All India Pre-Medical/Pre-Dental Entrance Examination (AIPMT) 2008 Questions from Electrostatics

Here are the two questions from electrostatics which appeared in the All India Pre-Medical/Pre-Dental Entrance Examination (AIPMT) 2008 question paper:

(1) A thin conducting ring of radius R is given a charge +Q. The electric field at the centre O of the ring due to the charge on the part AKB of the ring is E. The electric field at the centre due to the charge on the part ACDB of the ring is:

(a) E along OK

(b) E along KO

(c) 3E along OK

(d) 3E along KO

Since the ring is conducting, the charge on it gets distributed uniformly along it. The resultant electric field at the centre of the ring is zero (since a unit positive test charge placed at the centre will be pushed by equal radial forces all around). The field due to the charge on the part ACDB of the ring is therefore equal and opposite to the field due to the charge on the part AKB of the ring. So the answer is E along OK [option (a)].

[It would have been better if it is mentioned that K is the mid point of the arc AKB].

(2) The energy required to charge a parallel plate condenser of plate separation d and plate area of cross-section A such that the uniform electric field between the plates is E, is

(a) ε₀ E²/Ad

(b) ε₀ E²Ad

(c) ½ ε₀E²Ad

(d) ½ ε₀E²/Ad

The energy (U) required for charging a capacitor of capacitance C to V volt is given by

U = ½ CV² where C = ε₀A/d for a parallel plate capacitor with air (or vacuum) as dielectric.

Since E = V/d, we have V = Ed

Therefore, U = (½) (ε₀A/d)(Ed)² = (½) ε₀E²Ad

You can find more questions (with solution) in this section at AP Physics Resources.