Physics: Problems and Solutions

Kirchoff's laws

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Kirchoff's Laws are a set of two laws which enable one to analyse arbitrary circuits.

First law or Junction Rule
For a given junction or node in a circuit, the sum of the currents entering equals the sum of the currents leaving. This law is a statement of charge conservation.
Second law or Voltage Rule
The algebraic sum of the potentials in a closed path is zero.

In algebraic sum, we take a rise in potential as positive and fall in potential as negative. This implies that:

  1. If we pass through a resistor in the direction of current, the change in potential is -iR. If we pass through the resistor opposite to the current, the change in potential is +iR.
  2. If we pass through the cell in the direction of emf (i.e. from negative terminal to positive terminal), the change in potential is +E. The change in potential is -E, if we go from the positive to negative terminal (i.e. opposite to emf).

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