Download series resistors

Charging and Discharging a Capacitor
•Complete the activity on charging and discharging capacitors
located under Activities on the website
•sites.google.com/site/sienaphys140spring2011/activities/char
ging-and-discharging-a-capacitor
Tactics: Using Kirchhoff’s loop law
EXAMPLE 32.1 A single-resistor circuit
EXAMPLE 32.1 A single-resistor circuit
Energy and Power
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
EXAMPLE 32.4 The power of light
EXAMPLE 32.4 The power of light
Series Resistors
• 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.
EXAMPLE 32.7 Lighting up a flashlight
EXAMPLE 32.7 Lighting up a flashlight
Parallel Resistors
• 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.
Series and Parallel Resistors
EXAMPLE 32.10 A combination of resistors
QUESTION:
EXAMPLE 32.10 A combination of resistors
RC Circuits
• 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
Applications
EXAMPLE 32.14 Exponential decay in an RC
circuit
QUESTION:
EXAMPLE 32.14 Exponential decay in an RC
circuit
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 = 10,
R2 = 20 , and
R1 = 30 , 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