Transcript I. Energy & Work

Ch. 15 - Energy
I. Energy and Work (p.124-131)



Energy and Work
Energy
Conservation of Energy
Energy and Work

What is energy?
 Whenever work is
done, energy is
transformed or
transferred from one
system to another
 When is work done?
Energy is the ability to do work.
Energy and Work

How are energy
and work related?

REMEMBER: Work is done
only when an object
moves.

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.
SI Unit of Energy
Because the amount of energy
transferred is measured by how
much work is done – energy and
work are expressed in the same
unit.
Joules (J) is the unit of Energy.
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 same direction of force!
Energy and Work

Brett’s backpack weighs 30 N. How much
work is done on the backpack when he lifts
it 1.5 m from the floor to his back?
GIVEN:
F = 30 N
d = 1.5 m
W=?
WORK:
W = F·d
W = (30 N)(1.5 m)
W = 45 J
W
F d
Energy and Work

A dancer lifts a 40 kg ballerina 1.4 m in the air
and walks forward 2.2 m. How much work is
done on the ballerina during and after the lift?
GIVEN:
m = 40 kg
d = 1.4 m - during
d = 2.2 m - after
W=?
W
F d
WORK:
W = F·d
F = m·a
F =(40kg)(9.8m/s2)=392 N
W = (392 N)(1.4 m)
W = 549 J during lift
No work after lift. “d” is not
in the direction of the force.
Potential Energy
AKA – Energy of Position
Potential energy is energy that is
stored.
You can’t see it but
you know it’s there.
Types of Potential Energy
Type of Energy
Example
Gravitational Potential Energy (GPE)
Energy stored due to position (objects that are
above Earth’s surface such as apples in an apple
tree).
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.
Gravitational Potential Energy
AKA - GPE
 Depends
on mass
and height.
 GPE = m g h
Or

GPE = mass x free-fall acceleration x height
(mg = weight in Newtons)
Mass (m) = kg; Gravity (g) = 9.8 m/s2; Height (h) = m
Gravitational 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?
Example
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?
GPE = mgh
 Given:
m = 65kg
GPE = 65 x 9.8 x 35
h = 35 m
2
GPE = 22,295 J
g = 9.8m/s
GPE = ?
Kinetic Energy

is the energy of motion.
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
Kinetic Energy
AKA = KE
KE = ½ mass x velocity
OR
KE = ½ m v
mass (m) = kg
velocity (v) = m/s
2
2
Note:
Kinetic energy depends more
on speed than on mass.
Example
 What
is the kinetic energy of a
44kg cheetah running at 31 m/s?
Given:
KE = ½ m v 2
m = 44 kg
2
KE
=
½
(44)
x
(31)
v = 31 m/s
KE = 22 x 961
KE = ?
KE = 21142 J
Forms of Energy


Forms of Energy:
Kinetic
Potential
Energy in Fields
Mechanical
Thermal
Chemical
Electrical
Electromagnetic
Nuclear
Each of these forms of energy can be converted
into other forms of energy.
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
Energy
Electrical energy:
results from the
flow of charged
particles or
electrons. Electric
charges can exert
forces that do work.
Chemical Energy
is
the energy
stored in chemical
bonds – when the
bonds are broken,
the released
energy can do
work.
Energy
Mechanical Energy:  Nuclear 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).
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.
Energy
Electromagnetic
Energy:
a form of energy that
travels through
space in the form of
waves (visible light
and X-rays are
examples).
Thermal Energy:
energy
given off
as heat (friction).
The total potential
and kinetic energy
of all the
microscopic
particles in
an object.
Energy Conversions
The process of changing energy
from one form to another.
Law of Conservation of Energy
 Energy
can not be created or
destroyed, it can only be
changed.
 Energy can be transferred to
another object/system or to
another form.
Conservation of Energy

Law of Conservation of Energy
 Energy may change forms, but it
cannot be created or destroyed
under ordinary conditions.

Example:
 PE  KE
 mechanical  thermal
 chemical  thermal
E. Conservation of Energy
PE  KE
View pendulum animation.
View roller coaster animation.
E. Conservation of Energy
Mechanical  Thermal
View rolling ball animations.
View skier animation.
Ch. 5 - Energy
II. Thermal Energy
(p.134-137, 141-144)
 Temperature
 Thermal Energy
 Heat Transfer
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
because particles in gas collide less often.
E. Convection


Convection is the
transfer of thermal
energy when particles of
a fluid move from one
place to another (liquids,
air).
Convection currents are
important in many
natural cycles such as
ocean currents, weather,
and systems.
F. Radiation



Radiation is the
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
G. 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
G. 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