Temperature is a measure of the ability of a body to transfer heat energy to another body. As heat is added to a body, its temperature rises, but the magnitude of the rise depends on what it is made of. To put it another way, it depends on its specific heat capacity (SHC).

Specific heat capacity describes heat capacity per kg, whereas the term 'heat capacity' can be used to describe an entire object.

The energy required to raise the temperature of a body of mass m (kg) with SHC c (Jkg^-1K^-1) by Δ T degrees (Kelvin) is:

Laws of thermodynamics

These deal with the relationships between heat and other forms of energy. Much of the early work was associated with the development of the steam engine where a detailed understanding of the relationship between heat and mechanical energy was crucial to efficiency. It has relevance in anaesthesia because of its application to gases.

First law

The change in internal energy of a system is equal to the heat added minus the work done by the system (Equation 2).

An alternative way of thinking of this is in terms of the law of conservation of energy which states that energy can be transformed (changed from one form to another), but cannot be created or destroyed, i.e. in an isolated system there is no change in total energy. It is also a statement of the older concept of the mechanical equivalence of heat, which states that motion and heat are mutually interchangeable and that in every case, a given amount of work would generate the same amount of heat, provided the work done is totally converted to heat energy.

Second law

Heat cannot spontaneously flow from a colder location to a hotter location.

Again, this arose from work with steam engines as a way of expressing that none of the systems were 100% efficient and that energy was always lost. The term entropy describes an overall state of 'disorder' or 'energy dispersal' which always tends to increase.

Third law

It is impossible by any procedure, no matter how idealized, to reduce the temperature of any system to zero temperature in a finite number of finite operations (Wikepedia).

At absolute zero, the entropy of a system is zero. However,it is impossible to achieve this as (from the second law) there is a tendency towards energy dispersal i.e. energy always leaks into the system.

Temperature

Temperature represents the tendency of heat to move to or from an object. It correlates with the kinetic energy of its molecules.

Temperature scales exploit known, repeatable points of reference such as the boiling and freezing points of water (Celsius scale). At -273.15 ^°C all molecular movement ceases and this is defined as absolute zero or 0^° Kelvin. Each degree on the Kelvin scale has the same magnitude as degrees on the Celsius scale.

Temperature sensors measure physical properties which change with temperature, e.g. the expansion of mercury or alcohol in a thermometer or the change in resistance of metal oxides in a thermistor.

Objects may gain or lose heat by conduction, convection, radiation and evaporative loss. For patients, conduction is usually unimportant, while radiation predominates with losses of around 50 Wm² for an adult male.

Mechanical equivalent of heat

The principle of the mechanical equivalence of heat states that heat and mechanical work are two forms of energy and can be converted into each other in a quantifiable way. This principle was first established by James Prescott Joule in the mid-19th century. In his famous experiment, he used falling weights to rotate paddles inside a container of water. The mechanical energy from the rotating paddles was transferred to the water, causing its temperature to rise. Joule found that a consistent amount of mechanical work was needed to raise the temperature of the water by a specific amount

The principle can be mathematically expressed as: