Charge flows between two electrodes because of the potential difference between them.
Once the potentials are the same, no charge will flow.
To maintain a constant flow of charge, the potential difference between the electrodes needs to be maintained – this requires energy.
The amount of energy necessary to maintain a certain potential difference is called electromotive force.
The electromotive force in electrochemical cells comes from the energy changes that occur during the chemical reactions in each half-cell, keeping the potentials different.
As the reactants are used up, the reactions slow down meaning the electromotive force drops, resulting in less potential difference between electrodes, so a smaller current is produced. This is why batteries lose power and ‘go flat’.
Electromotive Force and Potential Difference
EMF is electromotive force and it’s commonly misunderstood by students as it appears to be the same as potential difference!
This is more background information that you will not be examined on, but I recommend you understand the difference between EMF and potential difference to avoid any potential confusion if either one is used in an exam question.
If there is a difference in potential between two connected surfaces, charge will flow from one to the other and an electrical current is formed. Touching a statically charged surface, for example, causes charged particles to move from the charged surface to the person touching the surface – this rapid movement creates a small electrical current that can be seen (and felt) as a spark. It is the difference in potential between the person and the surface that enables this to happen.
Once the potentials of both surfaces are the same however, no further charge will move. This is why the spark only happens once. If we want to maintain a potential difference between two connected surfaces, energy is required to generate and maintain this potential difference. This energy is electromotive force.
It’s a little bit like having two containers of water connected by a hose at the bottom. If the first container is full, water will go through the hose into the other container.
Water will flow until they both contain the same amount of water.
If we want to keep water moving between them, water must be taken out of the second container and added back into the first container – this maintains a difference in water between the two and ensures water keeps flowing. Electromotive force provides the energy to move the water back from container two to container one; the difference in water (potential difference) between the two containers causes water to flow from container one to container two
Now the amount of potential difference and electromotive force for electrochemical cells are the same value. The electromotive force in electrochemical cells comes from the chemical reactions between the metal and metal ions in each half-cell, and the resulting potential difference is the same as the electromotive force.
They are technically different concepts, but don’t let yourself get confused, if one exam question asks you to calculate the ‘EMF’ and another asks for the ‘potential difference’ of an electrochemical cell – they will give the same values.