A huge capacitor of capacitance is charged to a huge voltage \(\mathrm{V}\) and then discharged through a dc…

A huge capacitor of capacitance is charged to a huge voltage \(\mathrm{V}\) and then discharged through a dc motor. The motor raises a mass \(m\) through h comparable to earth's radius R. The capacitor energy is fully converted to the gravitational potential energy. If \(h=R / 2\), then \(V\) is equal to
  1. \(\sqrt{\frac{m g R}{2 C}}\)
  2. \(\sqrt{\frac{m g R}{C}}\)
  3. \(\sqrt{\frac{2 m g R}{C}}\)
  4. None of these

Solution

The capacitor energy is \(\frac{1}{2} \mathrm{CV}^{2}\) and \(\mathrm{GPE}\) is \(\left(\frac{m g h}{1+\frac{h}{R}}\right)\) So, we have \(\frac{m g h}{1+\frac{h}{R}}=\frac{1}{2} C V^{2}\) If \(h=R / 2\), then \(m g R / 3=\frac{1}{2} C V^{2}\). This implies that \(v=\sqrt{\frac{2m g R}{3 C}}\) ,

Asked in: JEE Mains - Capacitance - Chapter Test

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