Boardworks GCSE Science: Physics Heat Transfer

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Boardworks GCSE Science: Physics Heat Transfer
Autumn 2006

Boardworks GCSE Science: Physics Heat Transfer
Autumn 2006

How is heat transferred?
Boardworks GCSE Science: Physics Heat Transfer Where and how is heat transfer taking place at this seaside? Teacher notes This illustration contains several examples of heat transfer, including: Barbecue grill cooking burgers and sausages (conduction, radiation). The car bonnet is so hot that as a sideline to the barbecue, someone is cooking food on it (conduction). Picnic boxes labelled “cold” and “hot” contain items that have been kept cool and war, respectively, to prevent heat transfer taking place. The twins on the left are wearing identical clothing except that one is wearing a white t-shirt and keeping cool, while the other is wearing a black t-shirt and looking much hotter (radiation). The twin in the black t-shirt is trying to keep himself cool with a portable fan (convection). The fluttering sails on the boat out at sea and the fluttering flag on the flagpole show that it is windy at the seaside (convection). The ice-creams are melted very quickly by the heat of the Sun, much to the annoyance of the child near the ice-cream van (radiation). Autumn 2006

Why does heat transfer happen?
Boardworks GCSE Science: Physics Heat Transfer Heat is a type of energy called thermal energy. Heat can be transferred (moved) by three main processes: 1. conduction 2. convection 3. radiation During heat transfer, thermal energy always moves in the same direction: HOT COLD Heat energy only flows when there is a temperature difference from a warmer area to a cooler area. Autumn 2006

Why do objects get hotter or colder?
Boardworks GCSE Science: Physics Heat Transfer Temperature is a measure of how hot an object is. Heat transfer only takes place when there is a temperature difference. The heat energy flows from a warmer area to a cooler area. Why does an ice lolly melt on a warm tongue? There is a temperature difference between the tongue and the lolly, so heat energy flows from the warm tongue into the cold ice lolly. This heat transfer means that the ice lolly melts as it gets warmer, and the warm part of the tongue touching it gets cooler. How might climate change cause the polar ice caps to melt? Autumn 2006

So… about Thermal Energy What’s up with Temperature vs Heat?
Temperature is related to the average kinetic energy of the particles in a substance.

You know the SI unit for temp is Kelvin
K = C (10C = 283K) C = K – 273 (10K = -263C) Thermal Energy is the total of all the kinetic and potential energy of all the particles in a substance.

Thermal energy relationships
As temperature increases, so does thermal energy (because the kinetic energy of the particles increased). If the temperature stays the same, the thermal energy in a more massive substance is higher (because it is a total measure of energy).

Cup gets cooler while hand gets warmer
Heat The flow of thermal energy from one object to another. Heat always flows from warmer to cooler objects. Ice gets warmer while hand gets cooler

Specific Heat (c, sometimes s, but usually c)
Things heat up or cool down at different rates. Land heats up and cools down faster than water, and aren’t we lucky for that!?

Specific heat is the amount of heat required to raise the temperature of 1 kg (but in Chem we use g) of a material by one degree (C or K, they’re the same size). C water = 4184 J / kg C (“holds” its heat) C sand = 664 J / kg C (less E to change) This is why land heats up quickly during the day and cools quickly at night and why water takes longer.

Why does water have such a high specific heat?
water metal Water molecules form strong bonds with each other water molecule; (including H-bonds!)so it takes more heat energy to break the bonds. Metals have weak bonds (remember the “sea of e-) and do not need as much energy to break them.

How does energy affect materials?
Boardworks GCSE Science: Physics Heat Transfer Do different materials need the same amount of energy to increase their temperature by the same amount? To increase the temperature of 1 kg of water by 1°C, requires 4200 J. To increase the temperature of 1 kg of copper by 1°C, requires 390 J. Water and copper require different amounts of energy because they have different values for a property called specific heat capacity. Teacher notes This slide could be used to highlight the comparability between the specific heat capacity of different materials. It should be stressed that the specific heat capacity is the amount of energy required to produce a 1 °C temperature rise in 1kg of material. It is the amount of energy required to increase the temperature of 1 kg of a material by 1°C. So, the specific heat capacity for water is 4200 J/kg°C and for copper is 390 J/kg°C. Autumn 2006

Specific Heat Capacity can be thought of as a measure of how much heat energy is needed to warm the substance up. You will possibly have noticed that it is easier to warm up a saucepan full of oil than it is to warm up one full of water.

Approximate values in J / kg °K of the Specific Heat Capacities of some substances are:
Air Lead Aluminium Mercury Asbestos Nylon 1700 Brass Paraffin 2100 Brick Platinum 135 Concrete Polythene 2200 Cork Polystyrene 1300 Glass Rubber 1600 Gold Silver 235 Ice Steel 450 Iron Water 4200

What is specific heat capacity?
Boardworks GCSE Science: Physics Heat Transfer The specific heat capacity of a material is the amount of energy required to raise 1 kg of the material by 1 °C. It can be used to work out how much energy is needed to raise the temperature of a material by a certain amount: specific heat capacity temperature change energy = mass x x Energy is measured in joules (J). Mass is measured in kilograms (kg). Temperature change is measured in °C. Specific heat capacity is measured in J/kg°C. Autumn 2006

Specific heat capacity example
Boardworks GCSE Science: Physics Heat Transfer Using the specific heat capacity of water (4200 J/kg°C), how much energy is needed to increase the temperature of 600 g of water by 80°C in a kettle? Note: mass = 600 g = 0.6 kg specific heat capacity temperature change energy = mass x x energy = x x 80 = J Autumn 2006

Boardworks GCSE Science: Physics Heat Transfer
Autumn 2006

Boardworks GCSE Science: Physics Heat Transfer
What is conduction? Boardworks GCSE Science: Physics Heat Transfer How are the particles arranged in a solid, a liquid and a gas? solid liquid gas Particles that are very close together can transfer heat energy as they vibrate. This type of heat transfer is called conduction. Teacher notes The importance of these visual representations of the arrangement of particles in solids, liquids and gases should be stressed. Students could be asked to discuss the limitations of these models, which include: the models are static whereas the particles are actually moving the models are two-dimensional whereas the particles are actually 3D dimensional. Students could also be asked to discuss the good points of the models and why they are useful. Conduction is the method of heat transfer in solids but not liquids and gases. Why? What type of solids are the best conductors? Autumn 2006

How do non-metals conduct heat?
Boardworks GCSE Science: Physics Heat Transfer Teacher notes This animation shows how heat is conducted through a non-metal on a particle level. Whilst viewing the animation it should be emphasised that this process is relatively slow, which is why conduction in non-metals is slow compared to conduction in metals. Autumn 2006

How do metals conduct heat?
Boardworks GCSE Science: Physics Heat Transfer Metals are good conductors of heat. The outer electrons of metal atoms are not attached to any particular atom. They are free to move between the atoms. When a metal is heated, the free electrons gain kinetic energy. This means that the free electrons move faster and transfer the energy through the metal. This makes heat transfer in metals very efficient. Teacher notes Please note that this diagram is not drawn to scale. It should be stressed to students that an electron is tiny compared to a metal ion, especially the solid nucleus of a metal ion. This means that the electrons are able to pass between the metal ions. Insulators do not have free electrons and so they do not conduct heat as well as metals. heat Autumn 2006

Boardworks GCSE Science: Physics Heat Transfer
Autumn 2006

What happens a fluid is heated?
Boardworks GCSE Science: Physics Heat Transfer Liquids and gases can both flow and behave in similar ways, so they are called fluids. What happens to the particles in a fluid when it is heated? less dense fluid heat The heated fluid particles gain energy, so they move about more and spread out. The same number of particles now take up more space, so the fluid has become less dense Autumn 2006

Boardworks GCSE Science: Physics Heat Transfer
What is convection? Boardworks GCSE Science: Physics Heat Transfer Warmer regions of a fluid are less dense than cooler regions of the same fluid. The warmer regions will rise because they are less dense. The cooler regions will sink as they are more dense. This is how heat transfer takes place in fluids and is called convection. Teacher notes The diagram illustrates how temperature affects the density of particles in a fluid. When using the diagram it should be highlighted that the particles near the top are further apart and the particles near the bottom are closer together. The widely-space particles at the top of the diagram represent a warmer region of the fluid. They are floating on top of a cooler region of air (represented by the particles spaced closely together) because the warm region of the fluid is less dense. The dotted boxes have been included to help students visualise the density of the fluid. Students could be asked to count the number of particles in each box to compare the density of the different regions. It should be noted that the density of the regions of the fluid in this diagram have been exaggerated to increase the contrast and make the regions clearer to see. The steady flow between the warm and cool sections of a fluid, such as air or water, is called a convection current. Autumn 2006

How does convection in a liquid occur?
Boardworks GCSE Science: Physics Heat Transfer Teacher notes This three-stage animation uses the example of boiling water in an electric kettle to demonstrate how convection occurs in a liquid. Whilst showing the animation it could be highlighted that the hot water floats up through the cold water because the hot water is less dense. The denser cold water falls below the hot water, pushing the hot water up. Autumn 2006

How does convection in a gas occur?
Boardworks GCSE Science: Physics Heat Transfer Teacher notes This three-stage animation uses the example of seaside breezes to demonstrate how convection occurs in a a gas. Red arrows are used to represent the movement of warm air and blue arrows are used to represent the movement of cooler air. Suitable prompts include: Stage 1: Why is the hot air rising? Stage 2: What happens to the warmer air as it moves out to sea? Stage 3: Why has the convection current changed direction? Autumn 2006

Why is convection important in fridges?
Boardworks GCSE Science: Physics Heat Transfer Why is the freezer compartment at the top of a fridge? The freezer compartment is at the top of a fridge because cool air sinks. The freezer cools the air at the top and this cold air cools the food on the way down. It is warmer at the bottom of the fridge. This warmer air rises and so a convection current is set up inside the fridge, which helps to keep the fridge cool. Autumn 2006

Conduction and convection – summary
Boardworks GCSE Science: Physics Heat Transfer Teacher notes This completing sentence activity provides the opportunity for some informal assessment of students’ understanding of conduction and convection. Autumn 2006

Boardworks GCSE Science: Physics Heat Transfer
Autumn 2006

How does heat travel through space?
Boardworks GCSE Science: Physics Heat Transfer The Earth is warmed by heat energy from the Sun. How does this heat energy travel from the Sun to the Earth? ? infrared waves There are no particles between the Sun and the Earth, so the heat cannot travel by conduction or by convection. The heat travels to Earth by infrared waves. These are similar to light waves and are able to travel through empty space. Autumn 2006

What are infrared waves?
Boardworks GCSE Science: Physics Heat Transfer Heat can move by travelling as infrared waves. These are electromagnetic waves, like light waves, but with a longer wavelength. This means that infrared waves act like light waves: They can travel through a vacuum. They travel at the same speed as light – 300,000,000 m/s. They can be reflected and absorbed. Infrared waves heat objects that absorb them and are also known as thermal radiation. Autumn 2006

Investigating thermal emission
Boardworks GCSE Science: Physics Heat Transfer Teacher notes This virtual experiment uses a Leslie’s cube to investigate the emissions of heat radiation from different surfaces. It could be use as a precursor to running the practical in the lab or as a revision exercise. When using this activity, it should be made clear that the water inside the cube and the initial temperature of each side is the same in each experiment. The thermopile measures the amount of heat radiation emitted by the surface, which is then recorded in the table. Autumn 2006

Boardworks GCSE Science: Physics Heat Transfer All objects emit (give out) some thermal radiation. Certain surfaces are better at emitting thermal radiation than others. best emitter worst emitter matt black white silver Matt black surfaces are the best emitters of radiation. Shiny surfaces are the worst emitters of radiation. Which type of kettle would cool down faster: a black kettle or a shiny metallic kettle? Autumn 2006

Investigating thermal absorption
Boardworks GCSE Science: Physics Heat Transfer Teacher notes This virtual experiment investigates the ability of different surfaces to absorb heat. It could be use as a precursor to running the practical in the lab or as a revision exercise. To extend the activity, students could be asked to consider which variables need to be controlled to make this experiment fair and accurate. Autumn 2006

Boardworks GCSE Science: Physics Heat Transfer Infrared waves heat objects that absorb (take in) them. Certain surfaces are better at absorbing thermal radiation than others. Good emitters are also good absorbers. best emitter worst emitter matt black white silver best absorber worst absorber Matt black surfaces are the best absorbers of radiation. Shiny surfaces are the worst emitters because they reflect most of the radiation away. Why are solar panels that are used for heating water covered in a black outer layer? Autumn 2006

Infrared radiation – true or false?
Boardworks GCSE Science: Physics Heat Transfer Teacher notes This true-or-false activity could be used a plenary or revision exercise to check students’ understanding of infrared radiation. Students could be given coloured traffic light cards (red=false, green=true) to vote on the statements shown. To stretch students, they could be asked to explain their voting. Autumn 2006

Boardworks GCSE Science: Physics Heat Transfer
Autumn 2006

Boardworks GCSE Science: Physics Heat Transfer
Glossary Boardworks GCSE Science: Physics Heat Transfer absorber – A material that takes in thermal radiation. conduction – The method of heat transfer in solids. conductor – A material that lets heat flow through it. convection – The method of heat transfer in fluids, which occurs because hot fluids are less dense than cold fluids. emitter – A material that gives out thermal radiation. free electrons – Electrons in a metal that are free to move through the metal. heat transfer – The flow of heat energy from a hotter area to a colder area. radiation – Heat energy transferred by infrared waves. This method of heat transfer does not need particles. Autumn 2006

Boardworks GCSE Science: Physics Heat Transfer
Anagrams Boardworks GCSE Science: Physics Heat Transfer Autumn 2006

Which type of heat transfer?
Boardworks GCSE Science: Physics Heat Transfer Teacher notes Appropriately coloured voting cards could be used with this classification activity to increase class participation. Autumn 2006

Boardworks GCSE Science: Physics Heat Transfer
Multiple-choice quiz Boardworks GCSE Science: Physics Heat Transfer Teacher notes This multiple-choice quiz could be used as plenary activity to assess students’ understanding of heat transfer. The questions can be skipped through without answering by clicking “next”. Students could be asked to complete the questions in their books and the activity could be concluded by the conclusion on the IWB. Autumn 2006

Assignment 1.1 Research more and give 10 applications of thermal expansion to real life. Due on Friday.