Heating Things Up

Up Heating Things Up Heat Flow The Wood Stove Specific Heat

How can we heat things up?

  • We can add thermal energy to an object by doing work on the object.  If we rub an object, the force of sliding friction does work and changes ordered kinetic energy into thermal energy. 
  • We can burn something.  If fuel burns, chemical energy is converted into thermal energy.

 

Atoms in molecules and solids are held together by chemical bonds.  Chemical bonds are electromagnetic in origin, but can be modeled well by tiny springs.  Two atoms held together by a spring have an equilibrium position.  If they are pushed closer together, they repel each other.  If they are pulled farther apart, they attract each other.  If they are displaced in any way from their equilibrium position and then released, they start vibrating about their equilibrium position.  An atom can form different chemical bonds with a variety of other atoms.  Different bonds are represented by springs with different spring constants.  The stiffer the spring, the more work it takes to pull the atoms apart.  If enough work is done, then the spring is stretched too much and it breaks, i.e. the chemical bond breaks.  
At room temperature, gas molecules have random translational kinetic energy associated with the motion of their center of mass and random vibrational energy and rotational kinetic energy associated with the motion about their center of mass.  Collisions continuously transfer energy between the different degrees of freedom and the average energy in each degree of freedom is the same.  If work is done on the molecules which increases their vibrational energy, the amplitude of the vibrations increases, and eventually the chemical bonds break.  Most free atoms quickly form new bonds.  If the new bonds are stronger, i.e. if the new springs are stiffer, then they do more work pulling the atom towards their new equilibrium positions than was needed to break the old bonds and the atoms will have more kinetic energy as they pass through the equilibrium positions.  This kinetic energy is quickly shared with the other degrees of freedom, the energy of all degrees of freedom increases, i.e. the thermal energy increases.  Thermal energy is released by a chemical reaction.  The temperature increases.

To burn fuel, work must first be done to break the chemical bonds in the fuel.  This work provides the activation energy, the energy needed to start the chemical reaction.  The free atoms and molecules then bond with oxygen.  The new bonds with the oxygen atoms are much stronger than the broken bonds.  As the atoms form new bonds, they gain thermal energy.  When you strike a match, you first do work against friction to break the chemical bonds in some of the fuel on the head.  The free atoms and molecules now combine with oxygen from the air, forming stronger bonds and thus releasing thermal energy.  The random kinetic energy of these fast molecules is transferred in collisions to neighboring atoms and molecules, breaking their bonds, etc.

Video:  Activation Energy