P.3.7.5.4 Electromagnetic induction

A coil P is connected to a cell and a switch.

A second closed coil Q is parallel to P and is arranged on the same axis.

When the switch is closed, coil Q experiences a force.

Which row describes the force on Q?

• A

• B

• C

• D

Three identical magnets P, Q and R are released simultaneously from rest and fall to the ground from the same height.

P falls directly to the ground.

Q falls through the centre of a thick horizontal conducting ring.

R falls through a similar ring that has a gap cut into it.

In which order do the magnets reach the ground?

• P and R arrive together, followed by Q.

• P and Q arrive together, followed by R.

• P arrives first, followed by Q which is followed by R.

• All three magnets arrive simultaneously.

A coil with 20 circular turns each of diameter 60 mm is placed in a uniform magnetic field of flux density 90 mT.

Initially the plane of the coil is perpendicular to the magnetic field lines as shown in Figure X.

The coil is rotated about a vertical axis by 90° in a time of 0.20 s so that its plane becomes parallel to the field lines as shown in Figure Y.

Assume that the rate of change of flux linkage remains constant.

What is the emf induced in the coil?

• zero

• 1.3 mV

• 25 mV

• 100 mV

A vertical conducting rod of length l is moved at a constant velocity v through a uniform horizontal magnetic field of flux density B.

Which of the rows gives a correct expression for the induced emf between the ends of the rod for the stated direction of the motion of the rod?

• A

• B

• C

• D

The graph shows how the flux linkage, NΦ, through a coil changes when the coil is moved into a magnetic field.

The emf induced in the coil

• decreases then becomes zero after time t₀.

• increases then becomes constant after time t₀.

• is constant then becomes zero after time t₀.

• is zero then increases after time t₀.

A train is travelling at 20 m s^–1 along a horizontal track through a uniform magnetic field of flux density 4.0 × 10^–5 T acting vertically downwards.

What is the emf induced between the ends of an axle 1.5 m long?

• 3.0 × 10^–6V

• 5.3 × 10^–4V

• 1.2 × 10^–3V

• 7.5 × 10⁵V

In which one of the following applications does electromagnetic induction not take place?

• the generators at a nuclear power station

• the ac power adapter for a laptop computer

• the wings of an aircraft cutting through the Earth’s magnetic field

• the back up capacitor of an electric timer

When a magnet is dropped through an aluminium ring an emf is induced. A data logger connected to the ring records the variation of the induced emf ε with time t as shown below.

In a second experiment, the magnet is dropped from a greater height.

Which one of the following graphs best represents the induced emf in the second experiment?

• 
• 
• 
• 

A vertical conducting rod of length l is moved at a constant velocity v through a uniform horizontal magnetic field of flux density B.

Which line, A to D, in the table gives a correct expression for the induced emf for the stated direction of the motion of the rod?

• A

• B

• C

• D

A transformer, which is not perfectly efficient, is connected to a 230 V rms mains supply and is used to operate a 12 V rms, 60 W lamp at normal brightness. The secondary coil of the transformer has 24 turns.

Which line, A to D, in the table is correct?

• A

• B

• C

• D