Thermal Effects. IGCSE Physics

Size: px

Start display at page:

Download "Thermal Effects. IGCSE Physics"

Helena Reed
6 years ago
Views:

1 Thermal Effects IGCSE Physics

2 Starter What is the difference between heat and temperature? What unit is thermal energy measured in? And what does it depend on? In which direction does heat flow?

3 Heat (Thermal Energy) transfer of energy from hotter to colder as a result of temperature difference measured in joules (J).

4 Temperature measure of the average random kinetic energy of the molecules measured in degrees Celsius (ºC) or K.

5 Heat the heat energy depends on: temperature (speed of particles) mass (number of particles) material (type of particles)

6 Thermal Expansion When thermal/heat energy is applied to a substance, the molecules gain kinetic energy and begin to vibrate more vigorously. As a result, the average distance between the atoms/molecules of the substance increases, causing the substance to expand.

7 Expansion and Contraction

8 Bimetallic strip Strips of two different metals bonded together. When heated, one metal expands more than other, making it bend.

9 Expansion and Contraction in Solids

10 starter Expansion happens in solids, liquids and gases. Write the relative order of magnitude of the expansion of solids, liquids and gases. Explain it in terms of motion and arrangement of molecules.

11 Anomalous Expansion of Water 1. When you fill a plastic bottle with water and put it in the freezer, it expands. 2. Some houses get burst pipes in winter.

12 Anomalous Expansion of Water

13 Conduction movement of heat through a substance by the collision of molecules.

14 Conduction

15 Conduction More thermal energy is transferred every second if: temperature difference across the ends of the bar is increased cross-sectional area of the bar is increased length of the bar is reduced

16 Conduction Good Conductors e.g. Cu Metals Al Fe Silicon Graphite Poor conductors Glass Water Plastics Rubber Wood Materials containing trapped air (e.g. wool, plastic foam, polystyrene)

17 Conduction in Metals In metals, conduction happens faster because some electrons are free to move.

18 Water is a poor thermal conductor

19 Methods of Heat Transfer Which methods of heat transfer do you know? Your teacher has a series of demonstrations for a method of heat transfer. Provide an explanation for your observations.

20 Convection actual movement of the particles carrying thermal energy. occurs in liquids and gases.

21 Convection This cycle is called a convection current. heat heat Can you explain how the convection current moves using the idea of density?

22 Convection

23 Starter Explain how the convection current moves using the idea of density. As the fluid above the heat source becomes warmer (its particles gain energy and vibrate more rapidly), it expands and becomes less dense. It rises upwards as cooler, denser fluid sinks and displaces it. The results is a circulating stream, called a convection current. As the particles circulate, they transfer energy to other parts of the beaker.

24 Convection

25 Convection

26 Thermal Radiation heat transfer due to emission of electromagnetic waves is known as thermal radiation. all matter with a temperature greater than absolute zero emits thermal radiation

27 Thermal Radiation The higher the surface temperature and the greater surface area of an object, the more energy they radiate per second. Thermal radiation is a mixture of different wavelengths

28 Thermal Radiation Emitters and absorbers

29 DAILY QUESTION # 3 Identify and describe the heat transfer mechanisms: A B C

30 Evaporation Happens when a liquid below its boiling point changes into a gas. Ways of making a liquid evaporate more quickly: Increasing the temperature Increase the surface area Reduce humidity Blow air across the surface

31 Boiling Rapid form of evaporation. Vapour bubbles form deep in the liquid, expand, rise, burst and release large amounts of vapour. At the top of Mount Everest, where atmospheric pressure is less, water would boil at. only 70

32 Evaporation vs Boiling

33 Mario s Heat Problems In pairs, explain what happens in each of the situations given in the cards. Each group will have two questions* that they have to answer and explain to the class. When the time finishes, hand in your answers to your teacher. Groups * G Questions

34 DAILY QUESTION # 4 Which experiment would you use to compare: a) Emitters of thermal radiation b) Absorbers of thermal radiation Thermal radiation detector A B

35 Cooling effect of evaporation Why does the evaporation of sweat cool you down? If faster particles escape from the liquid, slower ones are left behind, so the temperature of the liquid is less than before. Explain why, on a humid day you may feel hot and uncomfortable.

A refrigerant evaporates and takes thermal energy from the food and air. Vapour is drawn away by the pump, which compresses it and turns it into a liquid.

36 A refrigerant evaporates and takes thermal energy from the food and air. Vapour is drawn away by the pump, which compresses it and turns it into a liquid. This releases thermal energy, so the liquid heats up. The hot liquid is cooled as it passes through the pipes at the back, and the thermal energy is carried away by the air. Refrigerators

37 Starter What are the factors which are going to affect the rate at which laundry dries due to evaporation? Folded/unfolded (surface area) Air temperature Relative humidity Wind (convection currents)

38 SAME amount of HEAT ENERGY Small TEMPERATURE RISE WATER SAND Large TEMPERATURE RISE Putting the SAME AMOUNT OF HEAT into some materials gives a BIGGER TEMPERATURE RISE than in other materials

39 Comparing water and cooking oil 30 rise 1kg of water 1kg of cooking oil 60 rise 20ºC 50ºC 80ºC 20ºC.heating Identical rings turned on for 1 minute The water heats up less than the oil. The SAME AMOUNT OF HEAT produces HALF the TEMPERATURE RISE in the water as in the oil

40 SAME AMOUNT OF HEAT PUT IN 5ºC rise 10ºC rise 20ºC rise 40ºC rise water cooking oil Silica(rock) copper Two ways to look at heat capacity 10ºC rise water 10ºC rise cooking oil 10ºC rise silica (rock) 10ºC rise copper We only get ¼ the TEMP RISE with water than with rock for the SAME AMOUNT of HEAT HEAT FOR 16 mins 8 mins 4 mins 2 mins We need to put in 4x the AMOUNT OF HEAT into water than rock to get the SAME TEMP RISE

41 Specific Heat Capacity The specific heat capacity of a substance is the amount of energy needed to change the temperature of 1 kg of the substance by 1 C. Energy transferred (J) Q mc T Temperature change (⁰C or K) Specific Heat Capacity (J/kg⁰C) Thermal capacity m c

42 Measuring SHC

43 Measuring SHC Energy supplied by heater = voltage x current x time Energy transferred to water = mass x specific heat capacity x temp rise V It = m c T Specific heat capacity (c)= VIt/m T

44 SHC Using a 40W heater, 250g of water is heated for 3.0 minutes. The temperature rise of the water is 5.9 C. Use these values to calculate a value for the specific heat capacity of water. Specific heat capacity of water = 4.9 J/(g C). The accepted value for specific heat capacity of water is 4.2 J/(g C). State one possible reason why the value you obtained is different from this. Thermal energy losses

45 Starter

46 Latent Heat Latent heat is defined as the heat absorbed or released when a substance changes its physical state completely at constant temperature heat of fusion heat of vaporization

47 Specific Latent Heat The specific latent heat of a substance tells us how much energy is needed to change the state of 1 kg of substance at constant temperature. Energy transferred (J) Q ml Specific Latent Heat (J/kg) mass(kg)

48 Specific Latent Heat of Fusion 1 kg 1 kg All at 0 C Water J absorbed

49 Specific Latent Heat of Vaporization 1 kg 1 kg kg 1 kg All at 100 C Water J input

50 Measuring Specific Latent Heat of: Fusion Vaporization

51 Latent Heat A 460watt water heater is used to boil water. Assuming no thermal energy losses, what mass of steam will it produce in 10 minutes? Specific latent heat of vaporization of water = 2300 kj/kg m = 0.12 kg

52 Starter Calculate the amount of heat required to completely convert 50 g of ice at 0 ºC to steam at 100 ºC. The specific heat capacity of water is 4.18 kj/(kg C) The specific latent heat of fusion of ice is 334 kj/kg The specific latent heat of vaporization of water is 2260 kj/kg. 50g 50g 0 C 100 C

53 Example Question: Heat is taken up in three stages: 1. The melting of the ice. 0 C 0 C 2. The heating of the water. 0 C 100 C 3. The vaporization of the water. 100 C 100 C

54 Stage 1 1. Heat taken up for converting ice at 0ºC to water at 0ºC 0 C Stage 2 2. Heat taken up heating the water from 0 ºC to the boiling point, 100 ºC 0 C 100 C 0 C mass of water x latent heat of fusion = (kg) x 334 (kj.kg -1 ) = 16.7 kj mass of water x specific heat capacity x temperature change = 0.05 (kg) x 4.18(kJ.kg -1. C -1 ) x 100 (ºC) = 20.9 kj

55 Stage 3 3. Heat taken up vaporising the water 100 C 100 C Answer The sum of these is = kj (151 kj) mass of water x latent heat of vaporization = 0.05 (kg) x 2260 kj.kg -1 = 113 kj

Chapter 1 Heating Processes

Chapter 1 Heating Processes Section 1.1 Heat and temperature Worked example: Try yourself 1.1.1 CALCULATING THE CHANGE IN INTERNAL ENERGY A student places a heating element and a paddle wheel apparatus