1. Look at the simple generator below.
a) Is the output voltage a.c. or d.c.?
a.c. – it is an alternating voltage.
b) What is the effect on the voltage of reversing the direction in which the coil rotates?
The voltage phase changes by 180 – i.e. it goes out of phase when the direction is swapped. If you said it is reversed, this is true but a bit simplistic.
c) When the coil turns faster, two features of the output voltage change. What are they?
The frequency and the peak value of the voltage.
d) When the output voltage is greatest, what is the position of the coil?
It is flat – i.e. the plane of the coil is parallel with the magnetic field.
e) When the output voltage is greatest, what is the direction of movement of the sides of the coil compared with the magnetic field?
The sides of the coil are moving at right angles to the field.
2. In the figure below, the electrons are moving from left to right. This is like a current flowing from right to left.
a) Why is the current in the opposite direction to the electrons’ movement?
The electrons have a negative charge. And conventional current is in the same direction as a flow of positive charge.
b) Explain why there is a force on a wire from which we take an induced current.
The wire is in a magnetic field and it is carrying a current. Therefore, there is a force on it.
c) The direction of the force opposes the motion that led to the induced current. Describe what would happen if this were not the case.
The wire would not be slowed down (indeed, it might even be speeded up). So we would be able to take out a current without the wire slowing down. Therefore, we would be getting something for nothing or we have the basis for a machine that creates energy from nothing. This goes against the idea of the conservation of energy.
d) Whose law is this?
3. Look at the circuit below. It shows a wire moving through a magnetic field. The wire is attached to a voltmeter. The magnetic flux density is 2 tesla and the wire is moving at 3 cm s-1. The length of the wire is 20 cm. Imagine we start timing the moving wire when it is 10 cm into the field.
a) What is the area of magnetic flux passing through the circuit?
Area = 0.4 × 0.1 = 0.04 m2
b) How much flux is passing through the circuit?
Φ = BA = 2 × 0.04 = 0.08 Wb
c) After 5 seconds, the wire will have moved through the field. How far will it have moved?
15 cm (speed × time) = 0.15 m
d) By how much will the flux have increased in those 5 seconds?
0.15 × 0.4 × 2 = 0.12 Wb
e) What is the EMF induced in the wire?
Rate of change of flux = 0.12 ÷ 5 = 0.024 V
Simple animation of d.c.generation. (Courtesy of Steven Carpenter.)