# Variance of resistance with significant current flow

• clearwaterkid
This is a really good answer. Resistors usually have a temperature coefficient of around 10-20%. Resistors with a high temperature coefficient will become much less resistive as the temperature increases. Resistors with a low temperature coefficient will remain relatively resistive even when the temperature is high.f

#### clearwaterkid

The value of a real-world resistor ( ) changes with significant time as the resistor conducts a current.

Never Rarely Sometimes Usually Always

Explain why.

I would say rarely.
If a resistor heats up significantly, it's resistance would change.
See: h ttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html
This question seems overly-vague to me.

I see. So the current causes the resistor to heat? Is this the case always, that current heats up all resistors with time as it conducts a current? I agree it is extremely vague. It's an odd question, however, he never hinted that heat was part of the solution or reasoning. Thanks for your help. Much appreciated.

Current always produces some heat, but it is negligible in real-world resistors by design, unless there is a power surge.

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I understand.

The value of a real-world resistor ( ) changes with significant time as the resistor conducts a current.

Never Rarely Sometimes Usually Always

Explain why.

I would give up in this folder and take your question to the Electrical Engineering forum.

I think maybe you mean to ask about the long term change in resistance: asking "how does the value of a real-world resistor change after a significant time conducting current?"

It's not absolutely clear whether you mean to ask about the change in resistance after it becomes hot, or instead, after it's been functioning in a circuit for a while.

Thank you. I did manage to find some information about real world resistors. The reason that resistors in the real world cannot function in perfect accordance with Ohm's Law is that in any given situation, a range of variables, including outside heat, external distortion, and ambient noise, all minutely affect the flow of current through a resistor.

Electrical energy is converted to heat when current flows through a resistor. Usually the effect is negligible, but if the resistance is low (or the voltage across the resistor high) a large current may pass making the resistor become noticeably warm. The resistor must be able to withstand the heating effect and resistors have power ratings to show this.

Well done. Usually you don't want to subject the resistor to more than half of it's power rating. But what you really want to talk about is this. In the real world, materials change resistance with temperature. Metals become more resistive. Carbon becomes less resistive. Semiconductors can be either. How much they change with temperature is called the temperature coefficent. A resistor that has increased resistance with temperature has a positive temperature coefficient. The whole idea, in most cases, is to keep the magnitude of the coefficent as low as possible. A part rated at 50 PPM, for instance, should change less than 50 micro ohms per ohm of initial resistance per degree centigrade over its range of operation.

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