When we are first taught electrical theory, we are often told that you need a complete circuit in order for an electrical current to flow. i.e. That you must have a complete conductive path between the terminals of the power supply, so that electrons can travel from the negative terminal to the positive terminal.
In fact this is not a completely accurate statement, and it is more correct to say, that you need a complete circuit to maintain a constant d.c. current, (i.e. a continuous current flow in the same direction). (You will see later that current flow in a.c. circuits is very different to d.c. current flow.) In this section you will discover that transient currents can flow in circuits without a complete conductive path.
The circuit we will initially consider, will consist of just a battery and a capacitor, (the properties and construction of the capacitor will be described as you progress through this section). To begin with we will first consider the characteristics of the battery’s operation that are relevant to understanding transient current flow.
Consider an imaginary battery in which we are able to switch its internal chemical reactions on and off. To begin with the chemical reactions are turned off and there is nothing connected to the battery. Therefore initially both terminals are electrically neutral, i.e. they have equal numbers of protons and electrons.
If we now “turn on” the battery’s chemical reactions, this immediately causes the transfer of electrons inside the battery, from the positive terminal, to the negative terminal. (Note: Electron flow in an external D.C. circuit is from the negative to the positive battery terminal. However to maintain the same direction of flow around the complete circuit, this means that the electrons move from positive to negative, inside the battery itself.)
As electrons carry negative charge, then this process obviously displaces charge. We clearly get an accumulation of negative charge at the negative terminal of the battery. We also get an accumulation of positive charge at the positive terminal. (i.e. the removal of electrons from the positive terminal, means that there are now less electrons than positively charged protons and so there is a net positive charge).
The rate of transfer of charge (i.e. current flow) starts off at its greatest rate but then reduces continuously until it stops completely after a brief period of time.
We know that like charges repel. Therefore as negatively charged electrons accumulate at the negative terminal, this repulsion builds up, making it more difficult to transfer more electrons and therefore reducing the current flow. Eventually the repulsion becomes strong enough to prevent the transfer of any more electrons and the current stops completely. (We can reach the same conclusion by considering the opposite process occurring at the positive terminal. i.e. The attraction of the positive charge builds up, resisting the removal of electrons, until eventually it is strong enough to prevent the removal of any more electrons from the positive terminal.)
(Another way of understanding this is to consider the electric fields that exist between charged particles. An electric field is created inside the battery, due to the positive and negative charge that has accumulated at the battery terminals. Transferring electrons through this field can be compared to raising masses in the Earth’s gravitational field. The important difference with the electric field however, is that it gets stronger as more charge accumulates at the battery terminals. In our analogy this is comparable to the Earth’s gravity getting stronger as we lift more masses, so that each identical mass feels heavier than the previous one that was raised! Eventually the masses would become too heavy to lift. Similarly as the electric field gets stronger, each subsequent electron requires more energy to transfer it across the battery and eventually the power generated by the battery is insufficient to transfer any more electrons.)
This comparison between electric and gravitational fields highlights an important aspect of this process. When lifting masses, the energy that is used is converted into gravitational potential energy, which is “stored” in the raised masses. With a battery, chemical energy is converted into electrical potential energy , “stored” in the displaced charge. The voltage that we measure between the battery terminals, is literally a measure of the electrical potential energy, per Coulomb of charge.
For our imaginary battery.
When we first start the chemical reactions, current flows inside the battery as electrons are transferred from the positive to the negative terminal. This begins to build up
charge at the terminals and develops a voltage across the battery. The current gradually reduces to zero as the terminal voltage builds up to its maximum value.
We cannot of course directly turn the chemical reactions on and off in a real battery but the final conditions described do reflect those of a real battery. i.e. there are (virtually) no chemical reactions taking place when it is not connected to a circuit, there is an accumulation of charge at the terminals and there is a voltage between them.
