Conservation of Charge
As charge flows through a circuit, it doesn't get used or lost. This means that the current that flows in is the same amount as the current that flows out. (Current is the rate of flow of charge)
Kirchhoff's first law
The total current entering a junction = the total current leaving it.
Conservation of energy
Energy is conserved and never lost. In electrical circuits, energy is transferred round the circuit. Energy transferred to a charge is e.m.f (electromotive force), and energy transferred from a charge is potential difference. In a closed loop, these two quantities must be equal if energy is conserved (which it is).
Kirchhoff's second law
The total e.m.f. around a series circuit = the sum of the p.d.s across each component
- There will be the same current at all points of the circuit (since there are no junctions)
- The e.m.f. is split between the components (by Kirchhoff's 2nd law)
- The current is split at each junction
- I = I1 +I2 + 13
- There is the same p.d. across all components - there are three separate loops and within each loop the e.m.f. equals the sum of the individual p.d.s