emf problem

[Daniel prince]

Thanks for answering my question, please i need help. Distinguish between potential difference and electromotive force and electric potential

[studiot]

Is this an exam question?

A lot of folks are doing physics exams at the moment.

 

Here is a hint, to explain the difference you should think about energy, both energy sources and energy sinks (dissipators).

[Delbert]

As far as I'm aware they are essentially all the same. They refer to a difference in the number of electrons between two points had there not been either an excess of deficiency difference in electrons at either place. An electrical potential is measured relative to another potential.

 

Sorry, but semantics give me a headache.

 

Workers can work on high voltage cables as long as they are at the same potential. Even seem to recall a TV prog some years ago whereby the workers operated from a helicopter! The helicopter was conductively connected to the high voltage cable via some sort of strapping whilst hovering! So, as long as they were at the same potential, and as far as they were concerned whilst so strapped, it was the ground and everything else that was at high potential.

 

Although, if it's an AC supply presumably there'll be a small AC current (depending of the capacity in Farads of the helicopter) flowing in and out of the helicopter. You know, for the same reason a neon voltage indicator lights up, or lights up brighter, when one touches the end with the blade end touching your AC domestic live.

[Enthalpy]

No absolute electric potential exists, so "the potential" should better be avoided. You may encounter the expression when the reference potential is at infinite distance, but I'd strongly prefer books and professors to formulate it more clearly, as this is a difficulty in understanding electromagnetism.

 

http://en.wikipedia.org/wiki/Electromotive_force

"For a time-varying magnetic flux impinging a loop, the electric potential scalar field is not defined due to circulating electric vector field, but nevertheless an emf does work that can be measured as a virtual electric potential around that loop."

[Delbert]

 

"For a time-varying magnetic flux impinging a loop, the electric potential scalar field is not defined due to circulating electric vector field, but nevertheless an emf does work that can be measured as a virtual electric potential around that loop."

But unless it's a superconductor, presumably its resistance will provide a potential (voltage) difference to be produced between two points according to ohm's law. But I suppose unless the two points are equally spaced halfway round, cancellation will presumably occur.

 

As for superconductors (slightly off subject I know), I've always puzzled as to how a supercurrent actually starts. Because when the first vestiges of current start and because of it being a superconductor of zero resistance, presumably a equal and opposite eddy current(s) will also start to flow causing cancellation. What in normal conductors would cause skin effect. In other words, equal and opposite cancellation apprehending the current from even starting.

 

Obviously, once a DC supercurrent is established, no such eddy currents and cancellation will occur. But my puzzlement is the starting process, because during the short period of current build up from zero, momentary eddy currents must presumably be produced, which being a superconductor must be equal and opposite due to the absence of resistive losses. To summarise: how does a superconductor current overcome this 100% cancellation at start up and actually get going?

[studiot]

 

As far as I'm aware they are essentially all the same.

 

No offence meant Delbert, but their difference would not be the subject of a physics exam or homework question if they were.

Look at the wording of the question, "distinguish between......" that is examspeak.

Edited October 18, 2013 by studiot