RL CircuitsAC Circuits

End of Ch 20, Ch 21.HW# 45, 47, 48

The time it takes for the current to rise depends on the time constant.In a RC circuit, the time constant (τ) depended on the resistance and the capacitance. τ = RC

In a RL circuit, τ = L/RSee graph of current vs. time on page 719

Ch. 21Alternating Current Circuits

(AC circuits)HW# 1, 5, 7, 9, 13, 19, 21a, 23, 28, 39, 45, 47

For the AC circuit with the resistor, the current and the voltage peak at the same time.See figures on page 734.

Capacitors in AC circuitCircuit with AC source and capacitor: Page 737

At initial time there is no charge on the capacitor. The current is maximum then because there is no charge to fight the new charges from moving onto the plates.

The current decreases as the charge build up. (the voltage across capacitor increases)

Circuit with AC source and capacitor: Page 737

When the current reverses direction, the voltage drops because the plates are losing their charge.

This process repeats over an over again.The current peaks before the voltage. The current and voltage are not in phase.The voltage lags behind the current by 900.

Impeding effect of a capacitor is called the capacitance reactance Xc.

See figure 21.7The voltage peaks before the current.