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EMF and PD
The electromotive force or EMF of a power supply is the work done on (or energy transferred to) one
unit of charge passing through the power supply.
The potential difference or PD of a component in a circuit is the work done by (or energy…
If you increase the amount of current drawn from a power supply, then you increase the "lost volts" in
the internal resistance and decrease the terminal PD supplied to the external circuit.
Measuring EMF and internal resistance graphs
If you connect the power supply to a variable resistor then you…
A very high voltage power supply (also known as an extra high tension or EHT supply) will have a large
resistor deliberately placed inside the supply to increase the internal resistance (usually to a value of
thousands of ohms).
The internal resistor will limit the maximum current that can be…
resistances. This means that if they're accidentally shortcircuited only a very small current
can flow, making them safer.
Using a circuit to measure internal resistance and e.m.f
The internal resistance (r) of a source in a circuit with a load resistance, ammeter and
voltmeter can be calculated using the following:…