Chapter Seven: Energy

1. Chapter Seven: Energy • 7.1 Energy and Systems • 7.2 Conservation of Energy • 7.3 Energy Transformations

2. 7.1 What is energy? • Energy measures the ability for things to change themselves or to cause change in other things. • Some examples are changes in temperature, speed, position, pressure, or any other physical variable.

3. 7.1 Units of energy • Pushing a 1-kilogram object with a force of one newton for a distance of one meter uses one joule of energy. A joule (J) is the S.I. unit of measurement for energy.

4. 7.1 Joules • One joule is a pretty small amount of energy. An ordinary 100 watt electric light bulb uses 100 joules of energy every second!

5. 7.1 Some forms of energy • Mechanical energy is the energy possessed by an object due to its motion or its position. • Potential energy and kinetic energy are both forms of mechanical energy.

6. Mechanical Energy • Energy associated with motion:

7. 7.1 Some forms of energy • Chemical energy is a form of energy stored in molecules. • Example: Batteries are storage devices for chemical energy.

8. Chemical Energy • Energy released or stored when atoms are bonded together. • Bonds can be broken to release this energy. The chemical bonds in a matchstick store energy that is transformed into thermal energy when the match is struck.

9. 7.1 Some forms of energy • Electrical energy comes from electric charge. • Caused by the flow of electrons

10. Electric Energy • Example: Power lines: Electric energy is easily transported through power lines and converted into other forms of energy

11. 7.1 More forms of energy • Nuclear energy is a form of energy stored in the nuclei of atoms. • In the Sun, nuclear energy is transformed to heat that eventually escapes the sun as radiant energy.

12. 7.1 More forms of energy • Radiant energy is energy that is carried by electromagnetic waves. • Light is one form of radiant energy.

13. 7.1 More forms of energy • The electromagnetic spectrum includes visible light, infrared radiation (heat), and ultraviolet light. • Light energy and heat energyare included in the electromagnetic spectrum.

15. QUIZ TIME! What type of energy cooks food in a microwave oven? ELECTROMAGNETIC ENERGY What type of energy is the spinning plate inside of a microwave oven? MECHANICAL ENERGY

16. QUIZ TIME! Electrical energy is transported to your house through power lines. When you plug an electric fan to a power outlet, electrical energy is transform into what type of energy? MECHANICAL ENERGY

17. QUIZ TIME! What energy transformation occurs when an electric lamp is turned on? ELECTRICAL ENERGY  ELECTROMAGNETIC ENERGY

18. What type of energy is shown below? Chemical Energy

19. What type of energy is shown below? Chemical Energy (yummy)

20. What type of energy is shown below? Thermal Energy

21. 7.1 Sources of energy • Without the Sun’s energy, Earth would be a cold icy place with a temperature of -273 C. • As well as warming the planet, the Sun’s energy drives the entire food chain.

22. 7.1 Sources of energy • All objects with mass feel forces in the presence of Earth’s gravity. • These forces are a source of energy for objects or moving matter such as falling rocks and falling water.

23. 7.1 Energy and work • In physics, the word workhas a very specific meaning. • A force is said to do work when it acts on an object & there is a displacement in the direction of the force. • Workis the transfer of energy that results from applying a force over a distance.

24. Work and Potential Energy • The work done on the ball gives the ball gravitational potential energy. • Gravitational potential energy = mgh

25. Potential Energy • Energy stored in an object due to its position. • Examples:

26. Gravitational potential energy • Depends on the weight of the object and its height above some reference point. • Formula: GPE = m x g x height

27. Gravitational Potential Energy Example: • Both blocks acquire the same gravitational potential energy, mgh. • The same work is done on each block. What matters is the final elevation, not the path followed.

28. Elastic Potential Energy • The potential energy due to the physical distortion (stretched or compressed) of an object. • Depends on: • Type of material • Amount of distortion

29. Elastic Potential Energy Example: • Work is done on the bow. • The work done is storedin the bow and string aselastic potential energy. • After release, the arrow issaid to have kinetic energy,1/2 mv2. • Energy is measured in the same units (joules) as work.

30. Potential Energy • Systems or objects with potential energy are able to exert forces (exchange energy) as they change. • Potential energy is energy due to position, which can change.

31. Potential Energy mass of object (g) height object raised (m) EP = mgh PE (joules) gravity (9.8 m/sec2)

32. Kinetic energy • Energy of motion is called kinetic energy. • A moving cart has kinetic energy because it can hit another object (like clay) and cause change.

33. Kinetic Energy • Energy of motion • All moving objects possess kinetic energy • Can change from one type to another

34. Factors Which Affect Kinetic Energy: • Mass&Velocity • KE = ½ mv2 • This makes the velocity much more important than the mass. • Doubling the velocity will quadruple the kinetic energy.

35. Kinetic Energy KE (joules) mass of object (kg) EK = ½ mv2 velocity (m/sec)

36. Solving Problems • A 2 kg rock is at the edge of a cliff 20 meters above a lake. • It becomes loose and falls toward the water below. • Calculate its potential and kinetic energy when it is at the top and when it is halfway down. • Its speed is 14 m/s at the halfway point.

37. Solving Problems • Looking for: • …initial EK, EPand EK, EP half way down. • Given: • mass = 2.0 kg; h = 20 m • v = 14 m/s (half way) • Relationships: • EP =mgh • EK = ½ mv2 • Assume rock starts from rest.

38. Solving Problems • Solution • Draw a free body diagram. m = 20 kg • EP= (2 kg)(9.8 N/kg)(20 m) = 392 J at top EP = mgh EK = 0 J h = 20 m • EP= (2 kg)(9.8 N/kg)(10 m) = 196 J half way • EK= 0 J, rock is at rest EP = mgh h = 10 m EK = ½ mv2 • EK= (1/2)(2 kg)(14 m/s)2 = 196 J half way

39. Chapter Seven: Energy • 7.1 Energy and Systems • 7.2 Conservation of Energy • 7.3 Energy Transformations

40. 7.2 Conservation of Energy • Systems change as energy flows and changes from one part of the system to another. • Each change transfers energy or transforms energy from one form to another.

41. 7.2 Energy flow • How can we predict how energy will flow? • One thing we can always be sure of is that systems tend to move from higher to lower energy.

42. 7.2 Sources of energy • 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.

43. Bell Work!Energy Transformation • 1. What is the formula for Gravitational potential energy? • 2. What is the formula for Kineticenergy? • 3. How does the Pe at the top of a hill compare to the Ke at the bottom?

44. Energy Transformation • The work done in lifting the mass gave the mass gravitational potential energy. • Potential energy then becomes kinetic energy. • Kinetic energy then does work to push stake into ground.

45. 7.2 Units of energy • Some units of energy that are more appropriate for everyday use are the kilowatt hour (kWh), food Calorie, and British thermal unit. oF

47. Chapter Seven: Energy • 7.1 Energy and Systems • 7.2 Conservation of Energy • 7.3 Energy Transformations

48. 7.3 Conservation of Energy • 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. • The law of energy conservation says the total energy before the change equals the total energy after it.

49. 7.3 Conservation of Energy • When you throw a ball in the air, the energy transforms from kinetic to potential and then back to kinetic.

50. EnergyConservation • Total energy is the sum of both types of energy.