Alternating Current — Practice

Question 1

A 220 V AC supply has what peak voltage?

Accepted Answer

V0 = V_rms x sqrt(2) = 220 x 1.414 = 311 V.

Question 2

An LCR circuit has R = 20, XL = 30, XC = 10 ohm. Find impedance and phase angle.

Accepted Answer

Z = sqrt(20^2 + (30-10)^2) = sqrt(800) = 28.28 ohm. tan(phi) = 20/20 = 1, so phi = 45 degrees (voltage leads).

Question 3

A 0.1 H inductor and 50 uF capacitor are in series. Find the resonant frequency.

Accepted Answer

omega0 = 1/sqrt(0.1 x 50e-6) = 1/sqrt(5e-6) = 447.2 rad/s. f0 = 447.2/(2 pi) = 71.2 Hz.

Question 4

A step-down transformer converts 220 V to 11 V with 1000 primary turns. Find the secondary turns.

Accepted Answer

Ns/Np = Vs/Vp = 11/220 = 0.05, so Ns = 50 turns.

Question 5

In an LCR circuit V = 200 V, I = 5 A, power consumed = 800 W. Find the power factor.

Accepted Answer

P = V I cos(phi): 800 = 200 x 5 x cos(phi), so cos(phi) = 0.8.

Practice — Alternating Current