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Crank Power What to do: Turn the crank to generate electricity. What happens: You can light up the bulbs or store electricity in a capacitor. How it works? 1. The generator is like a bike dynamo. When you turn a dynamo, a magnet spins round next to a coil of copper Bike Dynamo wire. The faster the magnet turns, the bigger the voltage generated between the ends of the coil, so the bigger the electric current the dynamo produces. In this generator, however, a strong magnet remains stationary and the coil of wire spins round next to it, so a similar voltage is set up across the coil’s ends. It doesn’t matter which one moves, the magnet or the coil. If it is the coil, sliding contacts have to be used to connect the spinning coil Wire coil to the rest of the circuit. Strong magnet Sliding contacts Electric current flows Electric current flows 2. The magnet is U-shaped, so that the ends are close together. This makes a strong magnetic field between the two ends and the coil is spun round in this strong field. There is a voltmeter (an electrical “pressure” gauge) connected across the coil so you can read the electrical pressure (voltage) which you are generating. The faster you turn, the higher the voltage you produce. Thin filament gets very hot (2500°C) Electric current Electrical pressure gauge (Voltmeter) Strong (but invisible) magnetic field 3. There are two bulbs in the exhibit, which can be connected to the generator. Each one has a very thin piece of tungsten wire inside, for the electric current to pass through. The electric current makes the thin wire get so hot that it glows brightly. To understand this, you need to know about electrons! The UK’s Leading Science Discovery Centre Stuart Street Cardiff CF10 5BW ©1998 T 029 20 475 475 F 029 20 482 517 e-mail: [email protected] Nucleus 4. An electric current is a flow of trillions of tiny particles, called electrons, which carry one of Nature’s mysteries, electric charge. All atoms have a “cloud” of electrons orbiting round the atom nucleus. The atoms of all metals (copper, iron etc) have a few of their electrons at the edge of the Metal atoms can easily lose the outermost electrons cloud which can easily become Wire cutting detached and wander off! Whenever a across piece of wire moves near a magnet, the field these unattached electrons are pushed along the wire by the magnetic field. The faster the wire moves past the magnet (or vice versa) the greater the pressure to drive the electrons. As the Electrons pushed along the wire electrons move between the metal atoms in the wire, they bang into these atoms and make them vibrate more - that means the wire gets warmer. If the wire is very thin, Electrons collide with atoms like the bulb filament, it gets so hot that it Too few glows! and jiggle them about Electrons leave this plate 5. The capacitor has a pair of metal plates which are very close together, with a thin layer of insulation between them. If you connect the capacitor to the generator and spin it round, electrons from one plate are pulled through the generator and piled up on the other plate - the capacitor is charged up! The faster you turn the generator, the more electrons you displace in the capacitor, so you charge it up to a bigger voltage. Bulb gets hot Voltage falls again Generator electrons so the + charge on the nuclei shows through Electrical pressure rises 6. Once the capacitor is charged, you can leave it as long as you like, until you want to discharge it. Connect the capacitor to the bulb (press the button) and the piled up electrons will flow through the bulb to the other plate until there is no electrical pressure left. You will notice that the bulb starts off very bright (large voltage, large current) and gradually gets dimmer as the voltage drops to zero. 7. You probably also noticed that you had to push the crank harder when you were lighting up the brighter of the two bulbs. Another of Nature’s mysteries is that you never get anything for nothing. Pushing a bigger current to make a brighter light means that you have got to push harder! Electrons flow back again Did you know? Michael Faraday made the first electrical generator in the 1830s. You can see a picture of it on a £20 note - it has a large U-shaped magnet and a copper disc (rather than coil of wire) which spins between the north and south poles of the magnet. Power stations use powerful engines (steam turbines, water turbines, gas turbines etc) to spin electromagnets round inside their generators. Generators are large - a single generator at Aberthaw Power Station (near Cardiff) can light 5 million light bulbs, each bulb producing 100 watts. The UK’s Leading Science Discovery Centre Stuart Street Cardiff CF10 5BW Capacitors are used instead of batteries in many computers to keep the memory, clock etc going for a few days after you switch off. T 029 20 475 475 F 029 20 482 517 e-mail: [email protected] ©1998