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The electric field in a certain region is acting radially outward and is given by E=Ar. A charge contained in a sphere of radius a centred at the origin of the field will be given by

Asked in CBSE AIPMT 2015 · Gauss's law and enclosed charge

Answer: (3) 4πε₀ A a³

Step-by-step solution

The field is radial and has the same magnitude everywhere on a sphere of radius a, so the flux through that sphere is just E times its area:

φ=E(a)×4π a²=(Aa)(4π a²)=4π A a³

Gauss's law then gives the enclosed charge:

φ=q/(ε₀)⇒ q=ε₀φ=4πε₀ A a³

Notice that the charge grows as a³, so the charge density is uniform - a field rising linearly with r is what a uniformly charged ball produces inside itself.

Why the other options are wrong

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