Two words that mean different things
In everyday speech, “heat” and “temperature” are used almost interchangeably. In physics they are different quantities.
Temperature describes how hot or cold something is. At the particle level, it reflects the average energy of motion of the atoms or molecules in a substance. In a hot gas the molecules move faster on average, and in a cold gas they move more slowly. In solids the particles mostly vibrate in place rather than travel around, but the same idea applies.
Heat is energy that moves from one place to another because of a temperature difference. Heat is not something an object “contains” in the sense of a stored fluid. Physicists usually speak of an object’s thermal energy, the total energy of the motion of all its particles, and of heat as the transfer of that energy.
A useful example compares a cup of boiling tea with a full bathtub of warm water. The tea has a much higher temperature, since its particles move faster on average. The bathtub contains vastly more water, so its total thermal energy is much larger. If you could extract energy from each, the bath would supply far more. This is why a large lake can warm a cold night’s air for a long time, while a small hot object cools quickly.
How heat moves
Heat flows from warmer regions to cooler ones until the temperatures even out. It travels in three main ways.
- Conduction is transfer through direct contact. A metal spoon left in a hot pot becomes warm along its length as fast-moving particles jostle their neighbors. Metals conduct heat well; wood, plastic and trapped air conduct it poorly, which is why they make good insulation.
- Convection is transfer by the movement of a fluid, meaning a liquid or gas. Warmer fluid is generally less dense and rises, while cooler fluid sinks, setting up circulation. This is a key idea in how weather systems form and in how a pot of water heats evenly.
- Radiation is transfer by electromagnetic waves, and it does not need any material in between. It is how warmth from the Sun crosses space to reach Earth. Part of that energy is later trapped by gases in the atmosphere, a process described in the greenhouse effect.
You can feel all three near a campfire: the air above it rises by convection, a metal poker conducts heat to your hand, and your face feels the radiation.
Temperature scales
People commonly use the Celsius and Fahrenheit scales. On Celsius, water freezes at 0 degrees and boils at 100 degrees at standard sea-level pressure. On Fahrenheit, those points are 32 and 212 degrees.
Scientists often use the kelvin scale, written K without a degree sign. A change of one kelvin is the same size as a change of one degree Celsius, but the zero point is different. To convert, add 273.15 to a Celsius value. So water freezes at about 273 K and boils at about 373 K. Using kelvins avoids negative temperatures in equations, because the scale starts at the lowest temperature that can exist.
What absolute zero is
Absolute zero is 0 K, which equals −273.15 degrees Celsius or about −459.67 degrees Fahrenheit. The idea emerged from the observation that the pressure of a gas drops as it is cooled, and that, extended backward, there should be a temperature at which the particle motion is as low as it can get.
Absolute zero is not simply “the particles stop.” Quantum physics shows that particles retain a minimum amount of motion, called zero-point energy, even in the lowest-energy state a system can have. Absolute zero is better described as the temperature at which a system has the least thermal energy it is allowed to have.
Why it can’t be reached
Physicists have shown that no real process can cool something all the way to absolute zero in a limited number of steps. This is usually stated as the third law of thermodynamics. Each cooling step removes some energy, but the closer you get, the harder it becomes to remove more. Laboratories have reached temperatures extraordinarily close to absolute zero, a tiny fraction of a kelvin, but never exactly zero.
Near these temperatures, matter can behave strangely. Some materials lose all electrical resistance, and certain atoms can form unusual states in which they act together as one. These effects depend on quantum physics, and ordinary ideas about solids, liquids and gases no longer capture everything. For the ordinary versions of those categories, see states of matter.
A sense of scale
Nothing in nature is known to be exactly at absolute zero. Even the emptiest regions of space are slightly warmer, at only a few kelvins, because they are bathed in faint radiation left over from the early universe. At the other extreme, there is no known upper limit to temperature in the same simple sense, though extreme conditions such as the interiors of stars reach millions of degrees.
Understanding the gap between heat and temperature also helps in everyday decisions, from why a metal handle feels colder than a wooden one at the same room temperature (metal conducts heat away from your hand faster) to why insulation keeps a house comfortable. The same ideas, combined with the way energy is conserved, form the basis of thermodynamics.