Leaving Cert Physics long questions on Electromagnetic Induction
2008 Question 8
(i) What is electromagnetic induction?
(ii) State the laws of electromagnetic induction.
(iii)A bar magnet is attached to a string and allowed to swing as shown in the diagram. A
copper sheet is then placed underneath the magnet.
Explain why the amplitude of the swings decreases rapidly.
(iv) What is the main energy conversion that takes place as the magnet slows down?
(v) A metal loop of wire in the shape of a square of side 5 cm enters a magnetic field of
flux density 8 T.
The loop is perpendicular to the field and is travelling at a speed of 5 m s–1.
How long does it take the loop to completely enter the field?
(vi) What is the magnetic flux cutting the loop when it is completely in the magnetic field?
(vii) What is the average emf induced in the loop as it enters the magnetic field?
2007 Question 12 (c)
(i) State Faraday’s law of electromagnetic induction.
(ii) Describe an experiment to demonstrate Faraday’s law.
(iii)A resistor is connected in series with an ammeter and an ac power supply. A current flows in
the circuit. The resistor is then replaced with a coil. The resistance of the circuit does not
change.
(iv) What is the effect on the current flowing in the circuit? Justify your answer.
2006 Question 11
Read the following passage and answer the accompanying questions.
The growth of rock music in the 1960s was accompanied by a switch from acoustic
guitars to electric guitars. The operation of each of these guitars is radically
different.
The frequency of oscillation of the strings in both guitars can be adjusted by
changing their tension. In the acoustic guitar the sound depends on the resonance
produced in the hollow body of the instrument by the vibrations of the string. The
electric guitar is a solid instrument and resonance does not occur.
Small bar magnets are placed under the steel strings of an electric guitar, as shown. Each magnet
is placed inside a coil and it magnetises the steel guitar string immediately above it. When the
string vibrates the magnetic flux cutting the coil changes, an emf is induced causing a varying
current to flow in the coil. The signal is amplified and sent to a set of speakers.
Jimi Hendrix refined the electric guitar as an electronic instrument. He showed that further
control over the music could be achieved by having coils of different turns.
(Adapted from Europhysics News (2001) Vol. 32 No. 4)
(a) How does resonance occur in an acoustic guitar?
(b) What is the relationship between frequency and tension for a stretched string?
(c) A stretched string of length 80 cm has a fundamental frequency of vibration of 400 Hz.
What is the speed of the sound wave in the stretched string?
(d) Why must the strings in the electric guitar be made of steel?
