# OCR - P3: Electric circuits

 P3.7 What is the process inside an electric generator? (separate science only) Background to the topic What you should be able to do: Mains electricity is produced using the process of electromagnetic induction. When a magnet is moving into a coil of wire a potential difference is induced across the ends of the coil; if the magnet is moving out of the coil, or the other pole of the magnet is moving into it, there is a potential difference induced in the opposite direction. If the ends of the coil are connected to make a closed circuit, a current will flow round the circuit. In a moving coil microphone sound waves cause a diaphragm to vibrate. The diaphragm is attached to a coil which is in the field of a permanent magnet. Sounds make the coil vibrate, inducing a changing potential difference across the ends of the coil. This potential difference drives a changing current in an electric circuit. In a generator, a magnet or electromagnet is rotated within a coil of wire to induce a voltage across the ends of the coil. The induced voltage across the coil of an alternating current (a.c.) generator (and hence the current in an external circuit) changes during each revolution of the magnet or electromagnet. To generate a d.c. split-ring commutator is used so that the current always passes from the same side of the generator. 1. Recall that a change in the magnetic field around a conductor can give rise to an induced potential difference across its ends, which could drive a current. 2. Explain the action of a moving coil microphone in converting the pressure variations in sound waves into variations in current in electrical circuits. 3. Recall that the direction of the induced potential difference drives a current which generates a second magnetic field that would oppose the original change in field 4. Use ideas about electromagnetic induction to explain a potential difference/time graph showing the output from an alternator being used to generate a.c. 5. Explain how an alternator can be adapted to produce a dynamo to generate d.c., including explaining a potential difference/time graph 6. Explain how the effect of an alternating current in one circuit in inducing a current in another is used in transformers 7. Describe how the ratio of the potential differences across the two circuits of a transformer depends on the ratio of the numbers of turns in each