11.1 Electromagnetic Induction

This question is about the motional e.m.f. which can be induced in different situations.

A circuit is set up as shown and then the switch is closed so that current can flow. Observations are made for the first 100 ms.

Explain the effects on the electromagnet of the switch being closed. The analysis should make reference to the expected changes in current, e.m.f. and energy.

An unmanned probe is in orbit around Jupiter at right angles to the planet's magnetic field. The probe launches a remote measuring device which remains connected to it by a conducting cable.

When the probe and the device are at a distance L the cable is held in a straight line which is also perpendicular to Jupiter's magnetic field. 

For the motion of the cable in the magnetic field

In an experiment two metal rings A and B are dropped from the same height between two magnets. The rings are identical, apart from a small slit cut in B.

Describe and explain the motion of A and B as they fall between the magnets. The use of sketches to illustrate is encouraged.

In a thought experiment the variation of induced e.m.f. ε in a coil with time is considered.

Students are asked to discuss the properties of the graph shown above to determine how it might be reproduced.

Use the axes provided to sketch a graph of the magnetic flux linkage NΦ through the conductor between t = 0 to t = t₄.

The graph in part (b) was produced using a coil moving at constant speed into and then out of a uniform magnetic field. The motion is represented in the diagram.

By analysing the stages of the motion explain why no induced e.m.f. is seen for the section t₂ to t₃ although the coil can be described as 'cutting magnetic field lines' at that time.

An experiment is designed to model an aeroplane flying parallel to the Earth's surface at a constant speed v.

The wingspan of the plane is modelled by a thin metallic rod of length L = 75.0 cm which is connected to a voltmeter. The Earth’s magnetic field is at 70° to the vertical and has a flux density of 1.8 × 10^–4 T.

Determine the speed at which the metal rod would need to be propelled to generate an emf of 0.15 mV across the ends.

A magnet is dropped through a vertical solenoid.

On the axes provided sketch a graph of the expected e.m.f. as time progresses.

Explain the shape of the graph from part (a).

A bar magnet attached to a spring is allowed to fall vertically from rest through a simple coil of conducting wire. The potential difference (pd) across the solenoid is measured using a datalogger.

The graph shows the variation of the pd across the coil with time.

Explain the shape of the graph by

The coil has 2 200 turns.   

Calculate the magnitude of the maximum rate of change of magnetic flux.

The magnet is displaced vertically so that it initially oscillates with simple harmonic motion. Apply Lenz's law to this situation.

Outline how the simple harmonic motion of the oscillations would be affected by replacing the magnet with one of equal mass but with a greater magnetic field strength.

A vertical copper rod is attached to the mass of a mass-spring system so that it can be made to oscillate with simple harmonic motion with a time period T at right angles to a uniform magnetic field.

The graph shows the variation with time t of the horizontal displacement x of the rod.

Sketch a graph to show

The length of the rod is 25 cm and the magnitude of the magnetic field is 68 μT. At the equilibrium position the rod is moving at 5.1 m s⁻¹.

Determine the magnitude of the maximum emf ε_max ­between the ends of the rod.

Explain your reasoning.

The frequency of the motion is doubled without any change in the amplitude of the motion.

State and explain the changes this causes to the time, t and the induced emf.

Conducting wire is wrapped around a circular disk of diameter 10 cm to create a loop with 145 turns. The loop is held in a constant magnetic field with field strength 0.36 T.

Determine the magnetic flux through the loop when the angle between the disk and the magnetic field is 10^o.

A toy car with a magnet attached so that the North pole faces upwards is released from the top of a ramp. The car rolls down the ramp at constant speed. An emf is induced in a coil of wire as the toy and magnet pass underneath it. 

The induced emf is recorded by a datalogger attached to the coil.

The graph shows the variation of induced emf.

The length of the magnet is 3.0 cm and the diameter of the coil is 1.5 cm.

Using the graph, determine the speed of the car between positions A and B.

The slope of the ramp is increased, making the car accelerate when released. It reaches point A at 1.86 s. 

Apply Faraday's Law to explain how this would affect the induced emf at points A and B.

Sketch a graph to show the new induced emf as the magnet moves from point X to point B.

An electromagnetic braking system uses a metal disk, attached to the wheel of the vehicle so that they rotate together. An electromagnet is placed such that the poles are on either side of the rotating disk, but do not touch it.

When the driver applies the brakes a direct current passes through the coil of the electromagnet.

Explain with reference to appropriate laws of electromagnetic induction how this design can produce a braking effect.

Conventional braking systems use friction pads which are brought into contact with a rotating disk to slow down vehicles.

Distinguish between the electromagnetic and conventional braking systems, identifying at least two advantages and two disadvantage of the electromagnetic system.

Induced current can be used in switching devices. A current in point P is made to flow in a clockwise direction when viewed from position O.

Outline how Lenz's law applies to the direction of the flow of induced current when the switch is opened and then closed.

The coil at P has 50 turns and an area of 15.8 cm². It produces a constant magnetic field of 0.26 T.  The two coils are identical. 

For coil Q