Transcript Work, Power, Energy, Heat

Ch 14, 15, 16
Work, Power,
Energy, and
Heat
Ch 14 WORK AND POWER
• Work – transfer of
energy
• through motion
• a. Force must be
exerted through a
distance
• b. Amount of
Work depends
on:
–1. Amount of
applied Force
–2. Distance
over which
the force is
applied
Work
F
d
Work
F
d
• c. W=Fd
(Force multiplied by distance)
• d. Unit is the Joule (1J = 1Nm)
• e. In order for work to be done:
–1. Something has to move
–2. Motion must be in the
direction of the force
• How much work is done when a car is
pushed with 600 N of force for a distance
of 50m?
• How far will a ball roll if 17J of work was
done to push it with 2 N of force?
Power – rate of
doing work
a. To increase
Power:
–1. increase
work done
– 2. do work in
less time
b. P=W/t
• (Work divided by time)
Work
P
c. SI unit is Watt
(1W=1J/s)
d. common unit is horsepower
(1hp=746W)
t
• How much power is needed to do 40
Joules of work in 5 seconds?
• How long will it take to do generate 70
Watts of power if I do 4900 Joules of
work?
• How much power will I need if I want to do
the same amount of work in half the time?
Reference table check
Work
P
Work
t
F
Fd
P
t
d
• You exert a vertical force of 72 N to lift a
box a height of 1 meter in a time of 2
seconds. How much power is used to lift
the box?
Star questions
• True or False
–Decreasing the amount of time it takes
to do work, increases the power
–Motion must occur in order for work to
be done
Ch 15-Energy
• Energy is the
ability of an
object to cause
change
• Basic unit is the
Joule (J) =Nm
Forms of energy
1. Mechanical = PE+KE
2. Thermal = heat
3. Chemical = wood,
gasoline
4. Electrical = lightning,
batteries
5. Electromagnetic =
visible light, x-rays
6. Nuclear = fission,
fusion
Kinetic Energy
– energy in the
form of motion
•a. Greater
mass, more
KE
•b. Greater
velocity, more
KE
mv
KE 
2
2
• A 20-kg kid slides down a slide at a
velocity of 2 m/s. What is the kinetic
energy of the kid?
Potential Energy – stored energy
a. Amount depends on:
1. Position
2. Shape
Types of PE:
1. Gravitational –
related to height
above Earth’s
surface
a. Higher up,
more PE
b. Greater mass,
more PE
2. Elastic –
object that is
stretched or
compressed
3. Chemical –
found in food or
fuels
• PE=mgh=Fgh
• SI unit is Joule=J=Nm
PE
mg
h
• A potted plant on a 2.5 m high window sill
has a mass of 3-kg. What is the potential
energy of the plant?
• A man weighing 950N stands at the top of
30m cliff. What is the man’s potential
energy?
3 people of equal mass
climb the mt using
different paths. Which
path shows the person
gaining the most PE?
Roller coaster
max height
PE
speed
ME
•Energy Conversion –
process of changing energy
from one form to another
Law of Conservation of Energy
states that matter cannot be
created nor destroyed
STAR Questions
• List the 6 forms of energy
– Mechanical, chemical, electromagnetic, thermal,
electrical, nuclear
• Work depends on what 2 things?
– Distance and force
• KE, chemical PE, elastic PE or gravitational PE?
–
–
–
–
–
Bouncing ball
Rock at edge of cliff
Glass of Orange Juice
Car battery
Compressed spring
KE
Gravitational PE
Chemical PE
Chemical PE
Elastic PE
Ch 16-HEAT
Heat =energy that flows from higher
temps to lower temps
What will
happen to the
temperature
in the beakers
over time?
30oC
90oC
Temperature –measure of
average KE of the particle
in a sample of matter
–a. Increase temp =
greater KE
–b. Decrease temp =
lower KE
–c. Absolute Zero=0K=
KE of all particles cease
Thermal Energy
– the total energy
of the particles in
a material (both
KE and PE)
a. More mass at
the same temp
means greater
thermal energy
KE of 3?
TE of 3?
50oC
70oC
70oC
200g
200g
400g
Thermal
Expansion &
Contraction
a. Hot temps
cause materials to
expand
b. Cold temps
cause materials to
contract
Specific Heat (C) – amount of energy
needed to raise the temp of 1g of a
material by 1oC
• Think of it as “resistance to temperature
change”
• The higher the c, the harder it is the change the
temp.
• An iron spoon and a silver spoon
have the same mass. Which
becomes hotter when both are left in
hot chocolate for one minute? Pg 476
for c values
• Why does a piece of steel heat up more
than an equal mass of wood when both
absorb the same energy?
The metal has a lower specific heat than the wood, so
its temp increases more as thermal energy is absorbed
Which substance will have the greatest
increase in temp when equal masses
absorb equal amounts of thermal energy?
•
•
•
•
•
Water (4.18 J/goC)
Ethyl alcohol (2.43 J/goC )
Ammonia gas (2.1 J/goC )
Aluminum (0.90 J/goC )
Lead (0.46 J/goC )
Answer : A
The same energy was added to each sample. Which material has the
highest specific heat?
Al, b/c Temp rises the least for a given amt energy added. Smaller ΔT
means a larger specific heat
Energy
Efficiency
no machine can be
100% efficient, why not?
It is impossible to build a
machine that does nothing
but convert thermal energy
into useful work.
Thanks Cambridge Physics Outlet for amazing graphics!
Conduction – transfer of
energy through matter
by direct contact of
particles
a. Energy is transferred
when particles moving
at different speeds
collide
b. Occurs in solids,
liquids, and gases
–1. Solids conduct
better
Convection – transfer of energy
by the movement of matter
a. Fluid – anything that flows
b. Occurs in liquids & gases
Radiation –transfer of
energy in form of
waves
a. Shiny materials
reflect radiant energy
b. Dull materials
absorb
c. Darker colors
absorb more
d. Lighter colors
absorb less
STAR Questions
•
•
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•
•
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•
•
•
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Decrease temperatures = ___________ KE
Decreased
Hot temperatures cause materials to ______
Expand
The higher the C-specific heat, the _________ it is to
change the temp
Harder
Dull materials ___________ radiant energy
Absorb
Lighter colors reflect ___________ radiant energy
less