Electrical current without an EMF?

[ben48300]

Homework Statement

Can there be an electric current without an e.m.f.?

(i noticed my mistake in the title, it should be e.m.f. not EMF)I was given this question and I don't know how to answer it, any help would be greatly appreciated

 

[Defennder]

I'm thinking of photocurrent from the photoelectric effect.

 

[ben48300]

Ok thankyou, I will research that and hopefully it will solve my problem.

 

[Topher925]

Would a coaxial cable count?

 

[Defennder]

I don't see how a coaxial cable induces current flow without applied emf. Care to elaborate?

 

[Topher925]

I miss understood the question. I thought he meant, how can you have a current flow without creating an emf, not the other way around. Coaxial cables due create emf in the conductors but ideally is limited to only the cable itself.

"In a hypothetical ideal coaxial cable the electromagnetic field carrying the signal exists only in the space between the inner and outer conductors"
http://en.wikipedia.org/wiki/Coaxial_cable

 

[ben48300]

Thank you all for helping, my problem is resolved now. (I don't know how to edit the thread to say it is resolved)

 

[MATLABdude]

Your problem might be resolved, but here's another neat example:

Current in a superconducting loop.

No resistance to current means no potential drop (e.m.f.), and ONCE YOU ESTABLISH A FLOW, it'll keep on going forever (as long as you don't disrupt the superconducting condition, whether by letting the temperature rise above the superconducting temperature, or establishing such a large current that the magnetic field is above the critical limit). You can still "bleed off" the current by inducing currents in other conductors (e.g. every time you take a measurement of the field).

 

[ben48300]

I see, that is actually a better answer than what I had in mind, thank you very much

 

[ben48300]

I have my final answer now:
Electromotive force (e.m.f.) is the force that drives electrons around the circuit, so you would expect an electric current to be impossible without an e.m.f. However, when certain metals are cooled below their “critical temperature” they exhibit a property known as superconductivity, which means the conductor has no electrical resistance at all. An electric current flowing in a loop of superconducting wire can persist indefinitely with no power source, this means that yes, there can be an electric current without an e.m.f. in certain situations.

Thank you very much MATLABdude you were a great help

 

[MATLABdude]

You're welcome! Enjoy intro to E&M! (At least, that's what I assume it is)

 

[salman213]

Your problem might be resolved, but here's another neat example:

Current in a superconducting loop.

No resistance to current means no potential drop (e.m.f.), and ONCE YOU ESTABLISH A FLOW, it'll keep on going forever (as long as you don't disrupt the superconducting condition, whether by letting the temperature rise above the superconducting temperature, or establishing such a large current that the magnetic field is above the critical limit). You can still "bleed off" the current by inducing currents in other conductors (e.g. every time you take a measurement of the field).

How would you establish the flow to begin with? (Capacitor? Inductor?)..

 

[MATLABdude]

How would you establish the flow to begin with? (Capacitor? Inductor?)..

Hmmm, never thought about that one... I'd assume that you can have the superconductor as part of a circuit with non-superconducting elements and then just short-circuit the superconducting bits so that current runs in a perpetual loop.

That or couple a changing magnetic field to it and just induce a current via Lenz's Law?

Would a real physicist weigh in here?

 

[Redbelly98]

. . . or couple a changing magnetic field to it and just induce a current via Lenz's Law?

I'm pretty sure that's how it's done.