The term voltage is often used in a rather casual way. In everyday life, the word is used in a less scientific and often incorrect sense – for example, ‘A big voltage can go through you and kill you.’ In this topic, we will consider a bit more carefully just what we mean by voltage and potential difference in relation to electric circuits.
Look at the simple circuit in Figure 8.11. Assume the power supply has negligible internal resistance. (We look at internal resistance later in Chapter 10). The three voltmeters are measuring three voltages or potential differences. With the switch open, the voltmeter placed across the supply measures $12 V$. With the switch closed, the voltmeter across the power supply still measures $12 V$ and the voltmeters placed across the resistors measure 8 V and 4 V. You will not be surprised to see that the voltage across the power supply is equal to the sum of the voltages across the resistors.
Earlier in this chapter we saw that electric current is the rate of flow of electric charge. Figure 8.12

voltmeter placed across the power supply measures the e.m.f. of the supply, whereas the voltmeters placed across the resistors measure the potential difference (p.d.) across these components. The terms e.m.f. and potential difference have different meanings, so you have to be very vigilant.
The term potential difference is used when charges lose energy by transferring electrical energy to other forms of energy in a component, such as thermal energy or kinetic energy. Potential difference, V, is defined as the energy transferred per unit charge.
The potential difference between two points, A and B, is the energy transferred per unit charge as it moves from point A to point B.

$Potential\,difference = \frac{{energy\,transferred}}{{charge }} \equiv V = \frac{{\Delta W}}{{\Delta Q}}$

This equation can be rearranged to calculate the energy transferred in a component:

$\Delta W = V\Delta Q$

A power supply or a battery transfers energy to electrical charges in a circuit. The electromotive force (e.m.f.), E, of the supply is also defined as the energy transferred per unit charge. However, this refers to the energy given to the charge by the supply. The e.m.f. of a source is the energy transferred per unit charge in driving charge around a complete circuit.
Note that e.m.f. stands for electromotive force. This is a misleading term. It has nothing at all to do with force. This term is a legacy from the past and we are stuck with it! It is best to forget where it comes from and simply use the term e.m.f.