Can electric field lines form closed loops? Explain your answer?

Question

I'm working on my homework by myself, but I can't seem to figure out the answer to this question.
The physics book explicitly states "Electric field lines begin at positive charges and end at negative ones."
So, in the case of electric fields created by point charges - the answer is definately no.
However, what about in the case of a closed circuit?? Are the electric field lines of a closed circuit in the same direction as the current?

My Physics book doesn't really say anything about electric field lines in a circuit.

Any help would be appreciated - thanks.

Time varying magnetic field?? Is that the case with a circuit? (We haven't done anything with magnetism yet, or more specifically electromagnetic induction, so I don't think I can use that answer.)

Thanks for the website strontium.

I have an answer down - I wrote no, because they go from postive to negative, and if you have a positive and negative charge overlap the charge with the stronger magnitude would cancel the weaker charge, so the electric field line would not go back to the starting point.

But from my understanding, an electric field line is the direction that a charge moves a long, and in a closed circuit, the charge moves in a closed loop...
But I don't know exactly how current relates to electric field lines.

My T.A. is a bastard... he grades us on what the correct answer is and doesn't give partial credit...

Awesome site strontium.

I found the answer out.

I used this applet:
http://www.falstad.com/emwave1/

I set it the applet to:
setup: current loop
1 loop src
show electric field (E)
and it showed circles emanating from it...
closed circles...

wow, thanks a lot everyone. Your explanations were great, and I think I have a better understanding of the concept now.

I wish my book explained stuff as well as you guys did.
The book I have basically just spits out formulas and how they were derived, and then has some example problems. But it doesn't teach the concepts. My professor is good at explaining concepts, but he has a thick russian accent, and I can't understand what he's saying half of the time.
(Like one time he said something that sounded like nyootanigations - and I was trying to think of what he really said, and when it finally hit me that he meant to say Newton Equations, I had missed a whole section of notes...)

Well, right now I'm not going to go with the closed-loop electric field in a time-varying magnetic field scenario - because we haven't touched magnetism at all yet. We haven't even peeked at it. And if the T.A. doesn't give me credit on it, then he really is a bastard.

Thanks to everyone though, that made the effort to explain it to me. I guess I'm going to be prepared for it, when it comes up in class.

Answer 1

Electric field lines form closed loops aroud a time-varying magnetic field.

I think you are getting confused between fields and currents in a circuit. Fields exist in a spacial medium. A circuit is a network of wires and components. There are fields inside the wire and the components, but the circuit does not constitute a field. If you are referring to static conditions only, then electic fields must start and end on charges. Only when time variations occurs can electric fields form closed loops.

Even in a closed circuit, there must be a source of potential. All electric fields will begin and end at that potential source (battery, power supply, etc.) This forms a closed circuit, and the electric current loops the circuit, but the "electric field", which we could imagine is the voltage from the source divided by the length of the circuit, starts and stops at the potential source (which contains charges), This is only an an analogy, because there is no real electric field involved.

NOTE: I checked that applet. I definitely involves time-varying fields (frequency is a parameter). Whenever time varying fields are involved, both magnetic and electric fields exist. (Based on Maxwell's equations.) Also do not confuse electric field lines with equipotential lines in the field. Equipotential lines in an electric field form closed paths. The applet shows equipotential lines also.

Answer 2

Yes, there can be a closed loop of electric field E if the loop path surrounds a changing magnetic field B. That's how a generator (dynamo) works!

The reason is that:
∇×E=-∂B/∂t (differential form)
∲E⋅dL=-(d/dt)∫B⋅dA (integral form)
This is one of the four Maxwell's equations, expressed in either differential or integral form. The equation, in either form, relates what is happening on a surface, to what is happening around the edge of that surface.

In the integral form, the left-side integral is a closed path (loop) integral, where the closed path is the edge (boundary) of the area over which the right-side surface integral is taken. The differential vector dL is everywhere on the path tangent to the path, and the differential vector dA is everywhere on the surface normal to the surface.

The left-side integral says that if you have a closed loop of wire, and the loop encloses a changing magnetic field, then an electric field will exist inside the wire parallel to the wire. A current will flow as a result of the existance of this electric field. If now you break the closed loop and insert a resistance across the break, the resulting voltage difference across the break (the integral of the electric field along the wire loop) will appear across the resistance and the loop current will flow through both the wire loop and the resistance. A current flowing through a resistance will cause power to be delivered in the form of heat into the resistance. This is how a generator works.

By the way, your TA is not being a bastard for not giving you partial credit for wrong answers. He's doing you a good, actually, because Nature is a real b*tch, who doesn't give partial credit for wrong answers. If you design a bridge or electric circuit or whatever and it falls down or sparks on fire or fails in some way, because you got it 'almost' right, the people depending on your design will not be so kind as to give you partial credit. Better to fail now, in school, and find out about it, than fail later, when lives, property and your career would be lost. Not to say your TA isn't a bastard, for other reasons, of course!

Answer 3

about the setup in your electromagnetic field simulator. ahh yes they can form closed lines, but based on your understanding and how far you are in the book can you explain it with stuff you learned in that chapter? Can you explain why the magnetic field has an effect on charged particles. your stumbling onto something big try the electrostatics applets then do the magnetic applets then do electromagnet applets. and see all that u can see.

One final note. make sure you know the difference between an equipotential line and an electric field line. many will mix this up. E field line is the path the particle could take. the equipotential lines are places where the voltage remain the same. if you follow an equipotential line the voltage will stay the same.

also note in the applet you used the current was alternating which is different from a battery which uses direct current. current you should learn later.

Answer 4

Electric Field Lines Applet

Answer 5

gp4rts gave you a correct answer.

In the case of an electric circuit, electric field lines are envisioned to follow the wiring from a + pole to a - pole of a voltage source. HOWEVER, if the circuit is in a time-varying magnetic field you can close the loop without a battery in it, and still have current flow. It is this fact that makes all electrical generators possible. This was first noted by Faraday, and later codified by Maxwell with his 4 field equations.

It is this fact that also drives me absolutely bonkers if I think about it too much!

Answer 6

In a closed circuit the electron moves from the negative terminal to the positive terminal. this movement of charge creates current in the direction opposite to the flow of electrons. the move ment of the charges creates a magnetic feild as being stated by biostavart's law. now in moving charges, the +ve charges are immobile. as the +ve charges are so known as holes. the holes creates an potential difference with the electron saturaed terminal and the electron while moves towards the +ve terminal with e.m.f. This whole phenomena is going on the base that opposite polarity attracts each other, and to talk about field lines, they are hypothetical lines that show the direction and inestity of this atractive force.

Answer 7

If you are referring to magnetic fields then yes