MadSci Network: Physics

Re: what are the properties of a conductor that affest the electric resistance?

Date: Sun Feb 1 17:21:37 1998
Posted By: William Beaty, Engineer / Interactive exhibit designer
Area of science: Physics
ID: 884405988.Ph

Hi Lorielyn!

A good way to think about electrical resistance is to visualize something similar. For example, the flow of electric charge through a wire is similar to the flow of water through a pipe that's full of sand. Electric current doesn't flow freely through wires. It must be pushed hard, and even with a large push, the charge still moves very slowly. Electric current doesn't flow like water through a pipe, instead it flows like water through a sand-filled pipe.

Q: How does the length of a conductor affect the electric resistance?

A: If we try to force water through a sand-filled pipe, we'll find that it's much easier to push the water through a short pipe than through a long one. In other words, the short pipe offers less resistance to flow. The same thing happens with wires, and the electrical resistance of a wire is proportional to its length. If you double the length, the resistance is doubled.
Q: How does the cross-sectional area of a conductor affect the electric resistance?
A: Again lets look at pipes. Suppose we force water through a particular sand-filled pipe. One way to get more water through per second of time would be to force water through several adjacent pipes at the same time. To simplify the situation, imagine square pipes that are next to each other. If we place two square pipes together, we'll get twice as much water flowing. If we create a pipe that is the same shape as two square pipes, that pipe will be 1x2 in size. It will have twice the cross-sectional area of a single pipe. When pressure is applied, it will give twice the flow. The electrical answer is: resistance is inversely proportional to cross-sectional area. If we double the area of a wire, we cut the electrical resistance in half. Or if we use two wires in parallel, we halve the total resistance.
Q: How does the temperature of a conductor affect the electric resistance?
A: The answer to this one cannot be found in water and pipes. We would expect that hot water would be more "runny", and cold water would freeze. Unfortunately, wires behave in the opposite way. It is EASIER to push charges through a cold wire. This is because heat is vibration, and the vibrating metal atoms in a wire tend to interfere with moving charges and slow them down. Cooling a wire makes the atoms vibrate less, which causes its electrical resistance to become less. Chilled wires are better conductors. It's not that simple through, because some specialized nonmetal materials behave opposite to wires, and their resistance increases when they are cooled.
Then there are the "superconductors" like mercury, lead, and some metal oxides. When copper or aluminum are cooled to absolute zero, their resistance decreases to a low level but not to zero. But when a "superconductor" material is cooled down, suddenly the material looses all resisistance entirely. It's as if the "sand" suddenly disappeared from the "pipe." It's even more than this, since the charges in a superconductor become totally frictionless. If something stirs the charges in a block of superconductor so they flow as an electric current, that current will continue essentially forever.

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