What is Energy?

What is Energy?

The ability to do work or cause change.

Work

• Easiest to start with the notion of work

• Work = Force X Distance

• Lift a box from the floor, you apply a force to overcome gravity

• Multiply that force by the distance through which you apply the force and you calculate the amount of work accomplished

Is this Work?

Work

• Unit is the JOULE

• A Joule is a newton-meter Kgm2 /s2

• A newton is a unit of force Mass times meters per second persecond times a distance in meters

Power

• The rate at which work is done

• Takes more power to run up the stairs than to walk up the stairs, but the energy consumed is the same in either case

power work done

time interval

Power

• Unit is the WATT

• A Watt is a newton--meter per second

• Think of 100-Watt light bulb

• Bigger units are kilowatts and megawatts

• Utility sells energy in kilowatt-hours

• 1 KWh = 1000 Joules/second times 3600 Seconds = 3.6 X 106 Joule

What are the different forms of Energy?

• Energy has a number of different forms, all of which measure the ability of an object to do work on or change another object.

• In other words, there are different ways that an object can possess energy.

What are the different forms of Energy?

• Kinetic Energy• Potential Energy• Elastic Potential Energy• Thermal, or heat energy• Chemical Energy• Electrical Energy• Electrochemical Energy• Electromagnetic Energy (light)• Sound Energy• Nuclear Energy

Mechanical Energy

Define Potential Energy

• Potential energy is energy due to position or stored energy. Potential energy is also called gravitational potential energy.

• If we lift an object from the floor into the air, it has the potential to do work for us (Gravitational PE)

Potential Energy

• This ability to do work is called POTENTIAL ENERGY

• Other forms of potential energy include the compression of a spring (Elastic PE), the stored energy in coal or oil, the stored energy in a uranium nucleus (Chemical PE)

Give an example of potential energy

By stretching a rubber band, or a spring you give it elastic potential energy. A vase on a shelf has stored potential energy. A football being held by a quarterback has potential energy until it is thrown and it turns into kinetic energy.

Potential Energy

• Gravitational potential energy is simple to calculate

• Gravitational Potential Energy = weight X height

PE mgh

Define Kinetic Energy

Kinetic energy is the energy of motion. Kinetic energy is measured by how much is work done to put an object in motion or to rest.

Give an example of kinetic energy

A basketball player has kinetic energy. The movements that she does show the energy that is being displayed while she is moving. When you are running, walking, or jumping, your body is exhibiting kinetic energy.

Kinetic Energy

• The energy of moving objects

• Kinetic Energy = 1/2 Mass X Speed2

KE 12

mv 2

Give an example of something that has both potential and kinetic

energy.

Work-Energy Theorem

• Work done on an object can give the object either potential or kinetic energy or both

• If we do work on an object to lift it into the air, we give it potential energy

• If we do work on an object and set it into motion, we give it kinetic energy

• The work-energy theorem relates to the second case

Work-Energy Theorem

• If we do work on an object and set it into motion without changing the object’s potential energy, the work done appears as kinetic energy of the object

Work KE

Describe Conservation of Energy

Energy cannot be created or destroyed; it can only be changed from one of form of energy to another form. 

The amount of energy stays the same. 

Conservation of Energy

• Perhaps the most important discovery of the past two centuries

• In the absence of external work input or output, the energy of a system remains unchanged. Energy cannot be created or destroyed.

• Remember from Einstein, that mass is a form of energy E mc 2

Give an example of something that has both

potential and kinetic energy.

A waterfall has both kinetic and potential energy. The water at the top of the waterfall has stored potential energy. Once the water leaves the top of the waterfall, the potential energy is changed into kinetic energy.

Kinetic-Potential Energy Conversions

• As a basketball player throws the ball into the air, various energy conversions take place.

Ball slows down Ball speeds up

Click this link for more information about potential &

kinetic energy. Remember to hit the “back” button on your

browser to return to this slide show.

http://www.usoe.k12.ut.us/curr/science/sciber00/8th/forces/sciber/potkin.htm

Describe Conservation of Energy

Click here for more examples of Conservation of Energy:

http://web.singnet.com.sg/~stepchin/Conserv.htm

Remember to hit the “back” button on your browser to return to this slide show.

Vocabulary Wordsenergymechanical energyheat energychemical energyelectromagnetic energynuclear energykinetic energypotential energygravitational potential energyenergy conversionLaw of Conservation of Energy