Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 12

# A Capacitor of Capacitance C is Given a Charge Q. At T = 0, It is Connected to an Uncharged Capacitor of Equal Capacitance Through a Resistance R. - Physics

Sum

A capacitor of capacitance C is given a charge Q. At t = 0, it is connected to an uncharged capacitor of equal capacitance through a resistance R. Find the charge on the second capacitor as a function of time.

#### Solution

Given:-

Initial charge on first capacitor = Q

Let q be the charge on the second capacitor after time t.

According to the principle of conservation of charge, charge on the first capacitor after time t = Q - q.

Let V1 be the potential difference across the first capacitor and V2 be the potential difference across the second capacitor after time t. Then,

$V_1 = \frac{Q - q}{C}$

$V_2 = \frac{q}{C}$

$\Rightarrow V_1 - V_2 = \frac{Q - q}{C} - \frac{q}{C}$

$= \frac{Q - 2q}{C}$

The current through the circuit after time t,

$i = \frac{V_1 - V_2}{R} = \frac{dq}{dt}$

$\Rightarrow \frac{Q - 2q}{CR} = \frac{dq}{dt}$

$\Rightarrow \frac{dq}{Q - 2q} = \frac{1}{RC}dt$

$\Rightarrow \frac{dq}{Q - 2q} = \frac{1}{RC}dt$

Integrating both sides within the limits time =0 to t and charge on the second capacitor varying from q=0 to q, we get:-

$\frac{1}{2} \left[ \ln \left( Q - 2q \right) - \ln Q \right] = \frac{- 1t}{RC}$

$\ln \frac{Q - 2q}{Q} = \frac{- 2t}{RC}$

$Q - 2q = Q e^{- \frac{2t}{RC}}$

$2q = Q\left( 1 - e^{- \frac{2t}{RC}} \right)$

$q = \frac{Q}{2}\left\{ 1 - e^{- \frac{2t}{RC}} \right\}$

Concept: Energy Stored in a Capacitor
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#### APPEARS IN

HC Verma Class 11, Class 12 Concepts of Physics Vol. 2
Chapter 10 Electric Current in Conductors
Q 83 | Page 203