in Kelvins are always positive. Absolute zero, 0 K, which is equivalent to –273ºC, is the lowest
theoretical temperature a material can have. Other than the placement of the zero point, the Kelvin
and Celsius scales are the same, so water freezes at 273 K and boils at 373 K.
Definition of Temperature
The temperature of a material is a measure of the average kinetic energy of the molecules that
make up that material. Absolute zero is defined as the temperature at which the molecules have
zero kinetic energy, which is why it is impossible for anything to be colder.
Solids are rigid because their molecules do not have enough kinetic energy to go anywhere—they
just vibrate in place. The molecules in a liquid have enough energy to move around one another—
which is why liquids flow—but not enough to escape each other. In a gas, the molecules have so
much kinetic energy that they disperse and the gas expands to fill its container.
Heat
Heat is a measure of how much thermal energy is transmitted from one body to another. We
cannot say a body “has” a certain amount of heat any more than we can say a body “has” a certain
amount of work. While both work and heat can be measured in terms of joules, they are not
measures of energy but rather of energy transfer. A hot water bottle has a certain amount of
thermal energy; when you cuddle up with a hot water bottle, it transmits a certain amount of heat
to your body.
Calories
Like work, heat can be measured in terms of joules, but it is frequently measured in terms of
calories (cal). Unlike joules, calories relate heat to changes in temperature, making them a more
convenient unit of measurement for the kinds of thermal physics problems you will encounter on
SAT II Physics. Be forewarned, however, that a question on thermal physics on SAT II Physics
may be expressed either in terms of calories or joules.
A calorie is defined as the amount of heat needed to raise the temperature of one gram of water by
one degree Celsius. One calorie is equivalent to 4.19 J.
You’re probably most familiar with the word calorie in the context of a food’s nutritional content.
However, food calories are not quite the same as what we’re discussing here: they are actually
Calories, with a capital “C,” where 1 Calorie = 1000 calories. Also, these Calories are not a
measure of thermal energy, but rather a measure of the energy stored in the chemical bonds of
food.
Specific Heat
Though heat and temperature are not the same thing, there is a correlation between the two,
captured in a quantity called specific heat, c. Specific heat measures how much heat is required to
raise the temperature of a certain mass of a given substance. Specific heat is measured in units of
J/kg · ºC or cal/g · ºC. Every substance has a different specific heat, but specific heat is a constant
for that substance.
For instance, the specific heat of water, , is J/kg · ºC or 1 cal/g · ºC. That means it
takes joules of heat to raise one kilogram of water by one degree Celsius. Substances
that are easily heated, like copper, have a low specific heat, while substances that are difficult to