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NEET PHYSICSMedium

A capacitor of 2μF2 \mu\text{F} is charged as shown in the figure. When the switch S is turned to position 2, the percentage of its stored energy dissipated is:

A

20%

B

75%

C

80%

D

0%

Step-by-Step Solution

This problem involves the redistribution of charge and the consequent loss of energy when two capacitors are connected, a concept detailed in NCERT Example 2.10.

  1. Initial State: A capacitor C1=2μFC_1 = 2 \mu\text{F} is charged to a potential VV. The initial stored energy is: Ui=12C1V2U_i = \frac{1}{2} C_1 V^2

  2. Final State: The switch is turned to position 2, connecting C1C_1 in parallel with an uncharged capacitor C2C_2 (implied to be 8μF8 \mu\text{F} based on the answer option). Charge flows until they reach a common potential VV'. V=Total ChargeTotal Capacity=C1VC1+C2V' = \frac{\text{Total Charge}}{\text{Total Capacity}} = \frac{C_1 V}{C_1 + C_2}

  3. Final Energy: The total energy stored in the combination is: Uf=12(C1+C2)(V)2=12(C1+C2)(C1VC1+C2)2=12C12V2(C1+C2)U_f = \frac{1}{2} (C_1 + C_2) (V')^2 = \frac{1}{2} (C_1 + C_2) \left( \frac{C_1 V}{C_1 + C_2} \right)^2 = \frac{1}{2} \frac{C_1^2 V^2}{(C_1 + C_2)}

  4. Energy Loss: ΔU=UiUf=12C1V212C12V2(C1+C2)=12C1V2(1C1C1+C2)\Delta U = U_i - U_f = \frac{1}{2} C_1 V^2 - \frac{1}{2} \frac{C_1^2 V^2}{(C_1 + C_2)} = \frac{1}{2} C_1 V^2 \left( 1 - \frac{C_1}{C_1 + C_2} \right) ΔU=Ui(C2C1+C2)\Delta U = U_i \left( \frac{C_2}{C_1 + C_2} \right)

  5. Percentage Dissipated: % Loss=ΔUUi×100=(C2C1+C2)×100\% \text{ Loss} = \frac{\Delta U}{U_i} \times 100 = \left( \frac{C_2}{C_1 + C_2} \right) \times 100 Substituting C1=2μFC_1 = 2 \mu\text{F} and C2=8μFC_2 = 8 \mu\text{F}: % Loss=82+8×100=810×100=80%\% \text{ Loss} = \frac{8}{2 + 8} \times 100 = \frac{8}{10} \times 100 = 80\%

This loss appears as heat and electromagnetic radiation [NCERT Class 12, Sec 2.15, Example 2.10].

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