Electromagnetic Devices (HT Triple Physics Only)
Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.
The cone of the speaker vibrates to make sound waves we can hear
The cone of the speaker is attached to a magnet
The magnet is close to a coil of wire
An alternating current representing the sound we want to produce flows in the coil
The current in the coil has its own magnetic field which attracts/repels the magnet making the magnet move backwards and forwards. This motion makes the cone move to produce sound waves.
Generator Effect ( HT Triple Physics Only)
If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect.
An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field.
The size of the induced potential difference/induced current can be affected by:
- The strength of the magnetic field
- The speed with which the wire moves relative to the magnetic field
- The length of the wire in the magnetic field
The factors that affect the direction of the induced potential difference/induced current are:
- The direction of the magnetic field
- The direction of motion of the wire
The generator effect is used in an alternator to generate ac and in a dynamo to generate dc. When the coil is moved near the magnet, a potential difference is induced in the coil. If the coil is in a complete circuit then a current will flow. The current will change direction every time the coil turns 180°.
Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits.
As the sound reaches the microphone, the membrane is pushed back wards and forwards.
The membrane is attached to a coil, which also vibrates backwards and forwards.
The magnet is near a coil of wire. When the coil moves, a potential difference is induced in the coil.
Because the coils in a completed circuit, an alternating current will flow.
This current is an electrical signal that represents the sound that reaches the microphone.
Question: Explain how a moving-coil microphone works. (4 marks)
- sound (waves) cause the diaphragm/membrane to vibrate
- the diaphragm/membrane causes the coil / wire to vibrate
- the coil / wire moves through the magnetic field
- a potential difference is induced (across the ends of the coil / wire)
Question: The number of turns of wire on the coil of an AC generator is increased. This increases the maximum induced potential difference.
State two other ways in which the maximum induced potential difference could be increased.
- increase speed of coil
- strengthen magnetic field
- increase area of coil
Transformers (HT Triple Physics Only)
A basic transformer consists of a primary coil and a secondary coil wound on an iron core. Iron is used as it is easily magnetised.
The ratio of the potential differences across the primary and secondary coils of a transformer Vp and Vs depends on the ratio of the number of turns on each coil, Np and Ns
How a Transformer Operates
An alternating current is supplied to the primary coil.
That alternating current has a magnetic field, which induces an alternating magnetic field in the iron core
The alternating magnetic field int he iron core induces an alternating current in the secondary coil
If transformers were 100% efficient, the electrical power output would equal the electrical power input.
Vs × Is = Vp × Ip
Where Vs × Is is the power output (secondary coil) and Vp × Ip is the power input (primary coil).
Question: A transformer has a primary coil with a potential difference of 4V and a secondary coil with a potential difference of 12V. Calculate the current in the primary coil of the current in the secondary coil is 3A.
Vs × Is = Vp × Ip
12 x 3 = 4 x primary current
primary current = (12 x 3) / 4
The primary current is 9 Amps
Question: Explain how a transformer works. (4 marks)
- an alternating current in the primary creates a alternating magnetic field
- so a magnetic field is induced in the (iron) core
- so an alternating current
- is induced across secondary coil
The ratio of the potential differences across the two coils depends on the ratio of the number of turns on each
Np/Ns = Vp/Vs
Where Np = number of turns on primary coil, Ns = number of turns on secondary coil, Vp = potential difference across primary coil and Vs = potential difference across secondary coil)
Question: A small step-down transformer is used in the charger for an electric screwdriver.
The input to the transformer is 230 V a.c. mains supply and the output is 5.75 V a.c.
There are 3200 turns on the primary coil.
Np/Ns = Vp/Vs
3200/Ns = 230/5.75
Ns = 3200 x 5.75 / 230
There are 80 turns on the secondary coil