Topic 19a Thermal physics (revision) 19.1 Temperature 19.1 - PowerPoint PPT Presentation

Topic 19a Thermal physics (revision) 19.1 Temperature 19.1 Kinetic model of matter 20.1 Transfer of thermal energy

2 0 .1 1 Tem perature Measuring temperature by sensation is very imprecise. That is why we need a temperature scale and a thermometer to measure temperature more accurately.  Temperature and Heat Flow  Temperature Scales  Thermometers  Chapter Review

tem perature and heat flow tem perature A measure of the degree of hotness of a body.  is measured using a thermometer heat The energy that is transferred from one region to another region due to a difference in temperature. higher lower temperature temperature heat flow hot cold

therm om eters therm om eters The choice of which thermometer to use depends on  range of temperatures to be measured  accuracy required  physical characteristics of the substances being examined

therm om eters therm om eters A thermometer must have a physical property that changes with temperature (thermometric property) Examples include:  the volume of a column of liquid in a capillary tube  the electrical resistance of a platinum wire  the e.m.f. of a thermocouple  the curvature of a bimetallic strip  the pressure of a gas at constant volume

therm om eters desirable features of a therm om eter  an easy-to-read scale  safe to use  sensitive to temperature changes  ability to measure a wide range of temperatures

therm om eters 1 . linearity The temperature reading changes linearly or proportionately with the length of the mercury column. • temperature against length graph – a straight line graph 2 . responsiveness The speed with which a reading can be obtained.  depends on the thickness of the glass bulb used to contain the mercury  responds faster (shorter time) through a thinner glass wall

therm om eters 3 . sensitivity The change in length of the liquid column per unit degree change in temperature, e.g. 2 cm/ ° C  increases if the thermometer has a larger mercury bulb or a narrower capillary tube  can detect a rise or fall of 0.2 ° C is more sensitive than one that can only measure a change of 1 ° C 4 . range The interval between the minimum and the maximum temperatures that it can measure.  a sensitive thermometer requires a longer capillary tube to cover the same range found on less sensitive thermometers.

tem perature scales tem perature scales To obtain a standard scale on a thermometer, two fixed points to be marked are chosen.  for the purpose of standardisation  always the same under given conditions These two fixed points are  Ice point and  Steam point.

tem perature scales the Celsius scale The two fixed points for the celsius scale: I ce Point Steam Point Temperature at which Temperature of pure boiling water changes melting ice at standard into steam at standard atmospheric pressure atmospheric pressure Assigned value of 0 ° C Assigned value of 100 ° C In Celsius scale (centigrade scale), the interval between the fixed points is divided into 100 equal divisions for easy reading. Each reading is 1 degree Celsius ( ° C)

tem perature scales the Celsius scale steam point ice point flask melting steam ice boiling funnel water finding the ice finding the steam point point

the Celsius scale corresponding Celsius height of 1. Note the position of the temperature column mercury in the tube at the ice point and the x 100 100 ° C steam point steam point 2. For any height x θ of the mercury, the x 100 – x 0 corresponding Celsius temperature θ is given by x θ θ x θ - x 0 x θ - x 0 θ = x 100 °C ice point x 100 – x 0 0 ° C x 0

the Kelvin or absolute scale The Kelvin scale has its zero at absolute zero, which is the lowest temperature that any substance can reach.  has a SI unit of kelvin (K)  absolute zero corresponds with –273 ° C on the Celsius scale Celsius Kelvin scale scale T (kelvin) = θ (celsius) + 273 473 K 200 0 C 373 K 100 0 C 100 K 100 0 C 0 0 C 273 K -100 0 C 173 K The magnitude of a unit -200 0 C 73 K on both scales is equal -273 0 C 0 K

therm om eters m ercury therm om eters A common type of thermometer in everyday use. safety chamber capillary glass stem with tube thickened wall mercury bulb with thin steam strengthened wall point vacuum in space ice point

com paring m ercury and alcohol therm om eters Properties Mercury Alcohol No ( irregular expansion but rather linearly over tem p. Uniform expansion Yes range encountered in science labs) Yes ( transparent, m eniscus No ( visible Stick to glass difficult to see, needs to be m eniscus) dyed) Reaction to tem perature Quick Slow changes Boiling point 3 5 7 ° C 7 8 ° C measure a lower ( upper lim it) temperature Range range than Freezing point -3 9 ° C -1 1 5 ° C mercury ( low er lim it) Cost Expensive Cheap Poisonous Yes No

therm om eters clinical therm om eters A type of maximum thermometer specially designed for measuring the temperature of the human body. curved constriction glass stem space holds bulb normal body mercury if temperature thermometer is overheated

therm om eters clinical therm om eters  short temperature range from about 35 ° C to 42 ° C  constriction in capillary tube located just above the bulb of the thermometer  Constriction prevents the mercury from flowing back into the bulb when the thermometer is taken out of the patient’s mouth.  After taking the reading, it is given a good shake to force the mercury back into the bulb.

therm ocouple  consists of two wires of different metals joined together at the end to form two junctions  if the junctions are at different temperatures, a voltage is produced w ire A w ire A w ire B cold hot sensitive junction junction voltm eter  the larger the temperature difference, the larger the voltage produced

therm ocouple  the temperature range depends on the two metals used for the wires  can operate over a very wide range of temperatures from –200 ° C to 1700 ° C  Advantages : suitable for measuring 1. wide temperature differences, 2. Temperature which vary rapidly due to its quick response and 3. the temperature at a point as the wire junctions are very small  To produce a larger voltage, several thermocouples are connected in series (thermopiles) to increase the sensitivity of the instrument

Temperature measured by Thermometer requires Scales: Physical properties Fixed points: Celsius scale that change ice point Kelvin scale with temperature steam point like Volume of fixed resistance e.m.f. (voltage) mass of liquid used in used in used in Mercury-in-glass Resistance Thermocouple thermometer thermometer has the advantages of Structure Sensitivity Range Linearity Responsiveness

2 0 .1 2 Kinetic Model of Matter Matter is made up of tiny particles called atoms and molecules. These particles are much too small to be seen by naked human eyes. How do we prove their existence?  States of Matter  Brownian Motion  Molecular Model of the Three States of Matter  Effects of Temperature  Pressure Exerted by a Gas

kinetic m olecular m odel of m atter The kinetic theory of m atter states that all matter is made up of a large number of tiny atoms or molecules which are in continuous motion.

brow nian m otion Using a microscope, smoke particles can be seen moving continuously and haphazardly, as a result of being hit by unseen fast-moving air molecules. dotted lines represent the path of smoke particles between collision Brownian motion provides evidence for the kinetic molecular model of matter (kinetic theory of matter).

brow nian m otion Dem onstrations • Gas molecules colliding with a smoke particle can cause the particle to trace out a zig-zag path. • Suspension of fat droplets (milk) in water • Pollen grains sprinkled on water

brow nian m otion diffusion If heat is supplied, the motion of the smoke particles becomes more vigorous. The smaller the smoke particles, the more rapid is their motion. Diffusion is a result of Brownian motion. smoke particles moving continuously and haphazardly (irregular or random motion)

brow nian m otion diffusion Diffusion is the spreading of molecules of their own accord without any external aid. Gas molecules move randomly all the time and they move into any available space. Hence they stay mixed and do not separate out. Rate of diffusion depends on  temperature of gases: higher temperature leads to faster diffusion  density of gases: greater density leads to slower diffusion

brow nian m otion diffusion in air vacuum bromine vapour diffused bromine vapour contained If bromine vapour is released into a similar space full of air  bromine molecules keep hitting air molecules which get in the way  bromine vapour spreads quickly throughout the space but much slower than in vacuum

brow nian m otion diffusion in liquid Diffusion also takes place in liquids, at a very much slower rate.  copper(II) sulphate solution and water become uniformly mixed after a while due to diffusion copper (II) lid sulphate solution water single uniform layer copper (II) diffusion of copper (II) sulphate solution sulphate solution in water