What is EMF? How do we test for it?

Electromotive Force (EMF) is the measure of how much energy a source like a battery gives to the charges in a circuit. As with most things, this energy source is not 100% efficient, and has an associated internal resistance. EMF is the ideal voltage of a power source, if there was no internal resistance.To find the EMF of a source, we can use Ohm's Law and Kirchoff's Law. From V = IR, and the fact that the voltage in a series circuit is divided between the components while the current is the same, we can see that the voltage across the whole battery is the difference between the EMF and the voltage across the internal resistor. That means that EMF = I(R+r) where R is the resistance of the circuit and r is the internal resistance. By varying R and plotting the voltage across the battery against the current in the circuit we can find the internal resistance: the slope of the line; and the ideal voltage output (EMF): the y-intercept.