What exactly is emf? i'm having a bit of trouble with a particular problem?

Question

I have a problem that wants me to find the potential difference vba=va-vb of a circut with 122 ohms and 21 ohms(internal resistance). can i use the formula v=emf-Ir and solve for emf? is that the same thing as potential difference? also, both point a and b occur before the internal resistance(21 ohm) resistor. the resistor between a and b is 122 ohm. when i add the resistors, should i include the internal resistor even though its not located between point a and b?

Answer 1

EMF is very much like a voltage: voltage is in fact a kind of EMF.



EMF is a more general term that's needed because some electrical fields are generated in such a way that the way of finding voltage doesn't cleanly apply. Recall that voltage is the electrical potential difference--you find it by adding up (i.e., integrating) the electrical fields over a distance. Some EMFs (such as those generated by moving magnets) occur in a loop: integrating around the loop would give zero voltage, when it is plain that whatever we're trying to find (electromotive "force") is not zero.



Whether that's understandable or not, in your problem the EMF is just the voltage around the whole circuit: emf = V_b + V_L, where V_b is the voltage drop across the battery (from the internal resistance) and V_L is the voltage in the rest of the circuit (the "load"). (Your v = V_L and your Ir = V_b.) The basic principle you need to use is that the total potential difference around the circuit is zero.



I don't know how you would use v = emf - Ir to solve for emf unless you also have the current or the voltage of the battery (to find the current, I, since V_b = Ir).



I'm having a hard time visualizing the setup. It sounds like something like this:



Battery-------a------Resistor----b----(loop back to Battery)



If this is the case, the resistance between points a & b is just 122 Ohms, so the potential difference between them is just the current times that resistance.

Answer 2

difference between EMF elctro motive force and Voltage:



Voltage is potential energy per unit charge. Joules per Columb.



No current no change in V, no motion of charge.



EMF is the electrical force from the chemistry in a battery or a spinning generator that RAISES the energy of the charge from the 0 refernce ground to V at the source terminal



V is the same everyplace on a conductor, charge moves as necessary



when there is an external load R and a complete circuit back to the V=0 point Current flows



E = IR to equal EMF.

for the same flow of positive current I amps = columbs/sec



Vr drops from =V to 0 across R.

V Rises through source (battery from 0 to Vsource

since wires are conductors Vs = Vr (neglecting very very small R in wire)



the total I depends on total R and V source. incude R internal.



usually R internal is very small and is not counted.



In a "short circuit" internal R and wire R are the only Restance to limit the current.



No physical device can have a V across 0 R ohms.



Must be electricity time in school , losts of Q



em my icon for more help

Answer 3

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