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Here is a way to think BIG with the idea of the lemon
battery. We did this using a galvanized (zinc-plated)
tub, a stew pot with a copper bottom, and a bottle of
lemon juice.
With scouring powder and steel wool, we cleaned some
of the cooking residue from the bottom of the pot so
that the copper would have a more direct contact in
the cell.
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 Next,
we poured filtered water into the tub to a depth of
about 1.2 cm (about 1/2 inch), and placed three plastic
film canister tops in the water for the pot to rest
on. You can use any small objects for the pot to sit
on as long as they are made of plastic, glass, wood,
stone, or some other material that does not conduct
electricity. So don’t use metal. The idea is to
keep the bottom of the pot slightly above, and not touching
the inside bottom of the tub.
Next, we placed the pot on top of the canister tops
so that the copper was in contact with the water. We
then stirred in about 35 mL (about 1/8 cup) of bottled
lemon juice. The exact amount is probably not important.
We used enough to make a diluted solution of lemon juice.
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Our “tub” electric cell was ready!
First, we used a multimeter to get an idea of
the electrical performance. Just as with the
lemon
electric cell, the copper on the bottom of the
pot was the positive (+) terminal, and the zinc-coated
tub was the negative (-) terminal. These are
shown
with the red and the black wires respectively
in setup in the photo below. |
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The results were rather interesting. The voltage was
about the same as that for a lemon cell, but a bit higher.
We expected this, because the voltage in such a cell
is really due to the differences in the electrical characteristics
of the copper and the zinc in the presence of an acid
(in this case, the citric acid and other acids in the
diluted lemon juice). In the photo below, the multimeter
is set on the 2.5 volt scale. The needle reads a little
over 1.0 v. In the lemon cell, we got about 0.9 v. Perhaps
we are getting more voltage here because there is only
liquid between the zinc and the copper in the tub. In
the lemon, there are fibers and other material besides
the lemon juice; this may produce some resistance to
the flow of electricity.
Things really turned impressive when we measured the
current. In the photo on the right above, the multimeter
is set for the 500 milliamp (mA) scale. This means that
the full scale reading would correspond to 0.5 amp.
According to the scale, we were getting more than 150
mA through the meter. With the lemon cell, we got about
1/1000th as much current! We think this is because we
have more zinc and copper in contact with the acid in
the tub cell than in the lemon cell.
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 Next,
we wanted to find some other way besides using a multimeter
to show that this setup produces a voltage and an electric
current. We hooked up an LED, and guess what? It didn’t
light!! The reason was the voltage was not high enough.
So we started thinking about the large electric current.
Maybe we could find or make something to respond to
that. The first thing we tried was wrapping some wire
around a magnetic compass. We knew that electrical current
passing through such loops creates an electromagnet.
If we could make an electromagnet strong enough, it
would move the needle of a magnetic compass. This will
work only if there is enough current in the wire. Here
is a photo of the wire wrapped four times around our
compass. The compass needle is lined up in a north-south
direction because of the earth’s magnetic field.
In the photo, the wrapped wires are also lined up in
a north-south direction, with the compass needle lined
up beneath them.
Next, we hooked this up to the terminals of the tub
cell.
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The needle moved! Here are photos of the compass needle
with the circuit incomplete (left photo) and the circuit
complete (right photo). Can you see the difference in
the angle of the needle?
The compass we used was filled with liquid. This compass
was designed this way to reduce the vibrations of the
needle so that it could be read more easily. However,
it also gave the needle some resistance to movement.
We thought of using a more “jittery” compass,
one whose needle would not need as much force to make
it move. We started out with a single loop of wire,
just for fun.
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When we connected the circuit, we saw the needle move!
Note: The needle in this photo is moving in the
opposite direction of the needle in the previous compass.
Can you tell why? Hint: Look at the way the wire is
looped.
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We then unwrapped the loop and tried it with just the
wire running over the top of the compass. Since the
wire did not loop, the current in the wire would not
make a very strong magnetic field. Would it be strong
enough to move the needle? The only way to find out
was to try it!
As you can see in the right photo above, the needle
moved a little when the circuit was connected! This
was truly amazing, because it let us experience the
same results as Hans Oersted did in 1819 when he first
discovered this connection between electricity and magnetism
using a single wire and a compass just as we did.
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See, for example, this
biography of Oersted. He didn’t have a tub cell
for his source of electricity! But who would have thought
that our tub of diluted lemon juice would connect us to one
of the most important moments in physical science?
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What do you think will happen if we take the copper-bottom
pot out of the tub and use a single penny instead?
- Do you think that we could get an LED to light if we connect
two lemon cells in series with the tub cell? Why or why
not?
- If we made two other tub cells and connected them in
series, do you think we could light a flashlight bulb? What
else should we try?
- What do you think will happen to the tub if we let the
tub cell run for several days?
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