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Fall 2008Physics 231 Lecture 6-1
Direct-Current Circuits
Fall 2008Physics 231 Lecture 6-2
Resistors in Series and ParallelAs with capacitors, resistors are often in series and parallel configurations in circuits
Series
Parallel
The question then is what is the equivalent resistance
Fall 2008Physics 231 Lecture 6-3
Resistors in Series
Since these resistors are in series, we have the same current in all three resistors
IIII 321
We also have that the sum of the potential differences across the three resistors must be the same as the potential difference between points a and b
ybxyaxab VVVV
Fall 2008Physics 231 Lecture 6-4
Resistors in SeriesThen using 321 ;; RIVRIVRIV ybxyax
We have that 321 RRRIVab
Now the equivalent resistor, R, will also have the same potential difference across it as Vab, and it will also have the same current I RIVab
Equating these last two results, we then have that
i
iRRRRR 321
The equivalent resistance for a sequence of resistors in series is just the sum of the individual resistances
Fall 2008Physics 231 Lecture 6-5
Resistors in Parallel
Here we have that the voltage across each resistor has to be the same (work done in going from a to b is independent of the path, independent of which resistor you go through)
abVVVV 321
Fall 2008Physics 231 Lecture 6-6
Resistors in ParallelWe now deal with currents through the resistors
At point a the current splits up into three distinct currents
We have that the sum of theses three currents must add to the value coming into this point
321 IIII
We also have that3
32
21
1 ;;R
VI
R
VI
R
VI ababab
The equivalent resistor, R, will have also have the current I going through it
Fall 2008Physics 231 Lecture 6-7
Resistors in Parallel
UsingR
VI ab
and combining with the previous equations, we then have
321 R
V
R
V
R
V
R
V abababab
or
i iRRRRR
11111
321
The inverse of the effective resistance is given by the sum of the inverses of the individual resistances
Fall 2008Physics 231 Lecture 6-8
Solving Resistor Networks
Make a drawing of the resistor network
Determine whether the resistors are in series or parallel or some combination
Determine what is being asked
Equivalent resistance
Potential difference across a particular resistance
Current through a particular resistor
Fall 2008Physics 231 Lecture 6-9
Solving Resistor Networks
Solve simplest parts of the network first
Then redraw network using the just calculated effective resistance
Repeat calculating effective resistances until only one effective resistance is left
Fall 2008Physics 231 Lecture 6-10
Solving Resistor Networks
Given the following circuit
What is the equivalent resistance and what is the current through each resistor
We see that we have two resistors in parallel with each other and the effective resistance of these two is in series with the remaining resistor
Fall 2008Physics 231 Lecture 6-11
Step 1: Combine the two resistors that are in parallel
yielding
Step 2: Combine the two resistors that are in series
624 effR yielding
Solving Resistor Networks
2;2
1
3
1
6
11 effeff
RR
Fall 2008Physics 231 Lecture 6-12
Solving Resistor NetworksCurrent through this effective resistor is given by
AmpsR
VI
eff3
6
18
The current through the resistors in the intermediate circuit of Step 1 is also 3 Amps with the voltage drop across the individual resistors being given by
VoltsV
VoltsV
623
;1243
2
4
Fall 2008Physics 231 Lecture 6-13
To find the current through the resistors of the parallel section of the initial circuit, we use the fact that both resistors have the same voltage drop – 6 Volts
AmpsVolts
I
AmpVolts
I
23
6
;16
6
3
6
Solving Resistor Networks
Fall 2008Physics 231 Lecture 6-14
Consistency CheckThere is a check that can be made to see if the answers for the currents make sense:
The power supplied by the battery should equal the total power being dissipated by the resistors
The power being supplied by the battery is given bywhere I is the total current
VIP
WattsVIP 54183
The power being dissipated by each of the resistors is given by RIP 2
WattsPWattsP
WattsPWattsP
Total 54;661
;1232;36432
6
23
24
Fall 2008Physics 231 Lecture 6-15
Two identical light bulbs are represented by the resistors R2 and R3 (R2 = R3 ). The switch S is initially open.
If switch S is closed, what happens to the brightness of the bulb R2?a) It increases b) It decreases c) It doesn’t change
Example 1
The power dissipated in R2 is given by R
VP2
When the switch is closed neither V nor R changes
So the brightness does not change
Fall 2008Physics 231 Lecture 6-16
Two identical light bulbs are represented by the resistors R2 and R3 (R2 = R3 ). The switch S is initially open.
What happens to the current I, after the switch is closed ?
a) Iafter = 1/2 Ibefore b) Iafter = Ibefore c) Iafter = 2 Ibefore
Example 2
Initially the current is given by 2RIbefore
After the switch is closed the net resistance is given by
32232
2111RR
RRRRnet since
The new current is then beforenet
after IRRI 222
22RRnet
Fall 2008Physics 231 Lecture 6-17
Kirchoff’s RulesNot all circuits are reducible
There is no way to reduce the four resistors to one effective resistance or to combine the three voltage sources to one voltage source
Fall 2008Physics 231 Lecture 6-18
Kirchoff’s RulesFirst some terminology
A junction, also called a node or branch point, is is a point where three or more conductors meet
A loop is any closed conducting path
Fall 2008Physics 231 Lecture 6-19
Kirchoff’s RulesKirchoff’s Rules are basically two statements
1. The algebraic sum of the currents into any junction is zero 0I
A sign convention:
0321 III
A current heading towards a junction, is considered to be positive,A current heading away from a junction, is considered to be negative
Be aware that all the junction equations for a circuit may not be independent of each other
Fall 2008Physics 231 Lecture 6-20
Kirchoff’s Rules2. The algebraic sum of the potential differences in any loop including those associated with emfs and those of resistive elements must equal zero
0VProcedures to apply this rule:
Pick a direction for the current in each branchIf you picked the wrong direction, the current will come out negative
Fall 2008Physics 231 Lecture 6-21
Kirchoff’s RulesPick a direction for traversing a loop – this direction must be the same for all loops
Note that there is a third loop along the outside branches
As with the junction equations not all the loop equations will be independent of each other.
Fall 2008Physics 231 Lecture 6-22
Kirchoff’s RulesStarting at any point on the loop add the emfs and IR terms
An IR term is negative if we traverse it in the same sense as the current that is going through it, otherwise it is positive
An emf is considered to be positive if we go in the direction - to +, otherwise it is negative
Need to have as many independent equations as there are unknowns
Fall 2008Physics 231 Lecture 6-23
Kirchoff’s Rules
For loop I we have 031432111 RIRIRI
For loop II we have 0324332 RIRI
Junction equation at a gives us 0321 III
We now have three equations for the three unknown currents
Fall 2008Physics 231 Lecture 6-24
Kirchoff’s Rules
Assume that the batteries are: 1 = 19 V; 2 = 6 V; 3 = 2 V
and the resistors are: R1 = 6R2 = 4 R3 = 4; R4 = 1
you should end up with: I1 = 1.5 A; I2 = -0.5 A; I3 = 2.0 A
The minus sign on I2 indicates that the current is in fact in the opposite direction to that shown on the diagram
Complete details can be found here
Fall 2008Physics 231 Lecture 6-25
RC CircuitsUp until now we have assumed that the emfs and resistances are constant in time so that all potentials, currents, and powers are constant in time
However, whenever we have a capacitor that is being charged or discharged this is not the case
Now consider a circuit that consists of a source of emf, a resistor and a capacitor but with an open switch
With the switch open the current in the circuit is zero and zero charge accumulates on the capacitor
Fall 2008Physics 231 Lecture 6-26
RC CircuitsNow close the switch Initially the full potential will be
across the resistor as the potential across the capacitor is zero since q is zero
The initial current in the circuit is then given by RI /0
As the current flows a charge will accumulate on the capacitor
At some time t, the current in the circuit will be I and the charge on the capacitor will be q
Initially the full potential is across the resistor
Fall 2008Physics 231 Lecture 6-27
RC CircuitsAccording to Kirchoff’s 2nd rule we have
Using a counterclockwise loop
0
0
C
qIR
VV capacitorresistor
Solving for the currentRC
q
RI
As time increases, the charge on the capacitor increases, therefore the current in the circuit decreases
Current will flow until the capacitor has a charge on it given by CQ
Fall 2008Physics 231 Lecture 6-28
RC CircuitsWe remember that
td
qdI
So we then have Cq
CRCR
q
Rtd
qd
1
Rearranging we haveRC
td
Cq
qd
Setting up the integration we have
tq
RC
td
Cq
qd
00
The resultant integration yieldsRCC
Cq 1ln
Fall 2008Physics 231 Lecture 6-29
RC CircuitsWe exponentiate both sides of this last equation and rearrange to obtain
RCtf
RCt eQeCq // 11
where Qf is the final charge on the capacitor given by C
We see that the charge on the capacitor increases exponentially
The constant RC is known as the time constant of the circuit
Fall 2008Physics 231 Lecture 6-30
Example 3
What will be the voltage across the capacitor a long time after the switch is closed?
(a) VC = 0 (b) VC = R2/(R1+ R2) (c) VC =
I1I3
I2
R2C
R1
After a long time the capacitor is completely charged, so no current flows through it
The circuit is then equivalent to a battery with two resistors in seriesThe voltage across the capacitor equals the voltage across R2 (since C and R2 are in parallel)
At t = 0 the switch is closed in the circuit shown. The initially uncharged capacitor then begins to charge.
Fall 2008Physics 231 Lecture 6-31
RC CircuitsThe current in the circuit is given by
RCtRCt eIeRtd
qdI /
0/
and looks like
Note that is also how the voltage across the resistor behaves
IRVresistor
Fall 2008Physics 231 Lecture 6-32
RC Circuits – Charging Summary
For the simple RC circuit we have the following for the voltage drops across the capacitor and the resistor
Fall 2008Physics 231 Lecture 6-33
RC Circuits
We now start from a situation where we have a charged capacitor in series with a resistor and an open switch
The capacitor will now act as a source of emf, but one whose value is not constant with time
Fall 2008Physics 231 Lecture 6-34
RC Circuits
We now close the switch and a current will flow
Kirchoff’s 2nd rule gives us
0C
qIR
Rearranging we haveRC
q
td
qd
To find q as a function of time we integrate the above equation
Fall 2008Physics 231 Lecture 6-35
RC Circuits
RC
t
Q
qtd
RCq
qdtq
Q
00
ln'1
'
'
0
Exponentiation of both sides of the equation on the right yields
RCteQq /0
We see that the charge on the capacitor decreases exponentially
Fall 2008Physics 231 Lecture 6-36
RC CircuitsThe current in the circuit is obtained by taking the derivative of the charge equation
RCt
eRC
Q
dt
dqI
0
The quantity Q0 / C is just the initial voltage, Vo , across the capacitor
But then V0 / R is the initial current I0
So we then have that RCt
eII
0
The voltage across the resistor is given by RCt
eVV
0
Fall 2008Physics 231 Lecture 6-37
The two circuits shown below contain identical fully charged capacitors at t = 0. Circuit 2 has twice as much resistance as circuit 1.
Compare the charge on the two capacitors a short time after t = 0
a) Q1 > Q2 b) Q1 = Q2 c) Q1 < Q2
Example 4
Initially, the charges on the two capacitors are the same. But the two circuits have different time constants: 1 = RC and 2 = 2RC
Therefore, at any given time, the charge on capacitor 2 is larger than that on capacitor 1
Since 2 > 1 it takes circuit 2 longer to discharge its capacitor
Fall 2008Physics 231 Lecture 6-38
Example 5The capacitor in the circuit shown is initially
charged to Q = Q0. At t = 0 the switch is connected to position a.
At t = t0 the switch is immediately flipped from position a to position b.
C
a b
R 3R
a) Which of the following graphs best represents the time dependence of the charge on C?
b) Which of the following correctly relates the value of t0 to the time constant a while the switch is at a?
(a) t0 < a (b) t0 = a (c) t0 > a
time
QQ0
t0 time
QQ0
t0 time
QQ0
t0
a) b) c)
Fall 2008Physics 231 Lecture 6-39
Example 5The capacitor in the circuit shown is initially
charged to Q = Q0. At t = 0 the switch is connected to position a.
At t = t0 the switch is immediately flipped from position a to position b.
C
a b
R 3R
a) Which of the following graphs best represents the time dependence of the charge on C?
time
QQ0
t0 time
QQ0
t0 time
QQ0
t0
a) b) c)
For 0 < t < t0, the capacitor is discharging with time constant t = RC
For t > t0, the capacitor is discharging with time constant = 3RC, i.e., much more slowly Therefore, the answer is a)
Fall 2008Physics 231 Lecture 6-40
We know that for t = a, the value of the charge is e-1 = 0.37 of the value at t = 0
Example 5The capacitor in the circuit shown is initially
charged to Q = Q0. At t = 0 the switch is connected to position a.
At t = t0 the switch is immediately flipped from position a to position b.
C
a b
R 3R
b) Which of the following correctly relates the value of t0 to the time constant a while the switch is at a?
(a) t0 < a (b) t0 = a (c) t0 > a
Since the curve shows Q(t0) ~ 0.6 Q0, t0 must be less than a
time
QQ0
t0 time
QQ0
t0 time
QQ0
t0
a) b) c)
Fall 2008Physics 231 Lecture 6-41
Capacitors Circuits, Qualitative
ChargingIt takes time to put the final charge on
Initially, the capacitor behaves like a wire (V = 0, since Q = 0).
As current starts to flow, charge builds up on the capacitor
it then becomes more difficult to add more charge
the current decreases
After a long time, the capacitor behaves like an open switch.
DischargingInitially, the capacitor behaves like a battery.
After a long time, the capacitor behaves like a wire.
Basic principle:
Capacitor resists rapid change in Q resists rapid changes in V