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Question 1
A coil of wire with area 4 x 10-4 m2 is placed in a magnetic field as shown below. The magnetic field has a field strength of 2 x 10-2 T. The coil has 50 turns. |
a) | The coil rotates in the magnetic field. Calculate the maximum flux linkage experienced by the coil.
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b) | Calculate the flux linkage of the coil at the position shown in the diagram.
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c) | The coil rotates at 240 rpm. As the coil rotates an emf is induced in the coil. Calculate the maximum emf induced in the coil.
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d) | The diagram below shows how the flux linkage varies with time as the coil spins in the magnetic field. Model answer Sketch a graph to show how the induced emf varies with time.
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Question 2
A student is investigating electromagnetic induction. |
a) | State Lenz's law.
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b) | A bar magnet is held stationary near a coil of wire connected to a sensitive ammeter. The bar magnet is moved away from the coil of wire rapidly and then brought to rest. State how the reading on the ammeter changes.
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c) | Suggest two ways the magnitude of the induced current can be increased.
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Question 3
This question is about electromagnetic induction. |
a) | State Faraday's law.
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b) | The graph below shows how the flux linkage varies with time. As the coil rotates an emf is induced. State when the induced emf is a maximum.
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c) | The field has a flux density of 0.4 T. The coil 50 turns. The area of the coil is 0.03 m2. Calculate the maximum emf induced in the coil.
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d) | The diagram below shows the coil at time t. Calculate the flux linkage in the coil at the instant shown.
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Question 4
A metal wire of length 20 cm is moved through a magnetic field of flux density 0.4 T at a constant velocity of 30 cm/s. |
a) | Explain the principle of electromagnetic induction and how it applies to a straight wire moving through a uniform magnetic field.
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b) | As the wire moves through the field an emf is induced across it. Calculate the magnitude of the induced emf.
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c) | Draw an arrow on the wire to show the direction of the induced current in the wire as it moves through the magnetic field.
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d) | The velocity of the wire is doubled and the magnetic flux density is increased by a factor of 3. Determine the new emf induced.
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Question 5
A coil is rotating in a uniform magnetic field. |
a) | State the value of θ that would give the greatest flux linkage in the coil.
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b) | State the value of θ when the induced emf is greatest.
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c) | The diagram shows the position of the coil at time t = 0 s. The magnetic flux density of the field is 0.75 T. The coil completes 120 revolutions per minute. The coil has dimensions 0.1 m x 5 cm. There are 100 turns on the coil. θ = 60° Calculate the maximum induced emf in the coil.
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d) | The diagram shows the position of the coil at time t= 0 s. Calculate the time taken until the emf reaches its maximum value again.
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Question 6
A solenoid is connected to an A.C signal generator. A search coil is placed near the solenoid and an emf is induced in the coil. |
a) | Explain why the solenoid is connected to an A.C signal generator instead of a D.C power supply.
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b) | The search coil has 500 turns and a cross sectional area of 1.13 x 10-4 m2. The signal generator has a frequency of 50 Hz. Calculate the magnetic flux density if the maximum induced emf is 8.88 V.
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c) | Describe what happens to the maximum induced emf as the search coil is rotated through 90° until it is parallel with the magnetic field.
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Question 7
A student is investigating the process of electromagnetic induction by dropping a bar magnet through a coil of wire. The coil of wire is connected to a sensitive ammeter. |
a) | Explain why there is a current in the coil of wire when the bar magnet is dropped.
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b) | State two ways in which the size of the induced current could be increased.
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c) | When the magnet is dropped a positive current is induced. Describe how the student could drop the magnet to induce a negative current.
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c) | The student attaches the magnet to a spring causing it to oscillate up and down through the coil. The graph below shows the induced current in the coil. Explain which feature of the graph shows that the current is alternating.
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Question 8
Some bike lights are powered by a device called a dynamo. A dynamo consists of a spinning magnet inside a coil of wire. |
a) | When the bicycle is moving it causes the magnet to spin inside the dynamo. Explain what happens when the magnet spins inside the dynamo coil.
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b) | Explain why the bicycle lights turn off when the bicycle stops.
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c) | State two ways in which the output current from the dynamo could be increased.
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d) | The graph below shows the change in flux in the coil as the cyclist travels at constant velocity. Calculate the maximum induced emf in the coil.
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Question 9
This question is about the alternating current generator. |
a) | Explain how the generator can be used to produce a voltage.
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b) | The diagram below shows the output of the generator when turned at a constant rate. State the maximum output current.
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c) | Calculate the frequency of output voltage.
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d) | The handle is slowed and is now rotated at a frequency of 1 Hz. Calculate the new maximum emf induced in the coil.
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e) | Describe how the output waveform would vary if the strength of the magnet used decreased.
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Question 10
A student is investigating how the number of turns on a coil effects the maximum current induced when a magnet is dropped through it. |
a) | Explain what happens when the magnet is dropped through the coil.
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b) | The table shows the student's results. Model answer Plot a graph of maximum induced current vs number of turns.
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c) | The data contains an anomalous result. State which result is anomalous.
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d) | Estimate the maximum output current when the number of turns is 18.
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e) | Describe the relationship between the number of turns on the coil and the maximum induced current.
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f) | Increasing the number of turns on the coil increases the maximum induced current. State one other way of increasing the maximum induced current.
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Question 11
An AC supply is connected to a load resistor R. An oscilloscope displays the potential difference across the load resistor R. |
a) | The A.C. supply has a 5.66 V rms voltage. Calculate the peak voltage of the a.c. supply.
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b) | The A.C. supply has a frequency of 40 Hz. Draw the trace shown on the oscilloscope when the y-voltage gain is 2 V div-1 and the time-base is 5 ms div-1.
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c) | The amplitude of the a.c. signal is increased. State one change to the oscilloscope controls that would enable the full waveform to be viewed.
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Question 12
This question is about alternating current. |
a) | The mean value of an alternating current is zero. Explain why heating occurs when there is an alternating current in the circuit.
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b) | An alternating current has an rms voltage of 4 V. Calculate the peak voltage of the alternating current.
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c) | Electricity is distributed via the National Grid using step up transformers and pylons. Explain why electricity is distributed using high alternating voltages.
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d) | The diagram below shows an oscilloscope trace of an alternating UK mains supply. State the time-base setting for the oscilloscope.
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