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A 100 μF capacitor in series with a 40 Ω resistance is connected to a 110 V, 60 Hz supply. (a) What is the maximum current in the circuit? (b) What is the time lag between the current maximum and the voltage maximum?

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Class 12 Physics
CBSE and UP Board
Alternating Current
Chapter-7 Exercise 7.15
Additional Exercise

NCERT Solutions for Class 12 Physics Chapter 7 Question 15

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1 Answer

  1. Capacitance of the capacitor, C = 100 μF = 100 x 10-6 F = 10⁻⁴ F

    Resistance of the resistor, R = 40 Ω

    Supply voltage, V = 110 V

    Ans (a).

    Frequency of oscillations, ν= 60 Hz

    Angular frequency, ω = 2π ν = 2π x 60 rad/s =120π rad/s

    For a RC circuit, we have the relation for impedance as: Z = √(R2 +1/ω2c2)

    Peak voltage, V0 = V√2 = 110√2. Maximum current is given as:

    I0= V/Z  =  V0/√(R2 +1/ω2c2)

    =110 √2 / √(R2 +1/ω2c2)

    =110 √2 / √(402 +1/(120π)2(10⁻⁴)2)

    =3.24A

    Ans (b).

    In a capacitor circuit, the voltage lags behind the current by a phase angle of φ. This angle is given by the relation:

    tan φ =(1/ωC)/R = 1/ωCR

    =1/(120 π10⁻⁴x 40)

    = 0.6635

    Therefore, φ =tan⁻¹ (0.6635) = 33.56º

    = 33.56π/180 rad

    Therefore ,Time lag = φ/ω = 33.56 π/(180 x 120 π) = 1.55 x 10⁻³ s

    =1.55 ms

    Hence, the time lag between maximum current and maximum voltage is 1.55 ms.

     

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