Potential difference and terminal potential difference

I really need to know the difference between the two as I have an exam tomorrow D:. What is the difference between the two and how do I calculate each one of them? The book says " IR is the potential difference that is measured across the load resistor R. This is the same as the potential difference that would be measured by a voltmeter connected across the terminals of the supply and is referred to as the terminal potential difference. Termina pd = EMF - Ir"
I = Current
R= Resistance
r= internal resistance
Can someone please word it better or help me out here?
Thank you
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 Recognitions: Gold Member Science Advisor I was not sure what you were talking about at first but I think you may be referring to the problem of making measurement where there are heavy currents involved. The actual contact resistance (Rc) between the resistor under test and the current carrying leads can cause a voltage drop of IRc. To get an accurate measurement of the true PD across the resistor, you need to put your voltmeter probe right on the resistor ends, after the power terminals. The voltmeter will then be measuring the PD across the resistor alone. Of course, the same current is flowing round the whole circuit so you have a good value I and a good value of Vr and you can calculate the resistor value without the contact resistance. Could that be what you are worrying about? If there is no contact resistance or supply lead resistance then the two terms in the title are the same.

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