415 + Interview Questions in ELECTROMAGNETISM IN GENERAL AWARENESS Page 2 InterviewSolution

51.	Why is a soft iron piece placed at the center of the coil of a moving coil galvanometer?
Answer» A soft iron core is placed at the center of the coil which intensifies the magnetic field and makes it radial by concentrating the magnetic field lines due to its high permeability as shown in next fig.

Discussion

52.	How is magnetic field due to solenoid carrying current affected, if a soft iron bar is introduced inside the solenoid?
Answer» The magnetic field increases when iron bar is introduced inside the solenoid.

Discussion

53.	Does this deflection fit with the direction of magnetic force found by right hand rule?
Answer» Yes. This deflection fits with the direction of magnetic force found by right hand rule.

Discussion

54.	Can we use this knowledge to construct an electric motor?
Answer» Yes. This is the principle of electric motor.

Discussion

55.	What happens when a current carrying wire is kept in a magnetic field?
Answer» Magnetic field applies force on current carrying wire. So it gets deflected and the direction of deflection is given by right hand rule. Or there will be no force acting on the wire when wire is in the direction of the field.

Discussion

56.	If a piece of soft iron is placed inside a current carrying coil, what is the name given to the device?
Answer» Aarmature is the name given to the device.

Discussion

57.	What happens to magnetic field if we reverse the current direction?
Answer» The magnetic field also gets reversed

Discussion

58.	What happens when a current carrying coil is placed in a uniform magnetic field?
Answer» It gets deflected since magnetic lines of force are perpendicular to the length of the coil.

Discussion

59.	Name the device which is used to reverse the direction of current in the coil of a A) commutator B) Split rings C) both A and B D) none
Answer» C) both A and B

Discussion

60.	In an electric motor which of the following remains fixed with the coil ? A) commutator B) brush C) both A and B D) none
Answer» Correct option is B) brush

Discussion

61.	Statement X : A current carrying coil rotates when it is kept in a uniform magnetic field.Statement Y: Net force on a current carrying coil when it is kept in a uniform magnetic field is zero. A) Both statements are true B) Both statements are false C) X is true, Y is false D) X is false, Y is true
Answer» A) Both statements are true

Discussion

62.	In an electric motor which of the following rotates with the coil ? A) commutator B) brush C) both A and B D) none
Answer» A) commutator

Discussion

63.	How could you make the coil rotate continuously in motor?
Answer» The direction of current through the coil is reversed every half rotation, the coil will rotate continuously and the same direction.

Discussion

64.	A 6.28m long wire is turned into a coil of diameter 0.2m and a current of 1 amp is passed in it. The magnetic induction at its centre will be -A. `6.28 xx 10^(5)` TeslaB. `0`C. `6.28` TaslaD. `6.28 xx 10^(-3)` Tasla
Answer» Correct Answer - A

Discussion

65.	A flat coil ABCD Is freely suspended between the pole pieces of a U – shaped permanent magnet with the plane of coil parallel to the magnetic field. a) What happens when current is passed in the coil? b) When will the coil come to rest? c) Name the instrument which makes use of the principle stated above.
Answer» a) The coil will experience a torque due to which it will rotate. b) The coil will come to rest when its plane becomes normal to the magnetic field. c) DC motor.

65.

A flat coil ABCD Is freely suspended between the pole pieces of a U – shaped permanent magnet with the plane of coil parallel to the magnetic field. a) What happens when current is passed in the coil? b) When will the coil come to rest? c) Name the instrument which makes use of the principle stated above.

Answer»

a) The coil will experience a torque due to which it will rotate.

b) The coil will come to rest when its plane becomes normal to the magnetic field.

c) DC motor.

Discussion

66.	In an electric motor the direction of current in the coil charge once in each ……………… A) two rotations B) one rotation C) half – rotation D) one – fourth – rotation
Answer» C) half – rotation

Discussion

67.	A resistance coil of `60 omega` is immersed in 42 kg of water. A current of 7 A is passed through it. The rise in temperature of water per minutes is :A. `4^(@)C`B. `8^(@)C`C. `1^(@)C`D. `12^(@)C`
Answer» Correct Answer - C

Discussion

68.	When can an electric charge give rise to a magnetic field?
Answer» When electric charge is in motion e. charge flows, it gives rise to magnetic field.

Discussion

69.	A flat rectangular coil is rotated between the pole pieces of a horse-shoe magnet In which position of coil with respect to the magnetic field, will the e.m.f (1) be maximum, (2) be zero and (3) change direction?
Answer» 1. The e.m.f. is maximum when the plane of coil is parallel to the magnetic field. 2. The e.m.f. is zero when the plane of coil is normal perpendicular to the magnetic field. 3. The e.m.f. will change direction when the plane of coil passes from the position normal to the magnetic field.

69.

A flat rectangular coil is rotated between the pole pieces of a horse-shoe magnet In which position of coil with respect to the magnetic field, will the e.m.f (1) be maximum, (2) be zero and (3) change direction?

Answer»

1. The e.m.f. is maximum when the plane of coil is parallel to the magnetic field.

2. The e.m.f. is zero when the plane of coil is normal perpendicular to the magnetic field.

3. The e.m.f. will change direction when the plane of coil passes from the position normal to the magnetic field.

Discussion

70.	Can a transformer work when it is connected to a d.c. source?
Answer» No. A transformer cannot work when it is connected to a d.c. source.

Discussion

71.	Which is the incorrect statement? In a step down transformer (a) number of turns in primary are more than the secondary coil (b) the primary coil is thinner as compared to the secondary coil. (c) the primary coil is thicker as compared to secondary coil. (d) the primary coil is more heavily insulated as compared to secondary coil
Answer» (c) the primary coil is thicker as compared to secondary coil.

Discussion

72.	What is the shape of magnetic lines due to straight current carrying conductor?
Answer» They are concentric circles.

Discussion

73.	A transformer contains 100 and 150 turns in its primary and secondary coils, respectively. The transformer is a (A) Step up tranformer (B) Step down transformerA. Only (A) is trueB. Only (B) is trueC. Both (A) and (B) are trueD. Both (A) and (B) are false
Answer» Correct Answer - A

Discussion

74.	A transformer contains 250 and 150 turns in its primary and secondary coils, respectively. The transformer is a__________. (A) Step up transformer (A) Step down transformerA. Only (A) is trueB. Only (B) is trueC. Both (A) and (B) are trueD. Both (A) and (B) are false
Answer» Correct Answer - B

Discussion

75.	In a transformer, the ratio of the number of turns in the secondary coil to that in the primary coil is 10:3. Such a transformer is called_______transformer.
Answer» Correct Answer - Electromagnetic induction

Discussion

76.	What is a transformer?
Answer» It is a device which increases or decreases the voltage.

Discussion

77.	State the energy change which takes place when a magnet is moved inside a coil having a galvanometer at its ends. Name this phenomenon.
Answer» When a magnet is moved in side a coil MECHANICAL ENERGY changes into ELECTRICAL ENERGY i.e. magnetic flux (Magnetic lines of force) is converted into e.m.f. or electric current. The phenomenon is called INDUCED CURRENT.

Discussion

78.	State the function of a split ring in a D.C. motor.
Answer» Function of split ring in a D.C. Motor to change the direction of current flowing after eveiy half rotation the coil.

Discussion

79.	State the two factors on which the magnitude and direction of induced e.m.f depend.
Answer» Two Factors are : 1. The change in magnetic flux and 2. The time in which the magnetic flux changes. DIRECTION of induced e.m.f depends on whether the magnetic flux increases or decreases.

79.

State the two factors on which the magnitude and direction of induced e.m.f depend.

Answer»

Two Factors are :

1. The change in magnetic flux and

2. The time in which the magnetic flux changes. DIRECTION of induced e.m.f depends on whether the magnetic flux increases or decreases.

Discussion

80.	Name the principle on which a transformer works.
Answer» A transformer works on the principle of electromagnetic induction.

Discussion

81.	The diagram below shows two coils X and Y. The coil X is connected to a battery S and a key K. The coil Y is connected to a galvanometer GWhen the key K is closed, State the polarity. 1. at the end B of the coil X. 2. at the end C of the coil Y. 3. at the end C of the coil Y if the coil Y is (a) moved towards the coil X, (b) moved away from the coil X
Answer» 1. Current at the end B of coil X is anti-clockwise therefore at this end there is a NORTH pole. 2. While closing the key, polarity at the end C of the coil Y will be north. There will be no polarity at the end C of the coil Y when the current becomes steady in the coil X. 3. (a) While the coil Y is moved towards the coil X, the polarity at the end C of the coil Y is NORTH. (b) While the coil Y is moved away from the coil X, the polarity at the end C of the coil B is SOUTH.

81.

The diagram below shows two coils X and Y. The coil X is connected to a battery S and a key K. The coil Y is connected to a galvanometer GWhen the key K is closed, State the polarity. 1. at the end B of the coil X. 2. at the end C of the coil Y. 3. at the end C of the coil Y if the coil Y is (a) moved towards the coil X, (b) moved away from the coil X

Answer»

1. Current at the end B of coil X is anti-clockwise therefore at this end there is a NORTH pole.

2. While closing the key, polarity at the end C of the coil Y will be north. There will be no polarity at the end C of the coil Y when the current becomes steady in the coil X.

3. (a) While the coil Y is moved towards the coil X, the polarity at the end C of the coil Y is NORTH.

(b) While the coil Y is moved away from the coil X, the polarity at the end C of the coil B is SOUTH.

Discussion

82.	Two coils A and B are placed as shown in Fig. The coil A is connected to a battery and a key K while the coil B is connected to a center zero galvanometer G.What will you observe in the galvanometer G when 1. the key K is closed. 2. the key K is opened. 3. with the key K closed, the coil A is move rapidly towards the coil B. 4. with the key K closed, the coil B is moved rapidly towards the coil A. 5. with the key K closed, the coils A and B are moved away from each other?
Answer» 1. As the key K is closed, a deflection is observed in the galvanometer for a short while (i.e. a momentary deflection). In other words, the galvanometer needle deflects for a moment and returns to zero. 2. As the key K is opened, again a momentary deflection (but more) is observed in the opposite direction. 3. With the key K closed, if the coil A is moved rapidly towards the coil B, a deflection is obtained in the galvanometer in the direction as in due to increase in magnetic flux through the coil B. But the deflection lasts so long as the coil A moves. 4. With the key K closed, if the coil B is moved rapidly towards the coil A, again a deflection is observed in the galvanometer in direction as in due to increase in magnetic flux through the coil B. The deflection lasts so long as the coil B moves. 5. With the key K closed, if the coil A and B are moved away from each other, a deflection is observed in the galvanometer in direction as in due to decrease in magnetic flux through the coil B. The deflection lasts so long as there is relative motion between the coils.

82.

Two coils A and B are placed as shown in Fig. The coil A is connected to a battery and a key K while the coil B is connected to a center zero galvanometer G.What will you observe in the galvanometer G when 1. the key K is closed. 2. the key K is opened. 3. with the key K closed, the coil A is move rapidly towards the coil B. 4. with the key K closed, the coil B is moved rapidly towards the coil A. 5. with the key K closed, the coils A and B are moved away from each other?

Answer»

1. As the key K is closed, a deflection is observed in the galvanometer for a short while (i.e. a momentary deflection). In other words, the galvanometer needle deflects for a moment and returns to zero.

2. As the key K is opened, again a momentary deflection (but more) is observed in the opposite direction.

3. With the key K closed, if the coil A is moved rapidly towards the coil B, a deflection is obtained in the galvanometer in the direction as in

due to increase in magnetic flux through the coil B. But the deflection lasts so long as the coil A moves.

4. With the key K closed, if the coil B is moved rapidly towards the coil A, again a deflection is observed in the galvanometer in direction as in

due to increase in magnetic flux through the coil B. The deflection lasts so long as the coil B moves.

5. With the key K closed, if the coil A and B are moved away from each other, a deflection is observed in the galvanometer in direction as in

due to decrease in magnetic flux through the coil B. The deflection lasts so long as there is relative motion between the coils.

Discussion

83.	The diagram given below shows two coils X and Y. The coil X is connected to a battery S and a key K. The coil Y is connected to a galvanometer G.When the key K is closed state the polarity. i) At the end B of the coil X. ii) At the end C of the coil Y. iii) At the end C of the coil Y if the coil Y is (a) moved towards the coil X, (b) moved away from the coil X.
Answer» i) On closing the key K, the current at the end B of the coil X is anti-clockwise, therefore at this end there is a north pole. ii) While closing the key, polarity at the end C of the coil Y will be north. But there will be no polarity at the end C of the coil Y when the current becomes steady in the coil X iii) a) With the key K closed, while the coil Y is moved towards the coil X, the polarity at the end C of the coil Y is north. b) With the key K closed, while the coil Y is moved away from the coil X, the polarity at the end C of the coil Y is south.

83.

The diagram given below shows two coils X and Y. The coil X is connected to a battery S and a key K. The coil Y is connected to a galvanometer G.When the key K is closed state the polarity. i) At the end B of the coil X. ii) At the end C of the coil Y. iii) At the end C of the coil Y if the coil Y is (a) moved towards the coil X, (b) moved away from the coil X.

Answer»

i) On closing the key K, the current at the end B of the coil X is anti-clockwise, therefore at this end there is a north pole.

ii) While closing the key, polarity at the end C of the coil Y will be north. But there will be no polarity at the end C of the coil Y when the current becomes steady in the coil X

iii) a) With the key K closed, while the coil Y is moved towards the coil X, the polarity at the end C of the coil Y is north.

b) With the key K closed, while the coil Y is moved away from the coil X, the polarity at the end C of the coil Y is south.

Discussion

84.	How do you increase the speed of rotation of coil in a DC motor?
Answer» The speed of rotation of coil can be increased by 1. Increasing the strength of current. 2. Increasing the number of turns in coil. 3. Increasing the strength of magnetic field. To increase the strength of magnetic field a soft iron core can be inserted within the coil.

84.

How do you increase the speed of rotation of coil in a DC motor?

Answer»

The speed of rotation of coil can be increased by

1. Increasing the strength of current.

2. Increasing the number of turns in coil.

3. Increasing the strength of magnetic field. To increase the strength of magnetic field a soft iron core can be inserted within the coil.

Discussion

85.	A vertical wire carries a current upwards. The magnetic field at a point due north of the wire is directed (a) upward (b) due south (c) due west (d) due east
Answer» Correct Answer is: (c) due west

Discussion

86.	Two parallel beams of protons and electrons, carrying equal currents, are fixed at a separation d. The protons and electrons move in opposite directions. P is a point on a line joining the beams, at a distance x from any one beam. The magnetic field at P is B. If B is plotted against x, which of the following best represents the resulting curve?
Answer» Correct Answer is: (c)

Discussion

87.	A horizontal ring of radius `r` spins about its axis with an angular velocity `omega` in a uniform vertical magnetic field of magnitude `B`. The emf induced in the ring isA. zeroB. `pir^(2) omegaB`C. `1/2 Br^(2) omega`D. `Br^(2) omega`
Answer» Correct Answer - A

Discussion

88.	A conducting disc of radius r spins about its axis with an angular velocity ω. There is a uniform magnetic field of magnitude B perpendicular to the plane of the disc. C is the centre of the ring.(a) No emf is induced in the disc. (b) The potential difference between C and the rim is 1/2 Br2ω. (c) C is at a higher potential than the rim. (d) Current flows between C and the rim.
Answer» Correct Answer is: (b, c)

Discussion

89.	A nonconducting ring of radius r has charge Q. A magnetic field perpendicular to the plane of the ring changes at the rate dB/dt .The torque experienced by the ring is(a) zero(b) Qr2 (dB/dt)(c) 1/2 Qr2 (dB/dt)(d) πr2Q (dB/dt)
Answer» Correct Answer is: (c) 1/2 Qr² (dB/dt) The same emf is induced in a conducting and a nonconducting ring, and it is equal to πr² dB/dt. At any point on ring, E = 1/2 r dB/dt . For any charge dQ on the ring, force = dF = EdQ, tangential to the ring Torque about the centre due to dF = dτ = rdF = rEdQ. Total torque = ΣrEdQ = r (1/2 r dB/dt) Q.

89.

A nonconducting ring of radius r has charge Q. A magnetic field perpendicular to the plane of the ring changes at the rate dB/dt .The torque experienced by the ring is(a) zero(b) Qr2 (dB/dt)(c) 1/2 Qr2 (dB/dt)(d) πr2Q (dB/dt)

Answer»

Correct Answer is: (c) 1/2 Qr² (dB/dt)

The same emf is induced in a conducting and a nonconducting ring, and it is equal to πr² dB/dt.

At any point on ring, E = 1/2 r dB/dt .

For any charge dQ on the ring, force = dF = EdQ, tangential to the ring

Torque about the centre due to dF = dτ = rdF = rEdQ.

Total torque = ΣrEdQ = r (1/2 r dB/dt) Q.

Discussion

90.	The magnetic field perpendicular to the plane of a conducting ring of radius `r` change ate the rate `(dB)/(dt)`A. The emf induced in the ring is `pir^(2)(dB)/(dt)`B. The emf induced in the rintg is `2pir(dB)/(dt)`C. The potential difference between diametrically opposite points on the ring is half of the induced emf.D. All points on the ring are at the same potential.
Answer» Correct Answer - A::D

Discussion

91.	A vertical conducting ring of radius `R` falls vertically in a horizontal magnetic field of magnitude `B`. The direction of `B` is perpendicular to the plane of the ring. When the speed of the ring is `v, A. no current flows in the ringB. `A` and `D` are the same potentialC. `C` and `E` are the same potentialD. the potential differece between `A` and `D` is `2BRv` with `D` at a higher potential
Answer» Correct Answer - C::D

Discussion

92.	The frequency of direct current is ………………… A) 0 Hz B) 50 Hz C) 60 Hz D) 100 Hz
Answer» Correct option is A) 0 Hz

Discussion

93.	Name some sources of direct current.
Answer» Dry cell, lead-acid battery.

Discussion

94.	If a copper rod carries a direct current, then where will be the magnetic field in the conductor?
Answer» It will be both inside and outside the rod.

Discussion

95.	A conductor ABCDEF, with each side of length L, is bent as shown. It is carrying a current I in a uniform magnetic induction (field) B, parallel to the positive y-direction. The force experienced by the wire is (a) BIL in the positive y-direction (b) BIL in the positive z-direction (c) 3BIL (d) zero
Answer» Correct Answer is a) BIL in the positive y-direction The force on BC and DE are equal and opposite, and cancel out. The force on CD is BIL.

95.

A conductor ABCDEF, with each side of length L, is bent as shown. It is carrying a current I in a uniform magnetic induction (field) B, parallel to the positive y-direction. The force experienced by the wire is (a) BIL in the positive y-direction (b) BIL in the positive z-direction (c) 3BIL (d) zero

Answer»

Correct Answer is a) BIL in the positive y-direction

The force on BC and DE are equal and opposite, and cancel out. The force on CD is BIL.

Discussion

96.	A small conducting circular ring of area 2 mm2 is placed coplanerly with a long straight current carrying wire at a distance of 10 cm from it. It current through the wire decreases from 4 A to zero in 0.1s, the average value of emf induced in the ring.A. `1.6 xx 10^(-10) V`B. `2.0 xx 10^(-10) V`C. `3.2 xx 10^(-10) V`D. Zero.
Answer» Correct Answer - A

Discussion

97.	A direct current is sent through a helical spring. The spring -A. tends to get shorterB. tends to get longerC. tends to rotate about the axisD. tends to move northward
Answer» Correct Answer - A

Discussion

98.	A long straight conductor carrying a current lies along the axis of a ring. The conductor will exert a force on the ring if the ring (a) carries a current (b) has uniformly distributed charge (c) has nonuniformly distributed charge (d) none of the above
Answer» Correct Answer is: (d) none of the above

Discussion

99.	A long straight conductor carrying a current lies along the axis of a ring. The conductor will exert a force on the ring if the ringA. carries a currentB. has uniformly distributed chargeC. has non uniformly distributed chargeD. non of the above
Answer» Correct Answer - D

Discussion

100.	Two long, thin wires distant a apart exert a force F on one another when current through each wire is i. The distance between the wires is doubled and the current is decreased to `i// 3`. The force they exert on one another now-A. `F//6`B. `F//9`C. `2F//3`D. `F//18`
Answer» Correct Answer - D

Discussion

Explore topic-wise InterviewSolutions in .

Why is a soft iron piece placed at the center of the coil of a moving coil galvanometer?

How is magnetic field due to solenoid carrying current affected, if a soft iron bar is introduced inside the solenoid?

Does this deflection fit with the direction of magnetic force found by right hand rule?

Can we use this knowledge to construct an electric motor?

What happens when a current carrying wire is kept in a magnetic field?

If a piece of soft iron is placed inside a current carrying coil, what is the name given to the device?

What happens to magnetic field if we reverse the current direction?

What happens when a current carrying coil is placed in a uniform magnetic field?

Name the device which is used to reverse the direction of current in the coil of a A) commutator B) Split rings C) both A and B D) none

In an electric motor which of the following remains fixed with the coil ? A) commutator B) brush C) both A and B D) none

Statement X : A current carrying coil rotates when it is kept in a uniform magnetic field.Statement Y: Net force on a current carrying coil when it is kept in a uniform magnetic field is zero. A) Both statements are true B) Both statements are false C) X is true, Y is false D) X is false, Y is true

In an electric motor which of the following rotates with the coil ? A) commutator B) brush C) both A and B D) none

How could you make the coil rotate continuously in motor?

A 6.28m long wire is turned into a coil of diameter 0.2m and a current of 1 amp is passed in it. The magnetic induction at its centre will be -A. `6.28 xx 10^(5)` TeslaB. `0`C. `6.28` TaslaD. `6.28 xx 10^(-3)` Tasla

In an electric motor the direction of current in the coil charge once in each ……………… A) two rotations B) one rotation C) half – rotation D) one – fourth – rotation

A resistance coil of `60 omega` is immersed in 42 kg of water. A current of 7 A is passed through it. The rise in temperature of water per minutes is :A. `4^(@)C`B. `8^(@)C`C. `1^(@)C`D. `12^(@)C`

When can an electric charge give rise to a magnetic field?

A flat rectangular coil is rotated between the pole pieces of a horse-shoe magnet In which position of coil with respect to the magnetic field, will the e.m.f (1) be maximum, (2) be zero and (3) change direction?

Can a transformer work when it is connected to a d.c. source?

What is the shape of magnetic lines due to straight current carrying conductor?

A transformer contains 100 and 150 turns in its primary and secondary coils, respectively. The transformer is a (A) Step up tranformer (B) Step down transformerA. Only (A) is trueB. Only (B) is trueC. Both (A) and (B) are trueD. Both (A) and (B) are false

A transformer contains 250 and 150 turns in its primary and secondary coils, respectively. The transformer is a__________. (A) Step up transformer (A) Step down transformerA. Only (A) is trueB. Only (B) is trueC. Both (A) and (B) are trueD. Both (A) and (B) are false

In a transformer, the ratio of the number of turns in the secondary coil to that in the primary coil is 10:3. Such a transformer is called_______transformer.

What is a transformer?

State the energy change which takes place when a magnet is moved inside a coil having a galvanometer at its ends. Name this phenomenon.

State the function of a split ring in a D.C. motor.

State the two factors on which the magnitude and direction of induced e.m.f depend.

Name the principle on which a transformer works.

How do you increase the speed of rotation of coil in a DC motor?

A vertical wire carries a current upwards. The magnetic field at a point due north of the wire is directed (a) upward (b) due south (c) due west (d) due east

A horizontal ring of radius `r` spins about its axis with an angular velocity `omega` in a uniform vertical magnetic field of magnitude `B`. The emf induced in the ring isA. zeroB. `pir^(2) omegaB`C. `1/2 Br^(2) omega`D. `Br^(2) omega`

A nonconducting ring of radius r has charge Q. A magnetic field perpendicular to the plane of the ring changes at the rate dB/dt .The torque experienced by the ring is(a) zero(b) Qr2 (dB/dt)(c) 1/2 Qr2 (dB/dt)(d) πr2Q (dB/dt)

The frequency of direct current is ………………… A) 0 Hz B) 50 Hz C) 60 Hz D) 100 Hz

Name some sources of direct current.

If a copper rod carries a direct current, then where will be the magnetic field in the conductor?

A direct current is sent through a helical spring. The spring -A. tends to get shorterB. tends to get longerC. tends to rotate about the axisD. tends to move northward

A long straight conductor carrying a current lies along the axis of a ring. The conductor will exert a force on the ring if the ring (a) carries a current (b) has uniformly distributed charge (c) has nonuniformly distributed charge (d) none of the above

A long straight conductor carrying a current lies along the axis of a ring. The conductor will exert a force on the ring if the ringA. carries a currentB. has uniformly distributed chargeC. has non uniformly distributed chargeD. non of the above

Two long, thin wires distant a apart exert a force F on one another when current through each wire is i. The distance between the wires is doubled and the current is decreased to `i// 3`. The force they exert on one another now-A. `F//6`B. `F//9`C. `2F//3`D. `F//18`