# Where does the heat (energy) come from?

#### art vandalay

Joined Sep 2, 2010
1
hello, i`ve just started getting into the science of electricity, and theres something i cant get my head around. From what i understand, in a simple DC circuit (two wires, battery, bulb), the energy that lights the bulb is not in the charges in the wires(moving electrons), its outside the wire in the electric and magnetic fields. Makes sense to me since in an AC system the electrons just wiggle.

What i dont get is in a DC system if the wire is too long, the system is going to lose lots of heat (energy) from the resistance of the wire. Where is all this energy coming from, just the kinetic energy of the electrons? Or are the electric and magnetic fields heating up the wires? Thanks!

#### t_n_k

Joined Mar 6, 2009
5,447
Excluding semiconductor, other active devices and sources for the moment, there are three circuit elements relevant to your question - resistance, inductance & capacitance.

Resistance accounts for energy (heat) loss in a circuit - whether the current is AC or DC or something in between.

Inductance which relates to the magnetic fields you mention, accounts in part for the energy storage that exists in a circuit when current is flowing. The remainder of the energy storage resides in the equivalent capacitance elements of the circuit which relate to the electric fields present in the circuit when energised. Magnetic and electric fields exist irrespective of whether the current is supplied by AC or DC.

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#### Papabravo

Joined Feb 24, 2006
12,770
It really doesn't come from anywhere. The question almost presupposes that energy can be created -- it can't. I can however be changed in form. There are two flavors of energy potential and kinetic. Potential energy comes from things like a battery or height above the ground in a gravitational field. Kinetic energy or the energy of motion occurs when things move like electrons or automobiles or Saturn-V rockets.

So in a DC circuit the potential energy in the battery is converted into kinetic energy of the electrons which give up that kinetic energy as heat when they collide with things -- like copper atoms in the wire.

#### Ghar

Joined Mar 8, 2010
655
The kinetic energy however is more complicated than in the mechanical situation.
It's no longer the simple mv^2 because of all of the strong interactions between particles. If you try to move one you need to move many because they all push on eachother.

#### Papabravo

Joined Feb 24, 2006
12,770
The kinetic energy however is more complicated than in the mechanical situation.
It's no longer the simple mv^2 because of all of the strong interactions between particles. If you try to move one you need to move many because they all push on eachother.
Kinetic energy is actually just (1/2)mv^2 There is special relativity to consider, and there are electric and magnetic fields to consider. Other than that I don't know what you're talking about -- some kind of second order terms that adjust the kinectic energy?!?!

#### Ghar

Joined Mar 8, 2010
655
Kinetic energy is actually just (1/2)mv^2 There is special relativity to consider, and there are electric and magnetic fields to consider. Other than that I don't know what you're talking about -- some kind of second order terms that adjust the kinectic energy?!?!
There's several interpretations that I've seen.

One is what you say, but that doesn't even remotely account for the energy in the system. This is where people say the energy "is in the fields".
Electrons travel at relativistic speeds as well so using the point mass equation seems a bit odd.
There are kinetic energy equations which involves integrating magnetic field over space.

Maxwell does this...

I've also seen some more quantum style equations which modify the point mass equation by an n^2 term because for every electron you add there is another force etc. making the mass seem larger.

#### Papabravo

Joined Feb 24, 2006
12,770
There's several interpretations that I've seen.

One is what you say, but that doesn't even remotely account for the energy in the system. This is where people say the energy "is in the fields".
Electrons travel at relativistic speeds as well so using the point mass equation seems a bit odd.
There are kinetic energy equations which involves integrating magnetic field over space.

Maxwell does this...

I've also seen some more quantum style equations which modify the point mass equation by an n^2 term because for every electron you add there is another force etc. making the mass seem larger.
For the most part I think classical mechanics with special relativity has a lot going for it. Remember that Maxwell is still classical physics. Fields by themselves don't actually do very much. It is only when they interact with things like waves and particles. I'm not convinced that quantum mechanics is much use until you start talking about what goes on in a vacuum tube or at a pn junction. Just my opinion though.

#### Ghar

Joined Mar 8, 2010
655
The only quantum I know is from infringing on it through semiconductor physics or electromagnetics. Not much... a few years ago I decided to not pursue it further.
Trying to read into it I ran into some open debates and the math gets beyond me and it's entirely the opposite direction of where I want to be.

#### Papabravo

Joined Feb 24, 2006
12,770
I know what you mean. I stopped at quantum chromdynamics, and I'm pretty sure there is no TOE (Theory of Everything) or a Grand Unified Field Theory.