All metals contain a movable substance called "electric charge". Even
uncharged wires are full of charge! After all, the atoms of the metal are
made half of positively-charged protons, and half of negative electrons.
special because their electrons don't stay connected to the metal atoms,
instead they constantly fly around inside the metal and form a type of
"liquid" inside the wires. All wires are full of electric fluid. Modern
scientists call this liquid by the name "electron sea" or "electron
gas," or the "sea of charge." The fluid
charge is movable, and this lets metals be electric conductors. The
movable charge-stuff is not
invisible, it actually gives metals their silvery shine. The electron gas
is like a silvery fluid. Sort of.
Whenever a circle of wire surrounds a magnetic field, and if the
magnetic field then changes, a circular "pressure" called Voltage appears.
The faster the magnetic field changes, the larger the voltage becomes.
This circular voltage trys to force the movable charges inside the wire to
rotate around the circle. In other words, moving magnets cause changing
magnetic fields which try to create electric currents in closed circles of
wire. A moving magnet causes a pumping action. If the circuit is not
complete, if there is a break, then the pumping force will cause no charge
flow. Instead, a voltage difference will appear at the ends of the wir
es. But if the circuit is "complete" or "closed", then the magnet's
pumping action can force the electrons of the coil to begin flowing. A
moving magnet can create an electric current in a closed circuit. The
effect is called
Electromagnetic Induction. This is a basic law of physics, and it is
used by all coil/magnet electric generators.
Generators don't have just one circle of wire.
Suppose that many metal circles surround the
moving magnet. Suppose that all the circles are connected in series to
form a coil. The small voltage from each circle will add together
to give much larger voltage. A coil with 100 turns will have a hundred
times more voltage than a one-turn coil.
If you want a DC generator, you'll have to add a special reversing switch
to the magnet shaft. It's a switch called a "commutator." All DC
generators have these. After every half-turn, it reverses the connection
to the coil. That way it comes out as pulsed DC. If you look up some DIY
projects for DC generators, you'll see how to build the commutator switch.
But those generators aren't Ultra Simple!
Now for the light bulb. If we connect the ends of the coil together, then
whenever the magnet moves, the metal's charges will move and a large
electric current will appear in the coil. The coil gets slightly warm.
What if we instead connect a light bulb between the ends of the coil? A
light bulb is really just a piece of thin wire. The charges of the light
bulb's filament will be pushed along. When the charges within the copper
wire pass into the thin light bulb filament, their
speed greatly increases. When the charges leave the filament and move
back into the larger copper wire, they slow down
again. Inside the narrow filament, the fast-moving charges heat the metal
by a sort of electrical "friction". The metal filament gets so hot that
it glows. The moving charges also heat the wires of
the generator a bit, but since the generator wires are
so much thicker, and since the bulb's thin filament is slowing the current
throughout the entire coil, almost all of the heating takes place in the
light bulb filament.
So, just connect a light bulb to a coil of wire, place a short powerful
magnet in the coil, then flip the magnet fast. The faster you spin the
magnet, the higher the voltage pump-force becomes, and the brighter the
light bulb lights up. The more powerful your magnet, the higher the
voltage and the brighter the bulb. And the more circles of wire in your
coil, the higher the voltage and the brighter the bulb. In theory you
should be able to light up a normal 3V flashlight bulb, but only if you
can spin your magnets inhumanly fast.
Disconnect one wire from the light bulb. Spin the magnet. While
still spinning the magnet, have a friend touch the wires together
so the bulb lights up again. Is the nail still easy to spin?
Keep spinning the magnet while your friend connects and disconnects
the bulb. Feel any differences in how hard you must spin the nail?
Also try spinning the magnets while your friend connects the generator
wires directly together (with no bulb connected.)
SO WHAT?When you crank the generator and make the lightbulb turn on, you are working against electrical friction in order to create the heat and light. You can FEEL the work you perform, because whenever you connect the bulb, it suddenly gets harder to crank the generator. When you disconnect the bulb, it gets easier.
Think of it like this. If you rub your hands together lightly, the skin
stays cool, but if you rub your hands together hard, your skin gets hot.
It takes more effort to rub skin hard so that it heats up;
it takes work. And in a similar way, it's hard to heat the lightbulb
filament, it takes work. You twist the generator shaft, the generator
pushes the wire's charge through the tiny filament, and if you don't keep
spinning the magnet, the magnet will be slowed quickly.
TURN OFF THE FIELDTry changing the magnets' position. Remove the magnets, then tape them around the nail so that the two stacks are clinging side by side, rather than stacked up in a line. Spin the magnets. Does the light bulb still light up? No. This happens because The N pole of one magnet stack is very close to the S pole of the other, and vice versa. The magnetic field is now stretching between the two stacks of magnets, and isn't spreading outward. Most of the field is trapped between the neighboring opposite poles, so the field doesn't extend out through the coil. When magnets are side by side like this, they form one larger but weak magnet. On the other hand, when you make a single stack of magnets instead, the field extends outwards for many inches. The stacked magnets form a larger but very strong magnet. If you spin the single magnet stack, the field cuts through the wires and pumps their electrons into motion.
MEASURE THE VOLTAGE AND CURRENTIf you can get a Digital Voltmeter or DVM, you can make some measurements. (Once you can see some numbers, you can perform some professional science experiments. This is great for science fair projects.) Spin the magnets to light up the bulb, then connect the meter leads across the light bulb connections. Set the meter for AC volts. Spin the magnets and see just how high a voltage your generator produces.
How high can you make the voltage just
by using fingers? Or using a hand drill? Try spinning the magnets just
fast enough to barely light the bulb in a dark room. How small a voltage
is needed? Also try
light bulb, then measure the AC voltage across the two ends of the coil.
Can you tell if it's still the same as when the bulb was connected? Hint:
to spin the magnets at a constant rate, use an electric drill with a
fully-charged battery. Or perhaps hook the nail to an electric motor and
connect the motor to a DC power supply with settable voltage.
Note: The light bulb has around 50 ohms resistance. Also, 250ft of #30
wire has around
21 Ohms resistance. Because of the wire resistance, the
generator can only create around 60 milliamps current at most (0.06
amperes.) If you wind extra #30 wire onto the generator, it will increase
the maximum voltage, and maximum power. But since this adds more
resistance it WON'T increase the maximum possible current. To increase
the maximum possible current, either replace the #30 wire with thicker
wire, spin the magnets faster, or use a stronger type of magnet material.
MOTOR CHALLENGE!There is a simple way to convert your generator into a motor. It involves using paint or tape to insulate a spot on one side of the nail, then using a 6V battery and using the generator's wires, touching the nail to form a switch. The rotating magnets turn the nail, which turns the coil on and off at just the right times. Can you discover the trick?
MAKING DCYou can change this generator so it makes DC rather than AC. The voltage is still very low, so it's not very useful. If spun very fast, you might be able to recharge a tiny 1.2v rechargeable battery. (Maybe you could add lots more turns of wire to the coil to increase the voltage?)
Converting to DC:
The hard way: add a spinning "commutator" switch and sliding metal "brushes," so that each time the magnets turn half way, the switch reverses the generator connections.
HISTORY OF "ULTRA SIMPLE" GENERATORWhile running the tech shop at the Museum of Science in Boston, I was working on new ideas for exhibits for the Electricity Hall in 1988. I knew that the Exploratorium had an electric generator exhibit where the museum visitor would yank a plastic-embedded coil plate through a row of huge magnets (magnetron horn-magnets from a military radar.) Doing so would light up a small bulb. I just knew that there HAD to be a way which uses more common magnets. So I stacked up a pile of 3" loudspeaker magnets (those black donut things) and waved it past various coils. Finally I wound about five pounds of #26 wire around a ring of nails pounded into a board, hooked up a #49 light bulb, then moved the stack of speaker magnets in and out. This easily lit up the bulb.
Around 1994 I was thinking about the ultra-simple electric motor which
later became known on internet as the "Beakman Motor." Wouldn't it be
cool if kids could also make an electric generator that simple?
But it should be possible with parts from a Radio Shack store, since Radio
Shack had the special light bulb as well as magnets and spools of
electromagnet wire. After a few hours of experimenting I fould that I
could just barely light up the 20 milliamps bulb by using a single spool
of #30 wire from radio shack. But the wire had to be VERY close to a fast
spinning magnet, and the magnet had to be composed of four powerful
ceramic magnets in a stack.
To impress all the Physics Teachers, I tried to make the parts be easily available, and the cost as low as possible. To make a popular project, I made sure no tools were needed except scissors. I refused to use ball bearings or plastic parts. So I made my own cardboard box for the coil, and used a nail for the spinning shaft. To avoid extra parts, the nail is just clamped by the powerful magnets. Here's a challenge: try to light a bulb, but do it with a generator which is even simpler.