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Electrical Systems · Part 2 of 4
9 exam-style questions with model answers, plus 12 quick multi-choice questions — every question on this part of the standard, grouped by the 3 pages of notes they come from.
Write a full answer before you reveal the model one. That comparison is where the learning happens.
A µF capacitor is charged to V.
Calculate the charge stored.
A parallel plate capacitor has plates of area m2 separated by mm of air.
Calculate its capacitance. Then state and explain what happens to the capacitance if the plate separation is halved.
A parallel plate capacitor is charged by a battery to V and then disconnected from the battery. A dielectric slab of relative permittivity is then slid between the plates.
Explain fully what happens to the charge, the capacitance, the voltage and the stored energy. Then explain how the answers would differ if the capacitor had been left connected to the battery throughout.
Capacitors of µF and µF are connected in parallel.
Calculate the total capacitance.
Capacitors of µF and µF are connected in series across a V supply.
Calculate the total capacitance and the voltage across each capacitor. Explain why the smaller capacitor has the larger voltage across it.
An engineer has three identical µF capacitors, each rated at a maximum of V. They need a combination that can safely withstand V.
Determine an arrangement that meets this requirement and calculate its capacitance and the energy it stores at V. Explain why the series arrangement raises the voltage rating, and discuss the trade-off involved.
A µF capacitor is charged through a kΩ resistor.
Calculate the time constant of the circuit.
A µF capacitor is charged through a kΩ resistor from a V supply.
Calculate the time constant and the initial current. Explain why the current decreases as the capacitor charges.
A camera flash uses a µF capacitor charged to V. The capacitor is charged through a kΩ resistor, and discharges through the flash tube, which has a resistance of about Ω.
Calculate the energy stored, the charging time constant and the discharging time constant. Explain fully why this arrangement produces a very bright but very brief flash, and why the photographer must wait between shots.