Induced potential, transformers and the National Grid

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.

Note the two separate ways of producing it: move the conductor, or change the field. Either will do, and both amount to the same thing — the conductor experiencing a changing magnetic field.

An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field. That opposition is why a generator becomes harder to turn when more current is drawn from it.

You should be able to recall the factors that affect the size of the induced potential difference or induced current:

• the speed of the movement — moving the conductor or magnet faster gives a larger induced potential difference • the strength of the magnetic field — a stronger magnet gives a larger induced potential difference • the number of turns on the coil — more turns give a larger induced potential difference.

You should also be able to recall the factors that affect the direction of the induced potential difference or induced current:

• the direction of movement — reversing it reverses the induced potential difference • the orientation, or polarity, of the magnetic field — reversing the poles reverses it too.

Reversing both at once leaves the direction unchanged.

You should be able to apply the principles of the generator effect in a given context. The reasoning is always the same: identify whether the conductor is experiencing a changing field, say what would make the change faster or larger, and remember that no relative movement means no induced potential difference at all. A magnet held still inside a coil induces nothing.

The generator effect is used in an alternator to generate ac, and in a dynamo to generate dc.

You should be able to explain how the generator effect is used in each, and draw or interpret graphs of potential difference generated in the coil against time.

Both work the same way: a coil is rotated in a magnetic field, so the coil continually cuts through the field lines and a potential difference is induced. The difference between them is entirely in how the coil is connected to the external circuit.

Reading the graphs: for both, the potential difference is zero when the coil is parallel to the field lines, because at that instant it is not cutting through any, and it is greatest when the coil is at right angles to them and cutting through them fastest. Turning the coil faster increases both the frequency and the peak potential difference, so the humps get taller and closer together.

Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits.

You should be able to explain how a moving-coil microphone works:

1. Sound waves reach a flexible diaphragm, and the pressure variations make it vibrate. 2. A coil of wire attached to the diaphragm vibrates with it, moving within the field of a permanent magnet. 3. The coil is moving relative to a magnetic field, so by the generator effect a potential difference is induced across it. 4. Because the movement reverses as the diaphragm goes back and forth, the induced potential difference alternates. 5. The frequency of the induced current matches the frequency of the sound wave, and a louder sound moves the diaphragm further, giving a larger current.

This is a loudspeaker running backwards. A loudspeaker uses the motor effect to turn a current into movement and then sound; a microphone uses the generator effect to turn sound and movement back into a current.

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. Knowledge of laminations and eddy currents in the core is not required.

You should be able to explain how the effect of an alternating current in one coil in inducing a current in another is used in transformers:

1. An alternating current in the primary coil produces a continually changing magnetic field. 2. The iron core carries that changing field to the secondary coil. 3. The secondary coil experiences a changing magnetic field, so by the generator effect a potential difference is induced across it.

The current must be alternating. A steady direct current would produce a steady field, nothing would be changing at the secondary, and no potential difference would be induced — which is why transformers only work on ac.

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:

Vp / Vs = np / ns

potential difference, Vp and Vs, in volts, V. This equation is given on the Physics equation sheet.

In a step-up transformer Vs is greater than Vp — the secondary has more turns. In a step-down transformer Vs is less than Vp — the secondary has fewer turns.

You should be able to explain how the ratio of the potential differences across the two coils depends on the ratio of the number of turns on each.

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 at the secondary coil and Vp × Ip is the power input at the primary coil. Power input and output are in watts, W. This equation is also given on the Physics equation sheet.

You should be able to calculate the current drawn from the input supply to provide a particular power output.

You should be able to apply the equation linking the potential differences and number of turns in the two coils of a transformer to the currents and the power transfer involved, and relate these to the advantages of power transmission at high potential differences.

That last point ties back to the National Grid. Because the power is essentially unchanged, stepping the potential difference up necessarily steps the current down. The power wasted heating the transmission cables is given by P = I² R, so a smaller current wastes far less energy — and because the current is squared, the saving is dramatic. The potential difference is stepped down again before it reaches homes, because hundreds of thousands of volts would be far too dangerous to use.