Physics of Technology
PHYS 1800
Lecture 30
Introduction
Electric Circuits
Section 0
Lecture 1
Slide 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 1
PHYSICS OF TOF
ECHNOLOGY
- PHYS 1800
PHYSICS
TECHNOLOGY
ASSIGNMENT SHEET
Spring 2009Spring
Assignment
Sheet
2009
Date
Day
Lecture
Chapter
Feb 16
M
Presidents Day
17
Tu
Angular Momentum (Virtual Monday)
18
W
Review
19
H
Test 2
20
F*
Static Fluids, Pressure
Feb 23
M
Flotation
25
W
Fluids in Motion
27
F*
Temperature and Heat
Mar 2
M
First Law of Thermodynamics
4
W
Heat flow and Greenhouse Effect
6
F*
Climate Change
Mar 9-13
M-F
Spring Break
Mar 16
M
Heat Engines
18
W
Power and Refrigeration
20
F*
Electric Charge
Mar 23
M
Electric Fields and Electric Potential
25
W
Review
26
H
Test 3
27
F*
Electric Circuits
Mar 30
M
Magnetic Force Review
Apr 1
W
Electromagnets
3
F
Motors and Generators
Apr 6
M
Making Waves
8
W
Sound Waves
10
F*
E-M Waves, Light and Color
Apr 13
M
Mirrors and Reflections
Introduction
Section
0 Lecture 1 Slide 2
15
W
Refraction and Lenses
17
F*
Telescopes and Microscopes
Apr 20
M
Review
22
W
Seeing Atoms
24
F
The really BIG & the really small
INTRODUCTION TO Modern Physics PHYX 2710
May
1
F
Final Exam: 09:30-11:20am
No Class
8
5-8
5-8
9
9
9
10
10
10
No Classes
11
11
12
12
13
9-12
13
14
9-12
14
15
15
16
17
17
17
1-17
18 (not on test)
21 (not on test)
Homework Due
-
6
7
8
-
9
10
11
No test week
12
Fall 2004
* = Homework Handout
*Homework Handout
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 2
Physics of Technology
PHYS 1800
Lecture 30
Electric Circuits
Introduction
Section 0
Lecture 1
Slide 3
Elements of Circuits
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 3
Can you get a flashlight bulb to light, with a battery and
a single wire?
Introduction
Section 0
Lecture 1
Slide 4
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 4
Electric Circuits and Electric Current
• Dennison’s Law of Circuit Analysis—Follow
the electrons with your finger Dummy!
•For the flashlight
bulb to light, there
must be a closed or
complete path from
the bulb to both ends
of the battery.
•Such a path is called
a circuit.Introduction Section 0
Lecture 1
Slide 5
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 5
Electric Circuits and Electric Current
In this circuit, the battery
is the energy source,
using energy from
chemical reactions to
separate positive and
negative charges.
•This leads to a voltage
difference, with an excess
of positive charges at one
end of the battery and an
excess of negative
charges at the other.
These charges will tend
to flow from one terminal
Introduction
0 Lecture
to the other
if weSection
provide
an external conducting
path (the circuit).
1
Slide 6
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 6
Electric Circuits and Electric Current
• A flow of electric charge is an electric current:
q
I
t
where I is electric current, q is charge,
and t is time.
• The standard unit for electric current is the ampere:
1A=1C/s
•For example, if 3 C of charge
flow through a wire in 2 s, then
the electric current I is 3 C / 2
s = 1.5 A.
•Positive charges
moving
to the
Introduction Section
0 Lecture
1 Slide
right have the same effect as
negative charges moving to the
left.
7
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 7
Electric Circuits and Electric Current
• A flow of electric charge is an electric current:
q
I
t
where I is electric current, q is charge,
and t is time.
• The standard unit for electric current is the ampere:
1A=1C/s
•The direction of current is
defined as the direction that
positive charges would flow.
•In reality, the charge
carriers inIntroduction
a metalSection
wire0 are
Lecture
negatively charged
electrons.
1
Slide 8
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 8
Physics of Technology
PHYS 1800
Lecture 30
Electric Circuits
Introduction
Section 0
Lecture 1
Slide 9
Resistance
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 9
Electric Circuits and Resistance
• In addition to an energy source and a conducting path, a circuit also
includes some resistance to the current.
•In the flashlight bulb, a very
thin wire filament restricts the
current because of its very
small cross-sectional area.
•The wire filament gets hot as
charges are forced through
this constriction.
Introduction Section 0 Lecture 1
•Its high temperature
makes it
glow, and we have light.
Slide 10
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 10
Two arrangements of a battery, bulb, and wire are shown below.
Which of the two arrangements will light the bulb?
a)
b)
c)
d)
Arrangement (a)
Arrangement (b)
Both
Neither
The bulb will light in arrangement
A in which the filament of the bulb
is connected to the two sides of
the battery
for a closed circuit. In
Introduction Section 0 Lecture 1 Slide
B there is no voltage across the
filament and thus no current in
the filament.
11
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 11
Physics of Technology
PHYS 1800
Lecture 30
Electric Circuits
Introduction
Section 0
Lecture 1
Slide 12
Analogy with Water Flow
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 12
+q, -q
charge
mass
wire
pipe
switch
valve
R
resistor
narrow pipe
I=(q/V)vA
Charge flow
rate
(current)
Mass flow
rate
V 
m
(m/V)vA
PEelectric
q
Introduction
Section 0
Lecture 1
Slide 13
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 13
• Water flowing in a pipe is similar to electric current
flowing in a circuit.
–
–
–
–
–
The battery is like the pump.
The electric charge is like the water.
The connecting wires are like the thick pipe.
The filament is like the nozzle or narrow pipe.
The switch is like the valve.
Introduction
Section 0
Lecture 1
Slide 14
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 14
• In a water-flow system, a high pressure difference will
produce a large rate of water flow or current.
– High pressure can be produced by raising the storage tank: this
pressure is related to the gravitational potential energy.
– Likewise, a large difference in potential energy between the
charges at the two ends of a battery is associated with a high
voltage and a greater tendency for charge to flow.
Introduction
Section 0
Lecture 1
Slide 15
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 15
In the circuit shown, the wires are connected to either side of a wooden block
as well as to the light bulb. Will the light bulb light in this arrangement?
a)
b)
c)
d)
Yes
No
Maybe
Impossible to tell
from the picture
The bulb will not light since
IntroductionisSection
Lecture 1poor
Slide 16
(dry) wood
a 0 very
conductor. The resistance will
be so high that virtually no
current is in the lamp circuit.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 16
In the circuit shown, could we increase the brightness of the bulb by
connecting a wire between points A and B?
a)
b)
c)
d)
Yes
No
Maybe
Impossible to tell
from the picture
No. Connecting A and B will
0 Lecture
Slide 17
provide Introduction
a shortSection
circuit
for1 the
battery that will damage it
while allowing virtually no
current in the bulb.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 17
Which of the two circuits shown will cause the light bulb to light?
a)
b)
c)
d)
Arrangement (a)
Arrangement (b)
Both
Neither
Diagram B will allow the
light bulb to light since
there is a closed circuit
providing current from
the batteryIntroduction
throughSection
the 0 Lecture 1 Slide 18
bulb. Whether the
switch is open or closed
is immaterial here since
it is in parallel with another conductor.
In diagram A no potential difference is in the closed circuit.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 18
Suppose we use an uncoated metal clamp to hold the wires in place in
the battery-and-bulb circuit shown. Will this be effective in keeping
the bulb burning brightly?
a)
b)
c)
d)
Yes
No
Maybe
Impossible to tell
from the picture
No. The metal clamp will provide a
conducting
path
across
the
battery
Introduction
Section
0 Lecture
1 Slide
19
causing the battery to discharge. If
we want to use a clamp we can put
insulating tape between one of its
jaws and the electrical connection.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 19
Physics of Technology
PHYS 1800
Lecture 30
Electric Circuits
Introduction
Section 0
Lecture 1
Slide 20
Ohm’s Law
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 20

Ohm’s Law and Resistance
• The electric current flowing through a given
portion of a circuit is directly proportional to the
voltage difference across that portion and
inversely proportional to the resistance:
Ohm's Law :
V
I
R
– Resistance R is the ratio of the voltage difference to the
current for a given portion of a circuit, and is in units of ohms:
1 ohm = 1  = 1 V / A.
– The resistance of a wire is proportional to the length of the
wire, inversely proportional to the cross-sectional area of the
wire,
and Section
inversely
proportional
to the conductivity of the
Introduction
0 Lecture
1 Slide 21
material.
R = L / (A σ)
– It also depends on the temperature of the material.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 21
Ohm’s Law and Resistance
• If we know the resistance of a given portion of a circuit
and the applied voltage, we can calculate the current
through that portion of the circuit.
• For example, consider a 1.5-V
battery connected to a light bulb
with a resistance of 20 ohms.
• If the resistance of the battery
itself is negligible, the current
can be found by applying Ohm’s
Law:
I Introduction
= 1.5 V / Section
20 0
= 0.075 A
= 75 mA
Lecture 1
Slide 22
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 22
Series and Parallel Circuits
• In a series circuit, there are no points in the
circuit where the current can branch into
secondary loops.
– All the elements line up on a single loop.
– The current that passes through one element must also
pass through the others.
Introduction
Section 0
Lecture 1
Slide 23
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 23
Physics of Technology
Next Lab/Demo:
Electric Charge
Electric Circuits
Thursday 1:30-2:45
ESLC 46
Ch 12 and 13
Next Class:
Wednesday 10:30-11:20
BUS
Slide 24318 room
Read Ch 13
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 30 Slide 24