2.06 Electro-motive force (e.m.f.)

Batteries displace electrons between their terminals raising their electrical potential similar to the crane raising a mass to increase its gravitational potential. To understand how this happens requires an understanding of the chemical reactions which take place within the battery. This situation becomes more complicated when we consider that batteries are not the only devices which are used to power electrical circuits. (Generators, photovoltaic cells, thermocouples etc). To simplify this situation we simply say that these devices possess an electro-motive force (e.m.f.) which raises the potential of the electrons. We can measure the voltage between the terminals of the power supply to determine the potential difference produced by the e.m.f.


A battery sweeps electrons inside the battery from the positive to the negative terminal. This increases the electrical potential of the electrons. If an external conductive circuit is connected between the terminals of the battery the electrons will flow back to the positive terminal of the battery where they are again swept back to the negative terminal through the inside of the battery. The difference in potential between the two terminals is called a potential difference (p.d.). Potential differences are measured in volts. The e.m.f. voltage supplies energy to the electrons as they are swept inside the battery from the negative to the positive terminal. The potential difference across the external circuit indicates the energy being dissipated by the electrons as they travel back to the positive terminal of the battery

simple electrical circuit

To help understand this consider another similar example that involves gravitational potential.
A pump is used to pump water from a one storage tank up a vertical pipe to a higher water tank. This increases the gravitational potential of the water. If a second pipe is connected between the two tanks the water will flow back to the lower tank loosing its gravitational potential as it falls.

simple hydraulic circuit

Electron flow and conventional current flow

As explained above electrical current in a wire is due to the movement of electrons flowing from the negative terminal of a power supply to the positive terminal. Unfortunately before the true nature of electricity was known it was assumed that current flowed from the positive terminal of a power supply to the negative terminal. This convention is still maintained in electrical theory as it does not affect the principals of some of the basic circuit laws. This "incorrect" direction of current flow is called conventional current flow. The true flow of current from negative to positive is called electron flow. We must often consider true electron flow when explaining the operation of electrical components (especially semiconductor devices).