JEE AdvancedPhysicsAlternating CurrentMCQ+3 / −1
An AC voltage source of variable angular frequency and fixed amplitude V0 is connected in series with a capacitance C and an electric bulb of resistance R (inductance zero). When is increased
- Athe bulb glows dimmer.
- Bthe bulb glows brighter.
- Ctotal impedance of the circuit is unchanged.
- Dtotal impedance of the circuit increases.
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Correct answer: B
-
Circuit elements and impedance
The circuit is a series combination of:
- resistance (the bulb),
- capacitance .
For a series circuit, the capacitive reactance is
Hence the total impedance is
-
Effect of increasing
As angular frequency increases, decreases.
Therefore, also decreases.
-
Current in the circuit
Since the source amplitude is fixed, the current amplitude is
Because decreases when increases, increases.
-
Brightness of the bulb
The bulb is purely resistive, so its average power is
Since current increases, the power dissipated in the bulb increases. Therefore the bulb glows brighter.
-
Checking options
- A: the bulb glows dimmer. ❌ False
- B: the bulb glows brighter. ✅ True
- C: total impedance of the circuit is unchanged. ❌ False
- D: total impedance of the circuit increases. ❌ False
Therefore, the correct option is B.
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