AQA GCSE 1 a-1 Heat Transfer AQA GCSE

AQA GCSE 1 a-1 Heat Transfer AQA GCSE Physics pages 24 to 37 AQA GCSE Science pages 228 to 241 August 21 st 2010

AQA GCSE Specification HEAT ENERGY TRANSFER 11. 1 How is heat (thermal energy) transferred and what factors affect the rate at which heat is transferred Using skills, knowledge and understanding of how science works: • to evaluate ways in which heat is transferred in and out of bodies and ways in which the rates of these transfers can be reduced. Skills, knowledge and understanding of how science works set in the context of: • Thermal (infra red) radiation is the transfer of energy by electromagnetic waves. • All bodies emit and absorb thermal radiation. • The hotter a body is the more energy it radiates. • Dark, matt surfaces are good absorbers and good emitters of radiation. • Light, shiny surfaces are poor absorbers and poor emitters of radiation. • The transfer of energy by conduction and convection involves particles and how this transfer takes place. • Under similar conditions different materials transfer heat at different rates. • The shape and dimensions of a body affect the rate at which it transfers heat. • The bigger the temperature difference between a body and its surroundings, the faster the rate at which heat is transferred.

Heat Energy • • Heat is energy that flows from a hot region to a cold region because of the temperature difference between the two regions. Heat energy flows by one or more of the processes of: 1. 2. 3. CONDUCTION CONVECTION RADIATION

Conduction This is how heat energy flows from a hot to cold region of a substance without the molecules of the substance moving from the hot to cold region.

Conduction by molecules • Molecules vibrate more when they are heated. • Intermolecular forces allow the molecules to pass their vibrations from one to another. • The stronger the forces the faster the vibrations are passed. • Solids are the best conductors and gases the worst. • A poor conductor is called an insulator. • A vacuum does not allow conduction.

Conduction through metals • Metals have an additional method of conduction. • Metals have electrons that can move from one atom to another. • These free electrons can pass energy through the metal very quickly. • Most of the best conductors are metals. • However, diamond, a non-metal, is the best conductor despite only using the molecule to molecule method.

Rate of heat transfer Heat transfer by conduction can increased by: 1. using a better conducting substance 2. decreasing the thickness of the substance 3. increasing the area of the substance 4. increasing the temperature difference across the substance 60°C 100°C copper glass copper 20°C

Finding the best conductor • All the rods have the same length and cross-sectional area. • They are all heated equally at one end with the bunsen burner. • When the other end of a rod reaches a certain temperature the paraffin wax melts and the match stick falls off. • A match stick will fall off the best conductor first. • This should be the rod made of copper.

Comparing brass & wood • The gummed paper singes and burns first on the wood side of the rod. • This is because the brass removes the heat away from the paper more quickly than the wood. • Brass is therefore the better conductor.

Water - a poor conductor • A boiling tube of water is heated near the top of the water. • Water boils at the top. • The bottom of the tube remains cool enough to hold. • This shows that water (and glass) only conduct heat relatively slowly.

Air - a good insulator Air trapped in feathers, cavity wall insulation, loft insulation, duvets and lagging greatly reduces heat loss by conduction.

Question 1 Choose appropriate words to fill the gaps below: heat Conduction is the main form of ______ transfer in solids. This is because the molecules are relatively _______ together. close move Extra heat energy makes the molecules _____ more. They pass on molecules their extra vibrational energy to neighbouring _______. good Metals are _____ conductors of heat energy because they contain electrons many free _____ which can move through the solid and transfer ______ energy. collide The electrons give up their energy when they ______ with other molecules. WORD SELECTION: close heat good collide molecules transfer move electrons

Question 2 Write down three insulating materials and three conducting materials as well as their uses in the table below: substance conductor or insulator use copper conductor cooking pan bases feathers insulator keeping birds warm water conductor fibre glass insulator steel conductor radiators air insulator clothing cooling hot substances roof insulation

Question 3 Explain why a) newspaper wrapping keeps hot things hot, e. g. fish and chips, and cold things cold, e. g. ice cream, b) fur coats would keep their owners warmer if they were worn inside out, c) a string vest keeps a person warm even though it is a collection of holes bounded by string, d) a concrete floor feels colder than a carpeted one even though they are at the same temperature.

Conduction Notes questions from pages 28/232 & 29/233 1. 2. 3. 4. 5. 6. Describe, with the aid of a diagram, an experiment to compare how well materials conduct heat. Explain the process of how conduction occurs in (a) metals and (b) non-metals. Why are metals generally better conductors than nonmetals? Copy and answer questions (a), (b), (c) and (d) on pages 28/232 and 29/233. Copy the Key Points on page 29/233. Answer the summary questions on page 29/233.

Conduction ANSWERS In text questions: (a) Plastic and wood are poor thermal conductors, so the handle doesn’t get hot. (b) The material the rods are made from is the independent variable. The dependent variable is the time taken for the wax to melt. (c) Felt. (d) The starting temperature. Summary questions: 1. (a) Fibreglass (b) Wood, plastic (c) Steel 2. (a) Felt or synthetic fur could be used, because they are good insulators. (b) Is your plan a fair test? 3. The free electrons that gain kinetic energy diffuse through the metal quickly, passing on energy to the other electrons and ions in the metal.

Convection This is how heat energy flows from a hot to cold region of a substance by the molecules of the substance moving from the hot to cold region.

Convection in fluids • A fluid is a liquid or a gas. • When molecules are heated they move more quickly and occupy more space. • Hotter fluids are therefore less dense than colder fluids. • Hotter fluids rise up to float on top of colder fluids. • A convection current is the path taken by rising hot fluids and sinking cold fluids. • Convection does not occur in solids.

Convection in water • Potassium permanganate crystals are used to dye water purple. • When the bottom of the flask of water is heated warm less dense water rises. • The potassium permanganate dye rises with the warmed water. • When the water cools it becomes denser and sinks down to the bottom of the flask.

House water heating system • The boiler heats the water. • Hot water rises to the top of the boiler and up to the top of the storage tank. • Colder water in the tank falls to the bottom of the boiler to be heater. • A hot water tap draws water from the top of the storage tank. • The cold water supply replenishes the hot water drawn off.

Engine water cooling system • Water heated by the engine rises to the top of the engine. • This water is pumped into the top of the radiator. • The fan cools the water in the radiator. • Cooled water falls from the radiator into the engine.

Hot air rises • Heated air provides lift for a hot air balloon. • Heated land causes rising air currents called thermals. • Thermals can be used by gliders to provide lift. • Many birds also use thermals for lift.

Sea and land breezes • During a hot day heated air rises up from land that is warmer than the sea. • Cooler air moves in from the sea to replace the heated air. This is the sea breeze. • At night the sea is often warmer than the land. • Air now flows to the sea. This is called a land breeze.

Simple mine ventilation hot air rising fresh air The fire causes hot air to rise up the shaft above it. Cooler, fresher air is drawn down the other shaft. .

Question 1 Choose appropriate words to fill in the gaps below: Convection is a method of heat transfer that only occurs in fluids gases ____ (liquids and ____). molecules When part of a fluid is heated the ______ in that region quickly move more ______ and take up more space. The heated fluid dense expands and becomes less _______ than the surrounding rises cooler fluid. The heated fluid _______ on top of the cooler fluid. The upward path of the heated fluid is called a convection ______current. WORD SELECTION: gases convection dense molecules rises fluids quickly

Question 2 Explain why a) a hot drink will cool more slowly if a lid is put on the top of the drink, b) clouds are often formed at the top of thermals, c) top loading freezers are more efficient than those with side doors, d) hot water radiators are best placedunder a window.

Convection Notes questions from pages 30/234 & 31/235 1. 2. 3. 4. 5. 6. 7. Describe, with the aid of a diagram, an experiment to demonstrate convection in air. Copy Figure 3 on page 30/234 and explain how hot water travels from the boiler to the hot water tap. Copy and answer question (a) on page 30/234. Draw a diagram and explain the cause of a sea breeze. Explain how convection occurs and why it only occurs in fluids. Copy the Key Points on page 31/235. Answer the summary questions on page 31/235.

Convection ANSWERS In text question: (a) The water from the taps would be cold. Summary questions: 1. Rises, mixes, cools, falls. 2. (a) It heats it and makes it rise. (b) The hot air passes through the grille into the room. (c) Cold air flows in at the bottom. 3. Drop the crystal into the beaker of water through a tube. Heat gently under one corner. the colour rises above the point of heating, travels across the top and falls at the opposite side of the beaker (where density of cooler water is greater). The colour then travels across the bottom of the beaker to replace lower density warmer water that rises above the Bunsen flame.

Thermal Radiation This is how heat energy moves from a hot to cold region by means of electromagnetic waves. Thermal radiation is also called: infra-red or IR radiation.

Properties of thermal radiation • Radiation travels equally in all directions. • Radiation travels far more quickly than the other forms of heat transfer. • Radiation is the only type of heat transfer that can travel through a vacuum. • Radiation travels best through a vacuum where its speed is that of light (300 000 km/s or 186 000 miles per second). • Gases tend to allow radiation through better than liquids, liquids better than solids.

Detecting thermal radiation • We can feel thermal radiation with our skin. • A thermopile is a device that converts thermal radiation into electricity. • Special cameras can be used to take infrared pictures (Thermal Camera Pictures)

Thermal radiation & temperature • All objects above absolute zero (-273 o. C) give off thermal radiation. • The hotter an object the more radiation it gives off.

Surface and emission Darker surfaces emit radiation better than bright surfaces. Rough surfaces emit radiation better than polished surfaces.

Absorption & Reflection Dark surfaces absorb radiation better than bright surfaces. Polished surfaces reflect radiation better than rough surfaces.

Silvered surfaces A metal kettle, a firefighter and a marathon runner make use of silvered surfaces.

The Greenhouse The ‘Greenhouse Effect’ is where gases such as carbon dioxide in the Earth’s atmosphere have the same effect as the glass of a greenhouse.

Question 1 Choose appropriate words to fill in the gaps below: heat Thermal radiation, also known as _____ radiation, is electromagnetic how heat travels by _______ waves. directions Radiation travels equally in all ____and most quickly vacuum 300 000 through a _____ where its speed is ______ km/s. All objects above absolute zero ( ____ o. C ) give off - 273 thermal radiation. dark Hot, _______ and rough surfaces emit radiation best. smooth Bright and ______surfaces reflect radiation best. WORD SELECTION: electromagnetic vacuum - 273 heat directions 300 000 dark smooth

Question 2 Explain why a) heat from the Sun can only reach us by thermal radiation, b) in hot countries houses are often painted white, c) car radiators are black, d) solar cells are black.

Thermal radiation Notes questions from pages 24/228 & 25/229 1. 2. 3. 4. 5. 6. 7. 8. Draw figure 1 on page 24/228 and describe the experiment. Why is thermal radiation sometimes called infra-red radiation? Copy the ‘Key points’ on page 25/229. In what form does energy travel from the Sun? Explain what is meant by the ‘Greenhouse Effect’ Copy and answer questions (a) and (b) on page 24/229. Copy the Key Points on page 25/229. Answer the summary questions on page 25/229.

Thermal radiation ANSWERS In text questions: (a) The water is cooler than the rhino, so there is less radiation from it. (b) The radiation from the rhino reflects at the water surface so the water surface acts like a mirror. Summary questions: 1. Hot iron: infra-red only; light bulb: both; TV screen: light only; Sun: both 2. Put your hand near it and see if it gets warm due to radiation from the iron. 3. (a) They lose less heat through radiation when they huddle together because they radiate heat to each other. (b) Is your design a fair test?

Surfaces and radiation Notes questions from pages 26/230 & 27/231 1. 2. 3. 4. 5. 6. Which surfaces are: (a) the best emitters; (b) the best absorbers; (c) the worst emitters; (d) the best reflectors of radiation. Draw diagrams to explain why rough surfaces absorb radiation better than smooth ones. Copy and answer questions (a) and (b) on page 26/230. Describe an experiment to compare how different surfaces absorb radiation. Copy the Key Points on page 27/231. Answer the summary questions on page 27/231.

Surfaces and radiation ANSWERS In text questions: (a) A sandy surface is rough, not smooth, so it absorbs solar radiation more than a smooth icy surface. The sand grains become warm and melt the ice. (b) They absorb solar radiation better than any other type of surface. Summary questions: 1. (a) White buildings stay cooler than dark buildings, are better reflectors than dark buildings and they absorb less solar radiation. (b) A black surface absorbs solar radiation better than a light surface. 2. (a) To ensure a fair test. (b) (i) D (ii) B (c) (i) B (ii) D (d) Greater accuracy, collects multiple sets of data at whatever time intervals you choose. 3. In direct sunlight black will absorb heat readily, but in shade black will emit heat more readily.

A model of heat transfer processes 1. CONDUCTION – book passed from student to student. 2. CONVECTION – book taken a student to the back of the room. 3. RADIATION – book thrown to the back of the room.

Heating a room using a water radiator • • • Hot water heats the metal radiator by conduction. The outer metal surface heats air in contact by conduction and radiation. Hot air circulates a room by convection.

Reducing heat transfer using a vacuum flask CONDUCTION – reduced by the vacuum, stopper, glass, cork and air spaces. CONVECTION – reduced by the vacuum, stopper and the trapped air spaces. RADIATION – reduced by the silvered glass walls.

Question 1 Choose appropriate words to fill in the gaps in below: heat travels _____ is the form of energy that ____ from a hot temperature place to a cold one because of the _____ difference between these two places. convection Heat moves by conduction, ______ and radiation. thermal _____ energy is an alternative name for heat energy. WORD SELECTION: thermal temperature travels heat convection

Question 2 Write down the ways in which a vacuum flask reduces heat transfer in the table below: part of flask outer cap / cup plastic cap processes reduced (eg ‘conduction’) conduction & convection & conduction shiny mirror surfaces radiation vacuum conduction & convection sponge conduction air conduction plastic case conduction

Question 3 Write down six different ways in which heat flow from a house can be reduced in the table below: device or part of a house draught excluders processes reduced convection double glazing conduction & convection reflecting strips behind radiators radiation loft insulation carpets cavity wall insulation conduction & convection

Question 4

Heat transfer by design Notes questions from pages 32/236 & 33/237 1. 2. 3. 4. 5. 6. 7. Explain how a car engine is kept cold. Copy and answer questions (a) and (b) on page 32/236. Copy Figure 2 on page 32/236 and explain how the different parts of the vacuum flask reduce heat transfer. Explain four ways of reducing heat loss from a house. Copy and answer question (c) on page 33/237. Copy the Key Points on page 33/237. Answer the summary questions on page 33/237.

Heat transfer by design ANSWERS In text questions: (a) To increase heat loss by radiation. (b) It increases. (d) Cavity wall insulation does not transfer heat by convection, because it is a solid but air would. Summary questions: 1. (a) Conduction (b) Convection (c) Radiation 2. (a) A metal frame conducts heat to the outside; plastic is a poor thermal conductor so it does not. (b) A vacuum would not transfer heat by convection, but air would. (c) Is your design a fair test? 3. Plastic cap – reduces convection and conduction; double walled plastic container – reduces conduction; silvered surfaces – reduce radiation; vacuum – stops conduction and convection.

Simulations Conduction of heat in solids - e. Chalk Relative conductivities of metal rods - e. Chalk Convection currents in a room & fridge - e. Chalk Sequential Puzzle on Heat Conductor order- by KT - Microsoft WORD Energy House Simulation. For instructions click here. Worksheet for Energy House Simulation (KT) Gas molecule simulation of convection - falstad Thermal Camera Pictures - falstad Black Body Spectrum - Ph. ET - How does the blackbody spectrum of the sun compare to visible light? Learn about the blackbody spectrum of the sun, a light bulb, an oven, and the earth. Adjust the temperature to see the wavelength and intensity of the spectrum change. View the color of the peak of the spectral curve The Greenhouse Effect - Ph. ET - Just how do greenhouse gases change the climate? Select the level of atmospheric greenhouse gases during an ice age, in the year 1750, today, or some time in the future and see how the Earth's temperature changes. Add clouds or panes of glass. BBC KS 3 Bitesize Revision: Temperature and heat Transfer of thermal energy - includes applet showing convection currents in a room BBC Bitesize Revision: Introduction Page on AQA Heat Transfer & Efficiency Heat transfer by conduction & convection - includes applet showing convection currents in a room Heat transfer by radiation Reducing heat loss Test bite on Heat transfer, Energy transfers & Efficiency

Hot issues Notes questions from pages 34/238 & 35/239 1. Answer part (a) from the activity on page 34/238. 2. Answer questions 1, 2, 3, 5, 6, and 7 on pages 34/238 and 35/239.

Hot issues ANSWERS 1. 2. 3. Activity part (a): (i) Add cavity wall insulation to reduce heat loss through the walls and loft insulation to reduce heat loss through the roof. (ii) 20 years. Thermal energy from a deep tumour would be conducted to the surrounding parts and would not reach the surface of the body. Appreciation that devices can assist with a range of treatments but they come at a cost. Is the money better spent on other things? The hospitals may be part of a research programme, they could be sponsored by manufacturer to test the technology or they could be given extra money by a private benefactor or they could even be charging for its use. 5. (a) white (b) black 6. (a) non-reversible (b) reversible 7. (a) The heat from the flame could escape, raising the temperature of the methane so that it ignites. (b) To make sure that the gauze was not damaged. (c) A modern flame safetylamp and electronic methanometers.

How Science Works ANSWERS a) b) c) d) Minimum should be that pins on the different rods are all the same distance along the rod. (Better: placed at intervals along the rods). For example: use the same amount of Vaseline each time. The metal rods should be heated in the same way each time by the Bunsen (e. g. in the same part of the flame). The rods should also be protected from direct radiant energy from the Bunsen’s flame which would make the results invalid. Repeat procedure. e) Are there accurate labels with a simple layout? f) Bar chart – categoric independent variable with a continuous dependent variable. g) A second person achieving the same results increases reliability. h) The results support the conclusion. However, the conclusion is invalid due to different initial temperatures.