Transcript What is energy?

Ch. 5 - Energy
I. Energy and Work (p.124-131)
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

Energy and Work
Energy
Conservation of Energy
A. Energy and Work

What is energy?
 Energy is the ability
to do work
 Whenever work is
done, energy is
transformed or
transferred from one
system to another
 When is work done?
A. Energy and Work

How is energy and
work related?

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
work is done only when an object
moves.
Because the amount of energy
transferred is measured by how
much work is done – energy and
work are expressed in the same
unit.
But - energy can be present in an
object or a system when nothing
is happening.
However – it can only be
observed when it is transferred
from one object or system to
another.
Measured in Joules
A. Energy and Work

Work
 transfer of energy through motion
 force exerted through a distance
W = Fd
W:
F:
d:
work (J)
force (N)
distance (m)
1 J = 1 N·m
Distance must be in direction of force!
B. G Potential Energy

Potential Energy (PE)
 stored energy
 Cannot be seen
 depends on position or
configuration of an object
• Which boulder has greater
gravitational PE?
• What other ways can an
object store energy?
B. G Potential Energy
Gravitational
Potential Energy
 Depends
on
mass and
height.
 GPE
Mass (m) = kg
 Free-fall (g) = m/s2
 Height (h) = m

GPE = mass x free-fall acceleration x height
B. G Potential Energy
A 65 kg rock climber ascends a cliff. What is
the climber’s gravitational potential energy
at a point 35 m above the base of the cliff?
B. G Potential Energy
Type of Potential Energy
Gravitational Potential Energy
Energy stored due to position
above Earth’s surface).
Example
(objects that are
Chemical Energy
Energy stored in chemical bonds such as food or
fuel.
Elastic Energy
energy stored by something that can stretch or
compress such as a rubber band or spring.
Energy stored in
chemical bonds such
as food or fuel.
C. Kinetic Energy

Kinetic Energy (KE)
 energy in the form of motion
 depends on mass and velocity
• Which has the most KE?
80 km/h truck
• Which has the least KE?
50 km/h motorcycle
80 km/h
50 km/h
80 km/h
C. Kinetic Energy
Kinetic energy depends more on
speed than on mass.
KE = ½ m v
mass (m) = kg
velocity (v) = m/s
C. Kinetic Energy
What is the kinetic energy of a 44kg
cheetah running at 31 m/s?
KE = 22 x 961
2
D. Other Forms of Energy
THERMAL
The ability to
cause change.
internal motion of
particles
MECHANICAL
NUCLEAR
ENERGY
motion of objects
changes in the
nucleus
ELECTRICAL
CHEMICAL
bonding of atoms
joules (J)
motion of electric
charges
D. Other Forms of Energy
Mechanical Energy

the energy associated
with the motion or position
of an object. The sum of
potential and kinetic
energy in a system
(Usually involves
movement of an object)
Chemical Energy
 is the energy
stored in chemical
bonds –
when the bonds are
broken,
the released energy
can
do work.
D. Other Forms of Energy
 Nuclear Energy:
energy stored in atomic
nuclei – nuclear fission
releases energy by
splitting
nuclei
apart, nuclear fusion
releases energy by
combining 2 nuclei into
a larger nuclei.

Electromagnetic
Energy: a
form
of energy that
travels
through
space in the form of
waves. Visible light
and X-rays
are
examples.
D. Other Forms of Energy
Electrical energy:
 results from the
flow of charged
particles or
electrons.
Electric charges
can exert
forces that do
work
Thermal Energy:
 energy given off
as heat (friction).
The total
potential and
kinetic energy of
all the
microscopic
particles
in an object.
F. Conservation of Energy
 How
is energy conserved in a
closed system?
 The process of changing energy
from one form to another.
F. Conservation of Energy

Law of Conservation of Energy
 Energy may change forms, but it
cannot be created or destroyed
under ordinary conditions.

EX:
 PE  KE
 mechanical  thermal
 chemical  thermal
Energy Transfer
Conservation of Energy
Roller Coaster Physics
Ch. 5 - Energy
II. Thermal Energy
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Temperature
Thermal Energy
Heat Transfer
A. Temperature
Thermal Energy
energy given off as heat (friction).
 The total potential and kinetic energy of
all the microscopic particles in an object.

A. Temperature

Temperature
 measure of the
average KE of
the particles in
a sample of
matter
B. Thermal Energy

Thermal Energy
 the total energy of the particles in
a material
 KE - movement of particles
 PE - forces within or between
particles due to position
 depends on temperature, mass,
and type of substance
B. Thermal Energy

Which beaker of water has more
thermal energy?
 B - same temperature, more mass
80ºC
A
80ºC
B
200 mL
400 mL
C. Heat Transfer
Heat
 thermal energy that flows from
a warmer material to a cooler
material
 Like work, heat is...
 measured in joules (J)
 a transfer of energy

C. Heat Transfer
What are 3 types of heat transfer?
 Conduction
 Convection
 Radiation
D. Conduction
Transfer of heat as a result of direct
contact
 Conduction in gases is slower than in
liquids and solids because the
particles in a gas collide less often

E. Convection
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Transfer of thermal energy when particles of
a fluid move from one place to another
Convection currents are important in many
natural cycles, such as ocean currents,
weather, systems, and movement of hot
rock in Earth’s interior
F. Radiation
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Transfer of energy by waves moving
through space.
All objects radiate energy
As an object’s temperature increases, the
rate at which it radiates energy increases
Summary
Quiz
C. Heat Transfer

Why does A feel hot and B feel cold?
 Heat flows from A to your hand = hot.
 Heat flows from your hand to B = cold.
80ºC
A
10ºC
B
C. Heat Transfer

Specific Heat (Cp)
 amount of energy
required to raise
the temp. of 1 kg
of material by 1
degree Kelvin
 units: J/(kg·K)
or J/(kg·°C)
Specific Heat Values
(J/(kg·K))
Water
4184
Alcohol
2450
Aluminum
920
Carbon (graphite) 710
Sand
664
Iron
450
Copper
380
Silver
235