What is the difference between emf and voltage?

admin 0 Comments March 9, 2021March 9, 2021 Ears Ringing (emf tinnitus)

Q: What is the difference between EMF and voltage?

A: Contrary to the other answers, and despite being measured in the same units – the voltage and EMF are not the same thing.

Consider a metal ring placed on the core of a coil:

When you energize the coil with an AC, there will be a current flowing in the ring. Obviously, the ring is resistive, so there must be some force pushing the electrons against the resisitance, otherwise they won’t flow. This is precisely an electromotive force, or EMF. But how about a voltage? There will be no voltage in a ring. The electric field generated by the magnetic flux will not be a potential field (it will have a swirl) and the voltage is by definition a difference of electric potentials, thus if a given field configuration has no potential the voltage cannot be defined.

If you opened the ring the electrons would still be pushed (thanks to the EMF still present) but they would have to gather up and stop at one end of the now-open ring, and that accumulation of the electrons would generate a voltage difference between the ends of the ring (you can connect the AC voltmeter probes to the ends of the ring and it would indicate some voltage). But here the voltage would be a side effect of the charges that are blocked from moving, so a secondary effect. Obviously if the charges are stopped that means the EMF and the force from the voltage difference created must balance each other and cancel out (that explains why the voltage and EMF will measure to equal values despite not being the same thing).

So – the EMF is what pushes you down the slope, and the voltage is a height difference between the top and bottom of the slope. But, not all of the slopes do have a top and bottom: