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# Chapter 7 Alternating Current

The seventh chapter of NCERT class 12 Physics has only nine topics in all. You get detailed knowledge and understanding about alternating currents and its application in resistor, LCR series circuit, peak and RMS value of alternating voltage, reactance, impedance, LC oscillations, resonance and power in AC circuits, AC generator and transformer, watt-less current, etc. At the end of the chapter, you will find exercises for you to solve.

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### Exercise 1

•  Q1 A 100 Ω resistor is connected to a 220 V, 50 Hz ac supply. (a) What is the rms value of current in the circuit? (b) What is the net power consumed over a full cycle? Ans: Resistance of the resistor, R = 100 Ω Supply voltage, V = 220 V Frequency, ν = 50 Hz (a) The rms value of current in the circuit is given as: I = V / R  = 220 / 100 = 2.20 A (b) The net power consumed over a full cycle is given as: P = VI = 220 × 2.2 = 484 W Q2 (a) The peak voltage of an ac supply is 300 V. What is the rms voltage? (b) The rms value of current in an ac circuit is 10 A. What is the peak current? Ans: (a) Peak voltage of the ac supply, V0 = 300 V Rms voltage is given as: (b) Therms value of current is given as: I = 10 A Now, peak current is given as: Q3 A 44 mH inductor is connected to 220 V, 50 Hz ac supply. Determine the rms value of the current in the circuit. Ans: Inductance of inductor, L = 44 mH = 44 × 10-3 H Supply voltage, V = 220V Frequency, ν = 50Hz Angular frequency, \begin{align}\omega = 2\pi v\end{align} Inductive reactance, \begin{align}X_{L}= \omega L = 2\pi vL = 2\pi\times50\times44\times10^{-3} \Omega\end{align} Rms value of current is given as: \begin{align}I = \frac{v}{X_{L}}\end{align} \begin{align} = \frac{220}{2\pi\times50\times44\times 10^{-3}} =15.92A\end{align} Hence, the rms value of current in the circuit is 15.92 A.

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