Heat is energy in transit. It consistently flows from a substance of a higher temperature to a substance of a lower temperature. In this process, heat will raise the temperature of the latter and lower that of the former, given that the volumes of the respective substances remain constant.
Unless another form of energy transfer (work) is also present, heat will not flow from a lower to a higher temperature.
B. Temperature and Heat. History
Up until the 19th century, researchers grappling with the precise nature of heat believed that heat's effect upon a body's temperature was attributable to the invisible substance known as the calorie.
The "Caloric Theory of Heat" stated that bodies at high temperatures contained more caloric mass than those at low temperatures. Furthermore, it explained that bodies at low temperatures lost caloric mass on contact with bodies of higher temperatures.
And while the caloric theory did satisfy some curiosity where heat transfer was concerned, it was contested in studies conducted in the late 1700s by American-born British physicist Benjamin Thompson and British chemist Sir Humphry Davy, who proposed the notion that, "heat, like work, was a form of energy in transit."
During the mid-1800s, the British physicist James Prescott Joule presented conclusive evidence that, "heat was indeed a form of energy in transit and that it had the ability to cause the same changes in a body as work."
C. Temperature and Heat. Effects
Temperature defines sensations such as warmth and coldness and the degree to which they occur within a substance.
Though it would certainly make things easier if we could compare temperature solely on the basis of touch, this fails to provide an accurate evaluation with respect to what is known as the "absolute magnitude of the temperatures."
When heat is added to a substance not only does it increase the temperature of the substance, it also causes a change in physical properties. These changes can be measured and doing so allows researchers to compare and contrast different forms of energy and their reactions with different types of substances.
When temperature is applied to a substance, there is the potential for the following physical changes to occur:
- The substance may either expand or contract.
- The electrical resistance may shift.
- In the gaseous form, it may exert varying amounts of pressure.
Note: The variances that occur in a given substance serve as a basis for the creation of an accurate numerical temperature scale.
As a whole, temperature is contingent upon on an averaged measurement of the kinetic energy exhibited by the molecules of a substance. According to Kinetic Theory, energy may exist in three different motion-based forms: rotational, vibrational, and translational.
Temperature, on the other hand, depends solely on translational molecular motion. Theoretically speaking, at the temperature referred to as absolute zero, the molecules of a substance exhibit no activity, they do not move.
D. Temperature Scales
At the present time, five different scales are employed to present accurate readings of the temperature of a substance.
- Celsius Scale (also known also as the Centigrade Scale) is widely accepted throughout the world as a standard measuring scale, it defines the freezing point of water as 0°C and the boiling point as 100°C.
- Fahrenheit Scale primarily is used in English-speaking countries for purposes other than scientific work. Based on the mercury thermometer, the freezing point of water is defined as 32°F and the boiling point as 212°F.
- Kelvin Scale is the most commonly used thermodynamic temperature scale, whereby zero is defined as the absolute zero of temperature, which is -273.15°C or -459.67°F.
- Rankine Scale is akin to the Celsius Scale, it also uses absolute zero as its lowest point. On the Rankine Scale, each degree of temperature is equivalent to one degree on the Fahrenheit scale. The freezing point of water on the Rankine scale is 492°R, and the boiling point is 672°R.
- International Thermodynamic Temperature Scale (ITTS) in the year 1933, based upon the Kelvin Scale and thermodynamic principles, scientists of 31 nations adopted a new international temperature scale with additional fixed temperature points. Based on the property of electrical resistivity, platinum wire serves as the ITTS standard for temperature between -190° and +660°C. The thermodynamic temperature is defined by the second law of thermodynamics.
Note: For temperatures in excess of 660°C reaching upwards to the melting point of gold which is 1063°C, a standard thermocouple (a device that measures temperature by the amount of voltage produced between two wires of different metals) must be used. Any temperatures beyond this point are measured by what is known as an "Optical Pyrometer," which uses the intensity of a wavelength of light for measurement purposes.
E. Heat Units
The principle method for measuring heat has been termed as the calorie. Defined, a calorie is the amount of heat necessary to raise the temperature of 1 gram of water at a pressure of 1 atm from 15° to 16°C. The terms large and small have been added to distinguish a "large" calorie (kilocalorie = 1000 cal used in nutritional matters) from a "small" or gram calorie (S.I.).
In mechanical engineering practices in both the United States and the United Kingdom, heat is measured in British thermal units, or Btu. One Btu is the quantity of heat required to raise the temperature of 1 pound of water 1°F. One Btu is equal to 252 cal.
It is possible to convert mechanical energy into heat by way of friction. The mechanical work necessary to produce 1 cal is known specifically as the "Mechanical Equivalent of Heat." It is equal to 4.1855 x 107 ergs/cal or 778 ft-lb Btu.
F. Law of Conservation of Energy
The Law of Conservation of Energy states that all the mechanical energy exerted to produce heat by friction appears as energy in the objects on which the work is performed.
The English physicist James Prescott Joule was the first to conclusively prove this statement when he conducted an experiment in which he heated water in a closed vessel by means of rotating paddle wheels and found the rise in water temperature to be proportional to the work exerted in the turning of the wheels.
The same principle applies when heat is converted into mechanical energy (an example being the internal combustion engine). In any type of machinery, however, some amount of energy will always be lost or dissipated in the form of heat because engineers have not yet found a way to build a totally efficient engine.
G. Phase Changes
Along with changes in temperature in a substance comes a range of physical changes. For instance, when heat is applied, the majority of substances expand in volume; when cold is applied to a substance, the majority contract in volume. The notable exception to this rule is water between 0°and 4°C (32°and 39°F), where water expands as it cools from 4°C to 0°C.
The term phase refers to a substance's presence as one of the three forms: a solid, liquid, or gas. Changes in phase (at least in pure substances) occur at set temperature and pressure levels. This is what is known as the Phase Rule.
As we have stated in previous chapters, the process of changing from solid to gas is known as sublimation; going from a solid to a liquid is melting; and moving from a liquid to a vapor is vaporization.
Should the levels of pressure remain constant then these processes will occur at consistent temperatures. The amount of heat required to produce a phase change is referred to as latent heat.
When heat is added to a substance, yet fails to change its temperature, this heat is not lost but rather absorbed. This is latent heat and when not contributing to changing the phase of the substance is stored for later use.
H. Specific Heat
The heat capacity, or the amount of heat required to raise the temperature of a unit mass of a substance by one degree, is referred to as specific heat. In instances when the heating process occurs while the substance remains at a constant volume or is subjected to a constant pressure, the measure is then referred to as, "specific heat at constant volume or at constant pressure."
Generally speaking, the specific heat of a substance is contingent upon the temperature.
I. Transference of Heat
There are two primary methods by which energy (in the form of heat) can be transferred between bodies, conduction and radiation. There is also a third method known as convection, which is akin to the first two in that it involves the motion of matter.
Conduction requires physical contact between the bodies (or parts of bodies) exchanging heat, whereas radiation requires neither this type of contact nor the existence of any matter between the bodies.
In contrast, convection occurs when a liquid or gas comes in contact with a solid body of a different temperature. This type of transference is always accompanied by the motion of the liquid or gas.