Q & A: temperature and resistance

Q:
How does the surrounding temperature affect the electric current through the wire? I am working on a science fair project now, and I want to work on the influence of temperature on the electric current. My idea is to connect a 9-V battery with an ammeter under different temperature conditions, measure the current, and compare the results. Does this make a good science fair experiment? If not, is there any way to improve it or to make it more in depth? Also, can you give me some background information on this topic?
- Sam (age 16)
A:
Sam- This could be the starting idea for a good project. What you are really asking here is whether the temperature (T) of the wire in the ammeter affects how easily current can flow through it. In other words, you're asking if T affects the resistance, R.
There are a few problems with your first plan. The resistance of an ammeter is deliberately made very low, so you don't want to connect them directly to a battery, which also has a low internal resistance. The current will be huge, and will drain the battery even if nothing gets fried. Also, the wires in the ammeter are out of reach, so it's hard to change their T, and you don't know what they're made of, so the interpretation would be difficult. Sending that much current through the wire and battery will almost certainly heat one or both of them up, making it hard to control and measure the temperature.

Why not mkae some coils of different types of wire and simply measure their resistance with an ohmmeter at different temperatures? You can see whether current flows more easily in hot or cold wires. You could try to do the same thing with salt water. You could check how much difference T makes for different types of wires, and see if any important patterns emerge.

I should let you know that many such experiments have already been done, and the results are well understood, and have provided lots of information about the electrical properties of materials. That may be fine- I don't know if your science fair requires new work or encourages students to explore any project new for the student.

Mike W.

(published on 10/22/2007)