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Physics of Technology
PHYS 1800
Lecture 31
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 31 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 31 Slide 2
Physics of Technology
PHYS 1800
Lecture 31
Electric Circuits
Introduction
Section 0
Lecture 1
Slide 3
Review
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 3
+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 4
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 4
• 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 5
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 5
• 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 6
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 6

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 inversely
proportional
to the conductivity of the
Introduction
Section 0 Lecture
1 Slide 7
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 31 Slide 7
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 8
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 8
Physics of Technology
PHYS 1800
Lecture 31
Electric Circuits
Introduction
Section 0
Lecture 1
Slide 9
Series Circuits
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 9
Series Circuits
• In a series combination of resistances, each resistance
contributes to restricting the flow of current around the
loop.
– The total series resistance of the combination Rseries is the sum
of the individual resistances:
Rseries  R1  R2  R3
– A common mistake is to think the current gets used up in
passing through the resistances in a series circuit.
– The same current must pass through each component much
like the continuous flow of water in a pipe.

Introduction
Section 0
Lecture 1
Slide 10
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 10
Series Circuits
• It is the voltage that changes as the current flows through
the circuit.
– Voltage decreases by Ohm’s Law: V = I R
as the current passes through each resistor.
• The total voltage difference across the combination is the
sum of these individual changes.
– If two light bulbs are connected in series with a battery, the
current will be less than with a single bulb, because the total
series resistance is larger.
– The bulbs will glow less brightly.
Introduction
Section 0
Lecture 1
Slide 11
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 11
Series Circuits
Two resistors are connected in series with a battery as shown.
R1 is less than R2. Which of the two resistors has the greater
current flowing through it?
a)
b)
c)
d)
R1
R2
Both
Neither
Introduction
Section 0
Lecture 1
Slide 12
The current is the same in each, since it
is a series circuit.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 12
Series Circuits
Two resistors are connected in series with a battery as shown. R1 is
less than R2. Which of the two resistors has the greatest voltage
difference across it?
a)
b)
c)
d)
R1
R2
Both
Neither
Introduction
Section 0
Lecture 1
Slide 13
The voltage difference is greater across R2. According to Ohm's
Law, V = IR, so for the same current, the larger the resistance
the greater the potential difference.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 13
Series Circuits
In the circuit shown, the 1- resistance is the internal resistance of the
battery and can be considered to be in series with the battery and the 9 load. What is the current flowing through the 9- resistor?
a)
b)
c)
d)
e)
0.1 A
0.3 A
0.9 A
3A
10 A
Rseries Introduction
9   1Section
  10
 1
0 Lecture
I series
Slide 14
Vtotal
3V


 0.3 A
Rseries 10 
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 14
Series Circuits
In the circuit shown, the 1- resistance is the internal
resistance of the battery and can be considered to be in series
with the battery and the 9- load. What is the voltage across
the 9- resistor?
a)
b)
c)
d)
e)
0.1 V
0.3 V
1.0 V
2.7 V
3.0 V
since I9Introduction
 Iseries :Section 0
Lecture 1
Slide 15
V9  IseriesR9  0.3 A9   2.7 V
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 15
Series Circuits
Three resistors are connected to a 6-V battery as shown. The
internal resistance of the battery is negligible. What is the
current through the 15- resistance?
a)
b)
c)
d)
e)
0.1 A
0.15 A
0.4 A
1.5 A
4.0 A
Rseries  15   20   25   60 
Introduction
I15  Iseries 
Section 0
Lecture 1
Slide 16
Vtotal
6V

 0.1 A
Rseries 60 
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 16
Series Circuits
Does this same current flow through the 25- resistance?
a)
b)
c)
Yes.
No.
It depends on
various things.
Introduction
Section 0
Lecture 1
Slide 17
Yes. Since all the resistors are in series, the same current must
flow through all three. There is no other path for the current
through the 15- resistor to follow, except to go through the 20-
resistor and then the 25- resistor.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 17
Series Circuits
What is the voltage difference across the 25- resistance?
a)
b)
c)
d)
e)
0.1 V
2.5 V
6V
25 V
60 V
since I25Introduction
 Iseries :Section 0
Lecture 1
Slide 18
V25  IseriesR25  0.1 A25   2.5 V
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 18
Physics of Technology
PHYS 1800
Lecture 31
Electric Circuits
Introduction
Section 0
Lecture 1
Slide 19
Parallel Circuits
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 19
Parallel Circuits
• In a parallel circuit, there are points at which the current
can branch or split up into different paths.
– The flow divides and later rejoins.
– The total cross-sectional area the current (or water) flows
through is increased, therefore decreasing the resistance to
flow:
1
Rparallel

1
1
1


R1 R2 R3
Introduction
Section 0
Lecture 1
Slide 20
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 20
Parallel Circuits
• In a parallel combination of resistances, the voltage
difference across each resistance is the same, since they
are all connected between the same two points.
– The currents can be different, since they divide: they add to
give the total current through the combination.
– A portion of the total current flows through each branch.
1
Rparallel

1
1
1


R1 R2 R3
Introduction
Section 0
Lecture 1
Slide 21
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 21
Parallel Circuits
Two 10- light bulbs are connected in parallel to one another, and this
combination is connected to a 6-V battery. What is the total current
flowing around the loop?
a) 0.6 A
b) 1.2 A
c) 6 A
d) 12 A
1
Rparallel
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Slide 22
e) 60 A
1
1
 
R1 R2
I

R
6V
5
1
1


10  10 

2
1


10  5 
 1.2 A
Rparallel  5 
Electric Circuits

Lecture 31 Slide 22
Parallel Circuits
How much current passes through each light bulb?
a) 0.6 A
b) 1.2 A
c) 6 A
d) 12 A
I
e) 60 A
V
R
6V

10 
 0.6 A
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Slide 23

Electric Circuits
Lecture 31 Slide 23
Parallel Circuits
Three identical resistors, each 24 , are connected in parallel with one
another as shown. The combination is connected to a 12-V battery
whose internal resistance is negligible.
What is the equivalent resistance of this parallel combination?
a)
b)
c)
d)
e)
0.0417 
0.125 
8
24 
72 
Introduction
Section 0
1
Rparallel
Physics of Technology—PHYS 1800
Spring 2009
1
1
1


R1 R2 R3
1
1
1



24  24  24 
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004

3
1


24  8 
Slide 24
Rparallel  8 
Electric Circuits
Lecture 31 Slide 24
Parallel Circuits
What is the total current through the combination?
a)
b)
c)
d)
e)
0.5 V
1.0 V
1.2 V
1.5 V
12 V
Itotal 
Vtotal
12 V

 1.5 V
Rparallel 8 

Introduction
Section 0
Lecture 1
Slide 25
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 25
Parallel Circuits
How much current flows through each resistor in the combination?
a)
b)
c)
d)
e)
The resistors are identical and are in parallel,
0.5 V
1.0 V
1.2 V
1.5 V
12 V
so the same current must flow through each resistor.
The total current is plit into three identical parts
Ione 24- resistor 
Introduction

Section 0
Lecture 1
1
1
Itotal  1.5 V  0.5 V
3
3
Slide 26
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 26
:
Parallel Circuits
In the circuit shown, R3 is greater than R2, and R2 is greater than R1.
 is the electromotive force of the battery whose internal resistance is
negligible. Which of the three resistors has the greatest current
flowing through it?
a)
b)
c)
d)
e)
R1
R2
R3
R1 and R2 are equal,
and greater than R3
They are all equal
Introduction
Section 0
Lecture 1
Slide 27
R3 has the greatest current since the current in it is the sum of
the currents in R1 and R2.
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 27
Physics of Technology
PHYS 1800
Lecture 31
Electric Circuits
Introduction
Section 0
Lecture 1
Slide 28
Power in Electric Circuits
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 28
Electric Energy and Power
• Energy is supplied to a water-flow system by the pump, which
increases the gravitational potential energy of the water by lifting
it up to a higher tank.
• As the water flows
down through pipes to a
lower tank, gravitational
potential energy is
transformed into kinetic
energy of the moving
water.
• Once the water comes
to rest in the lower tank,
the kineticIntroduction
energySection
is 0 Lecture
dissipated by frictional or
viscous forces which
generate heat.
1
Slide 29
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 29
Electric Energy and Power
• Similarly, in an electric circuit energy is supplied by a
battery, which draws its energy from the potential energy
stored in its chemical reactants.
– The battery increases the potential energy of electric charges
as it moves positive charges toward the positive terminal and
negative charges toward the negative terminal.
– When we provide an external conducting path from the
positive to the negative terminal, charge flows from points of
higher potential energy to points of lower potential energy.
• As potential
energy is lost,
kinetic energy is
gained by the
electrons.
• This kinetic
energy is Introduction Section 0
converted to heat
by collisions with
other electrons
and atoms.
Lecture 1
Slide 30
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 30
Electric Energy and Power
Energy source  potential energy  kinetic energy  heat
•
•
•
Since voltage is potential energy per unit charge, multiplying a
voltage difference by charge yields energy.
Since current is the rate of flow of charge, multiplying a voltage
difference by current yields power, the rate of energy use.
The power supplied by a source must equal the power dissipated
in the resistances.
P  I
 VI ;
V  IR 
P  ( IR ) R  I 2 R
I  I
Introduction
2
Section 0
Lecture 1
Slide 31
R
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 31
Electric Energy and Power
What is the power dissipated in a 20- light bulb powered
by two 1.5-V batteries in series?
   
1
a)
b)
c)
d)
e)
0.15 W
0.45 W
3.0 W
6.67 W
60 W
2
3V
R  20 
  IR

I 
R
3V
 0.15 A
20 
P  I  I 2 R  (0.15 A) 2 (20 )  0.45 W
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009

Slide 32
check :
P  I  (3 V)(0.15 A)  0.45 W
Electric Circuits
Lecture 31 Slide 32
Electric Energy and Power
• The ease with which electric power can be transmitted over
considerable distances is one of its main advantages over
other forms of energy.
– The source of the energy might be gravitational potential
energy of water, chemical potential energy stored in fossil
fuels, or nuclear potential energy stored in uranium.
– Power plants all use electric generators that convert
mechanical kinetic energy produced by turbines to electric
energy.
– These generators are the source of the electromotive force.
Introduction
Section 0
Lecture 1
Slide 33
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 33
Electric Energy and Power
• The unit of energy commonly used to discuss electric
energy is the kilowatt-hour, which is a unit of power (the
kilowatt) multiplied by a unit of time (an hour).
– 1 kilowatt equals 1000 watts
– 1 hour = 3600 seconds
– 1 kilowatt-hour equals 3.6 million joules
• The kilowatt-hour is a much larger unit of energy than the
joule, but it is a convenient size for the amounts of
electrical energy typically used in a home.
Introduction
Section 0
Lecture 1
Slide 34
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 34
Electric Energy and Power
How much does it cost to light a 100-watt light bulb for one day?
Assume an average rate of cost of 10 cents per kilowatt-hour.
a) 0.24 cents
b) 2.4 cents
Energy used = power x time
= (100 W)(24 hr)
= 2400 Wh
= 2.4 kWh
Introduction
Section 0
Lecture 1
c) 24 cents
d) $2.40
e) $24
Cost = energy used x rate of cost
= (2.4 kWh)(10 cents / kWh)
= (2.4 kWh)(10 cents / kWh)
= 24 cents
Slide 35
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 35
Physics of Technology
PHYS 1800
Lecture 31
Electric Circuits
Introduction
Section 0
Lecture 1
Slide 36
Alternating Current and Household Wiring
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 36
Alternating Current and Household Circuits
• The current we draw from a wall outlet is alternating current
(ac) rather than direct current (dc).
– Direct current implies that the current flows in a single
direction from the positive terminal of a battery or power
supply to the negative terminal
– Alternating current continually reverses its direction -- it flows
first in one direction, then in the other, then back again.
– In North America the ac goes through 60 cycles each second
(60 Hz).
Introduction
Section 0
Lecture 1
Slide 37
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 37
Alternating Current and Household Circuits
• The plot of electric current as a function of time for an
alternating current is a sinusoidal curve.
– The average value of an ordinary alternating current is zero.
– The power dissipated in a resistance is proportional to the
square of the current.
– The effective current or rms current is obtained by squaring
the current, averaging this value over time, and taking the
square root of the result.
– The effective current Ieff is 0.707 times the peak current Ipeak.
Introduction
Section 0
Lecture 1
Slide 38
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 38
Alternating Current and Household Circuits
• If we plot the voltage across an electrical outlet as a
function of time, we get another sinusoidal curve.
– The effective value of this voltage is typically between 110 and
120 volts in North America.
– The standard household power supplied in this country is 115
volts, 60 hertz ac.
– Household circuits are wired in parallel so that different
appliances can be added to or removed from the circuit
without affecting the voltage available.
Introduction
Section 0
Lecture 1
Slide 39
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 39
Alternating Current and Household Circuits
A 60-W light bulb is designed to operate on 120 V ac. What is the
effective current drawn by the bulb?
a)
b)
c)
d)
e)
P  60 W
0.2 A
0. 5 A
2.0 A
72 A
7200 A
Introduction
Veffective  120 V
P  IV 
I
Section 0
Lecture 1
Slide 40
Fall 2004
Spring 2009
60 W
120 V
 0.5 A
INTRODUCTION TO Modern Physics PHYX 2710
Physics of Technology—PHYS 1800

P
V
Electric Circuits
Lecture 31 Slide 40
Alternating Current and Household Circuits
• Household circuits are wired in parallel so that different
appliances can be added to or removed from the circuit
without affecting the voltage available.
– As you add more appliances, the total current drawn
increases, because the total effective resistance of the circuit
decreases when resistances are added in parallel.
– Since too large a current could cause the wires to overheat, a
fuse or circuit breaker in series with one leg of the circuit will
disrupt the circuit if the current gets too large.
– Appliances with larger power requirements (stoves, clothes
dryers, etc) are usually connected to a separate 220-V line.
Introduction
Section 0
Lecture 1
Slide 41
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 41
Physics of Technology
Next Lab/Demo:
Electric Circuits
Magnetism
Thursday 1:30-2:45
ESLC 46
Ch 13 and 14
Next Class:
Friday 10:30-11:20
BUS
Slide 42318 room
Read Ch 14
Introduction
Section 0
Lecture 1
INTRODUCTION TO Modern Physics PHYX 2710
Fall 2004
Physics of Technology—PHYS 1800
Spring 2009
Electric Circuits
Lecture 31 Slide 42