AQA GCSE 1a-1Heat Transfer AQA GCSE Physics pages 24 to 37 AQA GCSE Science pages 228 to 241 August 21st 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: • 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.
glass copper copper copper Rate of heat transfer Heat transfer by conduction can increased by: • using a better conducting substance • decreasing the thickness of the substance • increasing the area of the substance • increasing the temperature difference across the substance 60°C 100°C 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: Conduction is the main form of ______ transfer in solids. This is because the molecules are relatively _______ together. Extra heat energy makes the molecules __________ more. They pass on their extra vibrational energy to neighbouring ______________. Metals are __________ conductors of heat energy because they contain many free __________ which can move through the solid and ____________ energy. The electrons give up their energy when they ___________ with other molecules. heat close move molecules good electrons transfer collide 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: copper conductor cooking pan bases feathers insulator keeping birds warm water conductor cooling hot substances fibre glass insulator roof insulation steel conductor radiators air insulator clothing
Question 3 • Explain why • newspaper wrapping keeps hot things hot, e.g. fish and chips, and cold things cold, e.g. ice cream, • fur coats would keep their owners warmer if they were worn inside out, • a string vest keeps a person warm even though it is a collection of holes bounded by string, • a concrete floor feels colder than a carpeted one even though they are at the same temperature.
ConductionNotes questions from pages 28/232 & 29/233 • 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 non-metals? • 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.
In text questions: Plastic and wood are poor thermal conductors, so the handle doesn’t get hot. The material the rods are made from is the independent variable. The dependent variable is the time taken for the wax to melt. Felt. The starting temperature. Summary questions: (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. Conduction ANSWERS
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.
hot air rising fresh air Simple mine ventilation 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 ________ (liquids and ________). When part of a fluid is heated the ___________ in that region move more ______ and take up more space. The heated fluid expands and becomes less _______ than the surrounding cooler fluid. The heated fluid _______ on top of the cooler fluid. The upward path of the heated fluid is called a ____________current. fluids gases molecules quickly dense rises convection WORD SELECTION: gases convection dense molecules rises fluids quickly
Question 2 • Explain why • a hot drink will cool more slowly if a lid is put on the top of the drink, • clouds are often formed at the top of thermals, • top loading freezers are more efficient than those with side doors, • hot water radiators are best placed\under a window.
ConvectionNotes questions from pages 30/234 & 31/235 • 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.
In text question: 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. Convection ANSWERS
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 infra-red pictures (Thermal Camera Pictures)
Thermal radiation & temperature • All objects above absolute zero (-273 oC) 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: Thermal radiation, also known as __________ radiation, is how heat travels by _____________ waves. Radiation travels equally in all ________and most quickly through a __________ where its speed is ___________ km/s. All objects above absolute zero ( ________ oC ) give off thermal radiation. Hot, _______ and rough surfaces emit radiation best. Bright and ___________surfaces reflect radiation best. heat electromagnetic directions vacuum 300 000 - 273 dark smooth WORD SELECTION: electromagnetic vacuum - 273 heat directions 300 000 dark smooth
Question 2 • Explain why • heat from the Sun can only reach us by thermal radiation, • in hot countries houses are often painted white, • car radiators are black, • solar cells are black.
Thermal radiationNotes questions from pages 24/228 & 25/229 • 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.
In text questions: The water is cooler than the rhino, so there is less radiation from it. The radiation from the rhino reflects at the water surface so the water surface acts like a mirror. Summary questions: Hot iron: infra-red only; light bulb: both; TV screen: light only; Sun: both Put your hand near it and see if it gets warm due to radiation from the iron. (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? Thermal radiation ANSWERS
Surfaces and radiationNotes questions from pages 26/230 & 27/231 • 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.
In text questions: 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. 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. Surfaces and radiation ANSWERS
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: _________ is the form of energy that ________ from a hot place to a cold one because of the __________ difference between these two places. Heat moves by conduction, ____________ and radiation. _________ energy is an alternative name for heat energy. heat travels temperature convection thermal 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: outer cap / cup conduction & convection plastic cap 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: draught excluders convection double glazing conduction& convection reflecting strips behind radiators radiation loft insulation conduction& convection carpets conduction cavity wall insulation conduction& convection
Heat transfer by designNotes questions from pages 32/236 & 33/237 • 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.