Capacitance of a Capacitor
Simple Explanation
A capacitor's capacitance (C) measures how much charge it stores per volt applied across it, C=Q/V — a higher capacitance means the capacitor can hold more charge for the same voltage.
Why Do We Need It?
Capacitance is the single number that characterizes a capacitor — every other capacitor formula in this chapter (energy stored, combinations, the parallel-plate design) builds directly on this definition.
Formula
Definition of Capacitance
C = Q/V
A capacitor's capacitance measures how much charge it can store per volt of potential difference applied across it — a bigger capacitance stores more charge for the same voltage.
- C
- — capacitance, in farads (F)
- Q
- — magnitude of charge stored on each plate, in coulombs (C)
- V
- — potential difference across the capacitor, in volts (V)
When to use it: Whenever the capacitance of a capacitor needs to be found from the charge it stores and the voltage across it (or vice versa).
Worked Example
Find a capacitor's capacitance
A capacitor stores 0.002 C of charge when connected across a 100 V source. Find its capacitance.
Why Does This Work?
Capacitance is DEFINED as this ratio — it is a property of the capacitor's own physical construction (plate size, spacing, and material), so the same capacitor stores proportionally more charge as more voltage is applied, keeping Q/V constant regardless of the specific voltage used.
Real-Life Example
Choosing a capacitor for a circuit
An electronics engineer selects a capacitor with a specific capacitance rating (like 100 μF) printed on its casing for a particular circuit design.
That printed rating tells the engineer exactly how much charge the capacitor will store per volt applied — critical information for predicting how the circuit will behave.
Practice
A capacitor stores 0.0005 C of charge across a 50 V source. Find its capacitance, in μF.
MediumCommon mistake
Forgetting that a farad is an enormous unit for everyday capacitors — most real capacitors have capacitances measured in microfarads (μF, 10⁻⁶ F) or picofarads (pF, 10⁻¹² F), not whole farads.
Quick Review
- C = Q/V, in farads.
- Capacitance is a fixed property of the capacitor's construction.
- Real capacitors are usually μF or pF, not whole farads.