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2. Electric potential and capacitance — Physics STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 SciencePhysics2. Electric potential and capacitance1 Mark
MCQ
Assertion : The total charge stored in a capacitor is zero.

Reason : The field just outside the capacitor is $\frac{\sigma }{{{\varepsilon _0}}}$. ( $\sigma $ is the charge density).

  • A
    If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
  • B
    If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
  • ✓
    If Assertion is correct but Reason is incorrect.
  • D
    If both the Assertion and Reason are incorrect.
✓

Answer

Correct option: C.
If Assertion is correct but Reason is incorrect.
c
Charge stored on the two plates are $+Q$ and $-\mathrm{Q} \Rightarrow \mathrm{Q}+(-\mathrm{Q})=0$ and hence Assertion is correct. The field however, outside the plates is zero.

Draw a Gaussian surface $A B C D$ as shown.

The field $\overrightarrow{\mathrm{E}}$ is uniform on faces $\mathrm{AD}$ and $B C$

$ \Rightarrow \,\oint {\overrightarrow {\text{E}}  \cdot {\text{ds}} = 0} $ yields $\vec {\mathrm{E}}=0$

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