I was wondering why a resistor's resistance goes up when the temperature rises. Anyone care to explain?

Answer 1

As something gets hotter, the molecules and atoms vibrate more. Vibration of atoms and molecules interfere with the passage of electrical current, hence the resistance goes up.

If you plot the current versus voltage of a light bulb, you will see the curve is not a straight line; as the filament gets hotter the resistance increases!

Answer 2

The familiar model for deriving Ohm's law treats the conducting electrons in the material like a dilute gas. When a uniform electric field E is applied, each electron carrying charge e is accelerated to an average drift velocity:

u = (Force/mass) * t = (eE/m) * t

where t is the average time the electron is allowed to accelerate (i.e. the mean time between collisions). This is based on the simplifying assumption that when the electron collides with a fixed particle in the lattice its velocity is randomized, i.e. it loses all its forward momentum on average and has to start accelerating up from zero again.

It's like a long long pinball machine sloping gradually downward. If there were no obstacles each ball released at the top would move faster and faster as it went down the slope. But the obstacles (collisions with the lattice) limit the downward (average) speed to the terminal velocity u. The current is directly proportional to this drift velocity, so:

Resistance = (Voltage/Current) = [Constant]/u = [Constant]/t

So R ~ 1/t. The mean time between collisions is roughly t ~ 1/sqrt(T) so we would expect the resistance to rise roughly as sqrt(T). (T is the absolute temperature).

In other words, even though the random or thermal speed of electrons goes up as the temperature increases, this random motion does not contribute at all to the current since it's in all directions equally. Going back to the pinball analogy a ball released at the top with a very fast speed will actually take more time *on average* to get to the bottom than one released at the top with no initial motion.

Of course this is just a very crude way of looking at resistivity for all materials in general.

Answer 3

A very good question. I have been
in the electrical field over 75 years
and the answer has never been discussed. A ventured answer
may be as unproven as the flow of electrons theorized or the force of
the magnetic compass to the follow
the magnetic mass near the north pole . Certain assumptions are made until newer evidence supports
the older theory or revises it.

Answer 4

This is going to be a tough one....

ok, most metals (and yes, carbon too) exist, (while in the solid form) in a crystal lattice structure. the lattice structre is either hexagonal, face centered cubic, open faced cubic, and centered cubic. (there might be some others, I studied this a LONG time ago).

ok so you're going to have to do some of your own research. but maybe I can shed some light on certain things.

the way metals conduct electricity is that the electric field is distributed across the lenght of wire that is carrying that current. The current is the actual movement of electrons from one atom to another as the electrons move across the crystal lattices. The actual electrons move quite slow whereas the electric field moves at the speed of light (well close to the speed of light anyway, there are tables that give the speed of electric fields that are dependent upon the type of wire, the dielectric used (like in coax cables) etc.

anyway. when the metal increases in temperature, it is thought that the crystal lattice structures will increase in distance from each other. this requires the electron to literally "climb" out of its place on the valence position of the atom and have to literally go "higher" in order to traverse the distance between the two atoms. whearas if the atoms were closer, the next atom would pull the electron into itself at a lower energy state.

it's all a matter of energy. The electron has to climb out of its valence shell but when it finds the next atom it "falls" down into the shell of the next atom and when it does that, it releases energy in the form of heat. That loss of energy is directly related to the resistance of the wire.

Think of it this way: lets say you wanted to move a planet from this solar system to the next one. if you use the planet pluto, you have to first move it away from the sun then send it across thousands of light years to the next solar system. only to have it fall down into the well of gravity where the next time you have to pull that planet (electron) out of that gravity (electrical field) well, you need more energy to do it. it will require more energy not because of the distance but because you have to supply more energy to move it away from the sun before the sun from the next solar system starts pulling it in. I know, it's a poor analogy but you know something???

I have a bachelors degree in engineering, and a masters degree in engineering (both electrical) and I DON"T KNOW WHERE AN ELECTRIC FIELD COMES FROM.
I can manipulate it
I can control it.
I can make it do work for me.

but I don't know what it really is.

no one else knows either.

tom

Answer 5

It is really based on molecular and thermal properties. As the material increases in heat, it expands, thus producing less conductive material for the electrons to conduct through, therefore reducing the electron flow... or increasing resistance.

Answer 6

its so simple ..resistance is nothing but opposition to the current flowing thru the metal...if one increase the temp the vibration of the atoms and molecules inside the metal increase and so the opposition for the current gets higher ultimately resistance goes higher..since direction of current is determined by the opposite direction of the electrons..

Answer 7

dport ^ is absolutely correct as more heat is added the material expands which requires the electrons to pass through a more resistive material

Answer 8

carbon resistors, resist more current as the carbon becomes hotter