Energy and Work

1. Energy and Work Chapter 10

2. 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

3. 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

4. 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

6. 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.

7. 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.

8. 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.

9. 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.

10. 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

11. 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

12. 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.

13. Energy Transformations • Energy of one kind may transform into energy of another kind • We will use an arrow to represent an energy transformation

14. Echem Ug

15. K Eth

16. Echem Eth

17. Us K Ug

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

19. 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.

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

21. 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.

22. 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.

23. 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

24. 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

25. 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.

26. Redefining the system • System = ball + earth • Internal forces =Fg • External forces = T • System energies = K, Ug • ΔK + ΔUg = Fg

27. 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.

28. Mechanical Energy • The sum of the kinetic and potential energies is called the mechanical energy • K + U = K + Ug + Us • Energy is conserved

29. Equations • Work • The units of work are joules • 1 joule = 1J = 1 N m

30. Forces that do no work • If the object undergoes no displacement while the force acts, no work is done.

31. Forces that do no work • A force perpendicular to the displacement does no work

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

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

34. More Energy Equations • Gravitational Potential Energy depends only on the height of the object above a reference level

35. 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

36. 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