• Complete the activity on charging and discharging capacitors
located under Activities on the website
• sites.google.com/site/sienaphys140spring2011/activities/
charging-and-discharging-a-capacitor
The power supplied by a battery is
The units of power are J/s, or W.
The power dissipated by a resistor is
Or, in terms of the potential drop across the resistor
•  Resistors that are aligned end to end, with no junctions
between them, are called series resistors or, sometimes,
resistors “in series.”
•  The current I is the same through all resistors placed in
series.
•  If we have N resistors in series, their equivalent
resistance is
The behavior of the circuit will be unchanged if the N series
resistors are replaced by the single resistor Req.
•  Resistors connected at both ends are called parallel
resistors or, sometimes, resistors “in parallel.”
•  The left ends of all the resistors connected in parallel are
held at the same potential V1, and the right ends are all
held at the same potential V2.
•  The potential differences ΔV are the same across all
resistors placed in parallel.
•  If we have N resistors in parallel, their equivalent
resistance is
The behavior of the circuit will be unchanged if the N
parallel resistors are replaced by the single resistor Req.
QUESTION:
•  Consider a charged capacitor, an open switch, and a
resistor all hooked in series. This is an RC Circuit.
•  The capacitor has charge Q0 and potential difference
ΔVC = Q0/C.
•  There is no current, so the potential difference across the
resistor is zero.
•  At t = 0 the switch closes and the capacitor begins to
discharge through the resistor.
•  The capacitor charge as a function of time is
where the time constant τ is
QUESTION:
Junction Rule
General Physics 2
Circuits
24
Resistance, Voltage
•  Determine (a) the equivalent resistance of the
circuit and (b) the voltage across each resistor.
General Physics 2
Circuits
25
Rank in order of brightness
•  Rank bulbs 1 through 6 in
order of descending
brightness.
–  Brightness is
proportional to power
2
V
P = VI = I 2 R =
R
€
•  Now assume the filament in
B6 breaks. Again rank the
bulbs in order of descending
brightness.
General Physics 2
Circuits
26
Practice Problems
•  Determine the
equivalent
resistance and the
current through
R1 for the circuits
shown. Assume
R1 = 10Ω,
R2 = 20 Ω, and
R1 = 30 Ω, and
the battery is
12 V.
General Physics 2
Circuits
27
Activities
•  Exploration of Physics
–  E&M
–  Resistive circuits
–  Do each of the 5 circuits – set all to 50 ohms
–  Calculate I, V, and P for each resistor and then check answers in the
program
General Physics 2
Current & Resistance
28