Transcript Slide 1

This Week
Electric charge.
What is it? Where is it?
Voltage and energy
What is a battery?
Conductors and Insulators
Bad idea to wave a golf club in a thunderstorm
What is Lightning?
Electrocution
How does a microwave work?
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Electric charge
an atom has a central nucleus of protons and neutrons
surrounded by electrons
Each element in the periodic table has a different
combination of protons and neutrons. Hydrogen just has
one proton and one electron.
Electrons and protons carry electric charge and it is the
force between the charges that hold the atom together.
Charge comes in both negative (electrons) and positive
(protons) and each carry one unit of charge .
Normally objects have zero net charge but it is possible
for an object to have charge which is a multiple of the unit
charge either positive or negative.
It is quite easy to separate electrons from atoms.
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Electromagnetism
Almost every action in our everyday
lives is due to the electromagnetic force
caused by electric charges.
Everything that is not due to gravity is
electromagnetism.
 All transmission and use of electric
power is due to the flow of electrons
through wires.
A camera flash is an example of
separating positive and negative
charge, which requires work. This work
is then released as energy by letting the
charges combine (lightning)
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If there is a separation of
charge between two
objects there will be a
very strong attractive
force which will transfer
charge until the objects
are neutral
3
Positive and negative
Charge only occurs as a multiple of
the unit charge on an electron or
proton which is 1.6 x 10-19 coulombs
So any charge q is an integer multiple
And N can be positive or negative
q = N x 1.6 x 10-19 coulombs
Opposite charges attract
Same sign charges repel
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Coulomb’s Law
Coulomb’s Law
The force between two charges of one
coulomb each separated by 1 meter is
9 x 109 Newtons (your weight ~ 1000N)
The gravitational force between two 1 kg
masses 1 meter apart is 6.67 x 10-11 Newtons
a difference of ~ 1020.
To separate a negative charge from a
positive charge requires work which is
stored as potential energy. When the two
charges are released this energy will be
liberated as kinetic energy
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r
F
F = kq1q2/r2 (k = 9 x 109 N.m2/C2)
F
F
F
Fext
Held in place
5
Everyday charge
In order to separate electrons from atoms
requires work and frictional forces are sufficient
to do this.
Simple actions like rubbing two objects together
or walking across a carpet will often result in a
transfer of charge.
This transfer is caused by stripping electrons
from the atom since every atom has a few
electrons which are weakly bound.
Electrons can also be more easily moved since
(mass electron)/(mass proton) ~ 1/2000
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Conductors and insulators
There are two broad classes of substances
related to electrical phenomena.
Conductors – electrons are free to move
Insulators – electrons are not free to move
 If we bring a charged object near a neutral
object the electrons and the protons will feel
the force and in a conductor the charges will
separate.
Water is a very good conductor so for
example we usually only see static sparks in
the winter when the air is very dry and objects
can hold any excess charge that accumulates.
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5A-01 Static Electricity
Demonstration that positive and negative charge exists
++++++++
What
information
does the
electroscope
provide ?
Rubbing the hard rubber rod with fur produces a negative charge on the rod
Rubbing the glass with silk produces a positive charge on the glass
If we bring a charged rod close to the electroscope it will repel opposite
charge and the moving arm will be displaced. If we touch the
electroscope with a charged rod then charge will be transferred
THE ELECTROSCOPE CAN’T DETERMINE THE SIGN OF THE CHARGE BUT
IT DOES SHOW CHARGE EXISTS IN TWO KINDS + AND 7/21/2015
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5A-03 Two-by-Four Electroscope
Showing the strength of the electromagnetic force.
How does the
charged
ebonite rod
move the
board ?
++++++++++
------------Rubbing the rod produces a surface charge. When it is brought close to
the wood it attracts the opposite sign charge in the wood and there is an
attractive force. This is the same effect as “static cling”
THE GRAVITATIONAL FORCE IS MUCH WEAKER THAN THE ELECTRICAL
FORCE. THE RATIO OF THE TWO FORCES IS ~10-39 .
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5 A 04 Conductors and insulators
The difference between conductors and insulators
What
happens to
the balls?
The apparatus with the dome
produces electric charge.
If the rods connecting the balls are
conducting the balls become
charged and repel eachother
If the rods are insulators the
balls remain uncharged
All materials contain equal amounts of positive and negative charge. In
conductors charge, generally electrons, is free to move. In insulators the
charges on the constituent atoms or molecules is tightly bound and does
not move. Dry air is a very good insulator but water is a very good
conductor and damp air is also a good conductor. Even insulators can
“break down” for example lightning.
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5 A 06 Static electric charge and induction
Using an electroscope to see the effects of charge
++++++++++++
How do we
see charge?
++++++++
---------
+++++++++
When the rod is brought close it attracts negative charge meaning the
pivot rod becomes positively charged.
If the top plate is now touched negative charge flows out and the
whole electroscope becomes positively charged.
The movement of charge, usually electrons, is called a current and all
electrical systems work using currents.
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Electric field
Any distribution of charges will produce a
force field. That is if a test charge q is placed
anywhere it will feel a force.
Electric field E = F/q0
or F = q0E
By definition the test charge is positive and
both the electric field and the force are in the
same direction
The electric field at a point tells you what
force a charge +q would feel both in magnitude
and direction.
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Electric potential
Electric potential difference
is the change in potential energy per
positive charge.
That is it is the work done or energy
released in moving unit charge in an
electric field. The unit is
ΔV = ΔPE/q (joules/coulomb = volt)
In a battery charge is separated and
potential energy is stored. When it is
connected in a circuit charge flows and
the energy is released.
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Voltage
In a uniform field the work done in
moving a charge q a distance d
W = qEd = ΔPE
and the voltage difference is
ΔV = ΔPE/q = Ed
So if a positive charge q is moved
toward the positive plate the voltage
increases.
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Basic Force
Two point particles with electric charge
feel a force and the force between two
charges q1q2 is
F = kq1q2/r2 (k = 9 x 109 N.m2/C2)
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F
r
F
F F
15
Work and Energy
F
Objects with electric charge
also have mass and everything
we have done on work and
energy is the same. In fact
when I push an object across
the floor the force is
electromagnetic.
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d
F =ma
W =Fd
KE = 1/2mv2
16
Electric Fields
If there is some distribution of
charge that exists and I place a
charged object anywhere it will feel
a force that will depend on location
and what the value of q is. If I move
the charge around I can map out
the force field and we define the
force field by using q = 1coulomb
and we define electric field
E = F/q
So if I know E then we can
determine the force at any point for
any charge.
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F
q
The charge q we use to
map out the electric
field is always positive
So E is always in the
direction of the force a
plus charge feels
17
5A-10 Motion in an Electric Field
The effects of transferring charge
+
-
+
-
What is the
movement of
the balls ?
THE BALL IS ATTRACTED TO ONE TERMINAL THEN RECEIVES A
CHARGE AND THEN IS REPELLED TO THE OTHER TERMINAL, WHERE
IT PICKS UP THE OPPOSITE CHARGE AND IS REPELLED.
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5A-23 Electric Wind
The emittance of electrically charged particles from highly charged object
What causes
the arms to
turn ?
The metal arms are charged by an electrostatic generator and the forces
are greatest at the tips so charged particles are driven off by repulsion.
Conservation of momentum makes the arms turn in the “electric wind”
The “wind” can be indirectly seen by the extinguishing of a candle.
Before lighting strikes there is charge build up and lightning
conductors have sharp tips to “attract” the lightning. The sun also
has large electric and magnetic fields and emits the “solar wind”
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Simple fields and potential energy
The field lines show the direction of the force a
positive charge feels. If I place a charge
anywhere in the field it has potential energy.
That is if I release it will be accelerated and
gain kinetic energy.
v
F
F
v
Or if I start it with velocity v it will come to rest
and KE PE
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Voltage and potential energy
Electric potential difference (voltage)is the change in potential
energy per unit positive charge. That is it is the work done or
energy released in moving unit charge in an electric field.
ΔV = ΔPE/q (joules/coulomb = volt)
Uniform electric field E then force on charge q is Eq and work
done in moving the charge a distance d is Eqd so
ΔV = ΔPE/q = Ed
When you are going against the direction of the electric field the
voltage increases
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Conduction and Induction
A conducting solid contains electrons which can move and if
they are in an electric field they will move opposite to the
direction of E
If I rub two insulators together electrons will be removed from
one insulator and the other will get an excess.
If I bring an insulator with negative charge near a conductor
then the charge in the conductor will separate. If I now touch
the conductor where there is excess electrons they will repel
the electrons on the skin and electrons will flow into me. If I
now remove my finger the original conductor will be positively
charged. If I had touched the conductor where there was
excess positive charge electrons would have flowed from me
into the conductor. This is Induction
There is always charge conservation
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Summary of Chapter 12
Electric charge can be + or –
q = N x 1.6 x 10-19 coulombs
F = kq1q2/r2 (k = 9 x 109 N.m2/C2)
r
F
F
F F
Charge is carried by particles the most common are
electrons - light which can move easily
positive nuclei - more than 2000 times as massive
Conductors – electrons are free to move
Insulators – electrons are not free to move
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Fields and voltages
Electric field E = F/q
or F = qE
ΔV = ΔPE/q (joules/coulomb = volt)
W = qEd = ΔPE
voltage difference is ΔV = ΔPE/q = Ed
uniform field
So if a + charge q is moved toward the + plate
the voltage increases.
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Positive charge
In an atom the positive charge is carried by the protons in the nucleus.
For a conductor it is the nuclei which form the structure of the solid and
give it properties so in an electric field it is only the electrons which flow.
In the case of a gas electrons which are stripped from atoms can move
and in an electric field both the electrons and positive ions can move, but
F = ma so electrons are accelerated > 2000 times more than ions
In the case of the atmosphere strong updrafts and downdrafts cause
separation of charge with the bottom of clouds being negative and the
top positive. The negative cloud charge pushes negative charge away
from the earths surface and then there is lightning. The electric field is
highest at objects with sharp ends hence lightning conductors to attract
the lightning
+++++++++++++++ top of cloud
-------------------------- bottom of cloud
lightning
+++++++++++++++ earth
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Lightning
The air around a lightning bolt is superheated to about
54,000 degrees Fahrenheit (five times hotter than the sun!).
This sudden heating causes the air to expand faster than the
speed of sound, which compresses the air and forms a
shock wave; we hear it as thunder. Since the bolt is actually
several short bursts strung together, multiple shock waves
are created at different altitudes; this is why thunder seems
to rumble -- each shock wave takes a different amount of
time to reach your ear.
Every minute, there are more than a thousand
thunderstorms around the Earth causing some
6,000 flashes of lightning.
A lightning charge ~ 30 million volts at 100,000 amperes.
Insurance for being hit by lightning is a better bet than the
lottery but the downside is you can’t enjoy it.
http://www.space.com/scienceastronomy/101_earth_facts_030722-1.html
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Voltage and Current
Voltage is defined by the potential energy stored or released
as charge moves in an electric field. If I separate charge as
in a battery then there is a voltage across the terminals but
charge only flows if a conductor connects the two
terminals. As far as your body is concerned it can have
excess charge e.g. Static electricity or it can be part of a
circuit and charge would flow through the body. Since it is
charge and currents responsible for all muscle contractions
including the heart significant currents can cause death and
higher voltages give higher currents. If you touch a high
voltage source but there is no conducting path current will
not flow but if you are not insulated e.g. touching metal
connected to earth or in a wet environment you can be
electrocuted
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Microwave oven
A microwave oven generates electromagnetic radiation
which contains strong electric fields which change
direction 2450 x 106 times per second.
These electric fields exert a force on charge. The charge in
a H2O molecule is separated like a tylenol tablet so the
water molecules are shaken and the friction between
molecules causes the water to heat.
In conductors charge can move freely so the charge will
move and there will be large currents and sparking and it
acts like a short circuit.
Ice is not heated because of it’s crystalline structure and
the water molecules in air are not dense enough to heat.
The electric fields and therefore the heating is not uniform
in the oven which is why there is a rotating turntable or
mechanism which swirls the electric field
http://www.youtube.com/watch?v=8wmBXH_McDw&mode=related&search=
http://apache.airnet.com.au/~fastinfo/microwave/
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Questions Chapter 12
Q2 Two pith balls are both charged by contact with a plastic rod
that has been rubbed by cat fur,
A. What sign will the charges on the pith balls have? Explain.
B. Will the two pith balls attract or repel one another? Explain.
A.The sign of the charge is negative. B. They will repel
Q3 Two pith balls are charged by touching one to a glass rod that
has been rubbed with a nylon cloth and the other to the cloth itself,
A. What sign will the charge on each pith ball have? Explain.
B. Will the two pith balls attract or repel one another? Explain.
A. The first ball will have positive charge the second negative charge
B. The two balls will attract each other
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Q13 Will bits of paper be attracted to a charged rod even if they
have no net charge?
Yes because the charge will attract the opposite charge in the paper
Q14 Why are pith balls initially attracted to a charged rod and
later repelled by the same rod, even though they have not touched
any other charged object?
Because once they touch the charged rod they pick up some of
the charge and the ball is repelled
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Q18 If two charges are both doubled in magnitude without changing
the distance between them, will the force that one charge exerts on
the other also be doubled?
The force varies as q1q2/r2 so the force will increase by a factor of 4
Q19 Can both the electrostatic force and the gravitational force be
either attractive or repulsive?
No, the gravitational force is always attractive.
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Q20 Is it possible for an electric field to exist at some point in
space at which there is no charge?
Yes. Electric fields are created by charge but extend to infinity
Q21 Two charges, of equal magnitude but opposite sign, lie along a
line as shown in the diagram. Using arrows, indicate the directions
of the electric field at points A, B, C, and D shown on the diagram.
-q
C
•
-
q
A
•
B
+
•
D
•
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Q22 If we change the negative charge in the diagram for question 21
to a positive charge of the same magnitude, what are the directions of
the electric field at points A, B, C, and D? (Indicate with arrows.)
q
C
•
+
q
A
•
B
+
•
D
•
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Q23 Three equal positive charges are located at the corners of a
square, as in the diagram. Using arrows, indicate the direction of the
electric field at points A and B on the diagram.
A
q
+
•
+
q
•
B
q
+
Q25 If we move a positive charge toward a negative charge, does
the potential energy of the positive charge increase or decrease?
The potential energy decreases
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Q26 If we move a negative charge toward a second negative
charge, does the potential energy of the first charge increase or
decrease?
It increases because they repel each other it means as you get closer
the force gets larger. You have to do work to bring them closer
Q32 Would you be more likely to be struck by lightning if you stood
on a platform made from a good electrical insulator than if you stood
on the ground?
Standing on an insulator is the safest which is why the safest
place in inside a car because the tires are insulators. It’s also the
same with fallen power lines
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Ch 12 E 8
Electron and proton have charges of equal magnitude,
1.6×10-19C, but opposite signs. If the electron and
proton are separated by R=5×10-11m, what is the
electrostatic (vector) force b/w them?
P
+
e
r
F = kq1q2 / r2 = -(9×109)(1.6×10-19)2 / (5×10-11m)2
F = 9.22 × 10-8 N towards proton
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CH 12 E14
Charge q = -4 × 10-6 c placed in Electric field of
E = 8.5 × 104 N/C
Towards right. What is the electrostatic
(vector) force on charge q?
E
9m
F = qE = (4×10-6)(8.5×104)
F = 0.34 N
Since positive charge moves with field lines and q is
negative, q moves to the left
F = 0.34 N to left
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CH 12 CP 2
2 equal charges (positive) are near one another (see
diagram).
a) Using small arrows indicate the direction of the electric field at
labeled points on diagram. (Think about what a positive
test charge would do!)
b) Sketch electric field lines
E
F
D
B
A
+
C
+
I
G
H
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Ch 12 CP2 (cont)
E
D
a)
E
F
F
D
B
+
A
G
+
H
C
I
A
+
C
+
I
G
H
b)
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Ch 12 CP 4
4 equal positive charges located at corners of the square
(see diagram)
a) Use small arrows to indicate direction of electric field at
each labeled point
b) Would the magnitude of electric field be equal to zero at any
labeled point?
a)
+
+
E
+
+
+
B
+
+
A
D
C
+
b) Electric Field is equal to zero at the center of the
square (A).
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Summary of Chapter 7
Impulse equation p = Ft = mvf – mvi
F2
F1
Collision F1 = -F2 and Δp1 + Δp2 = 0
Total momentum is conserved
p = Ft = mvf – mvi
If we choose positive down
p will be negative
If we choose positive up
p will be positive
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Isolated systems
In each case
m1v1 = - m2v2
anim0010.mov
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Collisions
Elastic
1/2m1v2 = ½ mv1f2 + 1/2mv2f2
m1v = m1vf + m2vf
v
Pool balls
Bowling ball/tennis ball
v
v
Tennis ball/bowling ball
Inelelastic only momentum is conserved
mvi = 3mvfinal and vfinal = vi/3
Kinetic Energy before = 1/2mvi2
Kinetic energy after = ½ x 3mv2final
So KEbefore/KEfinal = 3
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Summary of Chapter 8
Rotational motion
Angular velocity ω = Δθ/Δt
Angular acceleration α = Δω/Δt
One full circle = 3600 = 2π radians
Circumference = 2πR
Time for one revolution = 2πR/v
2πr/v = 2π/ω then v = rω
Displacement
Velocity
Acceleration
Constant
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d
v = Δd/Δt
a = Δv/Δt
v = v0 + at
d = v0t + 1/2at2
v2 = v02 + 2ad
Physics 214 Fall 2012
v
R
57.30
θ
ω = Δθ/Δt
α = Δω/Δt
ω = ω0 + αt
θ = ω0t + 1/2αt2
ω2 = ω02 +2αθ
44
Torques
Torque = Fl where
where l is the perpendicular distance
to the line of action of the force
+ is counterclockwise and – is clockwise
net torque is sum of all torques
For the boy on the plank he will fall
when Wpdp > Wc dc
Torque = Fl = Iα
I plays the role of mass for rotation
Kinetic energy = 1/2Iω2
Work = Tθ ( full circle T2π = Fr2π
Angular momentum L = Iω
There is a point in the geometry of a
body at which all the mass appears to
act and one can balance the body with a
single force (center of mass/gravity)
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g
45
Conservation of angular momentum
In a closed system L = Iω
we can change I and ω will change like
a skater.
We can invert the bicycle wheel and ω
will change to keep L constant
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Conservation of angular momentum
In a closed system
L = Iω
and we can change ω by changing I
since L is constant in both magnitude
and direction.
If we change L for one object in a
system the rest of the system changes
to keep L constant.
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Summary of Chapter 9
P = F/A Pascals
F1/A1 = F2/A2
Work done = F1h1 = F2h2
1 Atmosphere = 1.013 105 Pa
and will support
76cm of mercury
32 feet of water
No Air
g
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Liquids
P = W/A = ρgh
Water is 1 gram/cc
1 gram/cm3 = 1000kg/m3
T
buoyant force = the weight of liquid displaced
Fb
T + FB = W = mg
g
For a floating object T = 0
mg
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Summary Chapters 10 and 11
W = PΔV for the piston shown
Wexternal = + Wsystem = ΔU = Q – Wsystem or ΔU = Q + Wexternal
Work done on gas = Fd = PAd
ΔV = Ad
W = PΔV
Work done by gas is -W
Temperature scales Celsius, Fahrenheit Kelvin
Mixture of ice and water 00C
320F 2730K
Boiling point of water 1000C 2120F 3730K
Tc = 5/9(Tf – 32)
Tf = 9/5Tc + 32 TK = Tc + 273
c = is the quantity of heat required to raise unit mass of a substance by one degree.
Q = mcΔT
1 calorie = heat required to raise 1 gram of water 10C = 4.186 joules
Change of state (internal energy changes temperature is constant)
80 cals/gm is the latent heat of fusion
540 cals/gm is the latent heat of vaporization
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Gases
Work done by gas = -Fd = -PAd
ΔV = Ad
W = -PΔV
External work done = W =P ΔV
Energy conservation
ΔU = Q – Wgas
Ideal gas law PV = NkT (T in degrees Kelvin)
Adiabatic no heat in or out Q = 0
Compression
work is + (ΔU, T increase)
Expansion
work is –
(ΔU, T decrease)
Isothermal T does not change ΔU = 0 Q = W = PΔV
put in heat gas expands take out heat gas must be compressed
Isobaric pressure is kept constant
Put in heat
T increases gas expands (hot air balloon)
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Heat engines
Efficiency = ε = W/QH
Change in internal energy in one cycle is zero
W = QH – Qc (c = environment)
2nd law
No engine working in a continuous cycle can
take heat at a single temperature and convert
that heat completely to work.
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Summary of Chapter 12
Electric charge can be + or –
q = N x 1.6 x 10-19 coulombs
F = kq1q2/r2 (k = 9 x 109 N.m2/C2)
r
F
F
F F
Charge is carried by particles the most common are
electrons - light which can move easily
positive nuclei - more than 2000 times as massive
Conductors – electrons are free to move
Insulators – electrons are not free to move
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Fields and voltages
Electric field E = F/q
or F = qE
ΔV = ΔPE/q (joules/coulomb = volt)
W = qEd = ΔPE
voltage difference is ΔV = ΔPE/q = Ed
uniform field
So if a + charge q is moved toward the + plate
the voltage increases.
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