Heat, Temperature and Thermometers

1. Heat, Temperature and Thermometers

2. Introduction to Heat • Heat is a form of energy. • A large amount of energy ends up as heat. • Example: in a car some of the chemical energy stored in petrol is turned into heat energy. • Heat can have a significant effect on solids, liquids and gases (such as expansion).

3. Effects of Heat • Heat can cause a solid to expand which can be seen in passing a metal ball through a ring. • When cold, the ball will pass through the ring but when heated it won’t pass through. • A bimetallic strip is made of two different metals riveted together. • When heated, one metal expands more than the other causing the strip to bend. It then straightens when cooled. • Bimetallic Coil

4. Temperature and Heat • Temperature is the measure of the hotness or coldness of a body. • Temperature determines whether or not heat will flow and which way it will flow. • Heat will flow from one body to another only if there is a difference in temperature between them. • When we heat a body we are giving it energy. • As the temperature rises, the kinetic energy of the molecules rises. Transfer of heat Hot Cold

5. Units of Temperature • There are a number of different units used in temperature. • The SI unit of temperature is the kelvin (K). • The kelvin scale is related to the celsius scale as follows: 0 °C = 273.15 K and 100 °C = 373.15 K.

6. Units of Temperature • Problem: • (i) Convert 310 K to degrees Celsius. • (ii) Convert 18 °C to kelvin. • Solution: • (i) 310 K = (310 – 273.15) °C = 36.85 °C. • (ii) 18 °C = (18 + 273.15) K = 291.15 K

7. Thermometers • The simplest type of thermometer contains mercury scaled in a glass tube. • It works on the principle that liquids expand when they are heated and contract when cooled. • Another liquid used commonly used is alcohol.

8. Thermometric Properties • For the accurate measurement of temperature, scientists choose some physical property that changes measurably as temperature changes. • This property is called a Thermometric Property. • There are a number of different Thermometric Properties: • Length of a column of liquid: When a liquid is heated it expands (its volume increases). In a thermometer the length of the column of liquid increases as the liquid expands. Thus as the temperature increases, the length of the column of liquid increases.

9. Thermometric Properties • Emf of a thermocouple: if two different metals are joined together to form a complete circuit and the two junctions maintained at different temperatures, a small emf appears in the circuit which causes a very small electric current to flow. • The emf can be measured using a voltmeter. • The greater the temperature difference between the junctions, the greater the emf.

10. Thermometric Properties • Electrical Resistance: The electrical resistance of a conductor changes with temperature. • For a metal, the resistance increases with increasing temperature. • For a semiconductor or carbon, the resistance decreases with increasing temperature. • Resistance is the thermometric property on which the resistance thermometer is based.

11. Thermometric Properties • Colour: The colour of certain crystals change with temperature. This is the basis of one form of thermometer used to measure the temperature of your body. • The colour of a very hot object changes as its temperature rises; e.g. a piece of iron changes from a dull brown to red, to bright red, then yellow, then white.

12. Thermometric Properties • Volume of a gas at constant pressure: The diagram shows a gas syringe. It contains a fixed mass of gas and a rubber cap seals one end. • The gas is at constant pressure due to atmospheric pressure acting on the piston at the other end. • If the temperature of the gas is increased by heating it, the volume of the gas will increase and push the piston out. • If the gas is cooled then the volume decreases and the piston moves in.

13. Thermometric Properties • Pressure of a gas at constant volume: A gas syringe can also be used to show how the pressure of a fixed volume of gas varies with temperature. • In the diagram the volume of the gas is noted. • The gas is then heated. • To keep the volume at the same value, weights will have to be placed on the end of the syringe. • The larger the rise in temperature, the greater the weights that must be added.

14. Using a Thermometric Property to Measure Temperature • Use an ungraduated mercury-in-class thermometer. • The length of the column of mercury is the thermometric property. • Place the thermometer in pure melting ice and mark the position of the top of the column of mercury. (0 °C). • Place the thermometer in the steam above pure boiling water and mark the position. (100 °C). • Remove the thermometer and measure the lengths of the columns of mercury; call these lengths Lice and Lsteam.

15. Using a Thermometric Property to Measure Temperature • On graph paper plot the points (Lice,0)and (Lsteam,100) like the graph below and draw a straight line through these points. • The temperature corresponding to any length Lθcan be found from the graph.

16. Using a Thermometric Property to Measure Temperature • Problem: • The length of the mercury column in a capillary tube is 3.2 cm when the tube is placed in melting ice. • In the steam above the boiling water its length is 22.3 cm. • If the length of the column when placed in a beaker of water is 10 cm, by using a suitable graph, calculate the temperature of the water in °C, according to this thermometer.

17. Disagreement Between Thermometers • If two different types of thermometer (mercury-in-glass and resistance thermometer) are created in this way, they will give the temperature as 0 °C in melting ice and 100 °C in steam. • However, in general, they will disagree at other temperatures. • Different thermometric properties do not change proportionally with the same change in degree of hotness. • To get agreement on the quantity temperature we therefore need a standard thermometer. • Mercury-in-glass thermometer is a school standard.

18. Practical Use of Thermometers • A clinical thermometer is used to measure body temperature. • The clinical thermometer has a narrow constriction in, so that when removed from the patient the mercury does not fall and can be read accurately. • An infra-red radiation thermometer is inserted into the ear and infra-red radiation emitted from the ear drum is detected and the patient’s temperature is obtained. • The plastic strip thermometer is also used.

19. Quantity of Heat:Heat Capacity • When you add heat energy to a substance its temperature usually rises. • When you take heat energy away, its temperature usually falls. • The amount of heat needed to raise the temperature of an object by 1 °C (1 K) is the same amount of heat given out if its temperature falls by 1 °C. • The heat capacity of an object is the heat energy needed to change its temperature by 1 K (1 °C).

20. Heat Capacity • The symbol for heat capacity is C. Its unit is joule per kelvin (J K-1). • If an object has heat capacity C, the heat energy Q needed to change its temperature by ∆θ degrees Celsius is given by: Heat = Heat capacity x Change in temperature Q = C∆θ

21. Specific Heat Capacity • The specific heat capacity (c) of a substance is the heat energy needed to change the temperature of one kilogram of that substance by one kelvin. • The unit of specific heat capacity is the joule is the joule per kilogram per kelvin. Its symbol is J kg-1 K-1.

22. Specific Heat Capacity • The heat energy Q needed to produce a given rise in temperature is directly proportional to the rise in temperature. • The heat energy needed to produce a given rise in temperature in a body is directly proportional to the mass of that body.

23. Specific Heat Capacity 1. How much heat energy is needed to raise 3 kg of water from 2 °C to 100 °C? • Ans: Q = m c ∆θ = (3)(4180)(100 – 2) = 1,228,920 J 2.A 2 kW electric heater raises the temperature of 10 kg from 10 °C to 70 °C. If there is no heat loss to the surroundings, how long does it take? • Ans: Heat supplied Q = m c ∆θ = (10)(4180)(70 – 10) = 2,508,000 J The heat is supplied at the rate of 2 kW = 2000 joules per sec. Time taken = = = 1254 s

24. Applications

25. Latent Heat • When a substance is changing state, it can take in or give out heat energy without its temperature changing. • Examples: to keep a beaker of water boiling, change in states (melting ice and boiling water). • The Latent heat (L) of a substance is the heat energy needed to change its state without a change in temperature. • The human body is cooled by perspiring. • Your body produces perspiration on your skin when it is hot. Perspiration is mainly water. As this water evaporates, it takes latent heat from you and you cool down.

26. Latent Heat • There is no change of temperature during a change in state. (The energy is used to change state). • The latent heat needed to change from a solid to a liquid is called the latent heat of fusion. • The latent heat needed to change a liquid to a gas is called the latent heat of vaporisation. • The symbol for latent heat is L. Its unit is the joule.

27. Specific Latent Heat • To compare the amounts of heat needed to change the state of various substances, we use the heat needed to change the state of 1 kg of the substance without a change in temperature. • The specific latent heat (l) of a substance is the amount of heat energy needed to change the state of 1 kg of that substance without a change in temperature. • The specific latent heat of fusion of a substance is the amount of heat energy needed to change 1 kg of that substance from a solid to a liquid without a change in temperature (melting point).

28. Specific Latent Heat • The specific latent heat of vaporisation of a substance is the amount of heat energy needed to change 1 kg of that substance from a liquid to a gas without a change in temperature (boiling point). • The same amount of heat energy is given out when the substance changes state (from a gas to a liquid at its boiling point) or (from a solid to a liquid). • The unit of specific latent heat of vaporisation and of fusion is the joule per kilogram (J kg-1).

29. Formula for Latent Heat • Since the heat energy Q needed to change the state of a substance is directly proportional to the mass m of that substance it follows that:

31. Questions • Q. 1 How much heat energy is needed to convert 18 kg of ice at 0 °C to water at 0 °C? • Q. 2 How much heat energy is needed to completely convert 85 grams of water at 10 °C to 85 grams of steam at 100 °C? • Specific heat capacity of water = 4180 J kg-1 K-1. • Specific latent heat of fusion of ice = 3.3 x 105 J kg-1. • Specific latent heat of vaporisation of water = 2.3 x 106J kg-1. • Equations: Q = mcΔθ (Heat needed to raise temp.). Q = ml (Heat needed to change state).

32. The Heat Pump • A heat pump transfers energy from a cooler region to a warmer one. • Examples: refrigerators, air conditioning systems, cars. • The circulating liquid has a high specific latent heat of vaporisation and a low boiling point. • The liquid is pumped around a closed circuit and on reaching the expansion valve, pressure drops, the liquid vaporises and takes in its latent heat from inside the fridge, thus cooling the fridge. • When the vapour reaches the compressor, its pressure is increased and turns back into a liquid, giving out its latent heat as it does so.

33. Heat Transfer • Convection • Hot air • rising • carrying • the • heat up • with it. Conduction -Transfer by vibrations Radiation -Transfer by Electro-magnetic wave

34. Heat Transfer • Heat can be transferred from one place to another by conduction, convection and radiation. • Conductionis the movement of heat energy through a substance by the passing on of molecular vibration from molecule to molecule.

35. Conduction • If one end of an object is at a higher temperature than the other, the molecular vibration is passed on from the hotter end to the cooler end. • Substances in which this happens easily are called good thermal conductors (e.g. metals). • Substances in which it only happens a little are called thermal insulators (e.g. air, water).

36. Convection • Convection is the transfer of heat through a fluid by means of circulating currents of fluid caused by the heat. • When the lower part of the fluid is heated, it expands and becomes less dense. It rises above the cooler fluid. • This movement is called a convection current. • Water in domestic heating systems may circulate by convection with a pump usually used to speed up the flow of water through the system.

37. Convection • Convection also occurs in gases. • In a room, warm air rises from the heater to a higher point in the room. • As the air cools, it drops down and the cycle is repeated.

38. Radiation • Radiation is the transfer of heat energy from one place to another in the form of electromagnetic waves. • The electrons at the surface of a substance vibrating with thermal energies emit electromagnetic waves. • The higher the temperature the shorter the wavelengths emitted. • The darker the colour of an object the better it is at radiating heat.

39. Solar Heating • Solar heating is using the sun’s energy to heat something. In some solar panels water flowing in black pipes under glass absorbs heat and heats the water. • In another method, a solar panel consisting of photocells converts the Sun’s energy into electrical energy which can be used for heating or other purposes.

40. U-Value • How well a part of a building (a roof or wall) conducts heat is given in terms of what is called its U-value. • The U-value of a structure is the amount of heat energy conducted per second through 1 m2 of that structure when a temperature difference of 1 °C (1 K) is maintained between its ends. • The unit of U-value is the W m-2 K-1. • A structure that is a good insulator has a low U-value. • Increasing the insulation reduces the U-value.

41. Read the following passage and answer the accompanying questions. • The sun is a major source of ‘green’ energy. In Ireland solar heating systems and geothermal systems are used to get energy from the sun. • There are two main types of solar heating systems, flat-plate collectors and vacuum-tube collectors. • A flat-plate collector is usually an aluminium box with a glass cover on top and a blackened plate on the bottom. A copper pipe is laid on the bottom of the box, like a hose on the ground; water is passed through the pipe and transfers the absorbed heat to the domestic hot water system. • In a vacuum-tube collector, each tube consists of an evacuated double-walled silvered glass tube in which there is a hollow copper pipe containing a liquid. The liquid inside the copper pipe is vaporised and expands into the heat tip. There the vapour liquefies and the latent heat released is transferred, using a heat exchanger, to the domestic hot water system. The condensed liquid returns to the copper pipe and the cycle is repeated. • In a geothermal heating system a heat pump is used to extract solar energy stored in the ground and transfer it to the domestic hot water system. • Why is the bottom of a flat-plate collector blackened? • How much energy is required to raise the temperature of 500litres of water from 20°C to 50°C? • The liquid in a vacuum-tube solar collector has a large specific latent heat of vaporisation. Explain why. • Name the three ways in which heat could be lost from a vacuum-tube solar collector. • How is the sun’s energy trapped in a vacuum-tube solar collector? • Describe, in terms of heat transfer, the operation of a heat pump. • Give an advantage of a geothermal heating system over a solar heating system.

42. Latent Heat of Fusion of Ice [2002] In an experiment to measure the specific latent heat of fusion of ice, warm water was placed in an aluminium calorimeter. Crushed dried ice was added to the water. The following results were obtained. Mass of calorimeter.......................................= 77.2 g Mass of water.................................................= 92.5 g Initial temperature of water...........................= 29.4 0C Temperature of ice ........................................= 0 0C Mass of ice.....................................................= 19.2 g Final temperature of water.............................= 13.2 0C Room temperature was 21 0C. • What was the advantage of having the room temperature approximately halfway between the initial temperature of the water and the final temperature of the water? • Describe how the mass of the ice was found. • Calculate a value for the specific latent heat of fusion of ice • The accepted value for the specific latent heat of fusion of ice is 3.3 × 105 J kg-1; suggest two reasons why your answer is not this value.

43. Latent Heat of Fusion of Ice

44. Latent Heat of Fusion of Ice • Heat lost to surroundings when the system is above room temperature would cancel out the heat taken in from the surroundings when the system was below room temperature. • Final mass (of calorimeter + water + ice) - initial mass (of calorimeter + water) • mcΔθAl +mcΔθwater=mlice +mcΔθmeltedice Fall in temperature = 16.2 oCAns= 3.2 × 105 J kg-1 . • Thermometer not sensitive enough, lack of insulation, lack of stirring, heat loss/gain to surroundings, too long for ice to melt, inside of calorimeter tarnished, splashing, heat capacity of thermometer.