حرارة وديناميكا حرارية المحاضرة الأولى د/عبدالرحمن لاشين قسم الفيزياء - كلية العلوم التطبيقية – جامعة أم القرى - المملكة العربية السعودية قسم الفيزياء - كلية العلوم – جامعة المنصورة – جمهورة مصر العربية

Temperature and thermometers

Temperature A basic physical quantity Temperature is the degree of hotness and coldness of an object. S.I. Unit for temperature is Kelvin (K). Measure by an instrument called thermometer

Thermometer Thermometer is a device used for measuring temperature, especially of the air or in a person's body. Thermometer is make use the fact that some physical properties of matter change proportionate to temperature change

Liquid thermometer The basic principle of liquid thermometer is liquid expansion. Liquid will expand if it is heated, and contract if it is cold. The expansion and contraction of liquid are used to make sign. The process make sign on the thermometer is namely calibration.

Liquid thermometer is make by capillary pipe, In the bottom of capillary pipe there are bigger container to save liquid. The liquid that usually used to fill thermometer is mercury or alcohol. Why are mercury or alcohol used to fill thermometer? Can water used to fill thermometer?

Make Liquid thermometer. ExpansionContraction HeatedCold

Calibration Putting a scale on the thermometer is done in three stages: 1. Make lower point. Immerse the thermometer in a mixture of ice and water and mark the point at the thermometer as lower fixed point. 2. Make upper point. Immerse the thermometer in a steam at one standard atmosphere (Pa= 760 mm Hg) and again mark the point indicated as upper fixed point. 3. Make Scales. (fundamental interval) Divide the distance between the lower point and upper point into equal portions each of them is called fundamental interval

Definitions The lower fixed point or ice point is the temperature at which pure ice melts at standard pressure. The upper fixed point or steam point is the temperature at which pure water boils at standard pressure.

Temperature scale Different scales can be used in the measurement of temperature, such as: – Celsius scale – Kelvin – Fahrenheit scale.

Celsius scale 2 fixed points at temperatures (0 o C and 100 o C) are chosen – ice point (0 o C) – steam point (100 o C) divide the range between ice point and steam point by 100 equal divisions

Calibrating a thermometer

Thermometers and Temperature Scales Household liquid-in glass thermometer A liquid-in-glass laboratory thermometer

Different type of thermometers liquid-in-glass thermometer – mercury-in-glass – alcohol-in-glass – Why not use water ??? Other materials which can vary with temperature – resistance, current, length, color, infra-red

Liquid-in-glass thermometer – Capillary tube ensure that a small change in volume causes a large change in length – Alcohol range –115 o C to 78 o C – Mercury range –39 o C to 357 o C

Mercury-in-glass thermometer Advantages – Expands evenly on heating – Responds quickly to temperature – A high boiling point, so used in hot places – It does not wet Disadvantages – Poisonous – Expensive – A high freezing point, so not used in cold places

Alcohol-in-glass thermometer Advantages – Expands about six times of mercury – Expand evenly on heating – A low freezing point, so used in very cold places – It is safe – It is cheap Disadvantages – It is dyed – It wet the tube – It does not respond quickly with temperature – A low boiling point, so not used in hot places

Constant-Volume Gas Thermometer The physical property in this thermometer is the variation of pressure of a fixed volume gas as its temperature changes. The volume of the gas is kept constant by raising or lowering the reservoir B to keep the mercury level at A constant. The pressure is indicated by the height difference between reservoir B and column A.

Constant-Volume Gas Thermometer The thermometer is calibrated by using a ice water bath and a steam water bath. The pressures of the mercury under each situation are recorded. – The volume is kept constant by adjusting A. The information is plotted.

Constant-Volume Gas Thermometer To find the temperature of a substance, the gas flask is placed in thermal contact with the substance. The pressure is found on the graph. The temperature is read from the graph.

Absolute Zero The thermometer readings are virtually independent of the gas used. If the lines for various gases are extended, the pressure is always zero when the temperature is – o C. This temperature is called absolute zero. Absolute zero is used as the basis of the absolute temperature scale. The size of the degree on the absolute scale is the same as the size of the degree on the Celsius scale. To convert: T C = T –

Absolute Temperature (Kelvin) Scale The absolute temperature scale is now based on two new fixed points. – One point is absolute zero. – The other point is the triple point of water Def. The triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance coexist. The triple point of water occurs at 0.01 o C and 4.58 mm of mercury. This temperature was set to be on the absolute temperature scale. – The units of the absolute scale are kelvin.

Absolute Temperature Scale. The absolute scale is also called the Kelvin scale. – Named for William Thomson, Lord Kelvin The triple point temperature is K. – No degree symbol is used with Kelvins. The Kelvin is defined as 1/ of the difference between absolute zero and the temperature of the triple point of water.

Fahrenheit Scale A common scale in everyday use in the US – Named for Daniel Fahrenheit Temperature of the ice point is 32 o F. Temperature of the steam point is 212 o. There are 180 divisions (degrees) between the two reference points.

Comparison between thermometer scales Celsius Fahrenheit Kelvin UFP LFP t o C t o F tK

The Relationship scale

Comparison of Scales Celsius and Kelvin have the same size degrees, but different starting points. T C = T – Celsius and Fahrenheit have different sized degrees and different starting points. To compare changes in temperature Ice point temperatures 0 o C = K = 32 o F Steam point temperatures 100 o C = K = 212 o F

RESISTANCE THERMOMETR The physical property is the electric resistance which varies with temperature The temperature is calculated from the formula: Where R 0, R  and R 100 are the resistance at temperatures 0, 100 and  respectively 

Thermal Equilibrium Two bodies are said to be at thermal equilibrium if they are at the same temperature. This means there is no net exchange of thermal energy between the two bodies. The top pair of objects are in contact, but since they are at different temps, they are not in thermal equilibrium, and energy is flowing from the hot side to the cold side. hotcold heat 26 °C No net heat flow The two purple objects are at the same temp and, therefore are in thermal equilibrium. There is no net flow of heat energy here.

Zero th Law of Thermodynamics Zeroth Law of Thermodynamics - If A is in thermal equilibrium with B, and B is in thermal equilibrium with C, then C is also in thermal equilibrium with A. Allows for the use of a thermometer to measure temperature.

Expansion Many substances expand when heated When heated, the particles (atoms or molecules) in a substance vibrate more This means they need more space So the substance expands Heat

Important note! The particles in a substance do not expand themselves! Heat Particles are the same size

Contraction The opposite happens when most substances are cooled The particles vibrate less They need less room The substance contracts Cools

34 Most solids and liquids expand when heated. Why? Internal Energy U is associated with the amplitude of the oscillation of the atoms Average distance between atoms Inter-atomic forces  “springs”

Solids The expansion of solids is small but strong! Like me!

Expansion in solids This means that we sometimes have to take into account expansion

Linear Expansion Assume an object has an initial length L. That length increases by  L as the temperature changes by  T. We define the coefficient of linear expansion as A convenient form is  L =  L i  T This equation can also be written in terms of the initial and final conditions of the object: L f – L i =  L i (T f – T i ) The coefficient of linear expansion, , has units of ( o C) -1 L LL

Some Coefficients Section 19.4

Area Expansion The surface area and volume also change with a change in temperature. The change in area is proportional to the original area and to the change in temperature:  A = 2  A i  T Areal expansion – solid cube A o = L o 2 A = L 2 = (L o +  L o  T) 2 = L o 2 (1 +   T) 2 A = L o 2 (1 + 2   T +  2  T 2 ) A= L o 2 (1 + 2   T) (ignoring higher order terms) A - A o =  A = 2  L o 2  T A AoAo

Volume Expansion The change in volume is proportional to the original volume and to the change in temperature.  V =  V i  T –  is the coefficient of volume expansion. – For a solid,  3  This assumes the material is isotropic, the same in all directions. – For a liquid or gas,  is given in the table Section 19.4 VoVo V

41 Volume expansion – solid cube V o = L o 3 V = L 3 = (L o +  L o  T) 3 = L o 3 (1 +   T) 3 V = L o 3 (1 + 3   T + 3  2  T 2 +  3  T 3 ) V = L o 3 (1 + 3   T) (ignoring higher order terms) V - V o =  V = 3  L o 3  T =  V o  T  coefficient of linear expansion  coefficient of volume expansion

EXample1 Eample2 Example5