Conversion of Electric Energy in Resistors

[Virogen]

Hello,

I was wondering if someone here could confirm or explain what happens in a resistor. My assumption is that when the electrons enter a resistor, they slow down, thus decreasing the strength of the electromagnetic field and causing the excess energy the field can no longer hold to be converted to heat. Correct, or completely off?

 

[Bloodthunder]

Strength of EM field?

Anyway, from what I remember, the electrons entering the resistor encounters the vibrating atoms, and defects in the material. Both of these scatters the electrons, so fewer electrons able to carry current though, so... resistance!

 

[SpectraCat]

Strength of EM field?

Anyway, from what I remember, the electrons entering the resistor encounters the vibrating atoms, and defects in the material. Both of these scatters the electrons, so fewer electrons able to carry current though, so... resistance!

The scattering picture is the right one, but the current has to be conserved, so there are no current losses. Rather what happens is that the scattering causes the electrons to do extra work as they flow through the region of higher resistance, so they have lower potential energy after flowing through the resistor .. this is why there is a voltage drop across a resistor. It's quite analogous to friction ... if a mechanical system has higher friction, you have to put more work into it to make it move .. if an electrical system has higher resistance, you have to do more work to push the electron current through it. In both cases one of the important by-products (EDIT: by-product=energy-dissipation mechanism in this context) is heat.

 

[Virogen]

But what about the energy in the electromagnetic field? That is the source of the electrical energy... So I'm really looking for the mechanism that explains why the electrical energy in the field becomes converted to heat.

 

[SpectraCat]

But what about the energy in the electromagnetic field? That is the source of the electrical energy... So I'm really looking for the mechanism that explains why the electrical energy in the field becomes converted to heat.

Huh? Not sure what you are asking about here ... the heat is generated by the electrons passing through the material as current interacting with the stationary electrons and ions in the lattice and scattering off of them. This scattering is inelastic, so some of the energy initially carried by the flowing electrons is transferred into vibrations of the atomic lattice, heating it up. This is not the only mechanism that would lead to electrical resistance, but it is the picture commonly used to explain resistance to electrical current flowing through electrical conductors.

 

[Virogen]

K, I think I found my answer here:

For example, whenever a battery powers a light bulb, the battery spews electrical energy into space. That EM field energy is then grabbed firmly by the wires and guided by them. The field energy flows parallel to the wires, and eventually it dives into the lightbulb filament. There it drives the metal's population of movable charges forward, against the resisting force of electrical "friction." Electrons in the metal momentarily speed up before colliding with tungsten atoms. In this way the electrical energy gets converted into thermal energy. As a whole, an electric circuit is like a duct for electrical energy, but this duct has no walls.

 

[SpectraCat]

K, I think I found my answer here:

For example, whenever a battery powers a light bulb, the battery spews electrical energy into space. That EM field energy is then grabbed firmly by the wires and guided by them. The field energy flows parallel to the wires, and eventually it dives into the lightbulb filament. There it drives the metal's population of movable charges forward, against the resisting force of electrical "friction." Electrons in the metal momentarily speed up before colliding with tungsten atoms. In this way the electrical energy gets converted into thermal energy. As a whole, an electric circuit is like a duct for electrical energy, but this duct has no walls.

Except that description seems to neglect current conservation. Since charge is conserved, and the circuit remains electrically neutral, electrons (not just energy) also have to flow out of the battery through the wires. The mechanism you posted would seem to allow operation of an electrical circuit with just one pole of the battery connected ...

 

[Drakkith]

K, I think I found my answer here:

For example, whenever a battery powers a light bulb, the battery spews electrical energy into space. That EM field energy is then grabbed firmly by the wires and guided by them. The field energy flows parallel to the wires, and eventually it dives into the lightbulb filament. There it drives the metal's population of movable charges forward, against the resisting force of electrical "friction." Electrons in the metal momentarily speed up before colliding with tungsten atoms. In this way the electrical energy gets converted into thermal energy. As a whole, an electric circuit is like a duct for electrical energy, but this duct has no walls.

You have an incorrect view on how electricity works. It is solely the movement of electrons through the wires and into the filament, which creates heat, that lights a light bulb. The "EM field energy" is not a "real" thing. In a light bulb, a difference in electric potential causes the electrons to flow through the circuit. Electrons are negatively charged particles that have their own electromagnetic field, as do the protons in the atoms in the conductors.

At the start of the circuit, whatever is providing the power behind it, such as a battery or generator, causes the initial "push" of electrons in the conductor. These electrons then push the ones next to them, and so on and so on down the wire. Energy simply means that something has the capability of causing something else to change.

 

[Virogen]

Really? This guy here has written a lot about the misconceptions of electricity and where I found that information:

http://amasci.com/elect/poynt/poynt.html

 

[Drakkith]

Really? This guy here has written a lot about the misconceptions of electricity and where I found that information:

http://amasci.com/elect/poynt/poynt.html

Don't use that site. It is the site of a single guy who thinks that he can explain things better than other people. Most of his stuff doesn't really follow physics very well and you will end up even more confused than you were before. Some of his stuff makes sense, but some of it is just confusing.

 

[Per Oni]

These electrons then push the ones next to them, and so on and so on down the wire. QUOTE]
This view is wrong.

I haven’t got much time at the moment to go through the calculation but some time ago I did just that here at this pf. It comes down to this: we know that the electronic speed in copper is very low say ~0.5 mm/s, now transport an average amount of power, say 100 W at 240 V, through a copper wire having a reasonable diameter for that amount of current. IE J~10^7 Amm-2. You will notice that for such a low speed the forces in the wire have to be very big, so big indeed that it surpasses young's module for copper.

Another problem is that your picture points to one defined direction of flow of power say from the negative terminal through the wire to the positive. That’s wrong as well.

At that time it was Dalespam who gave the correct solution using the Maxwell equations. Perhaps he could do that again so that we all can study once and for all the correct solution. In fact he should put it in the FAQ section.

 

[Drakkith]

These electrons then push the ones next to them, and so on and so on down the wire. QUOTE]
This view is wrong.

I haven’t got much time at the moment to go through the calculation but some time ago I did just that here at this pf. It comes down to this: we know that the electronic speed in copper is very low say ~0.5 mm/s, now transport an average amount of power, say 100 W at 240 V, through a copper wire having a reasonable diameter for that amount of current. IE J~10^7 Amm-2. You will notice that for such a low speed the forces in the wire have to be very big, so big indeed that it surpasses young's module for copper.

Dont mistake my explanation as 100% exact. It is a very basic way of explaining the flow of current. See here for more info: http://en.wikipedia.org/wiki/Speed_of_electricity

Another problem is that your picture points to one defined direction of flow of power say from the negative terminal through the wire to the positive. That’s wrong as well.

The movement of electrons is ALWAYS from negative to positive. The term "power" is something else. While the movement of electrons is the cause of everything in electricity, the EM fields of these electrons causes many different phenomena that can cause considerable confusion.

 

[Per Oni]

Dont mistake my explanation as 100% exact. It is a very basic way of explaining the flow of current. See here for more info: http://en.wikipedia.org/wiki/Speed_of_electricity



The movement of electrons is ALWAYS from negative to positive. The term "power" is something else. While the movement of electrons is the cause of everything in electricity, the EM fields of these electrons causes many different phenomena that can cause considerable confusion.

I made a mistake in post #11: J~10Amm-2.

My main concern was that you implied that the amasci.com website was wrong and your picture of “pushing electrons” is a better idea. Although I can’t possibly vouch for everything written in that website the ideas there are a far better proposal then yours.

 

[Drakkith]

I made a mistake in post #11: J~10Amm-2.

My main concern was that you implied that the amasci.com website was wrong and your picture of “pushing electrons” is a better idea. Although I can’t possibly vouch for everything written in that website the ideas there are a far better proposal then yours.

Considering that electricity is the movement of charges, I can't see myself being incorrect. Otherwise how do you explain, say, a battery?

 

[Drakkith]

Actually, after reading a little bit more on that site, he seems to know mostly what he's talking about. I think his description is sometimes a little confusing, but he definitely knows what electric charges are, and how current flows. I'm not sure why he's worded some of his articles the way he has.