Electromagnetic Induction & Alternating Currents (Edexcel A Level Physics): Exam Questions

The diagrams show the plan view and side view of a moving coil ammeter.

The fixed soft iron cylinder and magnets produce a uniform magnetic field of magnetic flux density B. The coil is able to rotate within this magnetic field. The coil has width $w$ and length $l$. There is a current $I$ in the coil in the direction shown in the side view diagram.

i) Explain which way the coil will rotate.

(2)

ii) Show that the moment M on the coil about the pivot, due to the magnetic field, is given by

M = BAIN

where

A is the cross-sectional area of the coil

N is the number of turns of wire on the coil.

(4)

An ammeter of this type has a resistance of 625 Ω and will measure a maximum current of 1.6 mA.

The ammeter can be adapted to measure potential difference by adding a resistor in series with the ammeter. This  resistor is known as a multiplier.

The ammeter is adapted so that it can measure potential differences up to 5.0 V as shown.

The following multipliers are available:

Deduce which multiplier should be used.

The coil within a very sensitive moving coil ammeter can be damaged when the ammeter is transported. The two ends of the coil are connected together when the ammeter is transported. This reduces the movement of the coil and makes it less likely to be damaged.

A student suggests that this is due to Faraday’s law and Lenz’s law.

Explain how these laws apply to this situation.

The diagram shows a generator used in a small wind turbine. It consists of a magnet on the shaft of the turbine's rotor, which is rotated inside a cavity in a soft iron core.

A coil wrapped around the iron core is connected to an external circuit. The graph shows how the output e.m.f. across the coil varies with time.

(i) Use Faraday's law to explain the shape of the graph.

[3]

(ii) Sketch a graph to show how the magnetic flux linkage of the coil varies with time.

[2]

The coil has $150$ turns. The maximum magnetic flux through the coil is $2.1\times {10}^{-2}\text{ Wb}$ when the magnet is rotated at a constant rate of $6.0$ revolutions per second.

Show that the maximum induced e.m.f. across the coil is approximately $120\text{ V}$.

The coil has total internal resistance $1.0\mathrm{\Omega }$. The output of the generator is connected to a load resistor of resistance $9.0\mathrm{\Omega }$.

Calculate the average power dissipated in the load resistor.

The maximum input power from the wind is given by

$P=\frac{1}{2}\rho A{v}^3$

where $A$ is the area swept out by the rotating blades, $\rho$ is the density of air and $v$ is the speed of the wind.

The wind turbine has blades of length $1.5\text{ m}$. The density of air is $1.2{\text{ kg m}}^{-3}$.

The manufacturer claims that the efficiency of the wind turbine is at least $35\%$ when the wind speed is $8.0{\text{ m s}}^{-1}$.

Deduce whether the manufacturer's claim is valid.

The Superconducting Maglev, or SCMaglev, is a Japanese high-speed magnetic levitation train system currently under construction between Tokyo and Nagoya.

Photo: Saruno Hirobano (opens in a new tab)

Powerful superconducting electromagnets are built into the sides of the train. As the train moves forward, these magnets pass metallic coils mounted in the walls of the track.

Explain, using the laws of electromagnetic induction, how this produces an upward levitation force.