Thermodynamics

Thermodynamics is the branch of Physics that deals with the conversion of heat into other forms of energy, or other forms of energy into heat.

Thermodynamics Definitions Absolute Pressure – The pressure measured above a perfect vacuum. It is the sum of atmospheric pressure and gauge pressure and is expressed as KPa (abs) or psia. Absolute Zero – The temperature at which all molecular vibrational motion ceases Atmospheric Pressure - The pressure exerted by the earth’s atmosphere. This is expressed Kpa, mm of mercury or meters of water. The standard atmospheric pressure is the average atmospheric pressure at sea level ( Kpa)

Thermodynamics Definitions Change of State – The altering of a substance from solid to liquid or liquid to gas or visa versa. Conduction – The flow of heat from molecule to molecule within a substance or from molecules of one body to those of another body in direct contact with it. Convection – The transfer of heat within a fluid by movement of the fluid whereby warm fluid is displaced by cooler fluid.

Thermodynamics Definitions Cooling – The process of removing heat from a substance, resulting in a decrease in temperature and/or a change of state. Density – The mass per unit volume of a substance expressed in Kg/m 3. Enthalpy or Heat Content – The amount of heat expressed in KJ/Kg, contained in a substance, relative to a base temperature at which the Enthalpy is determined to be zero. Gauge Pressure – The pressure in a closed vessel as registered on a pressure gauge. This is pressure above atmospheric pressure. It is expressed as kPa (gauge).

Thermodynamics Definitions Heat – This is a form of energy which when supplied to a body or substance will increase the internal energy of that body or substance. Latent Heat – Heat that causes a change of state of a substance without changing it’s temperature. Latent Heat of Evaporation – The amount of heat required to change a unit mass of a substance from liquid to vapor without changing it’s temperature. Latent Heat of Fusion - The amount of heat required to change a unit mass of a substance from solid to liquid without changing it’s temperature.

Thermodynamics Definitions Radiation – The transfer of heat by electromagnetic waves causing a rise in the temperature in the body that they strike by increasing the motion of the molecules of that body. Saturated Steam – Steam that is fully saturated with latent heat and has no water particles present. Saturation Temperature – The temperature at which a liquid reaches it’s boiling point. This temperature depends upon the atmospheric pressure.

Thermodynamics Definitions Sensible Heat – Heat that causes a change in the temperature of a substance without changing it’s state. Specific Heat – The amount of heat required to raise the temperature of a unit mass substance 1 0 C without changing the state of the substance. Specific Volume – the volume of a unit mass of a substance expressed in m 3 /kg. For gases this will depend on the temperature and pressure of the gas. Super Heat – The temperature increase of the gas or vapor above it’s saturation temperature after all the liquid has been evaporated.

Thermodynamics Definitions Vacuum – The reduction of pressure below atmospheric pressure. Wet Steam – Steam that does not have it’s full quantity of latent heat energy and has water present.

Laws of Thermodynamics The First Law of Thermodynamics – Heat and work are mutually convertible. Work = Heat In practice it is usually the case that only part of the heat is converted to work, or only part of the work generates heat. Q = delta U + W.D

Laws of Thermodynamics The Second Law of Thermodynamics – Unaided heat will only flow from a hot substance to a colder substance. If it is required to transfer heat from a cold substance to a hotter substance (as in refrigeration) then external work must be applied.

Temperature Temperature is the measure of the speed at which the body’s molecules vibrate. A high Temperature indicates a increased molecular velocity while a low Temperature indicates a decreased molecular velocity.

Temperature There are four different temperature scales in common use. –Fahrenheit – Freezing point of water is 32 F and Boiling point is 212 F –Rankine – 0 o is absolute zero which is – 460 F –Celsius - Freezing point of water is 0 C and Boiling point is 100 C –Kelvin – 0 0 K is absolute zero which is -273 C

Measuring Temperature Liquid in Glass Thermometers – A thick walled glass tube with a small bore and a bulb on one end is filled with a liquid (Alcohol or mercury) at the highest temperature of the thermometer and then is sealed. When the liquid is cooled it creates a vacuum and settles in the bulb.

Measuring Temperature Bimetal Thermometers – Two dissimilar metals are rigidly fixed together. Due to their different coefficients of linear expansion the bimetal strips will bend when subjected to temperature changes. This movement can be transmitted to a pointer on a graduated scale by linkage.

Measuring Temperature Pyrometers – Is an instrument made for measuring temperatures in the range above the suitable for the mercury thermometer. –Thermoelectric Pyrometer – makes use of thermocouple that produces an electric voltage which is proportional to temperature. –Optical Pyrometer – is suitable for measuring extremely high temperatures. Its operation is bases on the fact that the intensity of light emitted from a hot surface will very with the temperature of the surface.

Heat Mechanical Equivalent of Heat – One Joule is equivalent to the work done by the force of one Newton moving through a distance of one meter in the direction in which the force is applied. 1 Nm = 1 J In other words the unit of work (Nm) is equal to the unit of Heat (J).

Heat Heat Quantities – The quanity of heat absorbed depends upon 3 factors –The temperature rise –The mass of the substance –The Specific heat of the substance Q = mc(t 2 - t 1 ) Q – Heat absorbed by the substance (kJ) m – mass of the substance (kg) c - specific heat of the substance (kj/kg 0 C) T 1 – temperature of the substance before heating( 0 C) T 2 – temperature of the substance after heating ( 0 C)

Heat Specific Heat of common substances: –Water kJ/kg 0 C –Ice kJ/kg 0 C –Copper kJ/kg 0 C –Aluminum kJ/kg 0 C

Linear Expansion of Solids Practically all solids will expand when their temperature increases. If a rod or pipe of a given length is raise in temperature, it’s increase in length will be directly proportional to its initial length and to the rise in temperature. Coefficient of linear expansion – The change in length per unit length per degree rise in temperature.

Volumetric Expansion of Solids When a body is heated it will expand along all its dimensions. That is, it’s linear dimensions of length, width and height will all increase due to the increase in temperature.

Expansion of Liquids With the exception of water, all liquids will expand in direct proportion to their change in temperature when heated, and will contract similarly when cooled. Water is a special case, when cooled, it’s volume decreases until a temperature of about 4 0 C is reached. At this point there is a large increase in volume as the water cools further to 0 0 C and turns to ice.