Alternating Current

The rms value of AC which when passed through a resistor produces heat, which is twice that produced by a steady current of 1.414 A in the same resistor is

• 2 A

• 3.46 A

• 2.818 A

• 1.732 A

In a series LCR AC circuit, the current is maximum when the impedance is equal to

• the reactance

• the resistance

• zero

• twice the reactance

A transformer connected to 220 V mains is used to light a lamp of rating 100 W and 110 V. If the primary current is 0.5 A, the efficiency of the transformer is ( approximately)

• 60 %

• 35 %

• 50 %

• 90 %

A lamp consumes only 25 % of the peak power in an AC circuit. The phase difference between the applied voltage and the current is

• $\frac{\mathrm{\pi }}{6}$

• $\frac{\mathrm{\pi }}{3}$

• $\frac{\mathrm{\pi }}{4}$

• $\frac{\mathrm{\pi }}{2}$

In an L-C-R series circuit, at resonance

• the current and voltage are in phase

• the impedance is maximum

• the current is minimum

• the quality factor is independent of R

A step-down transformer increases the input current 4 A to 24 A at the secondary. If the number of turns in the primary coil is 330, the number of turns in the secondary coil is

• 60

• 50

• 55

• 45

An AC source of voltage E = 20 sin 100 t is connected across a resistance 20 Ω. The rms value of current in the circuit is

• 1 A

• $\frac{1}{2} \mathrm{A}$

• $\frac{1}{\sqrt{2}} \mathrm{A}$

• $2\sqrt{2} \mathrm{A}$

In an L-C-R circuit

• the impedance is equal to reactance

• the ratio between effective voltage to effective current is called reactance

• at resonance the net reactance is zero

• at resonance the resistance is equal to the reactance

A step-down transformer with an efficiency of 80% is used on a 1000 V line to deliver 10 A at 100 V at the secondary coil. The current drawn from the line is

• 1.5 A

• 2 A

• 3 A

• 1.25 A

Identify the wrong statement

• Eddy currents are produced in a steady magnetic field

• Eddy currents can be minimized by using laminated core

• Induction furnace uses eddy current to produce heat

• Eddy current can be used to produce breaking force in moving vehicles