Electronic – What happens to the circuit Voltage, Energy

capacitanceenergypower electronicsvoltage

I was recently asked this question to calculate the voltage across a capacitor. The problem is described as follows :-

Consider a capacitor "C" connected across a voltage source "V". Once the capacitor is charged fully, it is disconnected from the source. Now this fully charged capacitor is connected across another uncharged capacitor of capacitance "C". Assuming ideal conditions, what are the various parameters of the new circuit. Like the voltage, Equivalent Capacitance, Energy etc. at the equilibrium point?

Note :- Guys this is not a homework or assignment question. This was one of the question that was asked to test my ability in which I failed.

Best Answer

Solve the problem by first assuming that the capacitors are connected through a resistor with some resistance, say \$R\$ ohms. As long as there is any difference between the voltages across the capacitors, current will flow through that resistor and thus transfer charge from the first capacitor into the second capacitor. So the first capacitor discharges slowly (exponential decay!) into the second capacitor and the current ceases at \$t = \infty\$ when the two capacitors each have charge \$Q/2\$, and thus voltage \$V/2\$. Half the energy stored in the first capacitor is lost since when the discharge of one into the other is complete, they each have one-fourth of the initial energy. Where did this energy go? Will the answer change if we use a different resistor in this experiment?

After this, think about what happens if \$R\$ is infinitesimally small.