Physics
ys cs 132:
3 Lecture
ectu e 16
6
Elements of Physics II
A
Agenda
d for
f T
Today
d


Resistance
 Resistors
 Series
 Parallel
 Ohm’s law
Electric Circuits
 Current
 Kirchoff
Kirchoff’ss laws
Physics 201: Lecture 1, Pg 1
Clicker Question 1:

A light
g bulb is connected to a battery
y in case A.
In case B an identical light bulb is added in
parallel. What happens to the brightness of the
g
bulb?
original
 How do you suppose
(a) it stays the same
outlets in our homes are
(b) it decreases
wired?
(c) it increases
Physics 201: Lecture 1, Pg 2
Parallel wiring

If two objects are in a circuit such that both ends
of the object are connected, they are said to be in
parallel.


The current splits and travels through
R1and R2 separately
R1
R2
Voltage drop across resistors must be same as emf of the battery.
Physics 201: Lecture 1, Pg 3
Resistors in parallel
Two wires:
 Both ends of resistor are connected
 I1 + I2 = Ieq
 V1 = V2 = Veq

R1
R2
Req
Physics 201: Lecture 1, Pg 4
Clicker Question 2:
Current flows through a lightbulb. If a wire is now connected across the
bulb, what happens?
a) all the current continues to flow through the bulb
b) half the current flows through the wire, the other half continues through the
bulb
c) all the current flows through the wire
d)) none of the above
Question: If a path is present which offers less resistance than the other, will
all the current flow through that path of only part of it?
Physics 201: Lecture 1, Pg 5
Clicker Question 2:
Question: If a path is present which offers less resistance than the other, will
all the current flow through that path of only part of it?
Physics 201: Lecture 1, Pg 6
Summary: Resistors in Series and Parallel
Series
Parallel
V
Req
Parallel
Series
1
Req

1
R1
 R12
Req = R1 + R2
I
Ieq = I1 + I2
V1 = V2
Veq = V1 + V2
I1 = I2
Physics 201: Lecture 1, Pg 7
Example
p
What is the equivalent resistance of this circuit? What is
the current in all the resistors?
Physics 201: Lecture 1, Pg 8
Example:
A
B
10 
10 
18 
18 
5
C
8
3.1 
D
18 
13.1 
7.6 
Physics 201: Lecture 1, Pg 9
C
Parallel = Same Voltage!
18 
R ()
7.6
13 1 
13.1
13.1
Simplest circuit in the world!
D
12 V
7.6 
18
I (A)
1.6
0.92
0.67
V (V)
12
12
12
Physics 201: Lecture 1, Pg 10
B
Series = Same current!
C
10 
18 
18 
13.1 
3.1 
R ()
7.6
13.1
18
3.1
10
I (A)
1.6
0.92
0.67
0.92
0.92
V (V)
12
12
12
2.85
9.2
Physics 201: Lecture 1, Pg 11
A
Parallel = Same Voltage!
B
10 
10 
18 
18 
5
R ()
7.6
13.1
3.1 
8
18
3.1
10
5
8
I (A)
1.6
0.92
0.67
0.92
0.92
0.57
0.36
V (V)
12
12
12
2.85
9.2
2.85
2.85
Physics 201: Lecture 1, Pg 12
Clicker Question 3:
What is the equivalent resistance between points a and b?
A.
B.
C.
D.
E.
51 
42 
24 
13 
20 
Physics 201: Lecture 1, Pg 13
Clicker Question 3:
Physics 201: Lecture 1, Pg 14
Where to get help



Office hours: Meyertholen
 M 2-3 in my office 129A
 F 11
11-1pm
1pm in room 126 (tutorial style)
 Make appt. or just stop by
Office hours: Harlow T 12-1 F 10-11
The Drop In Help Centre is in MP125 (TA has red lanyard):
 Monday to Thursday - 12:00 to 3:00 pm
 Friday 11:00 am to 2:00 pm

Undergrad
g
tutors ((LM 204A))
 M 4-5pm
 T 10 am -12 pm
 R 11 am-12 pm
 W and F 3-5pm
Physics 201: Lecture 1, Pg 15
Clicker Question 4:
Now remove the diagonal 24 Ω resistor
resistor, leaving a total of
four resistors. What effect does this have on the voltage
difference across the 16 Ω resistor?
(a) The voltage difference is greater w/o the resistor.
(b) The voltage difference is unaffected w/o the resistor.
(c) The voltage is smaller w/o the resistor
resistor.
Physics 201: Lecture 1, Pg 16
Clicker Question 4:
Physics 201: Lecture 1, Pg 17
Energy Transfer in the Circuit, final



As the charge moves through the resistor
resistor,
it loses energy in collisions with the atoms
of the resistor
The energy is transferred to internal
energy
When the charge returns, the net result is
that some chemical energy of the battery
has been delivered to the resistor and
caused its temperature to rise
Physics 201: Lecture 1, Pg 18
Energy and Power
 The power supplied by a battery is:
 The units of power are J/s or W.
Physics 201: Lecture 1, Pg 19
Energy
and
Power
Energy
and
Power
 The power dissipated by a resistor is:
 Or, in terms of the potential drop across the resistor:
Question: What determines the brightness of the light bulbs? Current?
Voltage? Resistance? Or some other factor?
Physics 201: Lecture 1, Pg 20
Clicker Question 5:
Rank the lightbulbs in terms of brightness.
A.
B.
C.
D.
E.
A>B>C
A<B=C
A<B<C
A=B=C
A>B=C
Physics 201: Lecture 1, Pg 21
Clicker Question 6:
The lightbulbs are identical. Initially both bulbs are
glowing. What happens when the switch is closed?
A. Nothing.
B. A stays the same;
B gets dimmer.
C A gets
C.
t brighter;
b i ht
B stays the same.
D Both get dimmer.
D.
dimmer
E. A gets brighter;
B goes out.
Physics 201: Lecture 1, Pg 22
Clicker Question 9:
Which bulb is brighter?
g
A. The
e 60 W bu
bulb.
b
B. The 100 W bulb.
C Their brightnesses are
C.
the same.
D. There
There’s
s not enough
information to tell.
E. I want to g
go home.
Physics 201: Lecture 1, Pg 23