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Parents' Notes

Science Lecture for Unit 46: Heat and Energy

For Class

• Topic area: Physics: Forms of Energy
• Terms and concepts to know: Potential energy, chemical energy, electrical energy, light energy; convection, radiation, conduction
• See historical period(s): 18th-19th Centuries

Outline/Summary

• Thermodynamics and Energy
  • Forms of Energy
• Study/Discussion Questions
• Further Study

Science Topic: Thermodynamics and Energy

Forms of Energy

During the seventeenth and eighteenth centuries, scientists begin to distinguish different forms of energy.

• Potential energy is the energy stored in an object because of its relation to some other object. Mass has potential energy if it is in the gravitational field of another mass. An electrically charged object has potential energy when it is in the electrical field of some other charged object. We can think of potential energy as stored energy.
  • Chemical energy is the energy stored in the bonds that hold atoms together in molecules. These bonds form as a result of the electrical forces exerted by charged particles in the atoms, so the potential energy is the result of the particle's positions in the combined electrical fields of the atoms.
  • Nuclear energy is the energy used to hold the nucleus of the atom itself together against the repulsive forces of like charges (the nucleus contains positively-charged protons).
• Kinetic energy is energy of motion, and depends on mass m and velocity v in the proportion ½mv² at any given instant. If the mass or or the charged object is not restrained, it will start to move under the influence of the forces from the field it is in, and its potential energy is realized as kinetic energy, or the energy of motion.
  • Mechanical energy is the result of moving something in a particular direction. This kind of energy is called work, and is proportional to the force used to move the object, and the distance it moves.
  • Electrical energy is the energy used to move charged particles in a wire or other medium, creating current.
  • Light energy is the energy carried by electromagnetic radiation, even if the wavelength of the light is too long (radio, microwaves) or too short (ultra violet, x-rays) for us to see it with our eyes as visible light.
  • Heat energy is the energy caused by the motion or collision of atoms and molecules in solids, liquids, and gases.

Energy can change from one form to another. When we lift a book onto a shelf, moving it further from the center of the earth, we exert our own force of lifting against the gravitational forces attracting the book to the earth. The work we do is equal to the force (mass m * acceleration due to gravity g) times the distance d we move the book:

Work W = potential energy change ΔPE = force F * distance moved d = mgh

When we take the book off the shelf and let go, the force of gravity is no longer facing resistance by the shelf, and the book falls, expending kinetic energy ½mv², with increasing velocity until it hits the floor.

Work W = potential energy change ΔPE = force F * distance moved d = mgh = energy recovered when book falls = ½mv²

(In this case, v is the average velocity over the course of the fall, since the book starts with 0 velocity if we just drop it.)

The fact that we can change one form of energy into another makes most of modern technology possible. The steam engine is a mechanism to change heat energy to mechanical energy, and to direct that mechanical energy so that it will do useful work, that is, what we want it to do. A photocell is a way of changing radiation or light energy into electrical energy or current.

The underlying rules of changing forms of energy come from the study of heat as an energy source used to do work, or thermodynamics. One of the most important intellectual advances in science during the 19th century was the realization that heat is a form of kinetic energy--the energy of many many atoms and molecules in motion. Heat energy can flow from one substance to another in one of three ways:

• Radiation is the movement of energy through empty space in the form of electromagnetic radiation, or light.
• Convection is the movement of some heated material from one place to another.
• Conduction is the exchange of heat from one substance to another by direct contact.

A good example of the three forms of heat transfer is a pot of boiling water on an electrical stove. The burner heats up and begins to glow -- that's radiation. Using conduction, the bottom of the pan in contact with the burner begins to heat up by conduction: the molecules in the burner move and transfer energy to the molecules in the pan's bottom surface, which transfer energy to the molecules of water along the inside surface of the pan. Using convection, water molecules heated up along the surface of the pan move away from the surface and carry their energy to the center of the volume of water.

It is impossible to follow every atom through every motion, but using statistical methods, scientists could predict probable outcomes of different situations. As we'll see in a later unit, the statistical analysis lead to studies of how many states a give system can have. Some of the states we would consider ordered (the members in a particular symmetrical distribution, for example), more states are unordered or randomly distributed. So the likelihood of winding up in an ordered state is less than the probability of ending up in a disordered state.

Study/Discussion Questions

• Pick any substance and explain how energy is stored, used, or released by the substance.
• Be able to explain how energy changes form from potential energy (stored energy) to kinetic energy (able to do work).

Further Study On your Own (Optional)

• Explore the discussion of energy at the Physics Classroom.
• There is another useful presentation in this Kinetic and Potential Energy Guide.

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