EMF and PD
- In A2/A-Level Physics, 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.
Internal resistance
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, r.
- 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.
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 can use the variable resistor to alter the current drawn from the power supply.
The terminal PD is the EMF minus the lost volts i.e. V = E – Ir
The y-axis intercept of the graph must be the EMF, E.
The gradient of the graph line will be –r (note the minus sign).
Measuring EMF and internal resistance - CRO
You cannot measure the EMF with a normal voltmeter as the voltmeter needs to draw a current from the supply to measure a voltage. There will be some “lost volts” in the internal resistance due to this current and the voltmeter reading will be less than the EMF.
{A modern multimeter has a very high resistance – typically 10 million ohms or greater – and so you can measure a value very close to the true EMF as the current drawn is so tiny.}
A cathode ray oscilloscope does not draw a current. If you switch the time base off (for convenience) then the CRO acts as a simple voltmeter with the vertical displacement measuring voltage.
When the switch is open, the dot on the CRO screen indicates the EMF.
When the switch is closed and a current is drawn from the supply, the dot moves down the screen. This drop indicates the “lost volts” in the internal resistance. If the current is measured, then the internal resistance can be calculated from V = Ir i.e. internal resistance = lost volts/current
Car batteries
A car battery is used to power the starter motor to start the engine. The current required for the starter motor is very high indeed.
The internal resistance of the car battery has to be very low in order that the battery can supply this high current, and so that the terminal PD is high enough to allow the starter motor to work effectively.
EHT power supplies
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 short-circuits, and an incompetent user from electrocution.
And we're all done for today!
Drafted by Kin (Physics)