Download RC Circuits

RC Circuits
PH 203
Professor Lee Carkner
Lecture 14
Capacitance
 Remember that a capacitor stores charge:
Q = C DV

C = ke0A/d
 The potential across the capacitor can change with time


 The voltage across an uncharged capacitor is zero

 The voltage across a charged capacitor is equal to the
applied potential
 VC = e
Resistors and Capacitors
If you connect a uncharged capacitor to a
resistor and a battery the capacitor will go
from 0 to maximum charge over some time

The current in the circuit will start at a maximum
and them go to zero as the capacitor charges

A capacitor C paired with a resistor R has a
charging time related to a time constant (t)
t = RC
This is the time to charge a capacitor to about
63% of the final value
Charging a
Capacitor
Time Curve
Charge Over Time
If we charge a capacitor by connecting it to a
battery of voltage e, the charge and voltage on
the capacitor is:
VC= e[1-e(-t/t)]
Capacitor charges rapidly at first and then the
rate of charge separation slows

At about t = 4t the capacitor is nearly fully
charged
Discharging a Capacitor

Energy in the capacitor is dissipated
through the resistor

QC = CVC
VC= V0e(-t/t)
where V0 is the initial voltage on the
capacitor
Using an Ammeter

Must be placed in
series

Has low resistance
Otherwise would
decrease the current
Using a Voltmeter

Must be placed in
parallel

Has high resistance
So only very little
current flows through it
Next Time
Read 28.1-28.6
Problems: Ch 27, P: 47, 49, 56, Ch 28, P:
2, 12
Consider two identical light bulbs in series. If a
third identical light bulb is added in series,
what happens to the current through the
battery and the power that flows from the
battery?
A)
B)
C)
D)
E)
increase, increase
increase, decrease
decrease, decrease
decrease, increase
Both stay the same
Consider two identical light bulbs in parallel. If
a third identical light bulb is added in
parallel, what happens to the current
through the battery and the power that flows
from the battery?
A)
B)
C)
D)
E)
increase, increase
increase, decrease
decrease, decrease
decrease, increase
Both stay the same