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Book 4 Chapter 4 Electromagnetism

New Senior Secondary Physics at Work (Second Edition) Oxford University Press 2017

Which of the following statements about compasses and magnetic fields is incorrect?

A The N-pole of a compass points along the magnetic field.

B The N-pole of a compass points towards the magnetic south pole of the Earth.

C If two compasses are placed close together, their N-poles will point towards each other.

D Magnetic field lines run from the N-pole round to the S-pole outside a bar magnet.

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Answer C Explanation The geographic south pole of the earth is the magnetic north pole, while the geographic north pole of the earth is the magnetic south pole. Magnetic field lines run from the magnetic north pole to the magnetic south pole and the N-pole of a compass points along the magnetic field lines. Therefore, the N-pole of a compass points towards the magnetic south pole of the earth.

∴ A, B and D are correct. The N-poles of two compasses repel each other and will not point towards each other. ∴ C is incorrect.



Iron filings are sprinkled near some magnets. Which of the following is a possible resulting scenario? A

N N

B

N S S N

C

N N

D

N S N

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Answer D Explanation A magnetic field line never runs from a north pole to another north pole. ∴ A and B are impossible. A bar magnet cannot have north poles at both ends. ∴ C is impossible.



Refer to the following set-up.

N S

When the switch is closed, the bar magnet

A will be attracted by the solenoid and move towards it.

B will be repelled by the solenoid and move away from it.

C will rotate about the axis of the solenoid.

D will rotate about an axis pointing into the page.

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Answer A Explanation By the right-hand grip rule for solenoids, the magnetic field produced by the solenoid is in the direction as shown in the following figure. The right end of the solenoid behaves like an N-pole while the left end behaves like an S-pole.

N S

N S

The S-pole of the bar magnet and the N-pole of the solenoid attract each other. Therefore, the bar magnet moves towards the solenoid.



Two long straight wires are carrying the same current in opposite directions as shown in the following figure.

P Q

d d d

If the magnitude of the magnetic field at point P is B0, what is the magnitude of the magnetic field at point Q?

A 3

0B

B 3

2 0B

C 9

4 0B

D 9

5 0B

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Answer A Explanation Let I be the current in the two wires. By the right-hand grip rule for straight wires, the magnetic fields produced by the two wires are respectively in the directions shown below.

At P, the magnetic fields due to the two wires point towards the same direction.

Then by B = rIπµ2

0 ,

B0 = dI

dI

πµ

πµ

2200 + =

dI

πµ0

At Q, the magnetic fields due to the two wires point towards opposite directions.

∴ Magnitude of magnetic field at Q =)3(22

00

dI

dI

πµ

πµ

− =dI

πµ3

0 =3

0B



Which of the following can increase the strength of an electromagnet? (1) Increase the length of the solenoid without changing the total number of turns. (2) Increase the total number of turns of the solenoid without changing its length. (3) Increase the current flowing through the solenoid.

A (1) and (2) only

B (1) and (3) only

C (2) and (3) only

D (1), (2) and (3)

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Answer C Explanation The strength of an electromagnet increases if the number of turns per unit length increases. ∴ (1) is incorrect and (2) is correct. A larger current can produce a stronger magnetic field. ∴ (3) is correct.



Two long straight parallel wires P and Q are carrying currents in opposite directions as shown.

P Q

Which of the following statements is/are correct? (1) The magnetic field at any point on wire P due to wire Q points into the page. (2) The magnetic field at any point on wire Q due to wire P points into the page. (3) The two wires attract each other.

A (2) only

B (1) and (2) only

C (1) and (3) only

D (1), (2) and (3)

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Answer B Explanation By the right-hand grip rule for straight wires, the magnetic field at any point on wire P due to wire Q and the magnetic field at any point on wire Q due to wire P both point into the page.

∴ (1) and (2) are correct. By Fleming’s left-hand rule, the magnetic force acting on wire P points towards the left and the magnetic force acting on wire Q points towards the right.

magnetic force

magnetic field current magnetic force

magnetic field

current

∴ (3) is incorrect.



Two parallel iron rails are placed on a horizontal surface and separated by a distance d. A copper rod of length L is placed on the rails, making an angle of 60° with the rail. A uniform magnetic field of magnitude B is applied perpendicularly to the horizontal surface. The figure below shows the top-view of the set-up and the magnetic field points into the page.

d

I

L

60°

If a current I flows through the rails in the direction shown above, what is the magnitude of the magnetic force acting on the copper rod?

A BIL

B BIL sin 60°

C BId sin 60°

D °60sin

BId

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Answer D Explanation

The part of the copper rod carrying a current is of length °60sin

d .

I

60°

°60 sin

dd

Note that the copper rod is perpendicular to the magnetic field. By F = BIl sin θ,

magnetic force acting on the copper rod = °

°90sin

60sindBI =

°60sinBId



A current-carrying coil is placed in a uniform magnetic field and experiences a torque due to the magnetic force. Which of the following may increase the torque experienced by the coil? (1) Use a coil with more turns. (2) Use a coil with a larger area. (3) Rotate the coil to a position at which its plane is perpendicular to the magnetic field.

A (1) only

B (1) and (2) only

C (1) and (3) only

D (2) and (3) only

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Answer B Explanation The torque on the coil can be increased by increasing the current, increasing the number of turns in the coil, increasing the area of the coil (within the B-field), or increasing the magnetic field. ∴ (1) and (2) are correct. If the coil is rotated so that its plane is perpendicular to the magnetic field, the magnetic forces acting on the coil is along its plane and do not produce any torque. ∴ (3) is incorrect.



Consider the following set-up

Which of the following describes the motion of the coil?

A It will not rotate at all.

B It will vibrate a few times.

C It will keep rotating in one direction steadily.

D It will keep rotating in one direction at an unsteady rate.

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Answer B Explanation Initially, the magnetic forces acting on the coil are in directions as shown in the following figure. The magnetic forces produce a torque and the coil rotates.

magnetic force

After the coil shoots past the vertical, the directions of the magnetic forces are as shown below. The direction of the torque reverses and the coil rotates back.

magnetic force

The process then repeats and the coil will vibrate a few times.



Two particles X and Y have the same charge. They both enter a region of uniform magnetic field at the same speed. The following figure shows their paths after entering the field.

Y

d

d

X

If the mass of X is m, what is the mass of Y?

A 2m

B 2

m

C m2

D 2m

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Answer D Explanation When a charged particle enters a uniform magnetic field with a velocity perpendicular to the field, it will undergo uniform circular motion. The centripetal force for circular motion is provided by the magnetic force. Therefore,

BQv =r

mv 2

m =v

BQr

Particles X and Y have the same charge and travel at the same speed in the same magnetic field, i.e. same Q, v and B. Since the radius r of the path of Y in the field is twice that of X, the mass of Y is twice that of X.

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