Mr. Rogers' AP Physics C: IB Physics Topics

Syllabus 1st Quarter 2nd Quarter 3rd Quarter 4th Quarter
IB SL Thermo IB HL Thermo IB HL Waves AP Review  

Topic 9: HL Thermal Physics (SL optional) - 1st Law of Thermodynamics

1st Law 2nd Law Processes Heat Engines HL Equations


Energy can change forms but can neither be conserved or destroyed. In a closed system, the amount of energy remains constant. In other words, in a closed system energy is conserved. For a thermodynamic system, the first law can be expressed mathematically as follows:

DQ = DU + W

    +DQ  = thermal energy transferred into the system


increase in internal energy of the system


work done by the system

With respect to energy transfer in and out of thermodynamic systems: what goes in comes out or stays there. None of the examples in either IB SL or HL Physics has any energy storage, so what goes in must come out. For heat engines the first law can be expressed as shown below

  Energy in   Energy out



Qc +  W


Note that matter is just a super condensed form of energy as shown in Einstein's famous equation:

E = mc2
E = energy content of a given amount of mass
m = mass converted into energy. (Here mass would be defined as rest mass.)
c = the speed of light in a vacuum

Hence, conservation of energy includes conservation of mass

Of all the principals in all of science, the 1st law of Thermo is the closest absolute truth. We know of no exceptions. Conservation of energy is entwined in essentially all areas of science and engineering. For example, in chemistry, balancing chemical equations assumes conservation of mass, a subset of the 1st Law of Thermo. Finding an exception to the 1st Law of Thermo would be a major event. If the 1st Law of Thermo were discredited, it would cast doubt on the validity of all science.


Energy Transfer

Heat: thermal energy transferred from high to low temperature regions. There are 3 forms of heat transfer:

  1. Conduction - the transfer of heat through a solid or stationary fluid.
  2. Convection - heat transfer using a moving fluid.
  3. Radiant - heat transferred as electromagnetic radiation. This type is the only kind which works in a vacuum.

Work: mechanical energy transferred by a force acting through a displacement.


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