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Current Electricity — Physics STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 SciencePhysicsCurrent Electricity5 Marks
Question
  1. State the two Kirchhoff's laws. Explain briefly how these rules are justified.
  2. The current is drawn from a cell of emf E and internal resistance r connected to the network of resistors each of resistance r as shown in the figure. Obtain the expression for (i) the current draw from the cell and (ii) the power consumed in the network.
✓

Answer

  1. Junction Rule: At any Junction, the sum of currents, entering the junction, is equal to the sum of currents leaving the junction.
Loop Rule: The Algebraic sum, of changes in potential, around any closed loop involving resistors and cells, in the loop is zero.
$\sum(\triangle V)=0$
Justification: The first law is in accord with the law of conservation of charge.
The Second law is in accord with the law of conservation of energy.
  1. Equivalent resistance of the loop
$\text{R}=\frac{r}{3}$
Hence current drawn from the cell
$\text{I}=\frac{E}{\frac{r}{3}+r}=\frac{3E}{4r}$
Power consumed $\text{P}=I^2(\frac{r}{3})$
$=\frac{9E^2}{16r^2}\times\frac{4r}{3}=\frac{3E^2}{4r}$

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