# Is heat actually developed in resistors?

1. Apr 16, 2012

### Cromptu

We say that "heat is developed" in resistors when electric current flows through them..But we define heat as the tranfer of energy due to difference of temperature. So is saying that " heat is developed" technically correct?

2. Apr 16, 2012

### Highspeed

I mean this sincerely...are you asking this because you've never touched a hot resistor? There can be a huge temperature difference (relative to your finger, or atmosphere) on a load resistor, and it is 'developed' within the casing.

3. Apr 16, 2012

### Philip Wood

Cromptu. You are absolutely right. Internal (random thermal) energy is developed in resistors. So they become hotter than their surroundings. They then give out heat (energy in transit from higher to lower temperature). It is quite wrong to speak of heat being developed in a resistor. Heat is not energy possessed by a system, but energy in transit.

HighSpeed. No one can sensibly deny that resistors can become very hot. Their high temperature is associated with their raised internal energy.

The distinction I'm drawing between heat and internal energy may seem pedantic, but if you've ever tried to teach students the First Law of Thermodynamics, it is vital!

Last edited: Apr 16, 2012
4. Apr 16, 2012

### HallsofIvy

Staff Emeritus
No, we don't. If that were true one could never produce heat. Heat flow might be defined as you say but not heat itself.

5. Apr 16, 2012

### Staff: Mentor

I can't remember your profession, Halls, but if you check a dictionary, in physics the word does indeed require the context of transfer: objects can exchange heat, but they can't have heat. As an engineer, I don't use it that way though.

6. Apr 16, 2012

### K^2

Yes, you can't define how much heat an object has. Just think of Cv vs Cp for ideal gas. The amount of heat you can extract for same temperature change depends on how you extract it. So how can you define the amount of heat an object has?

With resistors, however, we can think of it as heat, because the internal energy of resistor changes by that quantity, just as if heat would flow into resistor.

Keep in mind that electric current can be treated as infinitely hot or absolutely cold, so you can sort of think of it as resistor drawing heat from infinitely hot source.

7. Apr 16, 2012

### Philip Wood

K^2. So – re your middle paragraph – you're replacing the irreversible work done electrically on the resistor by an equivalent inflow of heat. Perfectly valid, of course, though not (I'd venture to suggest) for beginners. But who said your comment was for beginners?