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Capacitors and/or TV (not Emf)
RC CIRCUITS
1. An empty capacitor does not resist
the flow of current, and thus acts
like a wire.
2. A capacitor that is full of charge
will not allow current to flow, and
thus acts like a broken wire.
RC Circuits
When the switch is closed, the
capacitor will begin to charge.
RC Circuits
If an isolated charged capacitor is
connected across a resistor, it
discharges:
1990B3
p.47
A battery with an emf of 24 V and an
internal resistance of 1 Ω is
connected to an external circuit as
shown above.
1990B3 p.47
a.Calculate the equivalent resistance of
the combination of the 4Ω, 8Ω, and
12Ω resistors
RS = 4 + 8 = 12 Ω
 1 1
RP    
 12 12 
1
= 6 Ω
1990B3 p.47
b.Calculate the current in the 5 Ω
resistor
RE = 5 + 1 + 6 = 12 Ω
V 24 V

I
RE 12 
= 2 A
1990B3 p.47
c. Calculate the terminal voltage, VAC
of the battery
VT = E - I r
= 24 V - (2 A) (1 Ω)
= 22 V
1990B3 p.47
d. Calculate the rate at which
energy is dissipated in the 12 Ω
resistor
Current splits at B with 1 A
through each branch
P = I2 R = (1 A)2 (12 Ω)
= 12 W
1990B3 p.47
e. Calculate the magnitude of the
potential difference VBC
VBC = I R = (2 A) (6) Ω
= 12 V
1998B4
p.58
In the circuit shown above, A, B. C,
and D are identical light bulbs. Assume
that the battery maintains a constant
potential difference between its
terminals (i.e., the internal resistance
of the battery is assumed to be
negligible) and the resistance of each
light bulb remains constant.
1998B4 p.58
a. Draw a diagram of the circuit in
the box , using the symbols to
represent the components in your
diagram. Label the resistors A, B. C,
and D to refer to the corresponding
light bulbs.
1998B4 p.58
b. List the bulbs in order of brightness,
from brightest to least bright. If any
two or more bulbs have the same
brightness, state which ones. Justify
your answer. A > D > B = C
A has the largest current
VD = VC + VB
so D is brighter than C or B
C and B are in series = same current
1998B4 p.58
c. Bulb D is then removed from its socket.
i. Describe the change in the brightness,
if any, of bulb A when bulb D is
removed from its socket. Justify your
answer.
The brightness of bulb A
decreases.
REq increases, IA decreases
1998B4 p.58
ii. Describe the change in the
brightness, if any, of bulb B when
bulb D is removed from its socket.
Justify your answer.
The brightness of bulb B
increases.
The current through bulb B
increases, voltage across B
increases
2003B2
p.47
A circuit contains two resistors (10 Ω
and 20 Ω) and two capacitors (12 μF
and 6 μF) connected to a 6 V
battery, as shown in the diagram
above. The circuit has been
connected for a long time.
2003B2 p.47
a. Calculate the total capacitance of
the circuit.
1
 1
1  
1
1


CT   
 

6
6
 C1 C2   12 10 F 6 10 F 
= 4x10-6 F
1
2003B2 p.47
b. Calculate the current in the
10 Ω resistor.
RE  R1  R2  10  20  30 
V
6V
= 0.2 A
I

RE 30 ½
2003B2 p.47
c.Calculate the potential difference
between points A and B.
VAB = V20 = IR20
= (0.2 A)(20 Ω)
= 4 V
2003B2 p.47
d.Calculate the charge stored on one
plate of the 6 μF capacitor.
Q = CTV
= 4x10-6 F (4 V)
= 16x10-6 C
2003B2 p.47
e.The wire is cut at point P. Will the
potential difference between points
A and B increase, decrease, or
remain the same? Justify your
answer.
___increase
___decrease

___remain the same
No current flows through A-B so it
does not affect the potential
difference