How Things Work II
(Lecture #20)
Instructor: Gordon D. Cates
Office: Physics 106a, Phone: (434) 924-4792
email: [email protected]
Course web site available through COD and Toolkit
or at http://people.virginia.edu/~gdc4k/phys106/spring07
March 12, 2007
Announcements
• The midterm will be this Wednesday, March 14th.
- The midterm will cover everything up to and including
chapter 11, and some of chapter 12 (Probably section 12.1).
• Problem Set #3 is posted, and is due this coming
Friday, March 2nd, at roughly 11:59PM.
06
0
2. 66
13 66 0
33 66
33 7
33
4.
3
26
5. 66 3.2
33 66
33 66
33 7
33
7.
3
46
8. 66 6.4
53 66
33 66
33 7
33
10
3
.6
11 66 9.6
.7 66
33 66
33 7
33
13
3
.8
1
14 66 2.8
.9 66
33 66
33 7
33
3
M
or
e
1.
Grade distribution
on
Quiz
#1
Quiz_1
45
40
35
30
25
20
Frequency
15
10
5
0
1
2
3
4
Mean = 9.57
5
6
7
8
9
10
11 12 13 14 15 16
Standard deviation = 0.19
Stuff to study
• The textbook, chapters 1, 2, 10, 11, and 12.1
- Do you know the stuff in the summaries ?
- Can you answer the questions in the “Check your
understanding boxes?
• The homeworks
- Can you do the problems?
- For homeworks 1 and 2, have you compared your work with
the solutions?
• The quiz
- Check out the solutions
- Do you know what you did wrong?
• Lectures, particularly stuff covered in the slides
Standard International Units
Length
meters
Time
second
Mass
kilogram
Speed
meters/second
Acceleration
meters/second2
Force
kilogram meters/second2
newton
Energy
newton - meter
Joule
Power
Joule/second
Watt
Charge
Coulomb
Current
Coulomb/second
Ampere
“electric potential”
joule/Coulomb
Volt
electric field
volts/meter = newtons/coulomb
From Problem Set #3
Physics 106 - How Things Work II - Spring 2007
Problem Set #3
You are at an amusement park, and decide to participate in a very unusual ride called
“Coulomb Bumper Cars”. The cars themselves are not propelled by motors. Instead, they
are given a large electrostatic charge of 2.5 Coulombs. Everything takes place in a large
circular track in the middle of which is a dome that is kept at a voltage of +10,000 Volts.
The outside edge of the track is kept at zero volts. At the beginning of the ride, everyone
pushes their car from the outside edge of the track in to the dome in the middle and keeps
their brakes on until the ride begins. When the ride begins, everyone releases their brakes
and are repelled away from the dome. You and the bumper car collectively have a mass of
500 kg.
0 Volts
1.
2.5 Coulombs
10,000 Volts
a. Assuming that your car is at 0 Volts when you start pushing it from the edge,
how much electrostatic potential energy does your car have once it reaches 10,000
Volts in the middle of the track?
b. Once your bumper car again reaches the outside edge of the track at zero volts,
what is the speed of the car? You may assume that your electrostatic potential
energy has been converted entirely into kinetic energy.
2. Why is it important that the filament of a light bulb have a much larger electrical
resistance than the supporting wires that carry current to and from that filament?
The outside edge of the track is kept at zero volts. At the beginning of the ride, everyone
pushes their car from the outside edge of the track in to the dome in the middle and keeps
their brakes on until the ride begins. When the ride begins, everyone releases their brakes
and are repelled away from the dome. You and the bumper car collectively have a mass of
500 kg.
From Problem Set #3
0 Volts
1.
2.5 Coulombs
10,000 Volts
a. Assuming that your car is at 0 Volts when you start pushing it from the edge,
how much electrostatic potential energy does your car have once it reaches 10,000
Volts in the middle of the track?
b. Once your bumper car again reaches the outside edge of the track at zero volts,
what is the speed of the car? You may assume that your electrostatic potential
energy has been converted entirely into kinetic energy.
2. Why is it important that the filament of a light bulb have a much larger electrical
Voltage
= electrostatic
potential
energy
/ current
unit charge
(joules/coulomb)
resistance
than the supporting
wires that
carry
to and from
that filament?
3. An interesting novelty toy consists of a small neon
So ....lampelectrostatic
potential
= lamp
Voltage
with a metal knob
on eachenergy
end. The
is a · charge
glass tube containing neon gas, with two sharp
metal V · 2.5 C
= 10,000
wires pointing toward one another and separated by a
= 10,000 joules/coulomb · 2.5 coulombs
small gap. In this toy, each of the neon lamp’s wires
= 25,000
is connected to a knob. You are negatively
charged joules
because you just got out of your antique car and your
The outside edge of the track is kept at zero volts. At the beginning of the ride, everyone
pushes their car from the outside edge of the track in to the dome in the middle and keeps
their brakes on until the ride begins. When the ride begins, everyone releases their brakes
and are repelled away from the dome. You and the bumper car collectively have a mass of
500 kg.
From Problem Set #3
0 Volts
1.
2.5 Coulombs
10,000 Volts
a. Assuming that your car is at 0 Volts when you start pushing it from the edge,
how much electrostatic potential energy does your car have once it reaches 10,000
Volts in the middle of the track?
b. Once your bumper car again reaches the outside edge of the track at zero volts,
what is the speed of the car? You may assume that your electrostatic potential
energy has been converted entirely into kinetic energy.
2. Why is it important that the filament of a light bulb have a much larger electrical
2 from that filament?
resistance than theKinetic
supporting
wires =that
carry
current
to and
Energy
1/2
mass
velocity
3. An interesting novelty toy consists of a small neon
lamp with a metal
knob on
each
The Energy/
lamp is a mass)
velocity
= √(
2 ·end.
Kinetic
glass tube containing neon gas, with two sharp metal
2 2
=
√((
2
·
25,000
kg
m
wires pointing toward one another and separated by/s
a )/500 kg)
2
= √of(100
m2/slamp’s
) = 10
m/s
small gap. In this toy, each
the neon
wires
is connected to a knob. You are negatively charged
because you just got out of your antique car and your