TEMPERATURE SCALES

In establishing a temperature scale, two fixed points are normally chosen as a reference. For example the points at which pure water freezes and boils. In the Centigrade system, the scale is divided into 100 graduated increments, known as degrees (0), with the freezing point of water represented by 00C and the boiling point 1000C. The Centigrade scale was renamed the Celsius scale after the Swedish astronomer Anders Celsius who first described the centigrade scale in 1742.

In another system, the Fahrenheit system, water freezes at 320F and boils at 2120F. The difference between these two points is divided into 180 increments. To convert Fahrenheit to Celsius, remember that 100 degrees Celsius

represents the same temperature difference as 180 degrees Fahrenheit.

Therefore, as 00C is the same as 320F it is first necessary to subtract 320 from the Fahrenheit temperature and then to either divide the result by 1.8, or multiply it by 5/9. 0 C = (0F - 32)  1.8 or 0C = 5/9 (0F - 32) Example 1: To convert 770F to Celsius 77 – 32 = 45 x 9 5 = 25ºC

To convert Celsius to Fahrenheit, you must multiply the Celsius temperature by 1.8 or, in other words, 9/5, and then add 320.

0 F = (1.8 x 0C) + 32 or 0F = (9/5 x 0C) + 32 Example 2: To convert 450C to Fahrenheit 45 x 5 9 = 81 + 32 = 1130C

JAR 66 CATEGORY B1 MODULE 2

PHYSICS

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uk

In 1802, the French chemist and physicist Joseph Louis Gay Loussac found that when you increased the temperature of a gas by one degree Celsius, it expands by 1/273 of its original volume.

Based on this, he reasoned that if a gas were cooled, its volume would decrease by the same amount. Therefore, if the temperature were decreased to 273

degrees below zero, the volume of a gas would decrease to zero and there would be no molecular activity. This point is referred to absolute zero. On the Celsius scale, absolute zero is –2730_{C. On the Fahrenheit scale it is –460}0

F.

Many of the gas laws relating to heat are based on conditions of absolute zero. To assist working with these terms, two absolute temperature scales are used. They are the Kelvin scale, which is based on the Celsius scale and the Rankine scale, which is based on the Fahrenheit scale. The relationship of the four scales can be seen in the chart below but the main points to remember are the following:

Fig 6.1 Temperature Comparison Chart Example 3: Convert 15ºC to Kelvin

15 + 273 = 288K

Note also that when thermodynamic principles and calculations are considered, it is usually vital to perform these calculations using temperatures expressed in Kelvin. The size of the units on the Kelvin and Celsius scales are the same. Note also that 0ºK is often termed absolute zero (it is the lowest temperature theoretically possible).

Issue 1 – 20 August 2001 Page 6-3 JAR 66 CATEGORY B1 MODULE 2 PHYSICS

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Heat is a form of energy. Heat is energy in the process of transfer between a system and it’s surroundings as a result of temperature differences. If two bodies, at different temperatures, are bought into contact, their temperatures become equal. Heat causes molecular movement, which is a form of kinetic energy and, the higher the temperature, the greater the kinetic energy of its molecules.

Heat is one of the most useful forms of energy because of its direct relationship with work. When the brakes on an aircraft are applied, the kinetic energy of the moving aircraft is changed into heat energy by the brake pad friction against the brake discs. This slows the wheels and produces additional friction between the wheels and the runway, which finally, slows the aircraft.

Petrol, diesel and gas turbine engines are forms of heat engines that burn fuel that produces heat that can be converted into mechanical energy.

Many different effects can be produced by the application of heat to a body:  Changes in chemical constitution

 Changes in electrical properties  Increase in temperature

 Increase in physical size  Changes in state

Thus when two bodies come into contact, the kinetic energy of the molecules of the hotter body tends to decrease and that of the molecules of the cooler body, to increase until both are at the same temperature.

There is a transfer of energy from the hotter to the cooler body and energy

transferred in this way is called heat. It must be emphasised that the term heat is applied ONLY to energy in transit and cannot describe stored energy. Heat transfer can occur in three ways, conduction, convection and radiation 6.3 HEAT CAPACITY AND SPECIFIC HEAT

In our introduction to heat, we discussed the difference between temperature and heat. Temperature is the degree of hotness of a body. Large dense objects are normally capable of absorbing large quantities of heat. We use the term Heat Capacity to describe the amount of heat energy contained within a body. In order to produce a change in temperature in a body, heat energy must be supplied to it or removed from it.

JAR 66 CATEGORY B1 MODULE 2 PHYSICS

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uk

Different materials require different amounts of heat to produce the same temperature rise.

The Specific Heat of a substance is defined as the heat (energy) required to raise the temperature of a unit mass of the substance by one degree.

The units concerned are:

Energy Joule J

Mass Kilogram kg

Temperature Kelvin K

So the Specific Heat of a substance will be identified in J/kg/K

The following table gives the Specific Heat of a number of typical substances including water:

Material Specific Heat J/kg/K

Lead ₁₂₇ Mercury 139 Zinc 386 Copper 389 Steel 481 Aluminium 908 Water 4200

Fig 6.2 Specific Heat of various materials