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Current Electricity question

2022 · 25 Jul · Shift 1 · Q42
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Current Electricity question

2022 · 25 Jul · Shift 1 · Q42

JEE MainPhysicsCurrent ElectricityMCQ+4 / −1
Which of the following physical quantities have the same dimensions?
  1. A
    Electric displacement (D→)(\overrightarrow{\mathrm{D}})(D) and surface charge density
  2. B
    Displacement current and electric field
  3. C
    Current density and surface charge density
  4. D
    Electric potential and energy
View written solutionFree

Correct answer: A

  1. Write dimensions of each quantity

We compare the dimensions option by option.


  1. Option A: Electric displacement D⃗\vec DD and surface charge density

Using D⃗=εE⃗\vec D = \varepsilon \vec ED=εE

Also, from Gauss's law in matter, ∮D⃗⋅dS⃗=qfree\oint \vec D \cdot d\vec S = q_{\text{free}}∮D⋅dS=qfree​ So, [D]=chargearea[D] = \frac{\text{charge}}{\text{area}}[D]=areacharge​

Dimension of charge: [Q]=[IT][Q] = [IT][Q]=[IT] Hence, [D]=[IT][L2]=[ITL−2][D] = \frac{[IT]}{[L^2]} = [I T L^{-2}][D]=[L2][IT]​=[ITL−2]

Now surface charge density σ=QA\sigma = \frac{Q}{A}σ=AQ​ so [σ]=[IT][L2]=[ITL−2][\sigma] = \frac{[IT]}{[L^2]} = [I T L^{-2}][σ]=[L2][IT]​=[ITL−2]

Thus, D⃗\vec DD and surface charge density have the same dimensions.

So, Option A is correct.


  1. Option B: Displacement current and electric field

Displacement current has dimensions of current: [Id]=[I][I_d] = [I][Id​]=[I]

Electric field: E=FqE = \frac{F}{q}E=qF​ So, [E]=[MLT−2][IT]=[MLT−3I−1][E] = \frac{[M L T^{-2}]}{[I T]} = [M L T^{-3} I^{-1}][E]=[IT][MLT−2]​=[MLT−3I−1]

Clearly, [I]≠[MLT−3I−1][I] \neq [M L T^{-3} I^{-1}][I]=[MLT−3I−1]

So, Option B is incorrect.


  1. Option C: Current density and surface charge density

Current density: J=IAJ = \frac{I}{A}J=AI​ Hence, [J]=[IL−2][J] = [I L^{-2}][J]=[IL−2]

Surface charge density: [σ]=[ITL−2][\sigma] = [I T L^{-2}][σ]=[ITL−2]

These differ by a factor of TTT.

So, Option C is incorrect.


  1. Option D: Electric potential and energy

Electric potential: V=workchargeV = \frac{\text{work}}{\text{charge}}V=chargework​ So, [V]=[ML2T−2][IT]=[ML2T−3I−1][V] = \frac{[M L^2 T^{-2}]}{[I T]} = [M L^2 T^{-3} I^{-1}][V]=[IT][ML2T−2]​=[ML2T−3I−1]

Energy: [U]=[ML2T−2][U] = [M L^2 T^{-2}][U]=[ML2T−2]

These are not the same.

So, Option D is incorrect.


  1. Final conclusion

Only Option A has the same dimensions in both quantities.

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