Slides
Lesson slides
Slides for the lesson
Open resourceLesson 04
Practice
Consolidate capacitance, stored energy, and capacitor discharge calculations.
Objectives
Lesson outcomes
- Retrieve the key definitions, equations, and graph features across capacitance.
- Practise mixed questions involving capacitance, stored energy, and capacitor discharge.
- Identify which capacitor skills need targeted follow-up before the test.
Syllabus
CIE 9702 syllabus points
7 linked
- 19.1.1 define capacitance, as applied to both isolated spherical conductors and to parallel plate capacitors
- 19.1.2 recall and use C = Q / V
- 19.2.1 determine the electric potential energy stored in a capacitor from the area under the potential–charge graph 1 1
- 19.2.2 recall and use W = 2 QV = 2 CV 2
- 19.3.1 analyse graphs of the variation with time of potential difference, charge and current for a capacitor discharging through a resistor
- 19.3.2 recall and use τ = RC for the time constant for a capacitor discharging through a resistor
- 19.3.3 use equations of the form x = x0 e –(t / RC) where x could represent current, charge or potential difference for a capacitor discharging through a resistor
Definitions
Required definitions
Capacitance
charge stored per unit potential difference.
Lesson Notes
Student guidance and lesson notes
Overview
This lesson consolidates the B2b capacitor sequence before assessment. You should be able to move between capacitance definitions, stored energy equations, discharge graphs, time constants, and exponential decay models.
What You Need to Know
- Use capacitance to connect stored charge, potential difference, and energy.
- Stored energy can be found from the area under a potential-charge graph or from capacitor energy equations.
- During discharge through a resistor, p.d., charge, and current decrease exponentially.
- The time constant tau = RC sets the rate of discharge.
- Prefixes such as microfarads, kilohms, and milliseconds must be converted carefully.
How to Work Through It
- Start with quick retrieval of chapter 19 equations and graph shapes.
- Work through mixed questions without sorting them by lesson first.
- Mark each response for model choice, unit conversion, graph interpretation, and algebra.
- Finish by writing a short target list for the capacitor test.
Check Your Understanding
- Can you choose between C = Q / V, stored energy equations, tau = RC, and exponential discharge equations?
- Can you interpret p.d.-time, charge-time, and current-time discharge graphs?
- Can you explain why a capacitor does not discharge completely after one time constant?
Common Mistakes
- Revising equations without practising when each one applies.
- Using microfarads or kilohms directly in SI calculations.
- Treating the time constant as the total discharge time.
- Mixing up charge, current, p.d., and energy on capacitor graphs.
Next Steps
- Use the linked lesson pages to target the weakest capacitor subtopic.
- Bring marked corrections and remaining questions into the topic test.
Lesson Resources
Materials for this lesson
Use these videos, slide decks, documents, or links to work through the lesson.