Question

Kirchhoff’s first and second law shows the conservation of:
A. linear momentum and angular momentum
B. charge and energy
C. mass and energy
D. charge and linear momentum

Answer 1

Hint: Recall the statements of Kirchhoff’s first law of electric current and Kirchhoff’s second law of voltage. Check which of the physical quantities remains the same according to Kirchhoff’s first law of electric current and Kirchhoff’s second law of voltage. Hence, determining Kirchhoff’s first and second law shows the conservation of which of the physical quantities given in the options.

Complete step by step answer:
Kirchhoff’s first law is also known as the junction rule.
According to Kirchhoff’s first law, the total current going inside a junction is equal to the total current coming out from the junction because there is no way that the current gets accumulated at the junction.
Since the current is the product of the charge and time, we can say that if the current going inside and coming outside the junction is the same, then the charge involved in this process due to the electric current is also the same.
Therefore, Kirchhoff’s first law states that the charge at the junction remains the same.
Hence, Kirchhoff’s first law of electric current shows the conservation of electric charge.
Kirchhoff’s second law is also known as the loop rule.
According to Kirchhoff’s second law, the total potential difference in a loop is zero or the algebraic sum of potentials of all the resistors (product of electric current and resistance of the resistor) and emf’s in a loop is zero giving the total potential inside a loop zero.
This shows that the total potential in the loop which is a form of energy is equal to a constant which is zero.
Hence, Kirchhoff’s second law of voltage shows the conservation of energy.
Therefore, Kirchhoff’s first and second law shows the conservation of charge and energy.

So, the correct answer is “Option B”.

Note:
The students may think that Kirchhoff’s first and second law states about the motion of electric current and potential difference related to the junction and loop. Hence, there must be conservation of velocity related with the motion of the electrons in the junction and loop. So, these two laws must show conservation of the linear or angular momenta. But this way of thinking is wrong.