Physics · Electric fields · 6 marks
A 220 μF capacitor is charged to 12 V and then discharged through a 47 kΩ resistor. (a) Calculate the energy stored before discharge. [2] (b) Calculate the time constant. [1] (c) Calculate the pd across the capacitor 15 s after discharge begins. [3]
Mark scheme
- (a) E = ½CV² = 0.5 × 220 × 10⁻⁶ × 12²
- (a) E = 1.58 × 10⁻² J
- (b) RC = 47 × 10³ × 220 × 10⁻⁶ = 10.3 s
- (c) V = V₀e^(−t/RC) = 12 × e^(−15/10.3)
- (c) exponent = −1.45
- (c) V = 2.8 V
Keep the time constant unrounded inside the exponential — rounding 10.34 to 10 shifts the final answer by about 2%.