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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