Spring 2003

Inquiry in the Natural World

Student Guide, Topic 4, What is Energy?

 

Topic Objectives

• Be able to identify the four basic forms of energy; i.e., kinetic, potential, electromagnetic, and mass energies.
• Be able to explain what is meant by conservation of energy and conservation of momentum.
• Be able to distinguish between temperature, heat, and internal energy (or thermal energy).
• Understand the failure of the caloric theory of heat.
• Understand the significance of the first and second laws of thermodynamics.
• Be able to distinguish between work, energy and power.

 

Wednesday’s Reading Assignment

Trefil and Hazen, The Sciences, 3rd  Ed., Ch 3 pp. 54 to 64 and 68 to 69 and Ch 4 pp 76 to 79 and 85 to 95.

 

Active Learning Exercises

1) List some forms of energy that are not in the above list of four basic forms and identify which of the basic forms are involved in each of the forms in your list.

2) Consider the possibility of using water from a faucet to turn a paddle wheel, which turns a generator that lights a lamp.

·        Explain all the energy transformations involved.

·        Can you see why this is not a source of "free" energy?

3) Give some examples, other than those on p. 92 of the text, of things, which would look silly when running a film or   videotape in reverse.

 

Reading for Friday

Chapter 1 of: P.W.Atkins, The Second Law, New York: Freeman 1984

http://web.sbu.edu/physics/faculty/dimattio/Clare102/physics.dimattio-sp03.htm

 

Questions for Discussion

 

• Identify the developers of thermodynamics who: (a) had more than one name (b) originally believed in the caloric theory (c) committed suicide.
• What was Joule's contribution to thermodynamics and how did

   it influence the work of Lord Kelvin?

• When does the author claim that the concept of energy became more important than that of force?
• What does the author mean by "nature's dissymmetry"?

Give some examples.

Mechanics

 

 

Conservation of Momentum

 

          Total momentum of an isolated system is a constant.

 

          Recall: force =  D(mv)/ D(t)

 

·        final momentum - initial momentum = force X time

 

Conservation of Energy

 

          Total energy of an isolated system is a constant.

 

          But: final energy - initial energy = work

                       Or: work = change in energy

 

Note: Total energy of the universe is conserved.

 

Note: Friction dissipates energy and thus decreases the

      kinetic energy of a moving body doing negative work.

 

 

 

Basic Forms of Energy

 

  (1) kinetic - energy a body has by virtue of its motion

 

  (2) potential - energy a body has by virtue of its position

 

  (3) electromagnetic - energy carried by electromagnetic

                        radiation

 

  (4) mass - energy a body has by virtue of its mass (E = mc²)

 

 

Examples of non-basic forms:

 

          mechanical energy - sum of kinetic and potential

 

          thermal energy (or internal energy)

- microscopic mechanical energy of the particles in a body

Thermodynamics

 

Heat (Q) - energy exchanged between two bodies due to a

                difference in temperature.

 

Internal Energy (U) - microscopic kinetic and potential energies

                    in the body.

 

Laws of Thermodynamics:

 

     First Law: Change in internal energy = heat + work

                Or: U_f - U_i = Q + W

         

          Note: Q is energy in transit not the energy in the body. U

      is the thermal energy of the body.

 

     Second law:

 

                    Kelvin Statement - There is no perfect heat engine;

          i.e., no cyclic device can completely convert

          heat into work,

 

                    Clausius Statement - There is no perfect refrigerator;

          i.e., no cyclic device which transfers heat from a

          cold reservoir to a hot reservior without work being

          done on it.

 

                    Entropy Statement - Total entropy of an isolated

          system can not decrease.

 

 

          Note: 1st law is a statement of conservation of energy.

 

          Note: entropy is a measure of disorder or an arrow which

                points in the direction of spontaneous processes.