# Joule heating /Ohmic heating

I have trouble understanding why Joule heating is called ohmic heating.

Also the joule heating effect establishes direct relation between current and resistance(as more current more collision more resitance more heat) while Ohm's law establishes inverse relationship i.e I=V/R why is that so?

I know I am missing something,but I can't specifically identify it.

• because it applies to resistors? (conductors and insulators don't have joule heating) Nov 13, 2020 at 20:34
• A perfect conductor has 0 resistance and a perfect insulator has infinite resistance. Both of those have 0 ohmic heating. You only get ohmic heating when you have some ohms, see what I mean? Nov 13, 2020 at 20:41
• Ah,yes that verifies the name but what about the relationship?
– user268541
Nov 13, 2020 at 20:50
• I thought the question was about the name. Ohmic heating is caused by ohms Nov 13, 2020 at 20:54

You have missed where the energy needed to cause heating comes from.

Electric current in materials causes heating because moving electrons shove material molecules and cause random vibrations. Those random vibrations are the generated heat. Electrons would lose their velocity soon if there were nothing keeping the movement going on. But there is - the electric field, the reason of the electric current. Without it there wouldn't be any current.

Practical electricians do not think the electric field, which is a 3D space vector field. They are not interested in how strong the field is in different points and what's its direction. They are only interested in how much the field is able to do work (=give energy to electrons) if they let it to move electrons in a conductive material.

The ability to do work is measured as voltage. If electric field moves so many electrons per second from point A to point B that the current is = I and it makes work (causes heating, rotates a motor, generates radiowaves etc...) so that the power (=work per time unit) is P, the voltage between A and B is =P/I. That's not magic it's the basic definition what voltage means. Most of us know that P=UI. It's very direct consequence of the definition of the voltage. Voltage is practical also because we have working voltage sources which can keep the voltage quite constant if the current taken out of the source is reasonable.

In metals and many other conductive materials the material brakes moving electrons so that to have current we need voltage which is proportional to the wanted current. That's the original Ohm's law. For resistors we write it U=IR where the proportionality factor R is the resistance.

We can combine the original definition of the voltage and Ohm's law in resistors and get P=(I^2)*R or as well P=(U^2)/R which both say the power which heats a resistor. The latter seems to tell that bigger resistance means less heating. That's true - it happens because the current is smaller if the resistance is bigger.

I'd say it's about naming stuff. Ideal resistors follow 2 main laws:

• Ohm's law which dictates the relationship between voltage and current. $$U = R.I$$
• The power dissipated accros the resistor (as heat). $$P = U.I$$ Dipoles following Ohm's law can be called 'ohmic conductors' and these are subject to heating given by the power formula above. 'Ohmic heating' thus refers to heating by appliying current to an ohmic conductor, or in simple terms: heating of a resistor traversed by a current.

I have trouble understanding why Joule heating is called ohmic heating.

I have never heard heating called "Joule heating" but the term would come from the unit of energy, the joule (lower case) or "J" (upper case). The unit of power is the watt (W) and 1 W = 1 J/s.

Also the joule heating effect establishes direct relation between current and resistance (as more current more collision more resitance more heat) while Ohm's law establishes inverse relationship i.e I = V / R why is that so?

You are using the wrong law. You should be using Joule's law, P = I2R. Heating will be proportional to R for a given current.

I know I am missing something,but I can't specifically identify it.

Any better now?

Joule's Law - Watt's law - Ohm's Law

Voltage - Current - Resistance - Power

Each of the three laws expresses a relationship among three of the quantities. No one equation encompasses all four quantities. You can hop among them and see that using Ohm's Law, you can make substitutions in Joule's Law to change it into Watt's Law.

For any given design situation, you have some variables defined and want to solve for one or more other variables. With the three Laws above (and permutations of each), you can solve for any one of the variables if two other quantities are known.

• Ohm's law is applicable at constant temperature conditions,while such a condition cannot be for joules law.So do they all three have different conditions ?
– user268541
Nov 13, 2020 at 22:23
• Neither half of that statement is correct. First, the equations do not take temperature coefficients into effect. That may or may not be important, depending on the design requirements. Second, Joule's Law says nothing about temperature, or component temperature coefficients. Resistors that vary by less than 1% over tens of degrees cost pennies. Nov 14, 2020 at 0:14