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physicsTRANSCRIPT
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Simple complicated
J. Manuel Physics 72 2nd Sem AY13-14
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Problem:
How do we analyze such kind of
complicated circuit?
J. Manuel Physics 72 2nd Sem AY13-14
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Direct-Current DC direction of the current does not change with time
Alternating-Current AC current oscillates back and forth
Direct-Current Circuits
J. Manuel Physics 72 2nd Sem AY13-14
26 Direct-Current Circuits
4 J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
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Objectives 1. Given a network of resistors connected in series and/or
parallel, evaluate the equivalent resistance, current and
voltage
2. Evaluate the voltage drop and current passing thru each
circuit element
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
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Resistor Resistance R (Ω)
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
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Four ways to connect 3 resistors
with resistances R1, R2, R3
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
8 Equivalent resistance
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
9
Resistors in Series
Current I must be the same in all resistors.
Their potential differences add.
Recall: Equivalent resistance of 3 resistors
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
10
Resistors in Series
Equivalent resistance of n resistors
In general, for n number of resistors,
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
11
Resistors in Parallel
Potential difference is the same for all resistors.
Their currents add.
Recall:
Equivalent resistance of 3 resistors
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
12
Resistors in Parallel
Equivalent resistance of n resistors
In general, for n number of resistors,
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
13
Example: Compute the equivalent resistance of the network
shown, and find the current in each resistor. The
battery has negligible internal resistance.
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
14
Seatwork: In the circuit shown, each resistor represents a light bulb.
Let R1= R2 = R3 = R4 = 1 Ω and ε = 9.00 V.
a) (0.2 pt) Find the equivalent resistance of the network.
b) (0.2 x 4 pt) Find the current (I1, I2, I3, I4) in each bulb.
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
15
Homework (0.5+0.5 pt): Consider the circuit shown. The current through the
6.00-Ω resistor is 4.00 A, in the direction shown. What
are the currents through the 25.0-Ω and 20.0-Ω
resistors?
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
16
Objective 1. Given a circuit diagram, calculate the current through and
voltage across a circuit element using Kirchhoff’s loop and
junction rules
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
17
Many practical resistor networks cannot be reduced to simple
series-parallel combinations.
Kirchhoff’s Rules
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
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Examples:
A junction in a circuit is a point where three or more conductors meet.
• (node or branch point)
A loop is any conducting path. J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
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1. Kirchhoff’s Junction Rule The algebraic sum of the currents into any junction is zero.
The junction rule is based on conservation of electric charge.
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
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Example: Consider the circuit fragment shown in the figure. What
is the current IX flowing out of node B?
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
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Example: Using the circuit diagram below use junction rule to
express the relationship of the assumed current
direction.
I1
I2
I3
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
22
2. Kirchhoff’s Loop Rule The algebraic sum of the potential differences in any loop,
(including those associated with emfs and those of resistive elements),
must equal to zero.
The loop rule is a statement that the electrostatic force is
conservative.
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
23
2. Kirchhoff’s Loop Rule
Sign Conventions:
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
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Example:
I. -I1 R1 - I2 R2 - V1 = 0
II. I1 R1 + I3 R3 + V2 = 0
III. -I2 R2 - I4 R4 - V2 = 0
Consider the circuit shown with the
assumed direction of each branch
current shown by the labeled arrows.
Assuming each inner loop is
travelled counterclockwise, which
of the following equations follows
Kirchhoff's voltage law?
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
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Example: In the circuit shown, find
(a) the current in the 3.00-Ω resistor;
(b) the unknown emfs ε1 and ε2;
(c) the resistance R.
Note that three currents are given.
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
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Seatwork: What is the internal resistance r in the 12-V battery?
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
27
In the circuit shown, find
(a)the current in resistor R;
(b)the resistance R;
(c)the unknown emf ε.
(d)If the circuit is broken at point x,
what is the current in resistor R?
Homework (0.5 x 4 pts):
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
28
Objective 1. Describe the behavior of current, potential, and charge as a
capacitor is charging or discharging in terms of the initial,
transient, and steady-state conditions
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
29
R-C Circuit A circuit that has a resistor and a capacitor in series
1. Current changes with time
2. Voltage changes with time
3. Power changes with time
Charging/Discharging a Capacitor
heart pacemakers
flashing traffic lights
automobile turn signals
electronic flash units
Applications:
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
30
Caution: Lowercase (i, q, v) means time-varying.
Instantaneous current i = i(t)
Instantaneous charge q = q(t)
Instantaneous voltage v = v(t)
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
31
Charging a Capacitor
1. idealized battery (or power supply)
constant emf ε (zero internal resistance, r = 0)
2. connecting conductors has negligible resistance
Assumptions:
Initially, capacitor C is uncharged. At t = 0, switch is closed.
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
32
Charging a Capacitor At t = 0, switch is closed.
vbc = q/C = 0 (since q = 0)
Current Io through the resistor R
ε = IoR
Io = ε/R
After some time t,
Charge q on the capacitor increases.
Current i decreases.
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
33
Charging a Capacitor After some time t,
Current i decreases.
Charge q on the capacitor increases.
Kirchhoff’s Loop Rule:
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
34
Charging a Capacitor
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
35
Time Constant τ measure of how quickly a capacitor charges
If τ is small, capacitor charges quickly.
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
36
Time Constant
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
37
Time Constant
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
38
Discharging a Capacitor Initially, capacitor is charged (q = Qo).
1. Remove the battery from RC circuit.
2. Connect point a and c to an open switch.
At t = 0, switch is closed.
Kirchhoff’s Loop Rule:
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
39
Discharging a Capacitor After some time t,
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
40
Discharging a Capacitor After some time t,
J. Manuel Physics 72 2nd Sem AY13-14
Resistors in Series and Parallel
Kirchhoff’s Rules
R-C Circuits 26
41
A 4.60-μF capacitor that is initially uncharged is connected in series with a
7.50-kΩ resistor and an emf source with ε = 125 V and negligible internal
resistance. Just after the circuit is completed, what are
(a) the voltage drop across the capacitor;
(b) the voltage drop across the resistor;
(c) the charge on the capacitor;
(d) the current through the resistor;
(e) the time constant?
(f) A long time after the circuit is completed (after many time constants)
what are the values of the quantities in parts (a)-(d)?
Example:
J. Manuel Physics 72 2nd Sem AY13-14