Transcript Ch 14, 15, 16

Ch 15 and 16
Energy and
Heat
• 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?
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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?
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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
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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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