Energy Stored in a Capacitor

Overview

Use potential-charge graphs and equations for energy stored in a capacitor.

What You Need to Know

• Determine the energy stored in a capacitor from the area under a potential-charge graph.
• Use stored-energy equations involving charge, capacitance, and potential difference.
• Connect the graph model to the algebraic equations for capacitor energy.

How to Work Through It

1. Start by sketching a potential-charge graph for a capacitor.
2. Find the stored energy as the triangular area under the graph.
3. Connect the graph area to equivalent stored-energy equations.
4. Practise choosing the most useful equation from the quantities given.

Check Your Understanding

• Why is the area under a potential-charge graph equal to stored energy?
• Where does the factor of one half come from?
• Which energy equation is most useful if C and V are known?

Common Mistakes

• Forgetting the factor of one half in stored-energy calculations.
• Using a rectangular area instead of the triangular area under the graph.
• Mixing up charge, capacitance, and p.d. symbols when rearranging equations.

Next Steps

• Practise one graph-area question and one algebraic energy question.
• Bring stored energy and charge ideas into capacitor discharge.