Energy and work
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Energy and Work. Chapter 10. Energy. Energy is the ability to do work An object that has energy has the ability to create a force. Energy is measured in units of joules 1 joule = 1 Newton meter There are many different types of energy. The Law of Conservation of Energy.

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Energy and work

Energy and Work

Chapter 10


Energy

Energy

  • Energy is the ability to do work

  • An object that has energy has the ability to create a force.

  • Energy is measured in units of joules

  • 1 joule = 1 Newton meter

  • There are many different types of energy


The law of conservation of energy

The Law of Conservation of Energy

  • Energy must be conserved

  • Energy can change forms

  • We must account for energy transferred into or out of a system.

  • Nature “balances the books” for energy

  • Our goal is to discover the conditions under which energy is conserved


Energy in a system

Energy in a System

  • Every system has a quantity associated with it called total energy.

  • The total energy of a system is made up of many kinds of energy


Kinetic energy k

Kinetic Energy (K)

  • Energy of Motion

  • All moving objects have kinetic energy

  • The heavier an object, and the faster it moves, the more K it has.


Gravitational potential energy u g

Gravitational Potential Energy Ug

  • GPE is stored energy associated with an object’s height above the ground.

  • As this roller coaster descends, stored potential energy is converted to kinetic energy.


Elastic or spring potential energy u s

Elastic or Spring Potential Energy (Us)

  • Us is energy stored when a spring or other elastic object is stretched.

  • This can later be transformed into kinetic energy.

  • The book may use U to represent potential energy when it is not

    important to distinguish

    between gravitational

    and spring.


Thermal energy e th

Thermal Energy (Eth)

  • Hot objects have more thermal energy than cold ones because the molecules are moving faster.

  • Eth = Sum of all the kinetic and potential

    energies of all the

    molecules

  • Some molecules have

    enough energy to escape

    as steam.


Chemical energy e chem

Chemical Energy (Echem)

  • Energy that is stored in the bonds of molecules

  • Energy is released as the bonds are rearranged during chemical reactions.

  • Fuel for car or food

    for our bodies


Nuclear energy e nuclear

Nuclear Energy Enuclear

  • Energy stored in the nucleus of an atom

  • If the nuclei is broken apart vast amounts of energy are released

  • Transformed into kinetic and thermal energy.

  • Source of energy for

    nuclear power plants

    and weapons


Total energy of the system e

Total Energy of the System (E)

  • E = K + Ug + Us + Eth + Echem + …

  • We will be most interested in these forms of energy although others may be present.


Energy transformations

Energy Transformations

  • Energy of one kind may transform into energy of another kind

  • We will use an arrow to represent an energy transformation


E chem u g

Echem Ug


K e th

K Eth


E chem e th

Echem Eth


U s k u g

Us K Ug


Energy transformations are changes of energy within the system from one form to another

Energy Transformations are changes of energy within the system from one form to another


Energy transfers

Energy Transfers

  • An exchange of energy between the system and the environment is called an energy transfer

  • Twp primary energy transfers

    • Work –The mechanical transfer of energy to or from the system by pushing or pulling on it

    • Heat-The non-mechanical transfer of energy to or from the system by the environment because of a temperature difference between the two.


Energy and work

Work

  • Work is the process of transferring energy between the system and the environment by the application of a mechanical force.


The energy transfer w k

The energy transferW K

  • The system is the shot

  • The environment is the athlete

  • As the athlete pushes on the shot, he is doing work on the system .

  • He transfers energy

    from himself to the ball.


The transfer w e th

The transfer W Eth

  • The system is the match and matchbox

  • The environment is the hand

  • As the hand pulls the match actross, the hand does work on the system, increasing its thermal energy.

  • The match head

    becomes hot

    enough to ignite.


The transfer w u s

The transferW Us

  • The system is the slingshot

  • The environment is the boy

  • As the boy pulls back the elastic bands, he does work on the system,

    increasing its elastic

    potential energy


Energy and work

  • Internal Forces are forces between objects within the system

  • Internal forces cannot do work on the system

  • External forces act on the system, but their agent is part of the environment

  • External forces Can do work on the system


Defining the system

Defining the system

  • System = the ball only

  • Internal forces = none

  • External forces = T, Fg

  • System energies = K

  • Since the earth is not part of the system, the system

    has no Ug.


Redefining the system

Redefining the system

  • System = ball + earth

  • Internal forces =Fg

  • External forces = T

  • System energies = K, Ug

  • ΔK + ΔUg = Fg


Redefining the system1

Redefining the system

  • System = ball + earth + crane

  • T, Fg Many internal forces of the crane

  • External forces = none

  • System energies = K, Ug. Echem

  • The system is isolated

  • Increased potential and

    kinetic energy comes

    from a transformation

    of chemical energy.


Mechanical energy

Mechanical Energy

  • The sum of the kinetic and potential energies is called the mechanical energy

  • K + U = K + Ug + Us

  • Energy is conserved


Equations

Equations

  • Work

  • The units of work are joules

  • 1 joule = 1J = 1 N m


Forces that do no work

Forces that do no work

  • If the object undergoes no displacement while the force acts, no work is done.


Forces that do no work1

Forces that do no work

  • A force perpendicular to the displacement does no work


Forces that do no work2

Forces that do no work

  • If the part of the object on which the force acts, undergoes no displacement, no work is done.


Energy equations

Energy Equations

  • Translational Kinetic Energy of an object moving along a line or some other path.


More energy equations

More Energy Equations

  • Gravitational Potential Energy depends only on the height of the object above a reference level


More energy equations1

More Energy Equations

  • Elastic potential energy

  • Us is the same whether x is positive (compressed spring) or negative (extended spring)

  • k is the spring constant


Thermal energy

Thermal Energy

  • When two objects slide against each other with friction present, mechanical energy is always transformed into thermal energy.

  • The surface must be included as part of the system

  • A system’s thermal energy increases as kinetic energy is transformed into thermal energy

  • ΔEth = Ki- Kf


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