Energy and Thermal Physics

Note on “warming things up” and "thermal energy"

Teaching Guidance for 11-14 14-16 PRACTICAL PHYISCS

All the energy transfers that you demonstrate will ultimately end up warming the atmosphere, or surroundings. There has been much debate about what is happening when a body is warmed up, at both the scientific and the education level. One of the teaching routes is suggested here.

A hot body has a high temperature and can store a lot of energy thermally. This hot body can be put into contact with a cooler body and some of the energy of the hotter body is transferred to the cooler body to increase the energy stored thermally, and the temperature, of the cooler body.The word ‘heat’ has been used as a similar word in physics to the word ‘work’. However, a better approach is to say that heating and working both transfer energy, and can change the temperature of a body or system.

When a body falls from a height, energy stored gravitationally (when the object is high up) is then stored kinetically (when it is at a lower height). Again, it is often said that 'gravitational potential energy changes into kinetic energy". A better approach is to describe how the energy is stored (e.g. gravitationally, or due to its position in a gravitational field, and then kinetically because it is moving). Energy is transferred due to the work done by a force, and it is driven by the difference in height through which the body falls.

There are many other processes in which energy is transferred. When a cell is used to light a light bulb:

  • there is a transfer of energy from energy stored chemically in an electrical cell to energy stored thermally in the hot filament of a lamp
  • this is called working (electrically) by means of an electric current.

When a light bulb is on:

  • there is a transfer of energy from energy stored thermally in a hot filament to energy stored thermally in the surroundings
  • this is called heating (by radiation).

In the Malvern energy kit energy is often transferred from one component to another by an elastic belt. The belt is used to do work.

While it is tempting to use terms such as "thermal energy" or "internal energy" for the energy stored in a hot body, it is misleading. A better approach is to use energy without an adjective.

The concept of "heat" also achieves ‘notoriety’ because when the energy transfer is not 100% efficient then it is attributed to being transferred to the Universe, so warming it up in some mysterious way. The model is one in which the kinetic theory of gases ‘explains’ the energy in a gas as being due to the random motion of its molecules. (Not only the translational kinetic energy of the molecules but also the rotational and vibrational energies as well. There is no potential energy stored up in a gas except the potential energy of the (P.E. + K.E.) of vibrational motion.)

Students sometimes think of a gas at high pressure as being like a compressed spring. Springs store energy elastically. If a gas is compressed by pushing a piston quickly into a cylinder, the gas grows hotter, and all the work done on the gas goes into the energy stored thermally in the gas. The molecules of the gas move faster. If the gas cools back to the original temperature, energy is transferred to the surroundings;the energy transferred to the gas has now 'escaped' to the outside world. The compressed gas, back at room temperature, has no extra energy by virtue of being compressed, yet it can transfer energy to other things by pushing the piston out with its high pressure. But the energy which it now transfers will be taken from the gas by cooling it down below room temperature.

If you transfer energy to a gas then you might want to calculate how much the energy stored kinetically by the molecules increases. An indication of that energy stored kinetically is the temperature of the gas. You can measure the energy experimentally by heating it up with an electrical heater. You might expect the energy transferred from an electrical supply to warm up a sample of gas agrees with the calculated increase of average energy stored kinetically by the gas molecules. You will find that to be true for a gas such as helium or neon, in which the molecules are single atoms that do not indulge in rotational or vibrational motion that can also be increased by heating. However, for other gases, such as ‘air’ or carbon dioxide, you will find that the electrical supply has to deliver more energy than goes simply in the energy stored kinetically by the molecules flying about in the gas. The extra energy goes to provide for these extra (rotational and vibrational) motions of the molecules.

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