that deal with just these situations.
Boyle’s Law
Boyle’s Law deals with gases at a constant temperature. It tells us that an increase in pressure is
accompanied by a decrease in volume, and vice versa:. Aerosol canisters contain
compressed (i.e., low-volume) gases, which is why they are marked with high-pressure warning
labels. When you spray a substance out of an aerosol container, the substance expands and the
pressure upon it decreases.
Charles’s Law
Charles’s Law deals with gases at a constant pressure. In such cases, volume and temperature are
directly proportional:. This is how hot-air balloons work: the balloon expands
when the air inside of it is heated.
Gases in a Closed Container
You may also encounter problems that deal with “gases in a closed container,” which is another
way of saying that the volume remains constant. For such problems, pressure and temperature are
directly proportional:. This relationship, however, apparently does not deserve a
name.
EXAMPLE 1
A gas in a cylinder is kept at a constant temperature while a piston compresses it to half its original
volume. What is the effect of this compression on the pressure the gas exerts on the walls of the
cylinder?Questions like this come up all the time on SAT II Physics. Answering it is a simple matter of
applying Boyle’s Law, or remembering that pressure and volume are inversely proportional in the
ideal gas law. If volume is halved, pressure is doubled.
EXAMPLE 2
A gas in a closed container is heated from 0ºC to 273ºC. How does this affect the pressure of the
gas on the walls of the container?First, we have to remember that in the ideal gas law, temperature is measured in Kelvins. In those
terms, the temperature goes from 273 K to 546 K; in other words, the temperature doubles.
Because we are dealing with a closed container, we know the volume remains constant. Because
pressure and temperature are directly proportional, we know that if the temperature is doubled,
then the pressure is doubled as well. This is why it’s a really bad idea to heat an aerosol canister.
The Laws of Thermodynamics
Dynamics is the study of why things move the way they do. For instance, in the chapter on
dynamics, we looked at Newton’s Laws to explain what compels bodies to accelerate, and how.
The prefix thermo denotes heat, so thermodynamics is the study of what compels heat to move in