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Electrical Systems · Part 4 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.
An alternating supply has a peak voltage of V.
Calculate its rms voltage.
A V rms AC supply is connected across a Ω resistor.
Calculate the peak voltage, the rms current and the average power dissipated. Explain why rms values, rather than peak values, are used for power calculations.
A student connects a Ω resistor first to a V DC supply, then to an AC supply whose oscilloscope trace shows a peak-to-peak voltage of V. They claim the AC supply must deliver more power because its trace reaches a higher voltage than V.
Calculate the average power in each case and evaluate the student's claim. Explain fully what the rms value represents and why the sine wave's average power is exactly half its peak power.
A H inductor is connected to a Hz supply.
Calculate its inductive reactance.
A Ω resistor is in series with a capacitor of reactance Ω across an AC supply.
Calculate the impedance of the circuit, and explain why the resistance and the reactance cannot simply be added together.
A series circuit contains a Ω resistor, a H inductor and a µF capacitor, connected to a V rms supply.
Calculate the impedance and the current at Hz and at Hz. Draw on phasor reasoning to explain how the circuit's behaviour changes between the two frequencies, and predict what would happen at a frequency between them.
A series LCR circuit has H and µF.
Calculate its resonant frequency.
A series LCR circuit contains a Ω resistor and is connected to a V rms supply.
State the impedance and current at resonance, and explain why the impedance is a minimum at that frequency.
A radio tuning circuit consists of a mH inductor, a variable capacitor and a Ω resistance, receiving signals of V rms.
Calculate the capacitance needed to tune to a station broadcasting at kHz, and the current at resonance. Explain fully how the circuit selects one station out of many, and discuss why a lower resistance improves the receiver.