# Electrical current without an EMF?

1. Aug 19, 2008

### ben48300

1. The problem statement, all variables and given/known data
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 dont know how to answer it, any help would be greatly appreciated

Last edited: Aug 19, 2008
2. Aug 19, 2008

### Defennder

I'm thinking of photocurrent from the photoelectric effect.

3. Aug 19, 2008

### ben48300

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

4. Aug 19, 2008

### Topher925

Would a coaxial cable count?

5. Aug 19, 2008

### Defennder

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

6. Aug 19, 2008

### Topher925

I miss understood the question. I thought he meant, how can you have a current flow with out 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

7. Aug 19, 2008

### ben48300

Thank you all for helping, my problem is resolved now. (I dont know how to edit the thread to say it is resolved)

8. Aug 20, 2008

### 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).

9. Aug 20, 2008

### ben48300

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

10. Aug 20, 2008

### 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

11. Aug 22, 2008

### MATLABdude

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

12. Aug 22, 2008

### salman213

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

13. Aug 27, 2008

### MATLABdude

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?

14. Aug 27, 2008

### Redbelly98

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