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Capacitor question

2017 · Shift 2 · Q51
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Capacitor question

2017 · Shift 2 · Q51

JEE AdvancedPhysicsCapacitorMCQ+3 / −1
Consider a simple RC circuit as shown in Figure 1. Process 1 : In the circuit the switch S is closed at t = 0 and the capacitor is fully charged to voltage V0 (i.e. charging continues for time T >> RC). In the process some dissipation (ED) occurs across the resistance R. The amount of energy finally stored in the fully charged capacitor is EC. Process 2 : In a different process the voltage is first set to V03{{{V_0}} \over 3}3V0​​ and maintained for a charging time T >> RC. Then, the voltage is raised to 2V03{{2{V_0}} \over 3}32V0​​ without discharging the capacitor and again maintained for a time T >> RC. The process is repeated one more time by raising the voltage to V0 and the capacitor is charged to the same final voltage V0 as in Process 1. These two processes are depicted in Figure 2. JEE Advanced 2017 Paper 2 Offline Physics - Capacitor Question 9 English ComprehensionIn Process 1, the energy stored in the capacitor EC and heat dissipated across resistance ED are related by
  1. A
    EC = ED ln2
  2. B
    EC = ED
  3. C
    EC = 2ED
  4. D
    EC = 12{1 \over 2}21​ ED
View written solutionFree

Correct answer: B

  1. Energy stored in a capacitor after charging to V0V_0V0​

For a capacitor CCC charged to voltage V0V_0V0​,

EC=12CV02E_C = \frac{1}{2}CV_0^2EC​=21​CV02​

This is the final electrostatic energy stored in the capacitor.


  1. Process 1: charging through a resistor from an ideal battery V0V_0V0​

When an initially uncharged capacitor is connected to a battery of emf V0V_0V0​ through a resistor RRR, the total energy supplied by the battery is

Ebattery=V0QE_{\text{battery}} = V_0 QEbattery​=V0​Q

Since the final charge on the capacitor is

Q=CV0,Q = CV_0,Q=CV0​,

we get

Ebattery=V0(CV0)=CV02E_{\text{battery}} = V_0(CV_0)=CV_0^2Ebattery​=V0​(CV0​)=CV02​

Out of this, the energy stored in the capacitor is

EC=12CV02E_C = \frac{1}{2}CV_0^2EC​=21​CV02​

So the heat dissipated in the resistor is

ED=Ebattery−EC=CV02−12CV02=12CV02E_D = E_{\text{battery}} - E_C = CV_0^2 - \frac{1}{2}CV_0^2 = \frac{1}{2}CV_0^2ED​=Ebattery​−EC​=CV02​−21​CV02​=21​CV02​

Thus,

ED=ECE_D = E_CED​=EC​

So,

EC=ED\boxed{E_C = E_D}EC​=ED​​


  1. Checking options
  • A: EC=EDln⁡2E_C = E_D\ln 2EC​=ED​ln2 ❌
  • B: EC=EDE_C = E_DEC​=ED​ ✅
  • C: EC=2EDE_C = 2E_DEC​=2ED​ ❌
  • D: EC=12EDE_C = \frac{1}{2}E_DEC​=21​ED​ ❌

  1. Final answer

The correct option is:

B\boxed{\text{B}}B​

with relation

EC=ED\boxed{E_C = E_D}EC​=ED​​

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