18.4 - Absolute zero
Absolute zero: As cold as it can get.
Absolute zero is a reference point at which molecules are in their minimum energy state (quantum theory dictates they still have some energy).
It does not get colder than absolute zero; nothing with a temperature less than this minimum energy state can exist.
Physics theory says it is impossible for a material to be chilled to absolute zero. Instead, it is a limit that scientists strive to get closer and closer
to achieving. Above, you see a photograph of scientists who chilled atoms to less than a hundred-billionth of a degree above absolute zero. At
that temperature, the atoms changed into a state of matter called a Bose-Einstein condensate.
Although we exist around the relatively toasty 293 K thanks to the Sun and our atmosphere, the background temperature of the universeí the
temperature far from any star í is only about 3 K. Brrrr.
Absolute zero
It cannot get colder
Molecules at minimum energy state
0 K, í273.15°C18.5 - Heat
Heat: Thermal energy transferred between
objects because of a difference in their
temperatures.
If you hold a cold can of soda in your hand, the soda will warm up and your hand will
chill. Energy flows from the object with the higher temperature í your hand í to the
object with the lower temperature í the soda. This energy that moves is called heat.
Ovens are designed to facilitate the flow of heat. In the diagram to the right, you see
heat flowing from the oven coils through the air to the loaf of bread. As with other forms
of energy, heat can be measured in joules. It is represented by the capital letter Q.
Physicists do not say an object has heat. Heat refers solely to the flow of energy due to
temperature differences. Heat transfers thermal energy that is internal to objects,
related to the random motion of the atoms making up the objects.
Heat is like work: It changes the energy of an object or system. It does not make sense
to say “how much work a system has”, nor does it make sense to say “how much heat
the system has”. Just as work is done by a system or on a system, heat as thermal
energy can enter a system or leave a system.
Having said that heat is measured in joules, we will backtrack a little in order to explain
some other commonly used units. These units measure heat by its ability to raise the
temperature of water. For example, a calorie raises one gram of water from 14.5°C to
15.5°C. The British Thermal Unit (BTU) measures the heat that would raise a pound of
water1° on the Fahrenheit scale.
The unit we use to measure one property of food í the calories you see labeled on the
back of food packages í is actually a kilocalorie (good marketing!). This is sometimes
spelled with a capital C, as in Calories. Food calories measure how much heat will be
released when an object is burned. A Big Mac® hamburger contains 590 Calories, or
590,000 calories. This amount of energy equals about 2,500,000 J. If your body could
capture all this energy, if it were 100% efficient and solely focused on the task, the
energy from a Big Mac would be enough to allow you to lift a 50 kg weight one meter
about 5000 times. We will skip the calculation for the french fries.
Heat
Energy flow due to temperature
difference
Not a property of an object