RL CircuitsAC Circuits

End of Ch 20, Ch 21.

The time it takes for the current to increase 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.

The larger the time constant, the longer it will take for the current to reach its maximum value.

The current becomes constant.

Ch. 21Alternating Current Circuits

(AC circuits)

Capacitor, resistor, inductor, or even a light bulb,…

The current alternates in direction. So it spends just as much time flowing in one direction as it does traveling in the opposite direction. Therefore the average current is zero.

Instead of average current, use “root mean square of the current”.

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

They are said to be “in phase”.We can apply Ohm’s Law to ∆Vrms and/or ∆vmax

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

At the 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. (As the voltage across capacitor increases, more charges are on the plates to resist the addition of new charges.)

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.

Ohm’s Law with reactance.

See figure 21.7The voltage peaks before the current.

AC circuit including a resistor, an inductor, and a capacitor.

We can calculate the phase shift between the potential difference and the current using the two reactances and the resistance.

The phase shift is Φ.

Simple transformer is made up of two coils wrapped around an iron core. Kind of like the experiment shown at the beginning of chapter 20, except the current in first coil is constantly alternating.

The voltages in each coil are proportional to the number of turns in the coil.