Kirchhoff's First & Second Law in Circuits
- Kirchhoff’s laws can be used to solve simple circuit problems
Using Kirchhoff's First Law
- Remember:
The sum of the currents entering a junction always equal the sum of the currents out of the junction
Worked example
For the circuit below, state the readings of ammeters A1, A2 and A3.
Using Kirchhoff's Second Law
- Remember:
The sum of the e.m.f’s in a closed circuit equals the sum of the potential differences
Worked example
For the circuit below, state the readings of the voltmeters V1, V2 and V3.All the lamps and resistors have the same resistance.
Examiner Tip
The best way to practice these calculations is to understand why you have made a mistakeCommon mistakes are:
- Thinking the current is the same through every branch in a parallel circuit
- Thinking the voltage is the same through all components in a series circuit
- Not taking into account multiple resistors
- Not calculating the total resistance using the appropriate parallel or series resistance equation
- The sum of the voltages of all the components in a series circuit not adding up to the e.m.f of the supply
- The current into a junction not being equal to the current out of a junction (Kirchhoff's First Law)
Don't be afraid to annotate circuit diagrams to help with this. The more information you have about all the components, the easier it is to calculate values that are missing
