Question 434: Electrostats & Capacitance

Question

A 60 pF capacitor is fully charged by a 20 V supply. It is then disconnected from the supply and
is conneced to another uncharged 60 pF capacitor in parallel. The electrostatic energy that is lost
in this process by the time the charge is redistributed between them is (in nJ) _____

Accepted Answer

U
i
= $${1 \over 2}CV_0^2$$
U
f
= $${1 \over 2} 	imes 2C 	imes {\left( {{{{V_0}} \over 2}} ight)^2}$$
$$\Delta $$E = $${1 \over 2}CV_0^2$$ - $${1 \over 2} 	imes 2C 	imes {\left( {{{{V_0}} \over 2}} ight)^2}$$
= $${{{CV_0^2} \over 4}}$$
= $${1 \over 4} 	imes 60 	imes {10^{ - 12}} 	imes 4 	imes {10^2}$$
= 6 $$ 	imes $$ 10
-9
= 6 nJ

Electrostats & Capacitance

A 60 pF capacitor is fully charged by a 20 V supply. It is then disconnected from the supply and is conneced to another uncharged 60 pF capacitor in parallel. The electrostatic energy that is lost in this process by the time the charge is redistributed between them is (in nJ) _____

Choose the correct answer: