What happens in a simple circuit with an EMF and a resistor?

  • #1
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Imagine a simple circuit, an EMF connected to a resistor which connects back to the EMF.

This is how I'm thinking about it

I know there is a current which means that there is charge moving around the loop which means that there must be some field because otherwise the charges wouldve never started moving. So lets look at the wire right after the EMF I'm assuming that the EMF produces some kind of electric field, perhaps by forcing some electrons into the wire, this causes the electrons that were originally at the beginning of the wire to move away from the EMF because of the new charges introduced. I'm imagining that this pattern continues down the wire until it reaches the resistor.

Now what?

I know that however many new charges were introduced into the wire by the EMF must make it back out at the end of the circuit (by the other side of the EMF) in the same amount of time (i.e. the current is the same around the loop) but what is happening at the resistor? Why is there a ΔV? is the resistor causing some sort of field? If it doesnt then why is there a ΔV? If it does then why havent the charges slowed down in response to this field?


Thanks
 

Answers and Replies

  • #2
davenn
Science Advisor
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So lets look at the wire right after the EMF
what do you mean " right after the EMF" ?


I'm assuming that the EMF produces some kind of electric field, perhaps by forcing some electrons into the wire,
The moving charges generate an electric and magnetic field

I know that however many new charges were introduced into the wire by the EMF must make it back out at the end of the circuit (by the other side of the EMF) in the same amount of time
yes but don't forget that the movement of electrons in the wire are very slow ... google electron drift velocity


(i.e. the current is the same around the loop) but what is happening at the resistor? Why is there a ΔV? is the resistor causing some sort of field? If it doesnt then why is there a ΔV? If it does then why havent the charges slowed down in response to this field?
The amplitude of the current flowing around the loop is determined by the value of the resistor ... I = V/R
There is a drop in voltage across the resistor because there is an energy loss in the resistor
electrical energy is converted to heat and radiated away.
The voltage dropped across the resistor is .... V = I x R
for a loop with a single resistor, the voltage drop across that resistor will equal the supply voltage
for a loop of multiple resistors, the voltage drop will equal the sum of the drops across all the resistors which will equal the supply voltage


Dave
 
  • #3
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First of all thanks for responding, unfortunately I think I am even more confused now (which is not necessarily a bad thing), what I thought would be simple is apparently not so simple.

what do you mean " right after the EMF" ?
Right after the EMF meaning I'm thinking of the beginning of the wire right out of the EMF

The moving charges generate an electric and magnetic field
I know, but the magnetic field wont push the charges forward (they do no work) and as far as I can see the only reason why the charges move is because there is an electric field caused by the new electrons in the wire. In other words, before the EMF was attached to the wire, it was neutral all around, however now the EMF puts more electrons in the beginning of the wire, now there is a net charge on the beginning of the wire, this causes repels the electrons further down the wire, so now there is a net charge a little further down the wire, which causes a field, which repels the electron even further down the wire.

I do not know if this is the right mental picture, am I wrong?

google electron drift velocity
Yeah I know, but the way I'm thinking of it is that the electrons which come in at the beginning are not the ones that come out at the end but the net amount of charge that enters the wire due to the EMF is the net amount that leaves out the other end of the wire.


As I'm writing this I thought of a (very silly) analogy that might illustrate the way I'm thinking of this.

There is a bunch of smelly people who hate the smell of all other smelly people (this is the electrons). They are all sitting in a ring of chairs five feet away from each other since this is the tolerable distance they can be from each other (this is the neutral wire) but here is a twenty foot gap in the ring. Now someone (whop we will call Mr EMF) pushes in a new smelly person up to the first guy after the gap, so he goes five feet down the line to get away from the new guy, but now the second guy has to get up and run because the first guy got within five feet of him, so he goes five feet down the line which causes the third guy to move.... (this is my current) all this goes on until the last guy at the gap. Now Mr EMF grabs this guy and ferries him along the entire gap to where the circle begins again and the whole process repeats.

This is my (very crude) analogy of a circuit.
1) How wrong is it?
2) I still dont understand what a resistor is, I know that there is a voltage drop but why?
3) I know that resistors take energy out of the system but where is that energy coming from, is it kinetic? but then the current must change. Is it potential? but then what is causing the field that changes the potential?
4) I know that the voltage drop is proportional to the current, but is the current the same along the whole circuit?
 
  • #4
davenn
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Right after the EMF meaning I'm thinking of the beginning of the wire right out of the EMF
That doesn't really make sense

Google What is an EMF and see what you come up with

Yeah I know, but the way I'm thinking of it is that the electrons which come in at the beginning are not the ones that come out at the end but the net amount of charge that enters the wire due to the EMF is the net amount that leaves out the other end of the wire.
in a DC circuit, the ones that go in one end do eventually come out the other end
The electron motion is slow, the charge/energy which is in the electromagnetic field travels close to the speed of light

In an AC circuit the electrons may never move any great distance, as they just oscillate back and forwards about a point

2) I still dont understand what a resistor is, I know that there is a voltage drop but why?
I answered that
Again, in a slightly different way. A resistor is made of a material that doesn't have so many electrons that are free to move as the copper wire does
that means electrons coming in find it more difficult to move through it. It takes more energy ( EMF) to push the same amount of electrons through.
Tho we all hate water analogies ... imagine a pipe with water flowing through it and you squeeze the pipe somewhere along the length. this impedes the
water flow ( a resistance) .... The pressure also drops as a result

4) I know that the voltage drop is proportional to the current, but is the current the same along the whole circuit?
in a series resistance circuit, yes it is. The current will be measured the same on either side of the resistor

cct1a.GIF




Dave
 

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