حرارة وديناميكا حرارية المحاضرة الأولى د/ محرز لولو

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

Thermometers and Temperature Scales Thermometer is make use the fact that some physical properties of matter change proportionate to temperature change. Especially expansion and contraction of liquid

How liquid expansion can be used to make 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.

Make Liquid thermometer. 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. Expansion Contraction Heated Cold

Calibration Thermometers can be calibrated either by: comparing them with other certified thermometers or checking them against known fixed points on the temperature scale. The best known of these fixed points are the melting and boiling points of pure water.

Calibration Activity The traditional method of putting a scale on a liquid-in glass or liquid-in-metal thermometer was 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 (101.325 kPa = 760 mm Hg) and again mark the point indicated as upper fixed point. 3. Make Scales. Divide the distance between these marks (lower point and upper point) into equal portions according to the temperature scale being used.

Thermometer scale Ice Point and Steam Point The lower fixed point or ice point is the temperature at which pure ice melts at standard pressure. (0oC). The upper fixed point or steam point is the temperature at which pure water boils at standard pressure.(100oC).

Calibration

Calibrating a thermometer

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

Temperature Scale 2 fixed points at temperatures (0oC and 100oC) are chosen ice point (0 oC) steam point (100 oC) divide the range between ice point and steam point by 100 equal divisions

He used the freezing point of water as his zero (zero degrees Celsius) and the boiling point of water as 100 and divided it equally in between.

In this way he calibrated his thermometer.

Ice Point and Steam Point Different scales can be used in the measurement of temperature, such as: Kelvin Celsius scale Fahrenheit scale.

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 –115oC to 78oC Mercury range –39oC to 357oC

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

Problems with Liquid-in-Glass Thermometers An alcohol thermometer and a mercury thermometer may agree only at the calibration points. The discrepancies between thermometers are especially large when the temperatures being measured are far from the calibration points. The thermometers also have a limited range of values that can be measured. Mercury cannot be used under –39o C Alcohol cannot be used above 85o C Section 19.2

Constant-Volume Gas Thermometer The physical change exploited 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. Section 19.3

Constant-Volume Gas Thermometer, cont. 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. Section 19.3

Constant-Volume Gas Thermometer, final 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. Section 19.3

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 –273.15o 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: TC = T – 273.15 Section 19.3

Absolute Temperature Scale The absolute temperature scale is now based on two new fixed points. Adopted by in 1954 by the International Committee on Weights and Measures One point is absolute zero. The other point is the triple point of water. This is the combination of temperature and pressure where ice, water, and steam can all coexist. The triple point of water occurs at 0.01o C and 4.58 mm of mercury. This temperature was set to be 273.16 on the absolute temperature scale. This made the old absolute scale agree closely with the new one. The units of the absolute scale are kelvins. Section 19.3

Absolute Temperature Scale, cont. The absolute scale is also called the Kelvin scale. Named for William Thomson, Lord Kelvin The triple point temperature is 273.16 K. No degree symbol is used with kelvins. The kelvin is defined as 1/273.16 of the difference between absolute zero and the temperature of the triple point of water. Section 19.3

Some Examples of Absolute Temperatures The figure at right gives some absolute temperatures at which various physical processes occur. The scale is logarithmic. The temperature of absolute zero cannot be achieved. Experiments have come close Section 19.3

Fahrenheit Scale A common scale in everyday use in the US Named for Daniel Fahrenheit Temperature of the ice point is 32oF. Temperature of the steam point is 212o. There are 180 divisions (degrees) between the two reference points. Section 19.3

The temperature scale used in the US is the Fahrenheit scale which is calibrated differently.

Comparison between thermometer scales 100 Celsius 212 32 Fahrenheit 373 273 Kelvin UFP toC toF tK LFP

The Relationship scale

Comparison of Scales Celsius and Kelvin have the same size degrees, but different starting points. TC = T – 273.15 Celsius and Fahrenheit have different sized degrees and different starting points. To compare changes in temperature Ice point temperatures 0oC = 273.15 K = 32o F Steam point temperatures 100oC = 373.15 K = 212o F Section 19.3

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. hot cold heat 26 °C 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.

Zeroth 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

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

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 L DL Assume an object has an initial length L. That length increases by DL as the temperature changes by DT. We define the coefficient of linear expansion as A convenient form is DL = aLi DT This equation can also be written in terms of the initial and final conditions of the object: Lf – Li = a Li (Tf – Ti) The coefficient of linear expansion, a, has units of (oC)-1 L DL Section 19.4

Some Coefficients Section 19.4

Linear Expansion, cont. Some materials expand along one dimension, but contract along another as the temperature increases. Since the linear dimensions change, it follows that the surface area and volume also change with a change in temperature. A hole in a piece of material expands in the same way as if the cavity were filled with the material. The notion of thermal expansion can be thought of as being similar to a photographic enlargement. Section 19.4

Area Expansion The change in area is proportional to the original area and to the change in temperature: DA = 2aAi DT Areal expansion – solid cube Ao = Lo2 A = L2 = (Lo +  Lo T)2 = Lo3(1 +  T)2 A = Lo2 (1 + 2 T + 2 T2) A= Lo2 (1 + 2  T) (ignoring higher order terms) A - Ao = A = 2 Lo2T Ao A Section 19.4

Volume Expansion Vo The change in volume is proportional to the original volume and to the change in temperature. DV = bVi DT b is the coefficient of volume expansion. For a solid, b = 3a This assumes the material is isotropic, the same in all directions. For a liquid or gas, b is given in the table V Section 19.4

Volume expansion – solid cube Vo = Lo3 V = L3 = (Lo +  Lo T)3 = Lo3(1 +  T)3 V = Lo3 (1 + 3  T + 3 2 T2 + 3 T3) V = Lo3 (1 + 3  T) (ignoring higher order terms) V - Vo = V = 3  Lo3T =  Vo T coefficient of linear expansion  coefficient of volume expansion