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© Boardworks Ltd of 32
© Boardworks Ltd of 32
© Boardworks Ltd of 32 How is heat transferred? Where and how is heat transfer taking place at this seaside?
© Boardworks Ltd of 32 Heat energy only flows when there is a temperature difference from a warmer area to a cooler area. Why does heat transfer happen? Heat is a type of energy called thermal energy. 1. conduction During heat transfer, thermal energy always moves in the same direction: HOT COLD 2. convection 3. radiation Heat can be transferred (moved) by three main processes:
© Boardworks Ltd of 32 Why do objects get hotter or colder? Temperature is a measure of how hot an object is. 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. How might climate change cause the polar ice caps to melt? Heat transfer only takes place when there is a temperature difference. The heat energy flows from a warmer area to a cooler area. This heat transfer means that the ice lolly melts as it gets warmer, and the warm part of the tongue touching it gets cooler.
© Boardworks Ltd of 32 How does energy affect materials? 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. and for copper is 390 J/kg°C. 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
© Boardworks Ltd of 32 What is specific heat capacity? The specific heat capacity of a material is the amount of energy required to raise 1 kg of the material by 1 °C. Energy is measured in joules (J). It can be used to work out how much energy is needed to raise the temperature of a material by a certain amount: energy specific heat capacity temperature change =massxx Mass is measured in kilograms (kg). Temperature change is measured in °C. Specific heat capacity is measured in J/kg°C.
© Boardworks Ltd of 32 Specific heat capacity example 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? energy = 0.6 x 4200 x 80 = J Note: mass = 600 g = 0.6 kg energy specific heat capacity temperature change =massxx
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© Boardworks Ltd of 32 How are the particles arranged in a solid, a liquid and a gas? What is conduction? solid liquid gas Particles that are very close together can transfer heat energy as they vibrate. This type of heat transfer is called conduction. What type of solids are the best conductors? Conduction is the method of heat transfer in solids but not liquids and gases. Why?
© Boardworks Ltd of 32 How do non-metals conduct heat?
© Boardworks Ltd of 32 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. How do metals conduct heat? heat This means that the free electrons move faster and transfer the energy through the metal. This makes heat transfer in metals very efficient. Insulators do not have free electrons and so they do not conduct heat as well as metals.
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© Boardworks Ltd of 32 What happens a fluid is heated? Liquids and gases can both flow and behave in similar ways, so they are called fluids. heat less dense fluid 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 What happens to the particles in a fluid when it is heated?
© Boardworks Ltd of 32 What is convection? Warmer regions of a fluid are less dense than cooler regions of the same fluid. called a convection current. The warmer regions will rise because they are more dense. The cooler regions will sink as they are less dense. This is how heat transfer takes place in fluids and is called convection. The steady flow between the warm and cool sections of a fluid, such as air or water, is
© Boardworks Ltd of 32 How does convection in a liquid occur?
© Boardworks Ltd of 32 How does convection in a gas occur?
© Boardworks Ltd of 32 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. This warmer air rises and so a convection current is set up inside the fridge, which helps to keep the fridge cool. Why is convection important in fridges? 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.
© Boardworks Ltd of 32 Conduction and convection – summary
© Boardworks Ltd of 32
© Boardworks Ltd of 32 infrared waves The Earth is warmed by heat energy from the Sun. 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. How does heat travel through space? How does this heat energy travel from the Sun to the Earth?
© Boardworks Ltd of 32 Heat can move by travelling as infrared waves. 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. What are infrared waves? Infrared waves heat objects that absorb them and are also known as thermal radiation. These are electromagnetic waves, like light waves, but with a longer wavelength.
© Boardworks Ltd of 32 Investigating thermal emission
© Boardworks Ltd of 32 Emitting thermal radiation All objects emit (give out) some thermal radiation. Matt black surfaces are the best emitters of radiation. white silver matt black best emitter worst emitter Certain surfaces are better at emitting thermal radiation than others. Shiny surfaces are the worst emitters of radiation. Which type of kettle would cool down faster: a black kettle or a shiny metallic kettle?
© Boardworks Ltd of 32 Investigating thermal absorption
© Boardworks Ltd of 32 Absorbing thermal radiation Infrared waves heat objects that absorb (take in) them. best emitter worst emitter best absorber worst absorber white silver matt black Matt black surfaces are the best absorbers of radiation. Certain surfaces are better at absorbing thermal radiation than others. Good emitters are also good absorbers. 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?
© Boardworks Ltd of 32 Infrared radiation – true or false?
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© Boardworks Ltd of 32 Glossary 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.
© Boardworks Ltd of 32 Anagrams
© Boardworks Ltd of 32 Which type of heat transfer?
© Boardworks Ltd of 32 Multiple-choice quiz
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