If two different half-cells are connected by a wire (and a salt bridge), an electrical current is created and electricity flows.
This is called an electrochemical cell.
Electrical current is the net movement of charge in one direction. As electrons are charged, if they are moving in one direction then an electrical current is produced.
The electrode that has the most electrons on its surface (more negative electrode potential) will force its electrons to flow to the electrode with fewer electrons on its surface (more positive electrode potential).
The greater the difference in potential (voltage) between the two half cells, the greater the amount of electricity produced.
It is the relative difference between potentials that is important in electrochemistry. It is not possible to measure the actual potential of a half-cell – just how different it is compared to other half-cells.
Two different half-cells connected together form an electrochemical cell.
Let’s take a half-cell of zinc and a half-cell of iron. We know that zinc is more reactive than iron, so the zinc metal (electrode) will have a greater number of electrons on its surface than the iron metal (electrode). See 'The Basics' if that is not clear.
If these two half-cells were connected, electrons would flow from where they are highest in number (zinc electrode) to where they are lowest in number (iron electrode). For charge to flow a circuit has to be complete, so a salt bridge is used between the half-cells to enable charged ions to move.
Key physics point! Electricity is the net movement of charge in one direction. If you have charged particles moving in one direction, you have an electrical current.
This means that a small electrical current is formed, because the electrochemical cell has a potential difference and produces ‘electricity’. The amount of electricity produced is a result of how the potentials of each half-cell differ (hence, potential difference – the difference in two potentials).
If there is a big difference between the potential of one cell and another, more electricity is produced; if there is a small difference between the potential of one cell and another, less electricity is produced. The potential difference between two half-cells can be measured using a voltmeter connected between them.
The key thing here is to acknowledge that the potential difference of an electrochemical cell is relative. It does not matter what the individual potentials of each half-cell are, the only thing that matters is the difference between the half-cells. If one half-cell has a potential of 10V and another 11V, the potential difference of the whole cell is 1V. Equally, if one half-cell has a potential of 0.1V and another 1.1V, the potential difference of the whole cell is also 1V.
This means we are only interested in the potentials of each half-cell relative to other half-cells. The actual potential of a half-cell is largely meaningless in electrochemistry, yet we need to know how one half-cell compares to another.
To overcome this, different half-cells are all connected up to the same half-cell, and the potential difference of the electrochemical cell is measured. The same half-cell that is used each time is the standard hydrogen electrode.