Capacitor charge law
Also known as the Definition of capacitance · Q = CV.
The charge stored on a capacitor is directly proportional to the voltage across it, the constant of proportionality being its capacitance; the current into it is proportional to the rate at which that voltage changes.
Q = C × V, i = C·dv/dt, E = ½CV²
- Q
- charge stored (coulombs)
- C
- capacitance (farads)
- V
- voltage across the plates (volts)
- i
- current into the capacitor (amperes)
- E
- energy stored in the field (joules)
In plain English: A capacitor is a bucket for charge — the fuller it gets, the higher the voltage rises. Current only flows while the voltage is changing, which is exactly why a capacitor blocks DC and passes AC.
Where you'll meet it: Sizing decoupling and smoothing capacitors, every RC timing calculation (τ = R × C), snubbers, and supercapacitor backup runtimes. The ½CV² form is why a charged high-voltage capacitor is genuinely dangerous long after the power is off.