Microphones and Speakers

Microphones and speakers are very similar in their internal structure and the manner in which they function. One is in effect the reverse of the other. Both convert between sound energy and electrical energy.

A microphone converts sound energy to electrical energy via electromagnetic induction.

A speaker converts electrical energy into sound energy via the motor effect.


A microphone is built with a paper or flexible plastic cone connected to a small magnet that can move if the cone moved up or down. This sits inside an iron core with a coil wrapped around it. This coil of wire is connected to the output wires.

Applications, figure 1

A sound wave is a series of compressions in the air and as these hit the flexible cone they make the cone vibrate. These vibrations then move the inner magnet. The moving magnet induces an alternating current in the coil and the output wires. The voltage variation in the output wires mirror the frequency and amplitude of the sound wave, thus the sound wave is converted to an electrical output that can be recorded, amplified etc.

__Speaker __

A speaker works in the opposite way to a microphone. The internal structure is almost identical, with the exception that the magnets and the flexible cone are generally larger in external loudspeakers and carry higher currents than a microphone.

The varying voltage in the input wires from the amplifier cause a varying magnetic field in the coil of wire, this in turn attracts or repels the inner magnet and makes the flexible cone vibrate. This vibration pushes on the air next to the cone and generates a sound wave whose amplitude and frequency mirror the voltage variation in the input wires.


A transformer can convert voltages in a circuit by using electromagnetic induction, transformers that increase voltage are known as step-up transformers, those that decrease the voltages are called step-down transformers.

Transformers can be found in the many domestic devices that need a lower voltage than the 230 V supplied by the mains. A laptop, for example, is connected to the mains via a slim black box, this is a step down transformer to changes the 230 V mains to the approximately 19 V which is the voltage used in many laptops.

Inside a transformer is a soft iron core, at either end of which is wrapped a coil of wire.

Note: there is no electrical connection between the two coils of wire. No electricity ever flows from one coil to the other, ____this is a common misunderstanding and error in exams.__ __

__An AC voltage __in the primary coil causes an alternating magnetic field to be produced in the iron core round the primary coil, just as it does in an electromagnet.

Applications, figure 1

This in turn produces an alternating magnetic field in the whole iron core.

The alternating magnetic field near the secondary coil induces an alternating voltage in the secondary coil.

The ‘connection’ between the coils is the alternating magnetic field, not a current passing through the iron core.

The magnitude of a voltage (potential difference) produced in the secondary coil is proportional the number of coils. The magnitude of the magnetic field produced in the primary coil is also proportional to the number of coils in the primary coil.

As a result the ratio of the voltage (pd) change is equal to the ratio of the number of turns in the primary coil compared to the secondary coil, and be expressed as an equations.

Applications, figure 2

Applications, figure 3


If the transformer in the diagram above had 230V AC across its primary coil, what would the pd be across the secondary coils?

Applications, figure 4

The output p.d. is lower than the input pd, so this is an example of a step-down transformer.

National Grid

The National Grid is the UK’s network of electrical supply cables, power stations and substations that ensures a reliable and consistent supply of electricity to all user. This involves transferring electrical energy across thousands of miles of cables to cover the whole of the UK, in fact the grid extends into our European neighbours of France and the Republic of Ireland.

Applications, figure 1

As electricity flows in any conductor, the conductor will heat up due to the current in the conductor, this heating effect results in the loss of energy as the electricity is transmitted across the thousands of miles of cables that form the National Grid. To reduce this energy loss the electricity is stepped up to very high voltages as it leaves the power stations. In the main National Grid overhead lines the voltage can be as high as 400,000 V. Before the electricity can be feed into the domestic supply it is stepped down to 230V for safety reasons.

High Voltage Power Transmission

To achieve the changes in voltages required for efficient transmission and then the safe delivery to homes, school and factories etc, the National Grid has a series of transformers placed at interchange points. These are known as substations. Every power station has a large substation to step up the voltage transmitted to the outgoing power lines. The high voltage lines from the main grid are stepped down near towns and cities to a local supply around 500 V and then the local substations step it down to 230V for delivery to homes etc.

In any conductor the heat loss will be higher if the current flowing in it is higher. The aim of the high voltages in the National Grid is to produce a low current for a given amount of power delivered to the consumer.

Recall that power is the energy transferred per second in Watts. A home in the UK for example has a 15 A supply at 230 V AC voltage.

As Power = Current x Voltage 15 x 230 = 3,450 Watts for a domestic supply.

But as energy can not be created and is conserved, any energy transferred to the house must be supplied by the Grid. if the grid ran at 230 V that would require a 15 A current in all the lines in the Grid. This would results in the wires being very hot, not only wasting energy but also potential a very dangerous situation.

However, as the voltage is 400,000 V, to deliver the 3.450 W would only need;

Power = Current x Voltage so Current = Power Voltage = 3,450 400,000 = 0.008625 A

This very low current results in very low levels of heating and makes the Grid more efficient.

The relationship between supply and the grid can be expressed for an individual transformer in the system as;

Power of primary coil = Power of secondary coil

Current in primary coil x pd across primary coil = Current in secondary coil x pd across secondary coil

Ip_ x V_p_ = I_s_ x V_s

Example: A transformer at a power station steps up a 50,000 V supply to an intermediate grid at 150,000 V at 0.023 A, What was the current supplied by the power station to the transformer?

Applications, figure 1

Explain how a speaker converts a data signal into a soundwave.
Your answer should include: alternating voltage / coil / fluctuating magnetic field / iron core
Explanation: The data signal is an alternating voltage supply and creates an alternating voltage across a coil wrapped an iron core inside the speaker. This creates a fluctuating magnetic field in the iron core. Within this core is a magnet connected to a flexible cone of paper or plastic. As the iron core’s magnetic field fluctuates the magnet is made to vibrate by the repulsion and attraction of the magnetic field. This in turn vibrates the cone, causing sound waves in the air in contact with it.
Why does the National Grid use high voltages across its main supply lines?
Your answer should include: low current / flow / supply lines
Explanation: The high voltages used in the main supply lines of the National Grid, allow a very low current to flow in these lines for any given amount of power delivered to a customer. As the heat lose from a conductor is proportional to the current, these low currents reduce the energy lost to heat in transmission making the Grid more efficient.