Glossary Definition for 16-19
- Level Advanced
EMF is an abbreviation for electromotive force. It is a measure of the ability to drive an electric current. The EMF of any electrical power supply, such as a cell, a battery or a dynamo, is defined as the work that the supply would do, per unit charge, to move charge around a circuit if the supply were connected into the circuit,
ℰ = WQ
where ℰ is the EMF of the supply, and W is the work done in moving charge Q around the circuit.
For historical reasons its name includes the word ‘force’, but it is important to recognise that an EMF is not a force.
An EMF may be generated, for example, by chemical reactions, as in a cell or battery, or by changes in the magnetic flux through a circuit, as in a dynamo or generator.
The potential difference, V, across the terminals of a real power supply that is driving a current around a circuit, is less than its EMF. This is because some of the energy supplied by the power supply is dissipated inside the power supply itself. This is often modelled by assigning an internal resistance r to the power supply, defined by
V = ℰ − Ir
where the quantity Ir is the difference between the EMF of a power supply and the potential difference across its terminals, and is sometimes referred to as the ‘lost volts’. See Figure 1.
Figure 1: A conceptual representation of a real power supply, modelled as an ideal power supply of EMF ℰ, and an internal resistance r.
Expressed in SI base units
A-1 kg m2 s-3
Other commonly-used unit(s)
- ℰ = WQ
- For a generator, ℰ = -d(NΦ)dt
- When a power supply drives a current, the terminal potential difference is less than the EMF according to the equation V = ℰ − Ir
is the EMF induced in a coil with N turns. Here, N is the number of turns in the generator coil and Φ is the magnetic flux through each turn of the coil.
where V is the terminal potential difference, I the current through the power supply and r the internal resistance of the power supply.
- Electrical potential difference
- Electric charge
- Magnetic flux
A typical fresh alkaline manganese cell with a terminal potential difference of 1.5 V, and an internal resistance of 0.21 Ω, driving a current of 250 mA has an EMF of approximately
1.55 V + (0.25 A × 0.21 Ω) = 1.55 V
Each of the six generators at Drax Power Station in Yorkshire produces an EMF of 23,500 V.