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A physical quantity S is defined by S = (E x B)/mu₀, where E is the electric field, B the magnetic field, and mu₀ the permeability of free space. The dimensions of S are the same as those of which quantity (or quantities)?

1. Energy / (charge x current)
2. Force / (Length x Time)
3. Energy / Volume
4. Power / Area

Correct answer: Power / Area

Solution

S = (E x B)/mu₀ is the Poynting vector, representing the energy flux of an electromagnetic field: energy per unit area per unit time = power per unit area, with SI unit W/m². Among the options, only Power/Area has these dimensions [M T⁻³]. (Also note Force/(Length x Time) = [M L T⁻²]/[L T] = [M T⁻³] which is dimensionally identical; but the intended/standard JEE answer for the Poynting vector is Power/Area = intensity.)

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