EnergyChapter 4Sections 1 & 2

What is Energy?• All changes involve energy.• Light bulbs heat air• Wind blows trees & leaves• You breathe, blink, move• Moving baseball hits a window…

Does change require energy?

• Energy is the ability to cause change.• Anything that causes change must have energy.

What are some different forms of energy?• Electrical- light bulb uses it• Chemical- stored in food• Radiant- comes from Sun• Thermal- heat energy from the movement of atoms &

molecules

How is energy like money?• You can change it into different forms. • Energy is the same no matter what form it is in.

Kinetic Energy• Kinetic energy is the energy a moving object has because of

its motion.• KE depends on the object’s mass & speed.• KE measured in unit joules.

Kinetic Energy• KE (J )= ½ mass (kg) X [speed (m/s)]2 or• KE= ½ mv2

• Energy is measured using the SI unit called the joule.• 1J= KE of a baseball dropped from 0.5 m

Kinetic Energy• A jogger whose mass is 60 kg is moving at a speed of 3 m/s.

• KE= ½ mv2 • =1/2 (60 kg)(3m/s)2 =1/2 (60 kg) (9 m2/s2) • = 30 9 kg m∙ ∙ 2/s2

• = 270 kg m∙ 2/s2

• The kinetic energy of the jogger is 270 J.

Potential Energy• Objects at rest have stored energy.• Stored energy due to position is called potential energy.• PE can change to KE when position changes.

What is elastic potential energy?• Elastic PE is stored by an object that can stretch or shrink like

a rubber band or a spring.• EPE can change to KE when released.

What is chemical potential energy?• Chemical PE is stored in chemical bonds in food and in

hydrocarbons like natural gas.• The energy is released when the substance is burned.

What is gravitational potential energy?• Anything that can fall has stored energy.• Gravitational potential energy (GPE) is energy that is stored

by objects that are above Earth’s surface.• It depends on mass & height.

What is gravitational potential energy?• GPE= mass (kg) X acceleration of gravity (m/s2) X height (m) or

• GPE=mgh, where g= 9.8 m/s2

• A ceiling fan has a mass of 7 kg & is 4 m high. What is its GPE?

How is potential energy stored?

Potential Energy Way It Is Stored

Elastic By an object that can stretch or shrink

Chemical In chemical bonds

Gravitational By objects that are above Earth’s surface

How does GPE change?• The acceleration of gravity is a constant or 9.8 m/s2.• The other two factors mass (m) and height (m) are variables

that can change.

What does GPE change into?• GPE changes into kinetic energy as height changes.• If mass is increased, GPE also increases so that when the

object falls it converts into more KE.

Section 2- Conservation of Energy• What You’ll Learn:• How energy can be changed from one form to another• How energy is conserved

Changing Forms of Energy• Many situations involve changing energy from one form to

another.• Cars use fuel for energy• Bodies digest food for energy• Sun warming Earth, etc.

How can electrical energy change?

“A Compact Fluorescent Light produces light when electricityis passed through a gas sealed in a glass tube, creating UV rays which react with a phosphor coating on the inside of the glass tube. CFLs use less power, release less heat, and last a lot longer than an incandescent light bulb.”

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Changes Between Kinetic & Potential Energy• Mechanical energy is the total amount of

potential & kinetic energy in a system.• The ME of a rubber band is the total of its PE +

KE at any one time. • ME comes from where an object is & the

movement of the object.

Changes Between Kinetic & Potential Energy• Mechanical energy is the total amount of

potential & kinetic energy in a system.• The ME of a rubber band is the total of its PE +

KE at any one time. • ME comes from where an object is & the

movement of the object.

How can electrical energy change?

In other words, electricity is changed to light and thermal energy or heat.

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How can chemical energy change?

• Chemical energy can be changed into KE. In an engine, an electrical spark causes a small amount of fuel to burn becoming thermal energy. The gases expand causing parts of the car to move (KE).

<http://www.geocities.com/MotorCity>

Do all energy changes result in motion?• Not all changes in energy result in motion that can be seen.• Nor do they all result in sound, heat, or light.• EX. Green plants change sunlight to CE; when you eat the

plant the CPE is changed to other forms of energy.

Does the mechanical energy of an object change?

• These apples have GPE; as they fall they lose height thus reducing GPE & raising KE. However, the total ME remains the same.

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How does energy change in projectile motion?

• A ball moves through the air in a curved path.• As the ball rises so

does its GPE but its KE goes down; as it falls the opposite occurs.

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What happens to energy during a swing?• Energy is constantly changing during a ride on a swing. The

push is KE; at the top of the path KE is low but GPH is high. Going down the speed increases as GPE changes to KE. As you swing KE & PE are constantly changing back & forth.

The Law of Conservation of Energy• Although KE & PE are always changing, mechanical energy

always stays the same.• No energy is destroyed; instead it is conserved.• The Law states that energy is neither created nor destroyed.

Is energy always conserved?• There are times when it seems energy is not conserved.• When you are coasting along on your bike you eventually

must pedal to keep going. • Why is this?

How does friction affect energy?• Friction and air resistance create thermal energy.• The total amount of mechanical energy remains the same, it

just changes form.

Where does the Sun get its energy?

• Nuclear fusion is the reaction that takes places when nuclei join together. It changes PE of a small amount of mass into huge amounts of energy.

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What is nuclear fission?

• In nuclear fission, the nucleus is broken apart. Once again mass is turned into energy, but energy is also conserved.

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Energy & the Human Body

• In order to move your body changes stored PE to KE. It also changes it to thermal energy to maintain body temp.• The total amount of

mass & energy do not change.

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How does your body store & use energy?• The PE stored in your

body as fat and other chemical compounds is the fuel for processes such as the beating of your heart, digesting of food, & moving muscles.

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What are calories?• The food Calorie (C) is a unit nutritionists use to measure how

much energy foods contain.• One Calorie = 4,184 J • These Calories produce the energy your body’s cells need to

function.

Calories Used in 1 Hour

Activity S body M body L body

Sleeping 48 56 64

Sitting 72 84 96

Eating 84 98 112

Standing 96 112 123

Walking 180 210 240

Tennis 380 420 460

Biking 500 600 700

Running 700 850 1000