B is ferromagnetic.
(2)The magnetic susceptibility (χm) of paramagnetic substance (i.e. material A) varies inversely with the absolute temperature.

Question. Define magnetic susceptibility of a material. Name two element, one having positive susceptibility and other having negative susceptibility. What does negative susceptibility signify?
Answer : Magnetic susceptibility The magnetic susecptibility (ᵡm) of a magnetic material is equal to the ratio of intensity of magnetisation and magnetising field i.e.
χm = I/H
Where,I = intensity of magnetisation
H =magnetising field
It has no unit.
Positive magnetic susceptibility possesses by paramagnetic substances, e.g. aluminium, sodium.
Negative magnetic susceptibility possesses by diamagnetic substance, e.g. bismuth, copper.
Negative susceptibility shows that substances gets magnetised in a direction opposite to the direction of magnetising field.

Question. Draw a plot showing the variation of intensity of magnetisation with the applied magnetic field intensity for bismuth. Under what condition a diamagnetic material exhibits perfect conductivity and perfect diamagnetism?
Answer : Bismuth, diamagnetic material gets feebly intensity of magnetisation with the applied magnetic field intensity in a direction opposite to it. (Image 25)
The diamagnetic material exhibits perfect conductivity (superconductivity) and perfect diamagnetism when metal is cooled below the critical temperature of the material (Meissner effect)

THREE MARKS QUESTIONS

Question. Two straight long parallel conductors carry currents I 1 and I 2 in the same direction. Deduce the expression for the force per unit length between them. Depict the pattern of magnetic field lines around them.
Answer : 

Question. Deduce the expression for the magnetic dipole moment of an electron orbiting around the central nucleus.
Answer :

Question. Using Ampere’s circuital law, obtain the expression for the magnetic field due to a long solenoid at a point inside the solenoid on its axis.
Answer : A solenoid is a long insulated copper wire wound in the form of a coil. If the solenoid is long 

Question. (a) Using Biot-Savart’s law, derive the expression for the magnetic field in the vector form at a point on the axis of a circular current loop.
Answer :

Question. a) Why is the magnetic field radial in a moving coil galvanometer? Explain how it is achieved.
(b) A galvanometer of resistance ‘G’ can be converted into a voltmeter of range (0-V) volts by connecting a resistance ‘R’ in series with it. How much resistance will be required to change its range from 0 to V/2 ?
Answer : (a) Need for a radial magnetic field:
The relation between the current (i) flowing through the galvanometer coil, and the angular deflection () of the coil (from its equilibrium position), is
∅=((NABI.sinθ)/k)
where θ is the angle between the magnetic field B and the equivalent magnetic moment µ _m of the current carrying coil.
Thus I is not directly proportional Φ to . We can ensure this proportionality by having θ=90⁰. This is possible only when the magnetic field, B , is a radial magnetic field. In such a field, the plane of the rotating coil is always parallel to B .
To get a radial magnetic field, the pole pieces of the magnet, are made concave in shape. Also a soft iron cylinder is used as the core. [Alternatively : Accept if the candidate draws the following diagram to achieve the radial magnetic field.

Question. Magnetic effect of current was discovered by
(a) Oersted
(b) Faraday
(c) Bohr
(d) Ampere
Answer: A

Question. Inside the magnet, the field lines moves
(a) from north to south
(b) from south the north
(c) away from south pole
(d) away from north pole
Answer: A

Question. Relative strength of magnetic field at a point in the space surrounding the magnet is shown by the
(a) length of magnet
(b) thickness of magnet
(c) degree of closeness of the field.
(d) resistance offered by the surroundings
Answer: A

Question. Which of the following statement is not correct about the magnetic field?
(a) Magnetic field lines form a continuous closed curve.
(b) Magnetic field line do not interest each other.
(c) Direction of tangent at any point on the magnetic field line curve gives the direction of magnetic field at that point.
(d) Outside the magnet, magnetic field lines go from South to North pole of the magnet.
Answer: D

Question. By which instrument, the presence of magnetic field be determined?
(a) Magnetic Needle
(b) Ammeter
(c) Galvanometer
(d) Voltmeter
Answer: D

Question. The pattern of the magnetic field produced by the straight current carrying conducting wire is
(a) in the direction opposite to the current
(b) in the direction parallel to the wire
(c) circular around the wire
(d) in the same direction of current
Answer: A

Question. The strength of magnetic field around a current carrying conductor is
(a) inversely proportional to the current but directly proportional to the square of the distance from wire.
(b) directly proportional to the current and inversely proportional to the distance from wire.
(c) directly proportional to the distance and inversely proportional to the current
(d) directly proportional to the current but inversely proportional the square of the distance from wire.
Answer: B

Question. A current through a horizontal power line flows from south to North direction. The direction of magnetic field line 0.5m above it is
(a) North
(b) South
(c) West
(d) East
Answer: A

Question. The nature of magnetic field line passing through the centre of current carrying circular loop is
(a) circular
(b) ellipse
(c) parabolic
(d) straight line
Answer: D

Question. The strength of each of magnet reduces to half when it cut along its length into the equal parts magnetic field strength of a solenoid. Polarity of solenoid can be determined by
(a) use of compass needle
(b) Right hand thumb rule
(c) Fleming left hand rule
(d) either (a) or (b)
Answer: D

Question. The factors on which one magnetic field strength produced by current carrying solenoids depends are
(a) Magnitude of current
(b) Number of turns
(c) Nature of core material
(d) All of the above
Answer: D

Question. A soft iron bar is introduced inside the current carrying solenoid. The magnetic field inside the solenoid
(a) will decrease
(b) will remains sam
(c) will increase
(d) will become zero
Answer: C

Question. Which one of the following is a temporary magnet
(a) Bar magnet
(b) Horse shoe magnet
(c) Electromagnet
(d) None of these
Answer: C

Question. What is the direction of magnetic field lines inside a solenoid?
(a) North to north
(b) South to north
(c) North to south
(d) East to south
Answer: B

Question. The size of concentric circles of magnetic field lines increases with distance from the conductor
(a) which shows that magnetic field decreases with distance.
(b) which shows that magnetic field increases with distance.
(c) which shows that magnetic field decreases with direction.
(d) which shows that magnetic field increase s with direction.
Answer: A

Question. MrNarasimhan a 65 year old person often complained of neck pain. One day his grandson Avinash, suggested that magnetic therapy is very effective in reducing such pains. He said that the permanent magnet/electromagnet, used in the device will help to produce Joule’s heating effects in the blood stream, which helps the blood flow better. He immediately contacted his friend in Chennai, who was running Magnetic Therapy Clinic. Mr. Narasimhan felt better.
a. What two values did Avinash exhibit towards his grandfather? Mention any two
Answer: Responsible behavior, concern and awareness

b. What is the SI unit of magnetic induction and define it?
Answer: Tesla (defn) 

Question. Ms Nita chander found that her son could not hear properly. Mother took her to the specialist who prescribed hearing aid for her son. Hearing aids consist of electromagnets in the loudspeakers used in the device.
a. What two values does Ms Nita exhibit towards her son and students? Mention any two (caring attitude, sensitive towards society, concern for others
c. What is an electromagnet? In what way its hysteresis curve is different from that used for permanent magnets?
nswer: Electromagnet- temporary magnet.Hysteresis curve has small area, small coercivity, small retentivity. 

Question. MsSumathy wife of Mr.Varadan complained about the non availability of gas cylinders and explained to him to look out for alternate methods for cooking. Mr.Varadan bought an induction stove to overcome the fuel problem. The next day Sumathy used her copper bottom cooker and kept it on the induction stove. But even after using it for half an hour she found that the cooker was not hot and food not cooked. As she was not aware of themethod to use the induction stove, she asked her elder daughter Dhanya, studying first year engineering about it. She told her, that some vessels cannot be used on this stove. She took the instruction manual and explained to her mother, that the stove works on magnetic induction, and copper being a dia magnetic material, will not respond to it.
a. What values did Mr.varadan and Dhanya exhibit towards Ms.Sumathy? Mention any two (Awareness, concern for conservation of energy and fossil fuels, sharing the knowledge)
b. Give few examples of diamagnetic materials and explain how their susceptibility varies with temperature?
Answer: Susceptibility is independent of temperature as they have no permanent dipoles. 

Question. Bala and Rama class X students were assigned a project based on magnetism. In their project work, they had calculated the value of earth’s magnetic field. When they submitted their project for verification. MrSantosh, their physics teacher, corrected the mistakes. He also suggested few books which could be of use to them.
a. What values did Mr.Santosh exhibit towards his students? Mention any two (Honesty, helpfulness, responsible behaviour towards students, concern for the student to create interest in the subject)
b. Mention the three magnetic elements required to calculate the value of earth’s magnetic field.
Answer: Magnetic declination, magnetic inclination and horizontal component of earth’s magnetic field. 

Question. MrSairam the chief development officer, in southern railway went on an official tour to attend a seminar on fast moving trains. He met his friend Ontosaki in Tokyo after he finished his seminar there. His friend explained to Sairam, how Japanese people are concentrating on energy conservation and saving of fossil fuels using Maglev trains. Mr sairam travelled from Tokyo to Osaka in maglev train and found that sound is less, traveliing is smooth and understood in what way we are lagging behind Japanese in mass transporting systems. This works on the principle of Meissner’s effect
(a) What values did Mr sairam found from Ontosaki? Mention any two. (Awareness about new technology, concern for energy conservation, decrease of noise pollution and air pollution i.e, concern for environment)(b) What is Meissner’s effect?
Answer:
(a) When a superconductor is cooled in a magnetic field below its critical temperature the magnetic field lines are expelled showing diamagnetic property. This is called Meissner effect. 
(b) concern for others, helpfulness, presence of mind, responsible citizen 

b. What are the roles played by Electric field and magnetic field in Cyclotron?
Answer:
The charged particles are accelerated by the electric field with the magnetic field bringing them again and again to the electric field that is the region between the ‘Dee’. 

Question. Mrs. Gupta family was fast asleep during Night. They had no clue that there living room has caught fire due to a short circuit. Suddenly they heard sound of alarm and woke up. They were surprised to see that the sound was coming from the model of fire alarm prepared by their son. They were all happy that a small science model has saved their life
(i) Give the values displayed by the parents and son.
(ii) Name the device use in the model.
Answer: 
(i) Knowledge, Scientific thinking
(ii) photo cell 

Question. Mrs Thakur left her car headlights on while parking. The car would not start when she returned. Seeing her struggle, Mohit went to her help. Not knowing much about cars, he ran and brought a mechanic Raju from a garage nearby. Raju realized that the battery had got discharged as the headlight had been left on for a long time. He brought another battery and connected its terminals to the terminals of the car battery to get the engine started. Once the engine was running, he disconnected this second battery. This is known as “JUMP STARTING”. Mrs. Thakur thanked both Mohit and Raju for helping her.
(i) What values did Mohit have?
(ii) A storage battery of emf 8.0 volts and internal resistance 0.5 ohm is being charged by a 120 volt DC supply using a series resistor of 15.5 ohms. What is the terminal voltage of the battery during charging? What is the purpose of having a series resistor in the charging circuit?
Answer:
(i) Helpful, aware of his weakness, decision making ability.