1. THERMAL PHYSICS

2.  Matter is most commonly found in solid, liquid or gas form. We will discuss the properties of these different states of matter. STATES OF MATTER

3.  Solids have a fixed shape and a fixed volume.  The molecules in a solid have a rigid structure.  The force of attraction between molecules is strong. SOLIDS

4.  Liquids have a fixed volume but change shape to fit the container.  The molecules in a liquid stay in contact but move around freely.  The force of attraction is strong enough to keep the molecules from completely moving away. LIQUIDS

5.  Gases do not have a fixed volume or shape.  The molecules in a gas are far apart and move quickly.  The forces of attraction between molecules is negligible. GASES

6.  The temperature of a gas is the average kinetic energy of its molecules.  Objects with a high temperature have molecules moving at a high speed. If the temperature is decreased the speed of the molecules decreases. TEMPERATURE IN GASES

7.  The pressure of a gas on a surface is due to the impacts of gas molecules with the surface.  When a molecule impacts the surface it exerts a tiny force. Billions of these impacts occur every second creating a steady pressure on the surface. PRESSURE IN A GAS

8.  The following diagram shows the names of the different changes of state. CHANGES OF STATE

9.  The changes of state can be described by the movement of molecules.  Melting – when a solid is heated the molecules begin vibrating to the point at which they break free of the rigid structure.  Freezing – When a liquid is cooled the molecules slow down and form a rigid structure. CHANGES OF STATE

10.  The changes of state can be described by the movement of molecules.  Boiling – When a liquid is heated the molecules move quickly and break free from each other.  Condensing – When a gas is cooled the molecules move more slowly and the force of attraction increases. CHANGES OF STATE

11.  Molecules in a solid are in a fixed structure.  Molecules in a liquid move in contact with each other.  Molecules in a gas are far apart and move at high speed.  Increasing the temperature of a gas increases the average speed of its molecules.  The pressure of the gas on a surface is caused by its molecules repeatedly hitting the surface.  Practice: pg 71 #1,2 and pg 73 #1,2 SUMMARY

12. PRESSURE, TEMPERATURE AND VOLUME IN GASES

13.  In 1785, Robert Brown observed pollen grains floating on water. He observed that the pollen grains moved randomly.  Using molecular theory, it has been explained that the small water molecules were constantly colliding with the large pollen grain. This caused it to move randomly.  This motion is called Brownian Motion. RANDOM MOTION OF PARTICLES

14.  If we had a container of sealed gas, what would happen to the pressure if we increase the temperature? (Note: we are keeping the same volume.)  The pressure would increase because the molecules would be moving faster and there would be more collision with the walls of the container. PRESSURE AND TEMPERATURE

15.  Imagine a piston filled with a gas.  The temperature and mass of the gas are constant.  If we force the piston down, what will happen to the pressure? GAS PRESSURE AND VOLUME

16.  If the piston is forced down, the volume decreases.  The pressure in the tube will increase because the molecules will impact the surface more often. GAS PRESSURE AND VOLUME

17.  What would happen to the piston if we lift the piston upwards? GAS PRESSURE AND VOLUME

18.  If the piston is forced up, the volume increases.  The pressure in the tube will decrease because the molecules will impact the surface less often. GAS PRESSURE AND VOLUME

19.  We see that when volume decreases the pressure increases. When the volume increases the pressure decreases.  This means that volume and pressure are “inversely proportional”. GAS PRESSURE AND VOLUME

20.  Pressure  Symbol – P  Unit – Pascal  Unit Symbol – Pa  Volume  Symbol – V  Unit – meter cubed or centimeter cubed  Unit Symbol – m 3 or cm 3 VOLUME AND PRESSURE

21.  For a fixed mass of gas at a constant temperature: pressure x volume = constant  When you are comparing two situations you can use the following equation: P 1 V 1 = P 2 V 2 BOYLE’S LAW

22.  A fixed mass of gas has an initial volume of 15cm 3. When the volume is increased to 45cm 3 the pressure is measured at 60kPa. What was the original pressure? BOYLE’S LAW EXAMPLE

23.  Brownian motion is the random motion of small particles due to the impacts of gas molecules on each particle.  The pressure of a gas in a sealed container increases if the gas temperature is increased.  For a fixed mass of gas at constant temperature, the pressure x volume = constant. SUMMARY OF PRESSURE, VOLUME AND TEMPERATURE IN GASES

24. EVAPORATION

25.  Evaporation is when high energy molecules in a liquid leave the surface of the liquid. EVAPORATION

26.  Why do we sweat when we’re hot?  The water is on the surface of our skin. The high energy water molecules will leave the surface. The average kinetic energy of the molecules on the skin will decrease. That means the temperature will decrease. EVAPORATION

27. 1.Increase the temperature because that increases the kinetic energy of the molecules. 2.Create a draught across it because the moving air molecules will collide with the water molecules. 3.Increase the surface area because there will be more molecules to escape from the surface. HOW CAN WE INCREASE EVAPORATION?

28.  Evaporation in a liquid occurs when high energy molecules leave the surface.  The average kinetic energy of the remaining liquid decreases which cools the liquid.  We can increase evaporation by increasing the temperature, passing a draught over the surface, or increasing the surface area. SUMMARY OF EVAPORATION

29. THERMAL EXPANSION

30.  Most substances expand when heated.  Some substances expand more than others.  Gases expand much more than liquids and solids. THERMAL EXPANSION

31. EXPANSION OF GASES

32. EXPANSION OF LIQUIDS - THERMOMETERS

33. EXPANSION OF SOLIDS – EXPANSION GAPS

34. EXPANSION OF SOLIDS – STEEL TIRES

35. EXPANSION OF SOLIDS – BIMETALLIC STRIPS

36.  When the temperature of an object increases the object expands.  Gases expand much more than liquids or solids.  Some applications of expansion include liquid thermometers, bimetallic strips, expansion gaps on bridges, and fitting steel tires. SUMMARY – THERMAL EXPANSION

37. THERMOMETERS

38.  Every thermometer depends on a physical property that changes with temperature.  For liquid-in-glass thermometers the thermometric property is the thermal expansion of liquids.  Amos Dolbear used the number of cricket chirps per minute to estimate the temperature. The thermometric property in that cases is the rate of cricket chirps. THERMOMETRIC PROPERTY

39.  To make a thermometer we need fixed points to define the scale.  The Celsius scale uses two fixed points:  0 o C, the temperature at which pure ice melts  100 o C the temperature at which pure water boils  Once these two points are marked on the thermometer we can divide the length into 100 equal intervals. HOW DO WE MAKE THERMOMETERS?

40.  When the bulb becomes warmer the liquid expands and moves up the capillary tube.  The liquid is usually mercury or coloured alcohol. HOW DOES A LIQUID-IN-GLASS THERMOMETER WORK? Thin glass bulb   Narrow glass capillary tube

41.  Thermocouples are electrical thermometers. The thermometric property is the voltage created when two different metals are in contact. This voltage varies with the temperature. THERMOCOUPLES

42.  Thermocouples are better for measuring temperatures that are changing rapidly.  They can also measure much higher temperatures than liquid-in-glass thermometers. ADVANTAGES OF THERMOCOUPLES

43.  For liquid-in-glass thermometers alcohol is better for lower temperatures because it freezes at -114 o C. Mercury freezes at -38 o C so would not be suitable for very low temperatures.  The range is the lowest to the highest temperature a thermometer can measure. RANGE OF THERMOMETERS

44.  The sensitivity of a thermometer is the extent to how much the thermometric property changes in a 1 o C temperature rise.  Alcohol expands about 5 times as much as mercury in a 1 o C temperature change. That means that alcohol has a higher sensitivity. SENSITIVITY OF THERMOMETERS

45.  The expansion of a liquid in a thermometer is not constant. That means that the temperatures between 0 o C and 100 o C and not exactly accurate.  A perfectly linear thermometer will make a straight line between 0 o C and 100 o C. The average thermometer will make a slightly curved line. THE LINEARITY OF A THERMOMETER

46.  The linearity of a thermometer tells how straight the line is on the graph. The greater the linearity the greater the accuracy of the thermometer. THE LINEARITY OF A THERMOMETER

47.  Each type of thermometer depends on a physical property that depends of the temperature.  The fixed points of the Celsius scale are 0 o C and 100 o C.  The range of a thermometer is from the lowest to highest temperature it can measure.  The sensitivity of a thermometer is the change in its thermometric property for a change of 1 o C.  The greater the linearity of a thermometer, the closer the readings are to a standard thermometer. SUMMARY OF THERMOMETERS

48. THERMAL CAPACITY

49.  The thermal capacity of an object is the energy that must be supplied to raise its temperature by 1 o C.  For example, if it takes 2000J to raise the temperature 1 o C, the thermal capacity is 2000J/ o C. THERMAL CAPACITY

50.  Thermal capacity is measured in J/ o C  E is the energy supplied in Joules   2 is the final temperature in o C   1 is the initial temperature in o C THERMAL CAPACITY thermal capacity = E  2 -  1

51.  The thermal capacity for a beaker with 1L of water will be different for a beaker with 2L of water.  This is a problem when comparing the thermal capacity of different substances.  The specific heat capacity will give a value for any mass of a substance for easy comparison. SPECIFIC HEAT CAPACITY

52.  The temperature rise depends on:  The amount of energy supplied  The mass of the substance  The nature of the substance  The specific heat capacity, c, is defined as the energy needed to raise the temperature of 1kg of the substance by 1 o C. SPECIFIC HEAT THERMAL

53.  c is the specific heat capacity J/kg o C  E is the energy in Joules  m is the mass in kilograms   2 is the final temperature in o C   1 is the initial temperature in o C SPECIFIC HEAT CAPACITY c = E m(  2 -  1 )

54.  2000J of heat energy is applied to 4kg of a substance. The temperature went from 20 o C to 23 o C. What is the specific heat capacity? EXAMPLE PROBLEM

55.  The thermal capacity of an object is the energy that must be supplied to raise its temperature by 1 o C.  The specific heat capacity, c, is defined as the energy needed to raise the temperature of 1kg of the substance by 1 o C. THERMAL CAPACITY SUMMARY thermal capacity = E  2 -  1 c = E m(  2 -  1 )

56. CHANGE OF STATE

57.  When ice is heated and it melts, its temperature remains at 0 o C until all the ice has melted.  This temperature is called the melting point.  When water is heated and boils, its temperature remains at 100 o C (the boiling point). Boiling occurs throughout the liquid at its boiling point but evaporation occurs at the surface when it is below the boiling point. CHANGING STATE

58.  Energy is supplied to an object to raise its temperature.  When an object is changing state (ie. melting) the temperature remains constant.  The energy supplied when it changes state is called latent heat.  Latent mean hidden. It is called latent because energy is supplied to the object but the temperature does not change. CHANGING STATE

59. LATENT HEAT GRAPH

60.  The specific latent heat of fusion is the energy needed to melt 1kg of a substance at its melting point. The unit is joule per kilogram (J/kg).  The specific latent heat of vaporisation is the energy needed to change 1kg of a substance at its boiling point from liquid to vapour. SPECIFIC LATENT HEAT l = E m l is the specific latent heat of fusion in J/kg E is the energy supplied in J m is the mass of substance melted in kg

61.  Energy is needed to melt a solid or boil a liquid.  Latent heat is energy used or released when a substance changes its state without changing its temperature.  Specific latent heat of fusion (or vaporisation) is the energy needed to melt (or boil) 1kg of the substance without change of temperature. CHANGE OF STATE SUMMARY

62. HEAT TRANSFER

63.  We will look into the three types of heat transfer; conduction, convection and radiation. THREE WAYS TO TRANSFER HEAT

64.  When you place hot water in a mug the fast moving water molecules bump into the molecules of the mug, causing the mug to heat up.  When heat is transferred through contact we call it thermal conduction. CONDUCTION

65.  Materials that transfer heat easily are called conductors. Materials that do not conduct heat easily are insulators.  Metals are good conductors. Wool is a good insulator. CONDUCTORS AND INSULATORS

66.  Metals contain electrons that move around freely. Why does this make them better conductors?  When metals are heated the free electrons transfer kinetic energy to the cooler parts.  In non-metallic solids energy is only transferred by atoms shaking each other. It is much less effective than free moving electrons in metals. CONDUCTORS AND INSULATORS

67.  Convection happens whenever a fluid (ie. gas or liquid) is heated.  The fluid expands where it is heated and becomes less dense. This causes it to rise.  As the hot fluid rises the cool fluid moves in to take its place, creating a convection current. CONVECTION

68.  Convection happens whenever a fluid (ie. gas or liquid) is heated. CONVECTION

69.  Convection happens whenever a fluid (ie. gas or liquid) is heated. CONVECTION

70.  Every object around us emits infra-red radiation. The hotter an object is, the more radiation it emits.  Different surfaces absorb and emit infra-red radiation differently. RADIATION

71.  When you wear a dark shirt in the sun you feel warmer than in a light shirt.  Dark surfaces absorb infra-red radiation better than light surfaces. RADIATION - COLOUR

72.  Shiny surfaces do not absorb infra-red radiation as well as matt surfaces.  Infra-red radiation reflects off of the shiny surface because it is smoother than a matt surface. RADIATION - SHAPE

73.  Thermal conduction is heat transfer through contact. Metals are better conductors due to the free electrons.  Convection takes place because when fluids are heated they become less dense and begin to rise.  All bodies emit infra-red radiation. Dark, matt surfaces are better emitters and absorbers of infra- red radiation than light, shiny surfaces. SUMMARY – HEAT TRANSFER