Thermal Physics

1. Thermal Physics

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

3. Solids • 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.

4. Liquids • 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.

5. gases • 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.

6. Temperature in gases • 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.

7. Pressure in a gas • 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.

8. Changes of state • The following diagram shows the names of the different changes of state.

9. Changes of state • 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.

10. Changes of state • 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.

11. Summary • 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

12. Pressure, temperature and volume in gases

13. Random motion of particles • 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.

14. Pressure and temperature • 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.

15. Gas pressure and volume • 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?

16. Gas pressure and volume • 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.

17. Gas pressure and volume • What would happen to the piston if we lift the piston upwards?

18. Gas pressure and volume • 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.

19. Gas pressure and volume • 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”.

20. Volume and pressure • Pressure • Symbol – P • Unit – Pascal • Unit Symbol – Pa • Volume • Symbol – V • Unit – meter cubed or centimeter cubed • Unit Symbol – m3 or cm3

21. Boyle’s Law • 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: P1V1 = P2V2

22. Boyle’s law example • A fixed mass of gas has an initial volume of 15cm3. When the volume is increased to 45cm3 the pressure is measured at 60kPa. What was the original pressure?

23. Summary of pressure, volume and temperature in gases • 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.

24. Evaporation

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

26. Evaporation • 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.

27. How can we increase evaporation? • Increase the temperature because that increases the kinetic energy of the molecules. • Create a draught across it because the moving air molecules will collide with the water molecules. • Increase the surface area because there will be more molecules to escape from the surface.

28. Summary of evaporation • 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.

29. Thermal Expansion

30. Thermal expansion • Most substances expand when heated. • Some substances expand more than others. • Gases expand much more than liquids and solids.

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. Summary – thermal expansion • 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.

37. thermometers

38. Thermometric property • 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.

39. How do we make thermometers? • To make a thermometer we need fixed points to define the scale. • The Celsius scale uses two fixed points: • 0oC, the temperature at which pure ice melts • 100oC 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.

40. How does a liquid-in-glass thermometer work? • When the bulb becomes warmer the liquid expands and moves up the capillary tube. • The liquid is usually mercury or coloured alcohol. Narrow glass capillary tube Thin glass bulb 

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

42. Advantages of thermocouples • Thermocouples are better for measuring temperatures that are changing rapidly. • They can also measure much higher temperatures than liquid-in-glass thermometers.

43. Range of thermometers • For liquid-in-glass thermometers alcohol is better for lower temperatures because it freezes at -114oC. Mercury freezes at -38oC so would not be suitable for very low temperatures. • The range is the lowest to the highest temperature a thermometer can measure.

44. Sensitivity of thermometers • The sensitivity of a thermometer is the extent to how much the thermometric property changes in a 1oC temperature rise. • Alcohol expands about 5 times as much as mercury in a 1oC temperature change. That means that alcohol has a higher sensitivity.

45. The linearity of a thermometer • The expansion of a liquid in a thermometer is not constant. That means that the temperatures between 0oC and 100oC and not exactly accurate. • A perfectly linear thermometer will make a straight line between 0oC and 100oC . The average thermometer will make a slightly curved line.

46. The linearity of a thermometer • 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.

47. Summary of thermometers • Each type of thermometer depends on a physical property that depends of the temperature. • The fixed points of the Celsius scale are 0oC and 100oC. • 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 1oC. • The greater the linearity of a thermometer, the closer the readings are to a standard thermometer.

48. Thermal Capacity

49. Thermal capacity • The thermal capacity of an object is the energy that must be supplied to raise its temperature by 1oC. • For example, if it takes 2000J to raise the temperature 1oC, the thermal capacity is 2000J/oC.

50. Thermal Capacity • Thermal capacity is measured in J/oC • E is the energy supplied in Joules • 2is the final temperature in oC • 1 is the initial temperature in oC E thermal capacity = 2 - 1