This is where it gets tricky! You may previously have used the term 'voltage' but now you need to understand the difference between the e.m.f. of a supply and the p.d. across a component. This also shows you where to put a voltmeter in a circuit and the definitions of all key terms. Please note that this analogy is not perfect, it doesn't really explain how energy can be transferred almost instantaneously or how alternating current works but it's a start.
Electromotive Force: The energy gained per unit charge by charges passing through a supply (from chemical to electrical).
Potential Difference: The energy lost per unit charge by charges passing through a component (from electrical to other forms).
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