Energy conservation
The principle of conservation of energy states that energy cannot be created or destroyed, but can be transferred from one form to another. Therefore, the total energy in a closed system stays constant.
An application of the principal of energy conservation and charge conservation is Kirchoff’s second law, which states:
- The sum of all the voltages in a series circuit is equal to the battery voltage or the sum of all the voltages in a loop is zero
Therefore, due to energy conservation, the distribution of potential differences in a circuit is as follows:
- In a series circuit -
- The battery p.d is shared across all elements in the circuit, therefore the total sum of the voltages across all elements is equal to the supply p.d.
- In a parallel circuit -
- The potential difference across each branch is the same.
This is easy to see with the series circuit as it is a direct application of Kirchoff’s second law (as described above) but takes a little more thought with the parallel circuit:
➔ Consider you are taking the blue path drawn on the circuit below.
➔ It is a closed loop so Kirchoff’s second law applies, and so the potential difference across the middle resistor must be equal to the supply potential difference.
➔ You can repeat this for each possible path in the circuit. This leads to the fact that the potential difference across each branch is the same.