What are some common mistakes students make with capacitance?
The main mistake is that capacitance is the total amount of charge that can be stored.
This mistake confuses capacitance with volumetric capacity. In that case the mistake leads to the belief that when a 100 µF capacitor is fully charged it will hold 1.2 mC of charge (for example) regardless of what voltage power supply is used to charge it.
In fact capacitance is the total amount of charge stored per unit voltage. Therefore if a capacitor is charged with a 12 V power supply it will store more charge than the same capacitor charged with a 6 V supply. Here are some examples for that 100 µF capacitor:
- 12 V supply:
#Q = CV = 100 ×10^(–6) × 12 = 1.2 mC#
- 6.0 V supply:
#Q = CV = 100 ×10^(–6) × 6.0 = 0.60 mC#