Electric current is the flow of electrons from the negative terminal to the positive terminal of a cell. This is because electrons are negatively charged and want to move away from the negative terminal and head towards the positive terminal.
When cells were first invented the theory of electron flow mentioned above was unknown. Rather it was incorrectly assumed that the movement was from the positive to negative terminal. Therefore, circuit diagrams showed the current moving from the positive terminal to the negative terminal. We have unfortunately stuck with this convention to this day and so current flow from positive to negative is called, “conventional current flow” and is used when drawing circuit diagrams.
Series and Parallel Circuits
Electrical components can be arranged in a circuit either in series or parallel.
Series Circuits
For components connected in series the following rules apply.
1. Electric Current
The electrical current through each component is the same, as all the current has to flow through everything in the circuit.
2. Potential Difference
The potential difference across each component adds up to the potential difference across the battery. This is because the energy transferred from the battery to the electrons must equal the amount of energy transferred by the electrons to the components.
The potential difference provided by cells connected in series is the sum of the potential difference of each cell.
3. Resistance
The total resistance across the components in series is equal to the sum of each resistance across the components. The potential difference is largest across the component with the greatest resistance as more energy is transferred by the electrons to overcome the resistance.
Parallel Circuits
For components connected in parallel the following rules apply.
1. Electric Current
The total current through the whole circuit is the sum of the current through each electrical component. The current in a parallel circuit branches out after leaving the battery and recombines before entering back in.
2. Potential Difference
The potential difference across each component is the same.
3. Resistance
The combined resistance across the components in parallel is less than either of the separate resistance across the components.
