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Kirchhoff’s Laws – Example Circuits
Kirchhoff’s 1st Law: The sum of the currents entering any point in a circuit is equal to the sum of the
currents leaving that point (a consequence of the conservation of charge)
Kirchhoff’s 2nd Law: In any closed loop in a circuit, the sum of the emf in the loop is equal to the sum
of potential difference around that loop (a consequence of the conservation of energy)
ADVICE: Beware of direction! When going around the loop, if you go through the power source from the
negative (short) side to the positive (long) side then this is classed as a positive emf; if you go in the opposite
direction through the power source, it is a negative emf.
Use Kirchhoff’s Laws to calculate the currents flowing where indicated:
1.
6V
2.
3
REMEMBER: To calculate
potential difference,
multiply resistance by
current (V = I × R)
6V
3
2
2
4V
I
I
3.
6V
4.
6V
I1
3
I2
2
3
4V
I3
I
6V
5.
6.
I1
3
2
I3
© FradleyPhysics 2016
4V
2
4V
6V
I1
3
I2
2
I3
4V
I2
ADVICE: If you end
up with a negative
current – that is ok!
It just means the
current is flowing in
the opposite
direction to the
arrow drawn