Heat is a form of energy associated with temperature and is more correctly termed thermal energy. In fact the term heat is used to describe the transfer of thermal energy from one substance to another. For example, the end product of a radiator is the production of thermal energy, however, when that energy is transferred from the radiator to the surrounding environment, it is felt as heat or indirectly measured as a change in temperature.
So what is heat? And what are the properties of heat?
Essentially, heat is the total amount of the kinetic energy produced by the random vibrating motion of molecules and atoms within substances and the measured temperature is the amount of energy per molecule. This relatively straight forward distinction between heat and temperature makes it easier to see that the more particles on object has, the more heat or thermal energy it contains but that doesn’t necessarily mean it has a higher temperature. So the more vibration or kinetic energy particle have, the more heat there is. The less kinetic energy there is, the less the particles will vibrate.
Incidentally, this also goes some way to explaining why substances expand and contract when heated up or cooled down. Depending on the substance in question, the vibration can be so great that individual particles begin to escape and this is what takes place during evaporation or boiling of a liquid for example. In general, the forces that hold molecules together in solids are stronger than in liquids so require a lot more heat for them to escape, instead they separate taking up more space but still remain bound toegther. The forces between gas molecules are much weaker and therefore require much less heat to exist in this liberated state.
Since heat is thermal energy, that energy can be transferred, however, one of the properties of heat is that it always flows from hot to cold until it reaches a state of equilibrium. For example, the heat from a hot drink transfers into the cup in which it is held and into the surrounding environment until the temperature of the drink and the cup are the same as the environment. The hotter drink cools down and the cooler environment becomes slightly warmer. Although the room itself has some heat, it is not the case that the heat from the room would transfer to hotter drink making it even hotter.
Heat energy itself can be transferred via three basic modes.
This is the transfer of heat energy by direct contact of substances. An example of this is placing a saucepan of hot water on a hot electric plate. The heat from the plate transfers directly from the plate to the saucepan.
This is the transfer of heat energy by diffusion or circulation of heated particles. Taking the saucepan example, the water molecules at the bottom of the pan would become hot and rise to the surface, the cool water molecules sink and become heated as they get closer to the heat source. This type of heat transfer also takes place in gases such as air in convection heaters where the air is warmed and circulated in a room.
This is a different from the other methods of heat transfer. All objects that have heat emit light in the infrared range; this infrared light is itself a form of energy. Some substances emit more infrared light than others but as light, it travels in straight lines and will travel through a vacuum. The light itself isn’t heat but it is a form of energy. When absorbed by other substances, it causes the molecules it strikes to vibrate more rapidly and in this way it is converted into heat energy.
Everything in the universe has some level of heat energy, even things that are considered to be cold. In fact cold is simply a relative term to describe something that has less heat than something else. Water ice at 0C is said to be cold but it does have heat, more heat than liquid nitrogen for example which is a liquid at between -195.8C and -210C.
In theory then, there should be some point at which a substance has no heat whatsoever, that is no energy or no temperature. In theory there is just such a temperature where a substance can be as cold as it can possibly get and that is absolute zero. On the Celsius scale, this would be -273.15C however, there is a better scale to use called the Kelvin scale. Absolute zero is 0K is equivalent to -273C.
It is impossible to cool a substance to 0K although close to it has been achieved. However, if it was possible to achieve a temperature of absolute zero, particles at that temperature would be completely at rest with no energy whatsoever and this would violate the laws of physics as we know them today.