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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 transferred
from) one unit of charge passing through the component.
Both EMF and PD are "voltage" i.e. energy transferred per unit charge. The difference is that a charge
gains electrical energy passing through an EMF but loses it passing through a PD.
An ideal power supply would be a pure source of EMF. Charge passing through it would gain electrical
energy and no other transfers would occur.
In a real power supply, there will be sources of resistance inside the power supply itself and so some of
the electrical energy gained from the EMF will be lost before the charge gets to the external circuit.
The internal resistance of a power supply is the resistance of the material and components inside
the power supply itself. It can also be defined as the PD lost (or voltage drop) per unit current
passing through the supply.
A real power supply is usually modelled as a box containing a perfect EMF, E, and internal resistance,
E r E.g. 12V 2
The terminal PD of the supply is the PD provided to the external circuit. It is equal to the EMF
minus the "lost volts" due to the energy losses in the internal resistance.
Terminal PD and current
The amount of "lost volts" in the power supply is not constant. It depends upon how much current you
draw from the supply, which (in turn) depends upon the circuit attached to the supply.
E.g. 12V 2 12V 2
2 external resistor 4 external resistor
Total circuit resistance = 4 Total circuit resistance = 6
Circuit current = 12/4 = 3A Circuit current = 12/6 = 2A
"Lost volts" = Ir = 6V "Lost volts" = Ir = 4V
Terminal PD = 6V Terminal PD = 8V
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.
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Measuring EMF and internal resistance graphs
If you connect the power supply to a variable resistor then you can use the variable resistor to alter the
current drawn from the power supply.
E.g. E r
The terminal PD is the EMF minus the lost volts i.e. V = E Ir
The yaxis intercept of the graph must be the EMF, E.
The gradient of the graph line will be r (note the minus sign).…read more
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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 drawn from the supply, protecting the
supply from accidental shortcircuits, and an incompetent user from electrocution.…read more