Electromotive force and potential difference

[sophiecentaur]

why must the potential need two points to be measured? a body can possesses a certain value of kinetic energy given any particular time in motion right? I don't understand about the infinity part. Sorry if I have a hard time understanding, I'm still in high school.

OK - back to you then. How would you define potential?


Potential basically represents Work Done. Work is defined as Force times Distance, so you have to move from place to place in order to do work. Without two points, you can't have a distance - so you have to use two points. The reason that people choose to use 'infinity' as one of the points is that it is the same, wherever you happen top be. The other alternative would have to be a certain spot on the Earth - say a platinum cross hair in the middle of Trafalgar Square.
One huge advantage of using Infinity for 'the other meter connection' is that the potential emerges from integrating the force over the distance. The potential is proportional to 1/R for large distances so you would have !/R1 - 1/R2 as the work done from point 1 to point 2. Having point 1 at infinity 1/∞ is zero, so you only have one term to calculate.

Actually, your point about Kinetic Energy is not strictly valid. The KE of a moving body relates to the damage it could do when it hits something. If that something happens to be going along beside it then the KE (in that particular reference frame) would be very low. If we referred the velocity to a planet going fast in the opposite direction then the KE, in the Planet's reference frame) would be enormous. So even KE is relative but, in this case it's not a relative position but a relative velocity that counts.

 

[NascentOxygen]

What's wrong with Potential Difference?

It's fine in writing, but way too cumbersome for lab class usage, all the time having to insert the +/- sign and make it clear which point it is referenced to. The "voltage drop" term is indispensable for use in oral presentations and quickly explaining circuit operation at the lab bench.

 

[NascentOxygen]

So just what is your point?

That there is not the universal definition of EMF that some would have OP believe.

As far as 2 batteries connected via a resistor, the one with the higher terminal voltage & lower resistance will supply charge to the other. The sourcing battery terminal PD is an emf.

And the same applies to a capacitor? When it is being charged, it exhibits a PD, but when it is losing charge this is attributable to an EMF?

The OP asked & I answered.

As did I.

Let's see what wikipedia says about relying on there being a universal definition for EMF:

However, there is not complete unanimity upon this usage. As Sydney Ross says, in excusing himself for avoiding the term emf:

We have refrained from using the term 'electromotive force' or 'e.m.f.' for short; for there is no consistency between different authors in the meaning of the term. To some authors it is synonymous with 'voltage.' To others it means the open-circuit voltage of a battery. To a third group of authors it means the open-circuit voltage of any two-terminal device. This use is met most often in connection with Thevenin's theorem in circuit theory. To a fourth group it means the work accounted for by agencies other than differences of the (not measurable) Galvani potentials. Such authors equate the current–resistance product of a circuit branch to the sum of voltage plus e.m.f. A fifth group extends this use to field theory. The authors of this group equate the product of current density and resistivity to the sum of electric-field strength plus an e.m.f. gradient. A sixth group applies the term to electromagnetic induction. These authors define e.m.f. as the spatial line integral of the electric-field strength taken over a complete loop. To them the term 'counter e.m.f.' means something.

It is common in some fields, such as circuit theory, to refer to the voltage created by the emf as the emf. Some authors do not distinguish between the emf and the voltage it creates. Some use emf to refer to the open-circuit voltage and voltage to the potential difference when current is drawn. Here is a quotation describing emf as an open-circuit voltage difference:

Doesn't leave much room to argue, does it? Believe in a universal definition of EMF at your peril.

 

[sophiecentaur]

Spoken explanations may be different but one needs to be careful to make sure some ephemoral remark is not misconstrued. 'Sloppy' speech is fine in the right context, of course. We all use it.