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  4. The potentials of the two plates of capacitors are +10 V and -10 V. The charge on one of the plates is 40 C. The capacitance of the capacitor is:

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Q. The potentials of the two plates of capacitors are $+10\, V$ and $-10\,V$. The charge on one of the plates is $40\, C$. The capacitance of the capacitor is:

AFMCAFMC 2005

Solution:

A capacitor is an arrangement which can store sufficient quantity of charge. Suppose, on given a charge q to a conductor, the electric potential of the conductor becomes $V$.
Then, capacitance of capacitor is
image
$C=\frac{q}{V}$
The potential difference across the parallel plate capacitor is $10\,V-(-10\,V)=20\,V$
$\therefore $ Capacitance $C=\frac{q}{V}=\frac{40}{20}=2\,F$
The unit farad $(F)$ is very much bigger unit as capacitance of earth, which is a huge body in the universe has a capacitance of $711\,\mu F.$ So, it is not reasonable to discuss such a bigger unit in questions.