Lecture 9/21 Chapter 21

Question 1

RC Circuits

Answer 1

Capacitors and Capacitance

A direct current circuit may contain
capacitors and resistors, the current will vary
with time

When the circuit is completed, the capacitor
starts to charge

The capacitor continues to charge until it
reaches its maximum charge (Q = Cε)

Once the capacitor is fully charged, the
current in the circuit is zero

Question 2

Charging Capacitor in an
RC Circuit

Answer 2

The charge on the capacitor varies with time

The time constant represents the time required for the charge to increase from zero to
63.2% of its maximum

In a circuit with a large time constant, the capacitor charges very slowly. The capacitor charges very quickly if there is a small time constant

After t = 10 τ, the capacitor is over 99.99% charged

Question 3

Charging Capacitor in an
RC Circuit

Equation

Answer 3

q = Q(1 – e-t/RC)

The time constant, τ=RC

Question 4

Discharging Capacitor in
an RC Circuit

Answer 4

When a charged capacitor is placed in the
circuit, it can be discharged

The charge decreases
exponentially

At t = τ = RC, the charge
decreases to 0.368 Qmax

In other words, in one time
constant, the capacitor
loses 63.2% of its initial
charge

Question 5

Discharging Capacitor in
an RC Circuit

Equation

Answer 5

q = Qe-t/RC