Chapter Seven: Energy Energy in a System ¾ 7.1 Energy and Systems

Chapter Seven: Energy
¾7.1 Energy and Systems
¾7.2 Conservation of Energy
¾7.3 Energy Transformations
Chapter 7.1 Learning Goals
¾Define energy as a description of an object’s
ability to change or cause change.
Investigation 7A
Energy in a System
¾Key Question:
How is energy related to motion?
¾Discuss examples of different forms of energy.
¾Distinguish potential and kinetic energy and
apply formulas to solve problems.
7.1 What is energy?
¾ Energy measures the ability for things
to change themselves or to cause
change in other things.
¾ Some examples are changes in
temperature, speed, position, pressure,
or any other physical variable.
7.1 Units of energy
¾ Pushing a 1-kilogram object with a force
of one newton for a distance of one
meter uses one joule of energy.
¾ A joule (J) is the
S.I. unit of
measurement for
7.1 Joules
¾ One joule is a pretty small amount of
¾ An ordinary 100
watt electric light
bulb uses 100
joules of energy
every second!
7.1 Some forms of energy
¾ Chemical energy is a form of energy stored in
¾ Batteries are storage devices for chemical
7.1 More forms of energy
¾ Nuclear energy is a form
of energy stored in the
nuclei of atoms.
¾ In the Sun, nuclear
energy is transformed to
heat that eventually
escapes the sun as
radiant energy.
7.1 Some forms of energy
¾ Mechanical energy is the energy possessed
by an object due to its motion or its position.
¾ Potential energy and kinetic energy are both
forms of mechanical energy.
7.1 Some forms of energy
¾ Electrical energy comes from electric
charge, which is one of the fundamental
properties of all matter.
7.1 More forms of energy
¾ Radiant energy is energy that is carried
by electromagnetic waves.
¾ Light is one form of radiant energy.
7.1 More forms of energy
¾ The electromagnetic spectrum includes
visible light infrared radiation (heat), and
ultraviolet light.
¾ Light energy and heat energy are
included in the electromagnetic
7.1 Sources of energy
¾ Without the Sun’s
energy, Earth would
be a cold icy place
with a temperature of
-273 °C.
7.1 Sources of energy
¾ All objects with mass feel forces in the
presence of Earth’s gravity.
¾ These forces are a source of energy for
objects or moving matter such as falling rocks
and falling water.
¾ As well as warming
the planet, the Sun’s
energy drives the
entire food chain.
7.1 Energy and work
¾ In physics, the word
work has a very
specific meaning.
¾ Work is the transfer of
energy that results
from applying a force
over a distance.
7.1 Potential energy
¾ Systems or objects with potential energy
are able to exert forces (exchange energy)
as they change.
¾ Potential energy is energy due to position.
7.1 Potential Energy
mass of object (g)
height object raised (m)
PE (joules)
EP = mgh
gravity (9.8 m/sec2)
7.1 Kinetic energy
7.1 Kinetic Energy
¾ Energy of motion is called kinetic energy.
¾ A moving cart has kinetic energy because it can
hit another object (like clay) and cause change.
KE (joules)
mass of object (kg)
EK = ½ mv2
velocity (m/sec)
Solving Problems
¾ A 2 kg rock is at the edge of a cliff
20 meters above a lake.
¾ It becomes loose and falls toward
the water below.
¾ Calculate its potential and kinetic
energy when it is at the top and
when it is halfway down.
¾ Its speed is 14 m/s at the halfway
Solving Problems
1. Looking for:
¾ …initial EK, EP and EK, EP half way down.
2. Given:
¾ mass = 2.0 kg; h = 20 m
¾ v = 14 m/s (half way)
3. Relationships:
¾ EP =mgh
¾ EK = ½ mv2
¾ Assume rock starts from rest.
Chapter Seven: Energy
¾7.1 Energy and Systems
¾7.2 Conservation of Energy
¾7.3 Energy Transformations
Solving Problems
4. Solution
m = 20 kg
EP = mgh
¾ Draw a free body diagram.
EP = (2 kg)(9.8 N/kg)(20 m)
h = 20 m
EK = 0 J
= 392 J at top
EP = (2 kg)(9.8 N/kg)(10 m)
= 196 J half way
h = 10 m
EK = 0 J, rock is at rest
EP = mgh
EK = ½ mv2
EK = (1/2)(2 kg)(14 m/s)2
= 196 J half way
Chapter 7.2 Learning Goals
¾Describe how energy changes as
systems change.
¾Discuss examples of energy
¾Explore the energy involved in
carrying out daily activities.
7.2 Conservation of Energy
¾ Systems change as energy flows and
changes from one part of the system to
¾ Each change transfers energy or
transforms energy from one form to
7.2 Energy flow
7.2 Flow of Energy
¾ How can we predict
how energy will flow?
¾ One thing we can
always be sure of is
that systems tend to
move from higher to
lower energy.
7.2 Sources of energy
7.2 Units of energy
¾ The chemical potential energy stored in the
food you eat is converted into simple sugars
that are burned as your muscles work against
gravity as you climb the hill.
¾ Some units of energy that are more
appropriate for everyday use are the kilowatt
hour (kWh), food Calorie, and British thermal
Chapter Seven: Energy
¾7.1 Energy and Systems
¾7.2 Conservation of Energy
¾7.3 Energy Transformations
Chapter 7.3 Learning Goals
¾Explain what it means when energy
is conserved.
Investigation 7B
Conservation of Energy
¾Key Question:
What limits how much a system may change?
¾Use energy conservation to solve
¾Discuss applications of energy
conservation in daily living.
7.3 Conservation of Energy
¾ The idea that energy tranforms from
one form into another without a change
in the total amount is called the law of
conservation of energy.
¾ The law of energy conservation says
the total energy before the change
equals the total energy after it.
7.3 Conservation of Energy
¾ When you throw a ball in
the air, the energy
transforms from kinetic
to potential and then
back to kinetic.
Solving Problems
¾A 2 kg car moving with a speed of 2
m/sec starts up a hill.
¾How high does the car roll before it
Solving Problems
Solving Problems
1. Looking for:
¾ …height of hill
2. Given
¾ … mass = 2 kg, v = 2 m/s
3. Relationships:
¾ Energy transformed from EK to EP
¾ EK = ½ mv2
1. Solution
¾ Find beginning EK
¾ EK = ½ (2 kg) (2 m/s)2 = 4 Joules
¾ Assume energy before = energy after
¾ EK = EP
¾ EP =mgh
4 J = mgh
¾ h = (4 N•m)/(2 kg)(9.8 N/kg) = .2 m
¾ EP =mgh
7.3 Conservation of Energy
¾ Many people are concerned about “running
out” of energy.
¾ What they worry about is running out of
certain forms of energy that are easy to use,
such as fossil fuels like oil and gas.
7.3 Conservation of Energy
¾ It took millions of years to
accumulate these fuels
because they are derived
from decaying, ancient plants
that obtained their energy
from the Sun when they were
¾ Because it took a long time
for these plants to grow,
decay, and become oil and
gas, fossil fuels are a limited
7.3 Conservation of Energy
¾ Regular (incandescent)
light bulbs convert
only 10% of electrical
energy to light.
¾ That means 90% of the
energy is released as
wasted heat.
7.3 Conservation of Energy
¾ Other forms of energy,
such as thermal
energy, flowing water,
wind, and solar energy
are not as limited.
Investigation 7C
Energy and Efficiency
¾Key Question:
How well is energy changed from one form to
A Matter of Survival
¾In 2005, the U.S. Defense
Advanced Research Projects
Agency (DARPA) launched
its VHESC. program.
¾The goal of the program is to
develop solar cells that
would operate at or above 50
percent efficiency.