Specific Latent Heat _ AQA GCSE Physics Revision Notes 2018.pdf

Join now GCSE Physics AQA Revision Notes 3. Particle M Specific Latent Heat (AQA GCSE Physics) Revision Note Download PDF Author Expertise Leander Physics...

Join now GCSE Physics AQA Revision Notes 3. Particle M Specific Latent Heat (AQA GCSE Physics) Revision Note Download PDF Author Expertise Leander Physics Specific Latent Heat A certain amount of energy is required to change the state of a certain mass of a substance This amount of energy is known as the latent heat The specific latent heat is defined as: The amount of energy required to change the state of 1 kg of a substance with no change in temperature There are two types of specific latent heat: Specific latent heat of fusion Changing the state between a solid and liquid Solid to liquid, or liquid to solid  Specific latent heat of vaporisation Changing the state between a liquid and gas Liquid to gas, or gas to liquid Latent heat is represented by the symbol L with units joules per kilogram (J/kg) The changes of state with heat supplied against temperature Latent Heat of Fusion The specific latent heat of fusion is defined as: The energy required to convert 1 kg of a substance between a solid and a liquid state with no change in temperature This applies when melting a solid or freezing a liquid When a solid substance is melted, its temperature stays constant until all of the substance has melted  The latent heat of fusion is the amount of energy needed per kg for all the particles in the substance to overcome the intermolecular forces of attraction holding them together in their solid state If a substance in its liquid state is frozen, the substance will solidify at the same temperature as its melting point In this case, the latent heat of fusion is the amount of energy per kg transferred away from the substance until all the particles in the substance have succumbed to the intermolecular forces of attraction that hold them together in their solid structure Latent Heat of Vaporisation The specific latent heat of vaporisation is defined as: The energy required to convert 1 kg between a liquid and a gaseous state with no change in temperature This applies when vaporising a liquid or condensing a gas When a liquid substance is vaporised, its temperature will stay constant until all of the substance has vaporised The latent heat of vaporisation is the amount of energy per kg needed for all the particles in the substance to overcome the intermolecular forces of attraction holding them together in their liquid state If a substance is a gas and is condensed, it will condense at the same temperature as its boiling point In this case, the latent heat of vaporisation is the amount of energy per kg transferred away from the substance until all the particles in the substance have succumbed to the intermolecular forces of attraction that hold them together in their liquid state Exam Tip The specific latent heat of fusion and vaporisation value of all substances will be provided for you in the exam question, so you do not need to memorise the value of any. However, make sure to include 'with no change in temperature' in your definition of specific latent heat to be awarded full marks. Use these reminders to help you remember which type of latent heat is being referred to: Latent heat of fusion = imagine ‘fusing’ the liquid molecules together to become a solid Latent heat of vaporisation = “water vapour” is steam, so imagine vaporising the liquid molecules into a gas But remember that the change of state can go in either direction! TEACHER TIP Leander Physics In my experience of teaching this topic to GCSE students, the graphs and the word ‘latent’ can cause some confusion. Latent just means hidden. So the energy being transferred into the system (by heating it) causes the temperature to rise to the melting of a substance, we can see this happening using a thermometer. But when the substance reaches its melting point, we can continue to transfer energy to the system but we don’t see the temperature rising any more, it stays at the melting point. So what happens to this energy? What is it being used for? It appears to be hidden or latent because we can’t see its effects. Once the substance has fully melted, the temperature begins to rise again. And when it reaches the boiling point, the same thing happens. The temperature stays constant even though energy is still being transferred into the system. What do these two events have in common? The period of latent energy transfers happened as the substance was changing state. Therefore, the energy transferred to the system at these points must be used for the state change. The energy is transferred to the molecules or particles and they use it to overcome the intermolecular forces of attraction holding them in their solid or liquid state. Physicists call this latent heat. Specific Latent Heat Equation The amount of energy E required to melt or vaporise a mass of m with latent heat L is: E = mL Where: E = thermal energy required for a change in state, in joules (J) m = mass, in kilograms (kg) L = specific latent heat, in joules per kilogram (J/kg) This equation can be rearranged with the help of a formula triangle:  For context, the values of latent heat for water are: Specific latent heat of fusion = 330 kJ/kg Specific latent heat of vaporisation = 2.26 MJ/kg Therefore, evaporating 1 kg of water requires roughly seven times more energy than melting the same amount of ice to form water Worked example Calculate the energy transferred to the surroundings as 0.60 kg of stearic acid changed state from liquid to solid.The specific latent heat of fusion of stearic acid is 199 000 J/kg. Step 1: List the known quantities Mass, m = 0.60 kg Specific latent heat of fusion, L = 199 000 J/kg  Step 2: Write down the relevant equation E = mL Step 3: Substitute in the values E = 0.60 × 199 000 = 119 400 J Exam Tip Remember that L is used as the symbol of specific latent heat of fusion or vaporisation. This equation will be given on your equation sheet, however, it is important you know how to use it! You've read 1 of your 10 free revision notes Unlock more, it's free! Join the 100,000+ Students that ❤️ Save My Exams the (exam) results speak for themselves: I would just like to say a massive This website is soo thank you for putting together such ever thank you eno a brilliant, easy to use website. I questions by topic really think using this site helped Furthermore, the n me secure my top grades in could not have be  science and maths. You really did appropriate as it l save my exams! Thank you. MY EXAMS! Beth Fathima IGSCE Student A Level Student Join now for free Test yourself Next topic Did this page help you? Yes No 1. Energy 2. Electricity 3. Particle Model of Matter 3.1 Changes of State & the Particle Model 3.2 Internal Energy & Energy Transfers  3.2.1 Internal Energy 3.2.2 Specific Heat Capacity 3.2.3 Latent Heat 3.2.4 Specific Latent Heat 3.2.5 Heating & Cooling Graphs 3.2.6 Specific Heat Capacity v Specific Latent Heat Topic Questions 3.2 Internal Energy & Energy Transfers Test yourself 3.3 Particle Model & Pressure 4. Atomic Structure 5. Forces 6. Waves 7. Magnetism & Electromagnetism  8. Space Physics Related Topic Questions Internal Energy & Energy Transfers Particle Model & Pressure Atoms & Isotopes Atoms & Nuclear Radiation Hazards & Uses of Radioactive Emissions & of Background Radiation Nuclear Fission & Fusion Forces & their Interactions Work Done & Energy Transfer Forces & Elasticity Moments, Levers & Gears Author: Leander Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. 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