Spark image
Kirchoff's laws

1. The algebraic sum of the currents at a junction is zero. In other words there is no build up of charge at a junction
2. The sum of the changes in potential round a closed circuit must be zero.

Rule 1 is about charge conservation while rule 2 is about energy conservation.

Rule 1
At the point B there is a junction
Current flowing from the cell (I) = Current in R1 (I1) + current in R2 (I2)

Rule 2
Round loop A,B,C,D,E: p.d across cell = - p.d across R1
This represents a gain of potential in the cell but a loss in R1

Round loop B,C,D,B: p.d across R1 = - pd across R2
In this equation there is a minus because we are moving 'against' the current in R2
© Keith Gibbs 2009