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 R
1 (I
1) + current in R
2 (I
2)
Rule 2
Round loop A,B,C,D,E: p.d across cell = - p.d across R
1
This represents a gain of potential in the cell but a loss in R
1
Round loop B,C,D,B: p.d across R
1 = - pd across R
2
In this equation there is a minus because we are moving 'against' the current in R
2