Part 08 · Circuits and magnetism · Card 6 of 12
Kirchhoff's loop rule
Around any closed loop, the voltage rises equal the voltage drops, because energy is conserved.
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Analogy
Going round a hilly walk and ending at the same height.
Example
Loop
9 V - 4 V - 5 V = 0
In practice
A 9 V battery with two resistors drops 4 V and 5 V in total 9 V.
Questions people ask
What sign do I give a voltage drop?
Choose a direction around the loop. Crossing a battery from negative to positive is a rise, so use plus. Crossing a resistor in the direction of the current is a drop, so use minus. The sum around the whole loop must be zero.
Why does the loop rule work?
The electric field of a steady circuit is conservative, so moving a charge once around a closed loop gives zero net energy change. Any energy a charge gains in a battery is lost in resistors. It is energy conservation in loop form.