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Chapter Seven: Energy

Chapter Seven: Energy. 7.1 Energy and Systems 7.2 Conservation of Energy 7.3 Energy Transformations. 7.1 What is energy?. Energy measures the ability for things to change themselves or to cause change in other things.

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Chapter Seven: Energy

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

  7. 7.1 Some forms of energy • Electrical energy comes from electric charge, which is one of the fundamental properties of all matter.

  8. 7.1 More forms of energy • Nuclear energy is a form of energy stored in the nuclei of atoms. • In the Sun, nuclear energyis transformed to heat that eventually escapes the sun as radiant energy.

  9. 7.1 More forms of energy • Radiant energy is energy that is carried by electromagnetic waves. • Light is one form of radiant energy.

  10. 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.

  11. 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.

  12. 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.

  13. 7.1 Energy and work • In physics, the word workhas a very specific meaning. • Workis the transfer of energy that results from applying a force over a distance.

  14. 7.1 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.

  15. 7.1 Potential Energy mass of object (kg) height object raised (m) EP = mgh PE (joules) gravity (9.8 m/sec2)

  16. 7.1 Kinetic energy • Energy of motionis called kinetic energy. • A moving cart has kinetic energy because it can hit another object (like clay) and cause change.

  17. 7.1 Kinetic Energy KE (joules) mass of object (kg) EK = ½ mv2 velocity (m/sec)

  18. 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.

  19. Solving Problems • Looking for: • …initial EK, EP and 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.

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

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