Kirchhoff's first law
- the algebraic sum of the currents meeting at a junction in a closed electric circuit is zero, i.e., 𝜮I= 0
- Consider a junction O in the electrical circuit at which the five conductors are meeting. Let I1, I2, I3, I4, and I5 be the currents in these conductors in directions,
- Let us adopt the following sign convention the current flowing in a conductor towards the junction is taken as positive and the current flowing away from the junction is taken as negative.
- According to Kirchhoff's first law, at junction O
(-I1 ) + (-I2 ) + I3 + (-I4 ) + I5 = 0
or -I1 + -I2 + I3 -I4 + I5 = 0
or 𝜮I = 0
or -I1 + -I2 + I3 -I4 + I5 = 0
or 𝜮I = 0
- I3 + I5 = I1 + I2 + I4 i.e., the total current flowing towards the junction is equal to the total current flowing out of the junction.
- Current cannot be stored at a junction. That is, there is no point/ junction in a circuit that can act as a source or sink of charge.
- Kirchhoff's first law supports the law of conservation of charge.
Kirchhoff's Second law
- The algebraic sum of changes in potential around any closed path of an electric circuit (or closed-loop) involving resistors and cells in the loop is zero, i.e., 𝜮∆ V= 0.
- In a closed loop, the algebraic sum of the EMFs and algebraic sum of the products of current and resistance in the various arms of the loop is zero, i.e., 𝜮ε + 𝜮 IR = 0.
- Kirchhoff's second law supports the law of conservation of energy, i.e., the net change in the energy of a charge, after the charge completes a closed path must be zero.
- Kirchhoff's second law follows from the fact that the electrostatic force is a conservative force and work done by it in any closed path is zero.
- Traverse a closed path of a circuit once completely in a clockwise or anticlockwise direction.
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