Chapter 7: Alternating Current

For an LCR circuit, the power transferred from the driving source to the driven oscillator is P = I2Z cos φ.

(a) Here, the power factor cos 0, 0. φ ≥ ≥ P

(b) The driving force can give no energy to the oscillator (P = 0) in some cases.

(c) The driving force cannot syphon out (P < 0) the energy out of oscillator.

(d) The driving force can take away energy out of the oscillator.

When an AC voltage of 220 V is applied to the capacitor C

(a) the maximum voltage between plates is 220 V.

(b) the current is in phase with the applied voltage.

(c) the charge on the plates is in phase with the applied voltage.

(d) power delivered to the capacitor is zero.

The line that draws power supply to your house from street has

(a) zero average current.

(b) 220 V average voltage.

(c) voltage and current out of phase by 90°.

(d) voltage and current possibly differing in phase φ such that |φ| < π/2.

If an LC circuit is considered analogous to a harmonically oscillating spring block system, which energy of the LC circuit would be analogous to potential energy and which one analogous to kinetic energy?

Draw the effective equivalent circuit of the circuit shown in Fig 7.1, at very high frequencies and find the effective impedance.

Study the circuits (a) and (b) shown in Fig 7.2 and answer the following questions.

(a) Under which conditions would the rms currents in the two circuits be the same? (b) Can the rms current in the circuit (b) be larger than that in (a)?

Can the instantaneous power output of an ac source ever be negative? Can the average power output be negative?

In series LCR circuit, the plot of I max vs ω is shown in Fig 7.3. Find the bandwidth and mark in the figure.

The alternating current in a circuit is described by the graph shown in Fig 7.4. Show rms current in this graph.

How does the sign of the phase angle φ, by which the supply voltage leads the current in an LCR series circuit, change as the supply frequency is gradually increased from very low to very high values.