Heat

Heat is often considered a form of energy associated with temperature. Based on the equation
above, we now see that a more accurate, though perhaps more abstract, way of looking at it is as a
means by which energy is transferred. More specifically, heat is energy transfer that occurs as a
result of a temperature difference between the system and its surroundings. This transfer will occur
spontaneously from a warmer system to a cooler system. Heat, being an exchange of energy, is
measured in the same units of energy, e.g., calories (cal) or joules (J), although kcal (kilocalorie,
equals 1,000 cal) or kJ (kilojoule, or 1,000 J) is often more convenient. The conversion between
calories and joules is done via the relation: 1 cal = 4.184 J; similarly, 1 kcal = 4.184 kJ. According to
convention, heat absorbed by a system (from its surroundings) is considered positive, while heat
lost by a system (to its surroundings) is considered negative. This is consistent with the first law:
heat absorbed would lead to a positive ∆E, meaning that energy has increased.

SPECIFIC HEAT AND HEAT CAPACITY

Heat is supplied to (or absorbed by) the system to raise its temperature; conversely, heat is released
if it cools. The heat absorbed or released by an object as a result of a change in temperature is
calculated from the equation:

q   =   mcΔT

where m is the mass of the object, ∆T is the change in temperature and is equal to the final
temperature minus the initial temperature, and c is a quantity known as the specific heat of the
substance, a notion that will be discussed further below.

It seems intuitively obvious that the more of the substance there is (the more massive it is), the
more heat is required to bring about a particular change in temperature. Recall from our earlier
discussion on the kinetic theory of gases that temperature is a measurement of the average kinetic
energy of the particles. This applies, at least conceptually, to other states of matter as well: Even
though motion is more restricted for particles in the condensed phases, they can still carry kinetic