Heat is a form of energy via molecular motion. In physics, heat is described as the most random, disordered, "degenerate" or "lowest" of all forms of energy because the natural tendency of all forms of energy is to become heat. Along with work, heat is a quantity that is a product of an energy transfer across the boundary of a system.

Examples: Electricity moving through a wire tends to heat the wire. Energy of motion used in transportation tends to become heat energy through friction.

## Properties of Heat

### Heat Transfer

As stated above, other forms of energy "naturally" tend to become heat. However, once energy has become heat, it requires specific engineering measures to convert it back into other forms. (Electricity moving in wires tends to become heat on its own. On the other hand, if we want to convert heat into electricity, we need to construct elaborate turbine generator or thermoelectricW systems.)

In devices and systems which are not specifically intended for heating or cooling, we maximize efficiency by minimizing the amount of energy converted to heat (lost or wasted). For example, the more efficient an electric motor or internal combustion engine, the less heat it produces. However, there are theoretical limits to the efficiency of all types of energy-transfer and energy-conversion processes, and some energy is always converted to heat (lost or wasted) through friction, electrical resistance, etc. 100% efficiency is not possible in the real world.

Heat always naturally tends to flow from a warmer area to a cooler one, never the reverse. However, by careful construction techniques and the application of additional energy (energy input), we may artificially force heat to move from a cooler area to a warmer area. Thermal insulation is often useful for minimizing unwanted heat transfer.

From the point of view of physics "cold" does not really exist - it is simply the absence of heat in an area (or less heat in an area than in another that we perceive as warmer.)

This is similar to the way that "darkness" does not "really" exist, but is only the absence of light. If we want to make an area darker, we can prevent light from entering, but we can't "add more darkness". If we want to cool an area, we can move heat out, but technically we can't "add cold". Adding cold materials (for example, adding ice to a drink) simply acts to absorb and equalize the heat which is already present in the material.

Things which we perceive as "cool" or "cold" contain less heat energy than others which we perceive as warmer or hotter, but all materials above the temperature "absolute zero"W contain some heat energy.

This is the principle of the heat pump: heat is moved from one area to another. If heat is transferred to an area, it will get warmer, and we say that the heat pump is being used for heating. If heat is transferred from an area (lowering the amount of heat energy there), it will get cooler, and we say that the heat pump is being used for cooling (as in refrigeration or air conditioning.)

### Heat Capacity and Specific Heat

Heat capacity can be defined as the amount of heat energy necessary to change the temperature of a substance. It can be described by the following equation:

C = Q/ΔT

Specific heat is more frequently used in calculations; it is the heat capacity per unit mass of a substance, and every unique substance has its own unique specific heat value. In other words, it is the amount of heat energy necessary to raise the temperature of 1 g of a substance by 1 degree Celsius. In this context, generally speaking, specific "anything" means something per unit mass.

Specific heat is often described using the equation: Q=mcΔT Where m is the mass, c is the specific heat, Q is heat, and ΔT is the change in temperature in Celsius.

Additionally, the specific internal energy of a substance can be considered as a function of temperature and specific volume. Specific enthalpy can be defined as $\displaystyle{ h=u+Pv }$, where:

Variable Defined
h specific enthalpy
u internal energy
P pressure
v specific volume

### Sensible and Latent heat

When heat is added to a substance, it can be categorized by its effect on the substance. Sensible heat refers to the heat necessary for raising a substance's temperature. Latent heat refers to the heat necessary to change the phase of a substance.

Example: When boiling water, the energy required to raise the water temperature to 100 C is sensible heat. When the water reaches 100 C, additional energy doesn't raise the temperature, but instead converts the water from liquid to vapor, so that additional energy is classified as latent heat.