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 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?

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

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 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 rectangular coil is rotated in a uniform magnetic field.

When the coil is rotated at a constant rate, an alternating emf ε is induced in it. The variation of emf ε, in volts, with time t, in seconds, is given by

ε = 20 sin (100 πt)

Which line, A to D, in the table gives the peak value ε₀ and the frequency f of the induced emf?

The magnetic flux through a coil of 5 turns changes uniformly from 15 × 10⁻³ Wb to 7.0 × 10⁻³ Wb in 0.50 s. What is the magnitude of the emf induced in the coil due to this change in flux?

Three vertical tubes, made from copper, lead and rubber respectively, have identical dimensions. Identical, strong, cylindrical magnets P, Q and R are released simultaneously from the same distance above each tube. Because of electromagnetic effects, the magnets emerge from the bottom of the tubes at different times.

resistivity of copper = 1.7 × 10^–8 Ωm

resistivity of lead = 22 × 10^–8 Ωm

resistivity of rubber = 50 × 1013 Ωm

The graph shows how the magnetic flux, Φ, passing through a coil changes with time, t.

Which one of the following graphs could show how the magnitude of the emf, V, induced in the coil varies with t?

Using the circuit shown, and with the switch closed, a small current was passed through the coil X. The current was slowly increased using the variable resistor. The current reached a maximum value and was then switched off.

The maximum reading on the microammeter occurred when

When a mobile phone is being recharged, the charger heats up. The efficiency of the transformer in the charger can be as low as 15% when drawing a current of 50 mA from a 230 V mains supply. If the charging current required is 350 mA, what is the approximate output voltage at this efficiency?

The output electromotive force (emf) of a simple ac generator can be increased by any of the four factors listed.

Which one of these factors should not be changed if the frequency of the output is to remain unaffected when the emf is increased?

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

The emf induced in the coil

A bar magnet is pushed into a coil connected to a sensitive ammeter, as shown in the diagram, until it comes to rest inside the coil.

Why does the ammeter briefly show a non-zero reading?

The above graph shows how the output emf, ε, varies with time, t, for a coil rotating at angular speed ω in a uniform magnetic field of flux density B. Which one of the following graphs shows how ε varies with t when the same coil is rotated at angular speed 2ω in a uniform magnetic field of flux density 0.5 B?

The graph shows how the magnetic flux passing through a loop of wire changes with time.

A coil rotating in a magnetic field produces the following voltage waveform when connected to an oscilloscope.

With the same oscilloscope settings, which one of the following voltage waveforms would be produced if the coil were rotated at twice the original speed?

The magnetic flux through a coil of N turns is increased uniformly from zero to a maximum value in a time t. An emf, E, is induced across the coil.

What is the maximum value of the magnetic flux through the coil?

An aircraft, of wing span 60 m, flies horizontally at a speed of 150 m s^–1, If the vertical component of the Earth’s magnetic field in the region of the plane is 1.0 × 10^–5 T, what emf is induced across the wing tips of the plane?

The diagram shows a square coil with its plane parallel to a uniform magnetic field. Which one of the following would induce an emf in the coil?

The magnetic flux threading a coil of 100 turns drops from 5 × 10^–3 Wb to zero in 0.1 s. The average induced e.m.f., in V, is

A rectangular conducting loop is pulled horizontally through the gap between two vertical magnets as shown in the diagram.

Which one of the graphs best represents the variation of loop current I with time t as the loop moves at a constant speed from JKLM to J'K'L'M'?

The diagram below shows the waveform obtained when the output of an alternator is connected to a cathode ray oscilloscope.

Which one of the following best represents the output when the speed of rotation of the generator is doubled and no adjustment is made to the oscilloscope?