Work Done By Elelctric field in a circuit

[Dale]

work against the electric field

I am no longer sure that I know what you mean by "work against the electric field". Can you explain precisely, perferably in terms of equations, what you mean by that?

 

[Antiphon]

Forgive me for jumping in but if you have a charge in a field where the force points along a certain direction but you mechanically cause the charge to move opposite the force; that is doing work on the charge against the electric field.

 

[tonyjk]

Forgive me for jumping in but if you have a charge in a field where the force points along a certain direction but you mechanically cause the charge to move opposite the force; that is doing work on the charge against the electric field.

That's what i am saying

 

[tonyjk]

I am no longer sure that I know what you mean by "work against the electric field". Can you explain precisely, perferably in terms of equations, what you mean by that?

If you take an emf inside it there's a force fs opposite to the electric field E that does
Work against the electric field. In the circuit the electric field applies an Electric force opposite to the electric field this force is equal to the charge of the electron(negative) X electric field. And the work is the integral of those forces by the lenght

 

[Sefrez]

I am unsure if I am correct, but the way I see it is that you first need some form of resistance in order for a potential difference to exist. That is, without it, no work is needed in the first place to move charges. So in a simple circuit with an EMF and a resistor the EMF first moves charges against an electric field - this gives them potential. That is, they now want to "fall back". Not much different than lifting a mass and then being able to drop it. Though, remove gravity, or the resistor, and there is no longer this said potential.

Once the charges have been forced up the electric field they then fall to the lower potential. The potential difference must exist only over the resistor as it is what allowed for it to begin with. Any wiring that may be in this simple circuit can either be real and considered as some resistance it self (being accounted for in the resistor) or ideal and no resistance. In the second case, no work is needed to move charges. The battery does work only on the resistor. And like wise, the resistor does "negative" work. The charges that pass the resistor no longer have said potential.

Again, I am unsure if I am correct.

 

[sophiecentaur]

"Jumping in" is what this forum's all about. No need to aplologise. :-)

 

[tonyjk]

last question:P we say that across a resistor there's a drop of potential is it because when an electron loses it potential energy and it is convert to heat?thanks

 

[thunderhadron]

It is because when a resistor is connected to battery, Battery's Higher potential and lower potential plates makes the same situation with the resistor also. An electron gets energy difference there hence moves.

 

[sophiecentaur]

I know they love to tell you at School that electricity is all about electrons moving but the problem is, they move very slowly and the explanation is, ifaiac, a bit of a cop out. It is actually quite difficult to justify those last two posts in detail when you are dealing with AC. The same electrons are moving backwards and forwards inside the resistor for hours on end and another set of them are moving in and staying in the cables. None of them do what those two posts are implying. i.e. they don't 'have energy' then 'give it up'.
You should really stick to 'Charge' as your conceptual carrier of energy. OK it may be a bit abstract but it avoids falling over yourself and coming to dodgy conclusions.

 

[Dale]

haha:P no seriously i want to know why it's the work AGAINST the electric field(across a resistance for example) i just don't get it

OK, then based on your clarification, the work done across a resistance is NOT against the field. The work done in a battery or in a generator is against the field.

 

[tonyjk]

OK, then based on your clarification, the work done across a resistance is NOT against the field. The work done in a battery or in a generator is against the field.

Im not saying that i am right i just don't know the answer and I am trying to find the right one! so across a resistance is it against or not? thanks

 

[tonyjk]

i think across the resistance there's drop of potential it means that the potential energy of the electrons is converted to heat? despite that the electron moves from a lower potential to a higher potential right?

 

[Sefrez]

I know they love to tell you at School that electricity is all about electrons moving but the problem is, they move very slowly and the explanation is, ifaiac, a bit of a cop out. It is actually quite difficult to justify those last two posts in detail when you are dealing with AC. The same electrons are moving backwards and forwards inside the resistor for hours on end and another set of them are moving in and staying in the cables. None of them do what those two posts are implying. i.e. they don't 'have energy' then 'give it up'.
You should really stick to 'Charge' as your conceptual carrier of energy. OK it may be a bit abstract but it avoids falling over yourself and coming to dodgy conclusions.

I think potential is a bit arbitrary anyway. So of course you could assign it with AC current, it would just be constantly changing and likely a bad way to think about it. The main point to be made is that the source driving current performs work and it can only do so because of the resistance within the circuit performing "negative work," if you will. That is, no net work is required to move charges from here to there if there is no intermediate resistances.

i think across the resistance there's drop of potential it means that the potential energy of the electrons is converted to heat? despite that the electron moves from a lower potential to a higher potential right?

For the sake of defining potential for an electron, no, it does not move from a lower to higher potential. That goes against the purpose of defining potential. In order for an electron to do positive work (and thus produce heat) it must lose potential energy.

Maybe you are still thinking of electric fields and are getting confused as to what an electric field means for an electron? An electric field is defined such that its direction points away from positive charges and toward negative charges. In consequence, it also describes the direction a positive charged particle will experience a force in. That is, an ideal positive charged particle will have field lines directed radially outward and placing another positive charged particle near it - you will find that the force it experiences is also radially outward - just as the field lines indicate. Place an electron in this vicinity however, and you get the opposite result.

In summary, a positive charged particle gains potential energy when an external force does positive work on the particle. That is, when this external force moves the particle in the opposite direction of the field. For a negative charged particle (electron) potential energy is gained when an external force does positive work on it also. However, in order to do positive work, the negative charged particle must be moved in the direction of the electric field. Why? Because the positive charged particle creating the radial outward fields actually attracts the electron rather than repelling it as would be so with another positive charge.

So
ΔU = W = -We = -∫F . dD = -∫qE . dD
{ΔU = change in potential energy, W = external work, We = work done by electric field (or source creating electric field, if you will), F = force on charge by field, q = charge of particle including sign, E = electric field created by source, D = displacement}

Being as simple as I can, when an electron goes over a potential difference of ΔV (positive change,) its potential energy change is -qΔV where I have explicitly place its negative charge sign. So it's potential energy change is explicitly negative in sign.

 

[tonyjk]

Im not understanding that EMF has a force fs that does work against the electric field so how they will gain energy if they are moving against the electric field? and all around the circuit the electrons are moving against the electric field (inside the resistor) I am confused

 

[Dale]

Im not saying that i am right i just don't know the answer and I am trying to find the right one! so across a resistance is it against or not? thanks

It is not. The work done across a resistor is always with the field, according to your description of what you mean by "against the field".

Btw, you should not think about electrons, only about charge and current. In many cases (i.e. electrolytes, electrolytic capacitors, batteries) the charge carriers are positive or even both positive and negative. That may be the source of your confusion. Ignore the flow of electrons, just think about current.

 

[sophiecentaur]

Im not understanding that EMF has a force fs that does work against the electric field so how they will gain energy if they are moving against the electric field? and all around the circuit the electrons are moving against the electric field (inside the resistor) I am confused

You keep ignoring the one thing here that answers your question: it's the answer you get when you multiply a negative number (electronic charge) by a negative number (the direction of the force on it, due to the field times the displacement). This gives a positive value of work done ON the electron when the conventional charge is transported from the positive end to the the negative part of the circuit "AGAINST the field". When the electron flows in the direction 'it wants to go or downhill' (sorry, but it's the best way to describe it) the sign of the work done BY IT on the substance of the resistor is Positive.

The essence of all this seems to be a confusion between Electric Potential (based on a positive charge) and Potential Energy (which involves the sign of the charge involved).