Transfer of Thermal Energy

Transcription

1 Transfer of Thermal Energy

2 Transfer of Thermal Energy Thermal Energy Transfer Conduction Convection Radiation

3 Transfer of Thermal Energy Thermal Energy Transfer Conduction Convection Radiation

4 Lesson Objectives Understand that thermal energy is transferred from a region of higher temperature to a region of lower temperature.

5 Thermal Energy If a hot object is placed next to a cold object, will both objects remain at their temperatures? Why? Will the transfer of thermal energy between the two bodies end? If so, when will it end? 20 o C 800 o C

6 Thermal Energy Transfer Thermal energy is transferred when there is a difference in temperature. Thermal energy always flows from a region of higher temperature to a region of lower temperature. When thermal equilibrium is reached between two bodies, i.e. both bodies are at the same temperature, there is no net flow of thermal energy between them.

7 Misconception! Thermal energy Heat. Heat is defined as energy in transit from a high temperature body to a lower temperature body. Amount of thermal energy transferred will be the amount of heat that flowed.

8 Processes of Thermal Energy Transfer When you put your hand above the pit, you feel the heat. Why? When you hold the skewer with your hand, you feel the heat. Why? When you stand near to the pit, you feel the heat. Why?

9 Processes of Thermal Energy Transfer Thermal energy is transferred by: Conduction Convection Radiation

10 Transfer of Thermal Energy Thermal Energy Transfer Conduction Convection Radiation

11 Lesson Objectives Define conduction. Describe some uses of conductors and insulators.

12 Conduction Conduction is the process of thermal energy transfer without any flow of the material medium.

13 Video Time!

14 Conduction All matter are made up of tiny particles. When thermal energy is supplied to one end of a rod, the particles gain energy and vibrate more vigorously. These particles collide with neighbouring particles, making them vibrate more as well. Thermal energy is transferred from the hot end to the cold end. There is no net movement of particles during the process of conduction.

15 Conduction in Solids Different materials conduct heat at different rates. Those that conduct faster are called good conductors and those slower are called poor conductors. Poor conductors are also known as insulators. Most metals are good conductors of heat. Materials such as wood, rubber, asbestos, plastic and glass are poor conductors of heat.

16 Touch Test At room temperature, certain objects feel warmer to the touch than others. This is because some objects are better conductors of heat than others.

17 Normal body temperature is about 37 C, while room temperature is about 30 C. Heat is conducted away from the body when it is in contact with cooler objects in the room. The carpet is a bad conductor of heat. It feels warm as it does not conduct heat away easily from the body. The marble floor is a good conductor, so it is cool to the touch. Both the carpet and the marble floor are at the same temperature.

18 Conduction in Liquids and Gases Thermal energy can be conducted from a hotter to a cooler region. Liquid particles are further apart and collisions of particles are less frequent and even lesser in gases. Thus, transfer of energy from particles to neighbouring particles is slower. Hence, air is poor conductor of heat compared to water, which in turn, is a poor conductor of heat compared to most solids.

19 Video Time!

20 Applications of Conduction

21 Transfer of Thermal Energy Thermal Energy Transfer Conduction Convection Radiation

22 Lesson Objectives Describe how energy transfer occurs in fluids via density changes during the process of convection.

23 Think About It Liquids and gases are poor conductors of heat. How does thermal energy gets transferred in liquids and gases?

24 Convection Convection is the transfer of thermal energy by means of currents in a fluid. A fluid can be a liquid or gas.

25 Video Time!

26 Convection When fluids are heated, they expand and become less dense than the surrounding fluids. The less dense fluids tend to rise from the heating source. The cooler fluids, being more dense, sink to replace the less dense fluids. This movement of fluid due to a difference in its density sets up a convection current.

27 Put On Your Thinking Cap! Why do the fluids become less dense when they expand? When they expand, volume increases. Mass remains the same. Given that density = mass / volume, density decreases. Hence, they become less dense and rises.

28 Put On Your Thinking Cap! Why does convection occurs only in liquids and gases, but not solids? Liquid and gas particles are able to move about. Liquids and gases can flow. Solid particles are held in fixed positions. Solids cannot flow.

29 Convection Convection involves the bulk movement of the fluids which carry with them thermal energy.

30 Applications of Convection Heating water in electric kettles: the heating coil is placed at the bottom to aid the heating of water by convection.

31 Applications of Convection Household hot water system: the heater is located at the bottom of the system.

32 Applications of Convection Air-conditioners: they are installed near to the ceiling of rooms to facilitate setting up convection currents as cooler air sinks.

33 Applications of Convection Refrigerators: freezing unit is placed at the top to cool the air and facilitate the setting up of convection currents.

34 Transfer of Thermal Energy Thermal Energy Transfer Conduction Convection Radiation

35 Lesson Objectives Explain energy transfer of a body by radiation. State the factors affecting the rate of energy transfer by radiation.

36 Radiation Radiation is the transfer of thermal energy in the form of waves such as infrared radiation without the aid of a medium. It can take place in vacuum.

37 Example of Radiation The Sun emits infrared radiation. Thermal energy from infrared radiation makes us feel warm. All objects emit infrared radiation. The hotter the object, the greater the rate of infrared radiation emitted.

38 Emission of Infrared Radiation Infrared radiation is emitted by all objects and surfaces. This emission causes the temperature of the objects to fall.

39 Absorption of Infrared Radiation Infrared radiation is absorbed by all objects and surfaces. The absorption of infrared radiation causes a temperature rise.

40 Emission and Absorption In general, a good emitter of infrared radiation is also a good absorber of infrared radiation. Conversely, a poor emitter of infrared radiation is also a poor absorber of infrared radiation.

41 Factors Affecting Rate of Radiation Colour and texture of the surface: Dull, black surfaces are better emitters of infrared radiation than shiny, silver surfaces. Dull, black surfaces are better absorbers of infrared radiation than shiny, silver surfaces.

42 Factors Affecting Rate of Radiation Surface temperature: The higher the temperature of the surface of an object relative to the surrounding temperature, the higher the rate of emission of infrared radiation.

43 Factors Affecting Rate of Radiation Surface area: The larger surface area will emit or absorb infrared radiation at a higher rate.

44 Who is Hotter?

45 Who is Cooler?

46 Applications of Radiation Teapots: shiny teapots can keep tea warm for a longer period of time as compared to black teapots. It can also keep cold liquids cool for a longer time than black containers.

47 Applications of Radiation Greenhouses: infrared radiation emitted by the contents in the greenhouse is trapped in the greenhouse.

48 Vacuum Flask Heat loss is minimized in four possible ways: Conduction Convection Radiation Evaporation

49 Vacuum Flask Plastic stopper reduces heat transfer by conduction, convection and evaporation. Air is trapped in the vacuum flask. Air is also a poor conductor of heat. The vacuum between the double-walled glass minimises conduction and convection. The glass walls are silvered and highly reflective to minimise heat transfer due to radiation. Cork is a poor conductor of heat.

50 Spacesuit Living conditions in space are very harsh. Not only is there no air to breathe, but temperatures can be very high in some places and very low in others. Astronauts on space shuttle missions usually have to work outside their shuttle. Thus, it is important that the astronauts spacesuits are able to keep temperature at a comfortable level.

51 Helmet This is made of a special insulating material to prevent heat transfer through conduction. A white outer layer Spacesuits are coloured white in order to reflect radiation from the Sun. This prevents the astronaut from absorbing too much radiation that heats up the body. Layers of insulating material These layers are made of a special insulating material, which prevents heat from conducting into or out of the spacesuit. Air trapped between these layers also prevents the conduction of heat.

52 Concept Map Transfer of Thermal Energy Conduction Convection Radiation Vibration and collision of particles; no flow of medium Fluid medium flows in currents due to density changes Transfer of infrared waves; no medium needed Affected by: Surface colour & texture Surface temperature Surface area