CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics CONTENTS 2.1 Simple Kinetic Molecular Model of Matter 2.1.1 States of Matter 2.1.2 Molecular Matter 2.1.3 Kinetic Theory 2.1.4 Evaporation 2.1.5 Pressure Changes 2.2 Thermal Properties & Temperature 2.2.1 Thermal Expansion 2.2.2 Measurement of Temperature 2.2.3 Temperature & Heat 2.2.4 Thermal Capacity 2.2.5 Specific Heat Capacity 2.2.6 Melting & Boiling 2.3 Thermal Processes 2.3.1 Conduction 2.3.2 Convection 2.3.3 Radiation 2.3.4 Consequences of Energy Transfer 2.1 SIMPLE KINETIC MOLECULAR MODEL OF MATTER 2.1.1 STATES OF MATTER © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 1 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Solids, Liquids & Gases Matter can exist in one of three different states: Diagram showing the 3 states of matter in terms of shape and volume Solids Solids have a definite shape (they are rigid) Solids have a definite volume Liquids Liquids have no definite shape – they are able to flow, and will take the shape of a container Liquids have a definite volume Gases Gases have no definite shape – they will take the shape of their container Gases have no fixed volume – if placed in an evacuated container they will expand to fill the container. They are also highly compressible © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 2 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.1.2 MOLECULAR MATTER Arrangement & Motion of Molecules Diagram showing the arrangement and motion of different states of matter In a solid: The molecules are very close together and arranged in a regular pattern The molecules vibrate about fixed positions In a liquid: The molecules are still close together (no gaps) but are no longer arranged in a regular pattern The molecules are able to slide past each other In a gas: The molecules are widely separated – about 10 times further apart in each direction The molecules move about randomly at high speeds © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 3 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only The Forces & Distances between Molecules In a solid: The molecules are held in place by strong intermolecular bonds These bonds prevent the molecules from moving, giving the solid its rigid shape and fixed volume In a liquid: The molecules have enough energy that they are able to break the bonds between them The bonds are still there, but they no longer hold the molecules in place As a result, the molecules can move around (by sliding past each other) allowing the liquid to flow In a gas: The molecules are now moving around randomly at high speeds The molecules have broken the bonds between them: They are widely separated with no long-range forces binding them together As a result the molecules are able to move freely and so the gas can flow freely Because of the large spaces between the molecules (along with the absence of long-range forces) the gas can easily be compressed and is also able to expand © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 4 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.1.3 KINETIC THEORY Motion of Molecules in a Gas The molecules in a gas move around randomly at high speeds The temperature of a gas is related to the average speed of the molecules: The hotter the gas, the faster the molecules move Gas molecules move about randomly at high speeds As the molecules move around, they collide with the surface of nearby walls Each collision applies a force across the surface area of the walls Pressure is the force per unit area: and hence a pressure will be exerted on those walls © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 5 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Molecule Momentum When molecules collide against a wall, they bounce off, changing their momentum When molecules (in a gas) collide against a wall, they undergo a change in momentum There are many such collisions every second, resulting in a large change in momentum each second This change in momentum each second results in a force being exerted against the wall: The pressure exerted by the gas is equal to this force divided by the area of the wall © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 6 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Brownian Motion When small particles (such as pollen or smoke particles) are suspended in a liquid or gas, the particles can be observed through a microscope moving around in a random, erratic fashion Brownian Motion: the erratic motion of small particles when observed through a microscope This movement is called Brownian Motion This motion is caused by molecules in the gas (or liquid) colliding at high speeds with the small particles © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 7 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Gas molecules move at high speeds, collide with larger particles, giving them a little nudge These collisions give the particles a little nudge, causing them to change their speed and directions randomly, each time they are struck by a molecule This effect provides important evidence concerning the behaviour of molecules in gases © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 8 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Massive Particles Moved by Small Molecules The small particles observed in Brownian motion are significantly bigger than the molecules that cause the motion The molecules are able to affect the particles in this way because they are travelling at very high speeds (much higher than the particles) and so have a lot of momentum, which they transfer to the particles when they collide © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 9 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.1.4 EVAPORATION Cooling by Evaporation Evaporation is a change in state of a liquid to a gas that can happen at any temperature from the surface of a liquid The molecules in a liquid have a range of different energies: Some have lots of energy, others have very little Evaporation occurs when more energetic molecules near the surface of the liquid have enough energy to escape Evaporation occurs when more energetic molecules near the surface of a liquid escape © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 10 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics When this happens energy is lost from the liquid: The average energy of the remaining molecules decreases This means that the temperature of the remaining liquid will also decrease © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 11 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Factors Affecting Evaporation A number of factors affect the rate of evaporation: The temperature of the liquid – At higher temperatures, more molecules have enough energy to escape The surface area of the liquid – If the liquid has a greater surface area there will be more area from which the molecules can escape The movement of air across the surface of the liquid – The presence of a draft can help to remove less energetic molecules (which might not have quite enough energy to escape) from the liquid The process of evaporation can be used to cool things down: If an object is in contact with an evaporating liquid, as the liquid cools the solid will cool as well This process is used in refrigerators and air conditioning units © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 12 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.1.5 PRESSURE CHANGES Pressure & Temperature The molecules in a gas move around randomly at high speeds, colliding with surfaces and exerting pressure upon them Gas molecules collide with surfaces, exerting pressure When the temperature of a gas is increased, the speeds of the molecules increases This causes them to collide harder against surfaces, increasing the pressure Increasing the temperature of a gas (whilst keeping its volume fixed) will increase the pressure © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 13 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Pressure & Volume When a gas is compressed, the density of the gas increases When a gas is compressed it becomes more dense, increasing the pressure This means there will be more collisions against surfaces, which will also increase the pressure Decreasing the volume of a gas (whilst keeping the temperature fixed) will increase the pressure © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 14 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Worked example showing the effect of compressing gas on its pressure © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 15 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Boyle's Law If a gas has an original pressure, P1, and volume, V1, then its final pressure and volume (P2 and V2) will be given by the relationship: P1V1 = P2V2 Note: The above equation only works if the temperature of the gas remains constant Worked example showing the change in pressure of gas as the result of compression © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 16 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Exam Question: Easy Exam Question: Hard © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 17 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics > CHECK YOUR ANSWERS AT SAVEMYEXAMS.CO.UK © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 18 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.2 THERMAL PROPERTIES & TEMPERATURE 2.2.1 THERMAL EXPANSION What is Thermal Expansion? When (most) materials are heated, they expand This expansion happens because the molecules start to move around (or vibrate) faster, which causes them to knock into each other and push each other apart When a solid is heated, the molecules vibrate more, pushing each other apart Note: When this happens, it is the space taken up by the molecules that increases. The molecules themselves remain the same size. © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 19 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Thermal Expansion in Solids, Liquids & Gases When solids, liquids and gases are heated: © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 20 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Applications and Consequences The thermal expansion of materials can have some useful applications, but also has some undesirable consequences Applications: The expansion of a liquid in a thermometer can be used to measure temperature A bimetallic strip, consisting of two metals that expand at different rates, can be made to bend at a given temperature, forming a temperature-activated switch The bimetallic strip will bend upwards when heated, closing the circuit Consequences: The expansion of solid materials can cause them to buckle if they get too hot This could include: Metal railway tracks Road surfaces Bridges Things that are prone to buckling in this way often have gaps built into them providing some room for them to expand into © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 21 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.2.2 MEASUREMENT OF TEMPERATURE Measuring Temperature When a substance is heated, some of its physical properties can change These properties include things such as: The volume (and density) of the substance The electrical resistance of the substance If these properties change in a well-defined way, by measuring the property you can determine the temperature In order to build a thermometer based on one of these properties, you need to start by measuring the property at some well-defined fixed points A fixed point is a temperature at which some easily identifiable change occurs, such as the melting of ice (at 0 °c) or the boiling of pure water (at 100 °c) Ice melts and water boils at well-defined temperatures (fixed points) which may be used to calibrate thermometers © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 22 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics These fixed points allow you to know the temperature without having to measure it directly Usually two fixed points are used: The lower fixed point: The melting temperature of ice The upper fixed point: The boiling temperature of pure water Once a property (such as electrical resistance) has been measured at these two fixed points, the values of that property at other temperatures can be worked out Measuring some property at two fixed points allows you to then work out the temperature for other values of that property © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 23 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Sensitivity, Range & Linearity The above description has made a big assumption about the way in which the property changes with temperature: That it does so at a steady rate Properties that change in this way are said to be linear, and linearity is a very useful property for something to have as it makes it easy to work out the relationship between that thing and the temperature Other important properties when designing a device such as a thermometer include: Sensitivity – if the thing you are measuring changes significantly when the temperature changes, it makes it easier to detect small changes in temperature: Your thermometer is more sensitive Range – What are the lowest and highest temperatures the thermometer can measure? If a liquid-thermometer is too short, there may not be enough room for the liquid to expand into © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 24 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Liquid Thermometers A liquid-in-glass thermometer consists of a thin glass capillary tube containing a liquid that expands with temperature At one end of the tube is a glass bulb, containing a larger volume of the liquid which expands when heated, moving into the narrower tube A scale along the side of the tube allows the temperature to be measured based on the length of liquid within the tube As the bulb is heated, the liquid expands and moves along the capillary tube © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 25 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only The Sensitivity of Liquid Thermometers The liquids used in liquid-in-glass thermometers are chosen for their thermal expansion properties: They expand linearly with temperature They expand significantly, giving them greater sensitivity They have low freezing points and high boiling points, allowing them to be used over a greater range of temperatures Sensitivity is another word for precision. Anything that makes it easier to detect small changes in temperature increases the precision, or sensitivity of a liquid thermometer Some of the factors which increase sensitivity are: Using a thermometer with a smaller glass bulb, as a smaller bulb contains less liquid and therefore, absorbs heat in a shorter time Using a narrow tube, as a small change in volume results in the liquid moving a larger distance along the tube Using a glass bulb with a thinner wall, as heat can be transferred to the bulb more easily and a small change in temperature can be more easily detected © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 26 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Thermocouples A thermocouple consists of two different types of wire attached together When the joint between the two wires is heated, a potential difference (voltage) is created between the two wires The greater the temperature, the greater the potential difference A thermocouple consists of two dissimilar wires connected together Thermocouples are not as sensitive as liquid-in-glass thermometers, but because the metals have high melting points, they can be used to measure very high temperatures Thermocouples are also very responsive to rapidly changing temperatures © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 27 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.2.3 TEMPERATURE & HEAT Internal Energy When a substance is heated, its internal energy (sometimes referred to as thermal energy or heat) increases As a substance’s internal energy increases, so will its temperature The higher the temperature of a substance, the more internal energy it possesses As the temperature of a substance is increased, the total energy of the molecules (the internal energy) increases © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 28 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Internal Energy & Molecules The molecules within a substance possess two forms of energy: Kinetic energy (due to their random motion/vibration) Potential energy (due to the bonds between the molecules) Together, these two forms of energy make up the internal (thermal) energy of the substance When a substance is heated, that heat will usually cause the kinetic energy of the molecules to increase The temperature of the material is related to the average kinetic energy of the molecules © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 29 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.2.4 THERMAL CAPACITY What is Thermal Capacity? The thermal capacity of an object is the amount of heat energy required to raise the temperature of that object by 1 °c The greater the thermal capacity of an object, the more heat energy it takes to raise its temperature The thermal capacity is also equal to the amount of heat energy an object will give out when it cools by 1 °c The energy, E, required to raise the temperature of an object by an amount T is given by the equation: E = thermal capacity × T © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 30 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Thermal Capacity & Specific Heat Capacity The thermal capacity of an object is related to the specific heat capacity (see next page) by the equation: thermal capacity=m × c (where c is the specific heat capacity of the material the object is made from, and m is the object’s mass) © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 31 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.2.5 SPECIFIC HEAT CAPACITY Extended Only What is Specific Heat Capacity? The specific heat capacity of a substance in the amount of thermal energy required to raise the temperature of 1kg of that substance by 1 °C Specific heat capacity has units of joules per kilogram per degree Celsius (J/kg °C) (Note: Different substances have different specific heat capacities) From the definition of specific heat capacity, it follows that if you have more than 1 kg of a material, you will need more thermal energy Likewise, if you want to raise its temperature by more than 1 °C, you will also need to add more thermal energy The amount of thermal energy needed is given by the equation: Note that in the above equation: ΔE is used to mean the change in internal energy C is used to mean the specific heat capacity of the substance ΔT is used to mean the change in temperature (The symbol Δ in maths is used to denote a change in value) © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 32 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Low v high specific heat capacity © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 33 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Measuring Specific Heat Capacity In your IGCSE examination you may be asked to describe an experiment to determine the specific heat capacity of a substance A method for carrying out such an experiment is given below In this experiment you need to use the following equation to determine the specific heat capacity of the substance: The following apparatus will be needed: A block of the substance (preferably 1kg in mass) or in the case of a fluid, a beaker containing a known mass of the fluid A thermometer An appropriate heater (e.g. an immersion heater) A power source A joule meter or a voltmeter, ammeter and stop-clock (I will assume we have the latter) © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 34 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Apparatus to determine the specific heat capacity of a 1 kg Aluminium block Start by assembling the apparatus and measure the initial temperature of the substance Turn on the power supply and start the stop-clock Whilst the power supply is on take several periodic measurements of the voltage and current, and calculate an average of these values After 5 minutes (300 seconds) switch off the power supply, stop the stop-clock and leave the apparatus for a few more minutes Monitor the thermometer and make a note of the highest temperature reached Calculate the rise in temperature The heat supplied to the substance can be calculated using the equation: energy = current × voltage × time (Note: the time must be in seconds) These values, along with the mass of the substance, can now be substituted into the top equation to find the specific heat capacity of the substance The biggest problem with the above experiment is that not all of the heat supplied by the heater will go into the substance – some will go into the surroundings and the substance will also lose heat whilst it is being heated This means that the value for the heat added will be too great which means that the calculated specific heat capacity will also be too great © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 35 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.2.6 MELTING & BOILING Changing State A change of state refers to a substance changing its state between solid, liquid or gas The various changes of state between solids, liquids and gases Usually, when a substance is heated its temperature increases However, whilst a substance changes its state (from solid to liquid or from liquid to gas) the temperature of that material remains constant, even though energy is still being added © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 36 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics When a substance changes state, its temperature remains constant, even though it is still gaining (or losing) internal energy This happens because the energy is being used to break the bonds between the molecules instead of increasing the kinetic energy of the molecules (and hence the temperature) When a substance turns from gas to liquid (condensation) or from liquid to solid (solidification) the opposite happens: The temperature remains constant whilst the bonds reform (giving out energy in the process) The melting point and boiling point of a substance is the temperature at which that substance changes state © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 37 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Boiling vs Evaporation Boiling and Evaporation both refer to a change in state from liquid to gas, however: Boiling occurs at a fixed temperature and happens throughout the liquid Evaporation can occur at any temperature and happens from the surface of the liquid Evaporation occurs when higher energy molecules near the surface of a liquid escape and become a gas. This can occur at any temperature © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 38 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Latent Heat Latent Heat is the energy required to change the state of a substance This energy is required to break the bonds holding molecules together (It is also given out again once the bonds reform) Latent Heat of Fusion is the energy required to turn a solid into a liquid Latent Heat of Vaporisation is the energy required to turn a liquid into a gas The Specific Latent Heat, L, of a substance is the energy required to change the state of 1 kg of that substance For an object of mass m, the total energy required to change its state is given by the equation: Energy = m × L © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 39 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Measuring Specific Latent Heat In your IGCSE examination you may be asked to describe an experiment to determine the specific latent heats for steam or ice A method for determining the specific latent heat for steam is given below In this experiment you need to use the following equation: The following apparatus will be needed: A vacuum flask containing pure water A top pan balance An appropriate heater (e.g. an immersion heater) A power source A joule meter or a voltmeter, ammeter and stop-clock (I will assume we have the latter) © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 40 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Image showing apparatus to measure the specific latent heat for steam © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 41 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Start by measuring (and recording) the mass of the vacuum flask along with the water Next place the heater into the water and connect it to the power supply Turn on the power supply and wait until the water starts to boil Once the water is boiling start the stop-clock and take several measurements of the potential difference (on the voltmeter) and current (on the ammeter), and calculate an average of these values After 5 minutes (300 seconds) switch off the power supply and stop the stop-clock Now measure the mass of the vacuum flask and water once more, and calculate how much the mass has decreased by. This will give the mass of water that has changed state (which you will use in determining your result) The heat supplied to the material can be calculated using the equation: energy = current × voltage × time (Note: the time must be in seconds) These values, along with the change in mass, can now be substituted into the top equation to find the specific latent heat for steam A similar experiment can be carried out for ice (in a beaker this time), but the ice must be measured separately from the beaker otherwise you’ll be measuring the mass of the water as well It is also important to ensure that the temperature of the ice is 0 °c at the start of the experiment. This will be the case if the ice has started to melt © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 42 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Exam Question: Easy © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 43 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Exam Question: Hard > CHECK YOUR ANSWERS AT SAVEMYEXAMS.CO.UK © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 44 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.3 THERMAL PROCESSES 2.3.1 CONDUCTION What is Conduction? Conduction is the main method of thermal energy transfer in solids Metals are extremely good at conducting heat Non-metals are poor at conducting heat whilst liquids and gases are extremely poor Poor conductors are called insulators Materials containing small pockets of trapped air are especially good at insulating, as air is a gas and hence a poor conductor © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 45 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only How does Conduction Occur? When a substance is heated, the atoms start to move around (vibrate) more As they do so they bump into each other, transferring energy from atom to atom Conduction: the atoms in a solid vibrate and bump into each other Metals are especially good at conducting heat as the delocalised electrons can collide with the atoms, helping to transfer the vibrations through the material © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 46 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Demonstrating Conduction A simple experiment to demonstrate the relative conducting properties of different materials can be carried out using apparatus similar to that shown in the diagram below The above apparatus consists of 4 different metal strips of equal width and length arrange around an insulated circle Ball bearings can be stuck to each of the strips and equal distance from the centre, using a small amount of wax The strips should then be turned upside down and the centre heated gently using a candle, so that each of the strips is heated at the point where they meet When the heat is conducted along to the ball bearing, the wax will melt and the ball bearing will drop By timing how long this takes for each of the strips, their relative thermal conductivities can be determined © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 47 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Exam Tip If a question on heat mentions metal, the answer will probably be to do with conduction. © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 48 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.3.2 CONVECTION What is Convection? Convection is the main way that heat travels through liquids and gases (Convection cannot happen in solids) When a liquid (or gas) is heated: The molecules push each other apart, making the liquid/gas expand This makes the hot liquid/gas less dense than the surroundings The hot liquid/gas rises, and the cooler (surrounding) liquid/gas moves in to take its place Eventually the hot liquid/gas cools, contracts and sinks back down again The resulting motion is called a convection current When a liquid or gas is heated, it becomes less dense and rises © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 49 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Demonstrating Convection A simple demonstration of convection in liquids involves taking a beaker of water and placing a few crystals of potassium permanganate in it, to one side, as shown in the diagram above When the water is heated at that side, the potassium permanganate will dissolve in the heated water and rise along with the warmed water, revealing the convection current Diagram showing an experiment with potassium permanganate to demonstrate convection Exam Tip If a question on heat mentions liquids or gases the answer will probably be about convection. Heat does not rise (only hot gases or liquids rise). © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 50 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.3.3 RADIATION Radiation: Basics All hot objects give off thermal radiation: the hotter they are, the more they emit Thermal radiation is part of the electromagnetic spectrum – infrared Thermal radiation is the only way in which heat can travel through a vacuum It is the way in which heat reaches us from the Sun The colour of an object affects how good it is at emitting and absorbing thermal radiation: © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 51 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Black objects are very good at absorbing thermal radiation (think about black leather seats in strong sunshine) but also very good at emitting it (when it goes dark those seats cool down quickly) Shiny objects reflect thermal radiation and so absorb very little. They also emit very little, though, and so take longer to cool down An image of a hot object taken in both Infrared and visible light. The black surface emits more thermal radiation (infrared) than the shiny surface © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 52 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Factors Affecting Emission of Radiation The amount of thermal radiation emitted by an object depends on a number of factors: The temperature of the object (hotter = more radiation) The colour of the object (black = more radiation) The surface area of the object (greater surface area = more area for radiation to be emitted from) © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 53 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Extended Only Demonstrating Radiation To demonstrate the absorption of thermal radiation: Take two conical flasks – one painted with silver paint, the other with black paint – and place thermometers and bungs in them Measure and record their initial temperatures Place the two flasks an equal distance from an incandescent light bulb (a good source of radiation) and switch the bulb on After a few minutes (between 2 and 5) switch the bulb off and record the new temperatures of the flasks (The black flask’s temperature should have increased by more A black and a shiny beaker can be used to demonstrate the effect of colour upon the emission and absorption of thermal radiation © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 54 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics To demonstrate the emission of thermal radiation: Fill the shiny beakers with boiling water Once each beaker reaches a set temperature (e.g. 90 °c) start a stopwatch and allow it to cool for a set amount of time (e.g. 10 minutes) After this time, take a new temperature measurement and record the change in temperature (The black beaker should have cooled by slightly more than the shiny beaker, because it emitted more thermal radiation) Exam Tip If a question on heat mentions the colour of something (black, white or shiny) then the answer will probably be about thermal radiation. Shiny things do not reflect heat (they reflect thermal radiation). Black things do not absorb heat (they absorb thermal radiation). © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 55 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics 2.3.4 CONSEQUENCES OF ENERGY TRANSFER Conduction The main means of thermal energy transfer in solids When heated, atoms vibrate more, knocking into each other and transferring energy from atom to atom as a result Metals are excellent conductors; Non-metals are poor; Liquids and gases are very poor If a question mentions metals, the answer will probably have something to do with conduction Trapped air is a very good insulator of heat. Air is a gas and so is a poor conductor. Trapping it prevents it from circulating and forming a convection current Convection The means of thermal energy transfer in liquids and gases When heated, a gas will expand and become less dense. This causes it to rise (a convection current). Cooler (denser) gas falls, replacing the hot gas If a question refers to a liquid or gas (that isn’t trapped) then convection currents will probably form Heat sources placed at the bottom of things will generally create convection currents. Likewise, cooling units placed high up will cool any rising air, causing it to sink again Thermal Radiation The only way in which heat can travel through a vacuum Thermal radiation is heat transfer by infrared (part of the electromagnetic spectrum) Black objects are good at emitting and absorbing thermal radiation; shiny objects are poor at emitting and absorbing it If a question refers to the colour of something (black, white or shiny) then the answer will probably have something to do with thermal radiation If a piece of apparatus contains a vacuum then radiation will be the only way heat can travel through that part of the apparatus © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 56 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Consequences Example Question In many hot countries it is common for houses to produce hot water using solar panels Diagram showing a section through a solar panel Explain the features of the solar panel that help it heat the water efficiently Answer: The thermal radiation (infrared) is able to pass through the glass sheet The black metal backing sheet absorbs the thermal radiation (sunlight) Being metal (an excellent conductor) it then conducts it into the copper pipes The copper pipes (also metal) then conduct the heat into the water The insulated material reduces the conduction of heat through the back of the panel, decreasing heat loss The glass also traps air which is a good insulator, preventing heat loss due to conduction from the front of the panel and preventing heat loss by convection (due to the air being trapped) © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 57 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Exam Tip A common mistake made by candidates when explaining how an insulator keeps something warm is to state something along the lines of “The object warms up the insulator which then warms the object up”. Avoid giving this kind of answer! The real explanation is: • The insulator contains trapped air, which is a poor conductor of heat • Trapping the air also prevents it from transferring heat by convection • This reduces the rate of heat loss from the object, meaning that it will stay warmer for longer Other things to watch out for: • Heat does not rise (only hot gases or liquids rise) • Shiny things do not reflect heat (they reflect thermal radiation) • Black things do not absorb heat (they absorb thermal radiation) And remember, a good answer will often include references to more than one method of thermal energy transfer. Exam Question: Easy © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 58 CIE IGCSE Physics Revision Notes savemyexams.co.uk YOUR NOTES ⬇ 2. Thermal Physics Exam Question: Hard > CHECK YOUR ANSWERS AT SAVEMYEXAMS.CO.UK © 2015-2021 Save My Exams Ltd CIE IGCSE Physics Resources REVISION NOTES TOPIC QUESTIONS PAST PAPERS Page 59
