Electromotive Force, Potential Difference And Voltage

• Moving electrons through a conductor as an electrical current requires energy because, as the electrons move they constantly dissipates energy (as heat). 
• This energy can be supplied by many means including batteries or generators. In the case of a battery chemical energy is converted into electrical energy, whereas a generator converts chemical energy to mechanical energy (through combustion) before turning it into electrical energy. 
• For historical reasons the amount of energy needed to move one coulomb of charge around a circuit is called the electromotive force or EMF (E). 
• The unit of EMF is the volt (V) and one volt is one joule per coulomb, thus:

                                              EMF (V) = Energy (J)/Charge (C)
                                                         E = W/Q
Example:

• A battery with an EMF of 6V can supply a current of 5A round a circuit for five minutes. How much energy is provided?

                    The total charge transferred in 5 minutes: Q = It = 5 * 5 * 60 = 1500C
                    The total energy supplied to do this: energy = E * Q = 6 * 1500 = 9000J

• When you lift a ball to a height of two meters above the ground you give it potential energy. When the ball is dropped this potential energy is converted to kinetic energy until, at the point when it hits the ground, there is no potential energy left. 
• A coulomb of charge travelling around a circuit behaves in a similar way. Each coulomb of charge leaving from a battery with an EMF of 6V has six joules of potential energy. 
• By the time the charge reaches the battery again, having travelled around the circuit, it will have dissipated all six joules and posses no potential energy. 
• The amount of energy expended by one coulomb of charge when travelling between any two points in a circuit is known as the potential difference (PD) between those points. Since the potential difference is a number of joules per coulomb the unit of PD is the volt (i.e. the same unit used for EMF). 
• The PD between two points is thus called the voltage drop and has the symbol U (the symbol V used to be used). Therefore, from the previous equation we can say:

                                                                               U = W/Q
Example:

• How much electrical energy is converted into heat each minute by an immersion heater which takes 13A from a 240V supply?

                       Energy = lost by each coulomb = 24J
                      Quantity of charge flowing per minute: Q = It = 13 * 60 = 780C
                      Energy converted in one minute = U * Q = 240 * 780 = 187 200 = 187.5kJ