Voltage-Current Graphs

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Voltage-Current Graphs

When metals are heated it causes the atoms in the metal to vibrate more.

Imagine an electron in a current travelling through heated copper. It's trying to flow through the metal but the atoms are vibrating more, so they are going to get in the way more, causing more collisions. More collisions gives more resistance. We say the atoms have a larger collision cross section.

Voltage-Current Graphs

So increasing temperature of a wire leads to increasing resistance (and of course a decrease in conductance).

But, it's a little more confusing than that because passing a current through metal causes it to heat up.

Why?

Collisions between the moving electrons and the metal atoms pass kinetic energy to the atoms, making them shake more. This makes collisions more likely. It's a vicious circle, isn't it?!

That means that the higher the current passing through a wire the greater its resistance will become. So most resistors don't obey Ohm's Law unless the temperature is kept constant.

Voltage-Current Graphs

Remember: the gradient of the line at any point = Copyright S-cool

If Copyright S-cool then the gradient = Copyright S-cool

An increase in resistance will show a reduction in gradient (it becomes less steep) - for example, the filament lamp.

Be careful: in some books or questions, the graphs are drawn with V on the y-axis. In this case, the gradient of the graph becomes R, not Copyright S-cool

Voltage-Current Graphs

Diodes behave like ohmic resistors when the current is travelling through them in the correct direction. However, if the current is reversed the resistance of the diode is extremely high.

Some devices, made from semiconductors, break the rule we've just explained (typical) and reduce their resistance as temperature increases. This is because the extra energy makes the atoms release electrons, allowing them to move more easily, this in turn reduces the resistance.

Voltage-Current Graphs

These devices are called thermistors. These are often used in temperature controls.

(This diagram shows a thermistor with a negative temperature coefficient. A positive temperature coefficient thermistor increases resistance with temperature).

Light-dependent resistors also decrease their resistance when energy is given, but this time the energy needs to be given as light energy.

Remember: The more energy given to a semiconductor the larger the number of charge-carriers that are able to flow, so the lower its resistance becomes.

You should know how to calibrate a thermistor for use in experiments.

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