Heat (Energy) Transfer and Thermal Equilibrium

"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones."

Albert Einstein

1 Calorie = 4.184 Joules
1 kcal = Heat required to raise 1 kg of water through 1 0C

exclamation Note that the calorie which descibes the energy content on food labels is actually 1 kcal.
The British Thermal Unit (Btu), still in use in the USA, is defined in a similar way to the calorie, but using British units,

1 Btu = Heat required to raise 1 lb of water through 1 0F

Note that,
1 kcal = 3.968 Btu = 4184 Joules = 3086.7 ft.lbs
  • Given these definitions, the amount of heat required to change the temperature of a mass, m by delta T is given by,


where c is the Specific Heat  (capacity) of the substance of which the mass is composed.  The units of c are J/kg.0C.  Notice that for water c = 1 kcal/kg.0C (= 4184 J/kg.0C), so that the value of c for other substances is always relative to that of water.
Also, due to the similar way in which calories and Btus are defined if the specific heat of a substance is  'x'  kcal/kg.0C  it is also 'x' Btu/lb.0F.
  • The Heat Capacity of an object, C, is defined by,

the heat needed to raise an object by one degree.
  • hotKnowledge of specific heats and/or heat capacities and the fact that energy must be conserved allows us to determine the equilibrium temperature of two objects initially at different temperatures by demanding that,
Heat lost by hot object = Heat gained by cold object

where we ignore heat gained or lost from/to the surroundings.  The study of heat gained and lost in this manner is often called Calorimetry.

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Change of Phase and Latent Heat

These phase changes occur at definite temperatures, e.g. water freezes at 0 0C and boils at 100 0C.
Latent heat of fusion of water/ice = 80 kcal/kg = 333 kJ/kg
Latent heat of vaporisation of water/water vapor = 540 kcal/kg = 2256 kJ/kg



There is this farmer who is having problems with his chickens. All of the sudden, they are all getting very sick and he doesn't know what is wrong with them. After trying all conventional means, he calls a biologist, a chemist, and a physicist to see if they can figure out what is wrong. So the biologist looks at the chickens, examines them a bit, and says he has no clue what could be wrong with them. Then the chemist takes some tests and makes some measurements, but he can't come to any conclusions either. So the physicist tries. He stands there and looks at the chickens for a long time without touching them or anything. Then all of the sudden he starts scribbling away in a notebook. Finally, after several gruesome calculations, he exclaims, 'I've got it! But it only works for spherical chickens in a vacuum.'


Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu