Lesson 05
Test
Assess understanding of capacitance, stored energy, and capacitor discharge.
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 test checks the B2b capacitor topic: capacitance, energy stored in a capacitor, discharge graphs, time constant, and exponential discharge equations.
What You Need to Know
- Show working clearly when using capacitance, energy, time constant, or exponential equations.
- Use SI units throughout, especially for capacitance, resistance, time, charge, and p.d.
- Interpret graphs carefully, including area under a potential-charge graph and exponential discharge curves.
- Explain physical meanings, not just equation substitutions.
How to Work Through It
- Read each question carefully and identify whether it is about capacitance, energy, or discharge.
- Write down known quantities, convert units, and choose the relevant equation.
- Use graph areas, gradients, and time constants where required.
- Check final answers for units, significant figures, and physical sense.
Check Your Understanding
- Can you define capacitance and use C = Q / V?
- Can you determine stored energy from a graph or equation?
- Can you analyse discharge graphs for charge, p.d., and current?
- Can you use tau = RC and exponential decay equations correctly?
Common Mistakes
- Choosing an equation from keywords without checking the physical situation.
- Forgetting the factor of one half in stored energy relationships.
- Treating one time constant as complete discharge.
- Losing marks through unit prefixes such as micro, milli, or kilo.
Next Steps
- Use the review lesson to correct mistakes and identify which capacitor skills need more practice.
- Keep working visible so feedback can target the exact weak step.