Thin and Thick Wires

Most recent answer: 01/17/2015

I did a science project with the question does the thickness of the wire affect the brightness of the light bulb. I used a 9volt battery and 12 14 16 20 gauges wire. Cut each 8 inches long. Used a automotive small light and hooked it to the battery. The light was the same for each wire. I looked on Internet and it says the light should have been dim with thinwire and bright with the thick wire. What have I done wrong?
- madison (age 10)
Ontario canada

This sounds like the start of a good experiment. My guess is that the wires you used just weren't thin enough to affect the brightness of the bulb. You need to use very thin wire (or a really long piece) to make the bulb dimmer. Here's why:

First, a little background that you may already know. A piece of thin wire has higher resistance than a piece of thicker wire with the same length, which means that it's harder for electric current to move through it. If you use the same battery but replace a thick wire with a thinner one, the electric current will get smaller and a light bulb in the circuit should get dimmer. Using a longer wire will also increase resistance.

The thinnest piece of wire you used, the 20 gauge, is less than a millimeter thick, which may seem small. But a typical light bulb still has about 2,000 times more resistance than the wire! That's why it glows—as the battery pushes current through the filament inside the bulb, the high resistance turns the electrons' energy into heat. The filament heats up to thousands of degrees and produces light. Compared to the light bulb itself, the wires you used just don't make much of a difference.

To get thinner wires, you can use some steel wool, which is made of tiny metal fibers (it's used for cleaning). Carefully separate out one of the fibers and use it as your wire. You could also try adding more steel fibers one by one, twisting them together, and see how that affects the brightness of the bulb. Can you think of a way you could measure, or quantify, the brightness?

Good luck!

Rebecca Holmes


(published on 01/17/2015)

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