emf
Electromotive force is defined as the work done to move one unit of charge around a complete circuit. In SI units, this would be the number of joules of work needed to move 1 coulomb (C) of charge round the circuit once.
The short form for electromotive force is emf. So we can write down a formula: e.m.f. = work done/charge, or
E = W / Q,
where E is emf, W is work and Q is charge. The unit of emf is volt (V).
Exercise. A battery is connected to a light bulb by wires to form a closed circuit. When 2 coulomb of charge flows through the whole circuit once, the battery has to do 3 J of work. What is the emf of the battery?
Answer. E = W / Q = 3 / 2 = 1.5 V.
Exercise. A 3 V battery is connected to a light bulb. When 4 C of charge flows round the circuit once, how much work has the battery done?
Answer. Rearrange the equation to make W the subject (by multiplying both sides by Q): W = E x Q. Then W = 3 x 4 = 12 J is the work done.
Exercise. A 5 V battery has done 10 J of work in moving charge round a circuit. How much charge has gone round the circuit once?
Answer. Make Q the subject: Q = W / E. So the charge is Q = 10 / 5 = 2 C.