Radio receivers are tuned using

Analyze the question: The question asks which component is used to tune a radio receiver, meaning to select a particular station's frequency.

Concept: Tuning relies on a resonant LC circuit (an inductor and a capacitor connected together). The circuit resonates at the frequency

f = 1 / (2π√(LC))

When the resonant frequency of the receiver equals the frequency of the radio station's signal, the circuit responds strongly to that signal and rejects the others.

Solution:

1. To pick different stations, we must change the resonant frequency of the circuit.
2. From f = 1 / (2π√(LC)), this is done by changing either L or C.
3. In a conventional radio receiver, the inductor (coil) is fixed, and the capacitance is varied by turning the tuning knob. This knob rotates a variable capacitor, whose plates move in and out of mesh, changing C and therefore f.
4. Option C (inductors) is part of the tuning circuit, but a fixed inductor alone does not tune the radio. The variable element is the capacitor.
5. Electromagnets, electric motors and variable resistors do not set the resonant frequency. A variable resistor controls things like volume, not frequency selection.

Answer: D. variable capacitors.

Exam tip: Remember "Tuning = Resonance = variable Capacitor." If you see "tuning a radio" in an exam, think of the LC circuit and the turning knob that changes the capacitance.