Edexcel IAL Physics
Capacitance
Big idea: A capacitor is a tiny electrical "bucket" that stores charge —
and how much it can hold per volt of push (its capacitance) determines how
much energy it stores and how fast it charges or discharges through a resistor.
Quick Summary
- Capacitance
C = Q/V — charge stored per unit potential difference, measured in Farads (F).
- Energy stored in a capacitor:
W = ½QV = ½CV² = Q²/2C — equal to the area under a Q–V graph.
- Charging: current starts high and decays exponentially to zero; charge and p.d. rise to a maximum.
- Discharging: current, p.d. and charge all decay exponentially from their starting values to zero.
- The time constant
τ = RC tells you how fast charging/discharging happens.
- Discharging capacitor:
Q = Q₀e^(–t/RC), and the same shape applies to V and I.
- Taking natural logs turns the exponential decay into a straight line — useful for experiments.
1. What Is Capacitance?
Think of a capacitor like an electrical water tank. The power supply is a pump
pushing water (charge) in. The "capacitance" is basically how wide the tank is — a wider tank
(bigger capacitance) can store more water (charge) for the same pump pressure (voltage).
Physically, a capacitor is just two metal plates facing each other, usually with an
insulating material called a dielectric sandwiched between them so charge can't jump
straight across. The circuit symbol is two parallel lines:
───┤ ├───
(two parallel lines = capacitor)
When you connect a capacitor to a battery, the battery doesn't push charge through the
capacitor (the dielectric blocks that) — instead it pushes electrons onto one plate
(making it negative) and pulls electrons off the other plate (making it positive).
The plates never touch, but they build up equal and opposite charge.
The Defining Equation
In plain English: capacitance is defined as the charge stored per unit potential
difference between the plates. The bigger the capacitance, the more charge you can
cram in for every volt you apply.
Watch out
1 Farad is a huge unit in practice — real capacitors are usually measured in
microfarads (μF, ×10⁻⁶), nanofarads (nF, ×10⁻⁹), or picofarads (pF, ×10⁻¹²). Always convert
to Farads before substituting into equations!
Common mixup
The letter "C" is used for two different things: the symbol for capacitance, AND
the unit of charge (Coulombs). Don't confuse "5 C of capacitance" with "5 C of charge" —
read the units carefully!
Practice Question 1
A parallel plate capacitor has a capacitance of 1 nF and is connected to
a voltage supply of 0.3 kV. Calculate the charge stored on the plates.
Practice Question 2
A capacitor stores 6 × 10⁻⁴ C of charge when the potential difference
across it is 12 V. Calculate its capacitance in μF.
2. Energy Stored by a Capacitor
constant
the p.d. across the
capacitor rises as the charge builds up
changing
average
Area Under the Graph
area under
this line
3. Charging & Discharging Curves
Charging
4. The Time Constant, τ
5. Core Practical: Investigating Charge & Discharge
Aim:
Method Outline