6 past paper MCQs on A.C. Through a Resistor, a Capacitor and an Inductor, from the Alternating Current unit of MDCAT Physics. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.
1.In a pure capacitance AC circuit, the current: (KMU MDCAT 2025)
Correct answer: (b) Leads the voltage by 90°
A capacitor must be charged before a voltage appears across it, so the current runs a quarter cycle ahead.
2.If frequency of AC is doubled, the inductive reactance will: (SIBA MDCAT 2025)
Correct answer: (c) Be doubled
Inductive reactance is $2\pi fL$, so it is proportional to the frequency.
3.A capacitor is connected to an AC source. If the frequency of the AC source is doubled the current in a purely capacitive circuit will: (SZABMU MDCAT 2025)
Correct answer: (a) Be doubled
Capacitive reactance is $1/2\pi fC$, so twice the frequency halves it and doubles the current.
4.Phase shift between voltage and current when AC flows through capacitor is: (NUMS MDCAT 2024)
Correct answer: (b) I leads V by 90°
The capacitor must be charged before a voltage appears, so the current runs a quarter cycle ahead.
5.What is the instantaneous power dissipated through a resistor of resistance 10 ohm if it is connected to an AC source of frequency 15 Hz at 1/180 sec if peak value is 20 V? (UHS MDCAT 2018)
Correct answer: (a) 10 W
At $t=1/180\,\mathrm{s}$ the phase is $30^{\circ}$, so $v=10\,\mathrm{V}$ and $P=v^2/R$ is $10\,\mathrm{W}$.
6.Tuning a radio is a best example of: (UHS MDCAT 2016)
Correct answer: (d) Electrical resonance
A radio is tuned by matching the natural frequency of its circuit to that of the station.