Make Electronics

Experiencing Electricity 27

Experiment 4: Varying the Voltage

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How much voltage does a wire consume?

Normally, we can ignore the resistance in electric wires, such as the little leads
of wire that stick out of resistors, because it’s trivial. However, if you try to force
large amounts of current through long lengths of thin wire, the resistance of
the wire can become important.

How important? Once again, we can use Ohm’s Law to find out.

Suppose that a very long piece of wire has a resistance of 0.2Ω. And we want to
run 15 amps through it. How much voltage will the wire steal from the circuit,
because of its resistance?

Once again, you begin by writing down what you know:

R = 0.2

I = 15

We want to know V, the potential difference, for the wire, so we use the version
of Ohm’s Law that places V on the left side:

V = I × R

Now plug in the values:

V = 15 × 0.2 = 3 volts

Three volts is not a big deal if you have a high-voltage power supply, but if you
are using a 12-volt car battery, this length of wire will take one-quarter of the
available voltage.

Now you know why the wiring in automobiles is relatively thick—to reduce its
resistance well below 0.2Ω. See Figure 1-69.

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Figure 1-69. When a 12-volt car battery runs some kind of electrical device through
a long piece of thin wire, the resistance of the wire steals some of the voltage and
dissipates it as heat.