Heat is energy in the form of particles moving in matter. This motion can cause a change in the internal energy of a body and transfer that energy to other bodies.
At a fixed pressure, the movement of particles in solids can increase until a stage is reached where the material changes from a solid to a liquid (melting). This process releases latent and sensible heat.
Radiation
Radiation is a method of heat transfer that uses electromagnetic waves. It is emitted by objects at temperatures higher than absolute zero and strikes the surrounding matter, which may be reflected, absorbed, or transmitted, depending on its reflectivity, absorptivity, and transmissivity respectively. Thermal radiation is found throughout the visible and infrared parts of the spectrum. It is proportional to the fourth power of the temperature of the object and has a characteristic spectrum whose shape depends on the emissivity of the material: a perfect emitter, jet black, has a value of 1, while tungsten light bulb filaments have an emissivity of about 0. Infrared radiation is also responsible for browning reactions in foods when it is used to cook.
When charged or uncharged particles of ionizing radiation interact with biologic tissues, they can ionize the atoms of molecules and disrupt chemical bonds, which leads to potentially irreversible biologic damage. The ionizing radiation must have enough energy, however, to penetrate to the cellular level and impact critical targets.
The biological effects of radiation depend on the type and duration of exposure and the nature of the radiation. Exposures can be external, such as from cosmic rays or radioactive materials, or internal, such as from medical and industrial sources. Regulatory dose limits have been established to ensure that an individual’s total exposure from regulated sources does not cause adverse health effects.
Gases and Liquids
While solids, liquids, and gases are all made of atoms or molecules that are tightly packed together, they have different properties. For example, a gas has no definite shape or volume and can expand to fill any container. Liquids are thicker than gases and can take on the shape of their container. They are also denser and can be squeezed. This is because the particles in a gas are widely spread out and cannot be compressed as easily as the tightly packed particles of a liquid or a solid.
When a solid is heated, its particles rattle around more and become less tightly packed. As they do this they lose some of their kinetic energy and begin to move more slowly. This is why when you heat water, it will boil. When the hot water cools, it will turn back into a liquid. A few solids (like dry ice, which turns directly to gas) have extra energy that allows them to jump straight from a solid state into a gas state without becoming a liquid first.
Both liquids and gases transfer heat by convection, which means the movement of the particles themselves transfers the heat. The particles in liquids and gases are farther apart than those in solids, which allows them to move more easily. They can also be compressed, which increases their density and causes them to move faster.
Solids
As solids get warmer, their tightly packed atoms and molecules vibrate more strongly. This movement takes up a greater area than the same number of atoms in a colder solid, causing the material to expand. This phenomenon is important for engineers, who make sure that bridges have gaps so that the metal can expand as the temperature rises.
Most solids are composed of atoms or molecules that are arranged in an orderly, repeating pattern known as a lattice. This class of solids is referred to as crystalline. Crystalline solids include common table salt (sodium chloride) and most metals. Noncrystalline solids are comprised of atoms and molecules that do not have a definite pattern, such as glass and plastics. Finally, quasicrystalline solids exhibit novel atomic symmetries not found in crystals.
Most solids, like liquids and gases, can change states as they are heated or cooled. When a solid is slowly heated, it may start to melt, or it might boil and turn into a liquid, such as water. If a solid is rapidly heated, it will likely turn into a gas. The rate of expansion for solids varies, depending on their chemical composition and structure. In particular, metallic solids display a different rate of thermal expansion than nonmetallic materials because of the difference in their bonding. The direction of a solid’s atoms also affects the extent to which it expands, a property known as anisotropy.
Pressure
The pressure exerted on a gas by its molecules colliding with each other and the walls of its container is a function of its average kinetic energy, which in turn is proportional to its temperature. The more kinetic energy the molecules have, the faster they will move and the more forceful each collision will be. This is why a tennis ball that is hit hard bounces off the wall with more force than one that is hit lightly.
When a gas is compressed (doing work on it) its molecules have less time to move between collisions with the walls of the container. This means that the molecules will collide with the walls more frequently and each collision will be more powerful. This is what causes the gas to heat up.
In a closed system where the volume of the gas is constant, the pressure is directly proportional to its Kelvin temperature. This principle is known as Boyle’s Law.
A more general description of pressure is its force per unit area, F / A. The SI unit for pressure is the Pascal, which is equal to 1 newton per square meter. It is also sometimes given the name bar (1 bar = 105 N/m2), millibar (1 mbar), or hectopascal (hPa). Pressure is usually measured with a manometer, which is a device that measures a liquid’s column of mercury.