Discharging a Capacitor

Overview

Analyse capacitor discharge graphs, time constant, and exponential decay equations.

What You Need to Know

• Analyse how p.d., charge, and current vary with time as a capacitor discharges through a resistor.
• Use the time constant tau = RC.
• Use exponential decay equations for current, charge, or potential difference during discharge.

How to Work Through It

1. Start by describing what happens to charge, p.d., and current during discharge.
2. Identify the time constant from R and C, or from a graph.
3. Practise exponential decay calculations for charge, current, and p.d.
4. Compare the shapes of p.d.-time, charge-time, and current-time graphs.

Check Your Understanding

• What does the time constant tell you about a discharge curve?
• Why do p.d. and charge have the same exponential shape?
• What fraction of the initial value remains after one time constant?

Common Mistakes

• Treating one time constant as complete discharge.
• Forgetting that current direction during discharge is opposite to the charging current direction.
• Using non-SI units for R, C, or t in exponential equations.

Next Steps

• Practise reading time constants from graphs.
• Bring all capacitor equations into the practice lesson.