Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Note on “warming things up” and thermal energy

All the energy transfers that you demonstrate will ultimately end up warming the atmosphere. 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 a high level of thermal energy. This hot body can be put into contact with a cooler body and some of the thermal energy of the hotter body is transferred to the cooler body to raise the thermal energy, and the temperature, of the cooler body. The word ‘heat’ is used as a similar word in physics to the word ‘work’. 
 
When a body falls from a height it is said to transfer energy from gravitational potential energy to kinetic energy. The energy transferred is called ‘work’ and it is driven by the difference in height through which the body falls. Heat and work are not descriptions of energy themselves but rather energy being transferred. 
 
The process of transferring thermal energy is known as heating. Many teachers consider that ‘heat’ should never be used as a noun (heat in a hot body) but only used as a verb as in the transfer process. 
 
There are many other processes in energy transfer chains. 

  • Transferring gravitational potential energy to kinetic energy is called work or working. 
  • Transferring thermal energy from one body to another is called heat or heating. 
  • Transferring energy from a hot filament to the surroundings is called radiating. 
  • Transferring chemical energy from an electrical cell to a lamp is by means of an electric current.  

In the Malvern energy kit energy is often transferred from one component to another by an elastic belt. 
 
Many teachers have adopted terms such as thermal energy or internal energy for the energy stored in a hot body. Later on students just need to talk about 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 level 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 with strain energy. If a gas is compressed by pushing a piston quickly into a cylinder, the gas grows hotter, and all the energy transferred to the gas goes into thermal energy of molecular motion. If the gas cools back to the original temperature, it transfers all its thermal energy to the atmosphere so all 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 supplies 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 that increase in molecular kinetic energy is. You can measure the thermal 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 kinetic energy of motion of 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 kinetic energy of molecules flying about in the gas. The extra energy goes to provide for these extra motions of the molecules.