9702 · 19.2
Energy stored in a capacitor — FAQ
Frequently asked questions for 9702 Energy stored in a capacitor. Direct answers first, then deeper explanation — then practise with marking.
Why is the energy stored formula $\frac{1}{2}CV^2$ and not $CV^2$?
The voltage across a capacitor isn't constant during charging. It starts from zero and increases to V. The factor accounts for this average voltage, representing the area of a triangle under the Q-V graph, which is how we calculate the total work done to charge it.
What is the significance of the time constant in capacitor discharge?
The time constant () provides a critical measure of how quickly a capacitor charges or discharges. It's the time required for the charge, voltage, or current to fall to approximately 37% of its initial value, giving insight into the circuit's transient response.
Can a capacitor store energy indefinitely?
In an ideal scenario, yes. However, in reality, all capacitors have some internal leakage resistance, meaning the charge will slowly dissipate over time, even if disconnected from a circuit. This self-discharge can be very slow but is not truly indefinite.