Question

A charge of +5µC is placed in free space. P and Q are two points at 3 mm and 5 mm respectively from 5µC.

1. The work done to bring a + IC charge from infinity to a point ‘P’ is called

• capacitance
• dielectric constant
• potential energy
• electric potential

2. Calculate the work done in above process.

3. Calculate the work done to move a +IC charge from ‘P’to ‘Q’.

Answer 1

1. Electric potential.

2. Work done w = VQ

\(=\frac{1}{4\pi\Sigma_0}\frac{q}{r}\times Q\) = \(\frac{9\times10^9\times5\times10^{-6}\times1}{3\times10^{-3}}\) = 15 J

 3. The potential energy at p,

PE₁ = \(\frac{9\times10^9\times5\times10^{-6}\times1}{3\times10^{-3}}\) = 15 V

The potential energy at Q

PE₂ = CQ = \(\frac{9\times10^9\times5\times10^{-6}\times1}{5\times10^{-3}}\)

PE₂ = 9V

Work done to move IC from P to Q, W = PE₂ – PE₁ 

= 9 – 15 

w = -6 J