The quantity that tells how warm or cold an object is with respect to some standard is called temperature. The temperature of an object is expressed by a number that corresponds to a certain degree of hotness on a chosen scale. In the United States, we mostly use the Fahrenheit scale. In this scale, 32 degrees corresponds to the freezing point of water, and 212 degrees to the boiling point of water. Most of the rest of the world uses the Celsius scale, with 0 degrees and 100 degrees corresponding to the freezing and boiling points of water. Kelvin is a third temperature scale. 0 degrees on the Kelvin scale is called absolute zero-the place where all the molecules of a substance stop moving. The temperature of a substance is proportional to the average kinetic energy of the molecules of the substance.

Consider a hot stove. If you touch the stove, energy flows from the stove to your hand. Now consider an ice cube. If you touch it, energy passes out of your hand to the ice. The direction of the spontaneous energy transfer is always from a warmer substance to a colder substance. Energy transferred from one thing to another because of temperature difference is called heat. An object cannot contain heat, just like an object cannot contain work. Heat is a transfer of energy, not energy itself.

Internal energy is the grand total of all energy inside an object. This includes kinetic, potential, rotational, and pressure energy. When an object absorbs or gives off heat, its internal energy changes. If it gives off heat, its molecules slow down. If it absorbs heat, its molecules will move faster. How much temperature change an object undergoes when it absorbs a given amount of heat depends on the amount of the substance. A large bucket of water will have a lower temperature than a small bucket of water after they absorb the same amount of heat. This is because the large bucket of water has more molecules. So each molecule gains less kinetic energy, so the average molecular kinetic energy is lower.

Since heat is a form of energy, it can be measured in Joules. A more common unit of heat measurement is the calorie. A calorie is the amount of heat required to change the temperature of one gram of water by one Celsius degree. This calorie is not to be confused with a food Calorie (spelled with a capital "C"). A food Calorie is equal to one thousand heat calories. It is a kilocalorie.

Different substances have different capacities for storing internal energy. Water on a stove may take 10 minutes to raise from room temperature to boiling temperature. Iron on the stove might take 1 minute to go through that same temperature range. This difference in the amount of energy needed to heat something occurs because different substances absorb heat in different ways. In some substances, heat makes the molecules of the substance move more, increasing the kinetic energy and therefore the temperature of the substance. In other substances, heat may increase internal vibration, which increases potential energy and doesn't increase temperature. We call the amount of energy an object absorbs to change its temperature its specific heat capacity. The specific heat capacity of any substance is defined as the quantity of heat required to change the temperature of a unit mass of the substance by one degree Celsius. So water has a higher specific heat capacity than iron.

When the temperature of a substance is increased, its molecules and atoms move faster and tend to move further apart. The result of this is an expansion of the substance. With a few exceptions, all matter expands when heated and contracts when cooled. In many cases the changes in size of substances are not very noticeable, but liquids expand appreciably with increases in temperature. In most cases, the expansion of liquids is greater than the expansion of solids. In a thermometer, mercury expands more than glass on a hot day, so the mercury rises.

Most liquids expand with increasing temperature. But not water! At 0 degrees Celsius, water contracts when heat is added. It continues to contract until the temperature of the water reaches 4 degrees Celsius. Then it starts to expand.

This explains why water freezes from the top down. If the density of water was the greatest at its freezing point, the cold water would sink to the bottom and water would freeze from bottom up. But water at 4 degrees Celsius is the densest, so it sinks to the bottom while colder water remains on top, freezing therefore from top down.

Heat is conducted. Hold one end of a nail in a fire and the other end will heat up. The fire causes molecules at the front of the nail to increase their kinetic energy. They bump into molecules and electrons next to them, which bump into other molecules and electrons, and this bumping transfers the heat from one end of the nail to the other. How well an object conducts heat depends on its molecular structure. Metals with loose outer electrons conduct heat well. Substances with strongly attached outer electrons don not conduct heat well. Poor conductors are called insulators.

Liquids and gases transmit heat by a process called convection, which is heat transfer by the actual motion of the fluid-currents. Convection can occur in all fluids, both liquids and gases. Here's how it works: If the fluid is heated from below, the molecules at the bottom increase their speed; the heated fluid becomes less dense than the surrounding fluid and is buoyed up (remember Archimedes!). The colder fluid moves to the bottom to take the heated fluids place.

Rising warm air expands because atmospheric pressure on it decreases. as it expands, the air cools. This is because the molecules of air are having less head-on collisions. Remember momentum-If two molecules are moving toward each other when they hit, a greater impulse is imparted. A greater impulse means a greater speed, which means a greater temperature. But expanding air molecules only collide with molecules moving away from them. This makes a lesser impulse, which makes a lesser speed, which makes a lesser temperature.

Heat from the sun reaches earth through radiation. Radiant energy is in the form of electromagnetic waves. All objects emit radiation. The wavelength of the radiated waves depends on the temperature of the object. Higher temperature objects emit shorter wavelengths.

Absorption and reflection are opposite processes. So, a good absorber is not a good reflector. A perfect absorber reflects no radiant energy and appears black. a perfect reflector absorbs no radiant energy and appears white. Emission is the re-radiation of absorbed electromagnetic waves. Good absorbers are good emitters.

An object at a different temperature from its surroundings will eventually come to a common temperature with its surroundings. The rate of cooling of an object depends on how much hotter the object is than the surroundings. A bowl of hot soup cools at a faster rate than a bowl of luke-warm soup. The rate of cooling of an object is proportional to the temperature difference between the object and its surroundings, or rate of cooling~T. This is known as Newton's Law of cooling. It also holds for cold substances heating up.

The sun radiates short wavelength radiation to Earth, which warms the Earth. In turn, the earth radiates out long wavelength radiation. This cools the earth. So the earth would not be a hot enough place to live on except for one thing-the greenhouse effect. The atmospheric gases around earth do not absorb short wavelength radiation like that the sun gives off. It does absorb, and then re-radiate, long wavelength radiation that the earth gives off. So heat is trapped on the earth, making it hot enough to live on.