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Doc Brown's Physics AQA GCSE Science-PHYSICS Revision Notes

Physics Unit P1.1 The transfer of energy by heating processes and the factors that affect the rate at which that energy is transferred

PHYSICS UNIT 1 Physics 1 P1 for GCSE Science or GCSE Physics Study Notes

REVISION NOTES GUIDE SUMMARY: What do you need to know for the examinations? What do you need to able to do in the exams? In AQA GCSE Science A examinations HT means for higher tier students only. Sorry, but I don't have much time to answer questions, but if you see any apparent errors or wish to comment, please email me. All my notes, learning objectives, comments for exam revision are based on the official AQA GCSE Science A Key Stage 4 syllabus specification.

AQA GCSE Science PHYSICS 1 Unit 1 Unit P1

Unit P1.1 The transfer of energy by heating processes and the factors that affect the rate at which that energy is transferred

• Energy can be transferred from one place to another by work or by heating processes.

• You  need to know how this energy is transferred and which heating processes are most important in a particular situation.

• Heat energy must always flow from hotter material at a higher temperature to cooler material at a lower temperature.

• The bigger the temperature difference between a body and its surroundings eg a hot object (mug of coffee) standing in a cold room, the faster the heat energy is transferred from the hotter material to the cooler material (eg surrounding air).

• The bigger the temperature difference the bigger the rate of heat energy transfer.

• Heat energy can be transferred by conduction, convection and radiation.

• Conduction: Conduction involves heat transfer by particles vibrating against each other in a solid or collisions between particles in a gas or liquid. Conduction is the main mode of heat transfer in a solid. In a solid the hotter particles vibrate more strongly (more KE) and bang into neighbouring cooler lower KE particles and so transfer KE to them, so heat energy is transferred from a higher temperature region to a cooler region. The more dense the solid, generally speaking the better the conductor. In materials where the particles are further apart the rate of heat transfer (rate of conduction) is reduced eg gases like air are much poorer conductors than solids like stone.

• Metals are particularly good conductors because of free moving electrons - a different heat transfer mechanism to that described here, which applies to all solids. Because the electrons are free to move in the solid metal, they can rapidly transfer kinetic energy by particle movement. The 'hot' electrons in the higher temperature region collide with neighbouring cooler electrons and so rapidly transferring heat energy (KE) - much faster than vibrating atoms in non-metals which are held in fixed positions.

• Incidentally if you pick up a cold poor conductor like a stone and then pick up an equally cold metal object at the same cool temperature, the metal object feels colder (but it isn't) because it conducts heat from your fingers faster than the stone!

• Convection: Convection also involves heat transfer via particles but this involves bulk movement of in liquids or gases and cannot take place in solids.

• Convection occurs when hotter/warmer less dense fluid (gas/liquid) rises and is replaced by cooler more dense fluid moving downwards. This is called a convection current. When a material is heated the particles have more KE, move faster and tend to push each other further apart, ie the material expands, becoming less dense. It is this change in density that cause convection to happen.

• This is how hot water is produced in the hot water tank in the home, or the heating of water in a kettle where convection currents flow from the heating element so enabling all the water to be heated up. Note that the heating element must be near the bottom of the tank or kettle to produce the convection circulation to heat up all the water! If you put the heating element at the top there is no convection and all you do is heat up the top layer of water!

• Despite the name, radiators on the walls heat rooms up mainly by convection (there is some radiation too). Warm air rises from the radiator towards the ceiling, cools and falls on the other side of the room and cooler air is drawn in at the base of the radiator to replace it - hence you get a convections current situation that gradually warms up all of the room.

• Radiation: Heat radiation is emitted by all materials, gases, liquids or solids and the hotter the material the more strongly it gives out heat radiation which is called infrared radiation (IR).

• Your knowledge of examples of heat transfer situations should include the ...

• design of a vacuum flask,

• how to reduce the energy transfer from a building,

• how humans and animals cope with low temperatures.

• You need to be able to use your knowledge and understanding to ...

• Compare ways in which energy is transferred in and out of objects by heating and ways in which the rates of these transfers can be varied.

• Evaluate the design of everyday appliances that transfer energy by heating, including economic considerations.

• Examples you should be familiar with include radiators and heat sinks.

• Evaluate the effectiveness of different types of material used for insulation, including U-values and economic factors including payback time.

• You should have studied loft insulation and cavity wall insulation.

• Evaluate different materials according to their specific heat capacities.

• The heat specific heat capacity in simple terms is how much energy (J) is needed to heat a specific mass (1 kg) by one degree oC.

• Examples to be studied include the use of water, which has a very high specific heat capacity, oil-filled radiators and electric storage heaters containing concrete or bricks.

AQA GCSE Science PHYSICS Unit P1.1.1 Infrared radiation

• a) All objects continuously emit and absorb infrared radiation from their surface, whatever their temperature.

• b) The hotter an object is the more infrared radiation it radiates in a given time, the higher the temperature of the material, the more intense is the infrared radiation.

• An object that is hotter (higher temperature) than its surroundings will emit more radiation than it absorbs and an object that is cooler than its surroundings will absorb more radiation than it emits.

• You notice this effect in bright sunlight by feeling the warmth on your hand or standing near a fire.

• When an object cools down to the same temperature as its surroundings emitted infrared radiation equals the absorbed heat radiation.

• c) Dark, matt surfaces are good absorbers and good emitters of infrared radiation eg rough black surfaces.

• Solar panels for hot water comprise of pipes carrying water to be heated set under a black surface to efficiently absorb the infrared radiation from the Sun. You can even just use matt black painted water pipes. You may even have a silvered surface under the pipes so more infrared ins reflected onto the black surface rather than becoming waste heat radiation. The pipes are made of copper which allows efficient conduction of the surface heat energy to the incoming cold water., so the hot water can be used as part of the households domestic heating or washing etc.

• d) Light, shiny surfaces are poor absorbers and poor emitters of infrared radiation eg white gloss paint, silver surface used in vacuum flask ('thermos flask').

• e) Light, shiny surfaces are good reflectors of infrared radiation, this maybe to keep heat in to keep things warm or to minimise heat radiation in to keep things cool eg a vacuum flask.

AQA GCSE Science PHYSICS Unit  P1.1.2 Kinetic theory

• a) The use of kinetic particle theory to explain the three different states of matter.

• You should be able to recognise simple diagrams to model the difference between solids, liquids and gases - the three states of matter.

• Gases: There are almost no forces of attraction between gas particles, they have the most kinetic energy of the three states, the particles are completely free to move around at random, and they move at high speeds in all directions. The free moving particles have kinetic energy of movement and there is much empty space between the particles.

• Liquids: There are weak forces of attraction between liquid particles (if there wasn't, you couldn't have a liquid!), the particles are relatively close together but free to move around at random but with lower speeds than in the gas. The free moving particles still have kinetic energy of movement.

• Solids: In solids there are stronger forces of attraction between the particles which prevents the particles moving around and passing each other. The particles are held in fixed positions in a regular arrangement. Their even lower kinetic energy is due to the particles (atoms or molecules) vibrating around their mean or average positions in the crystal structure.

• b) The particles of solids, liquids and gases have different amounts of kinetic energy (KE).

• You do not need to know about latent heat.

• When you heat a solid, the kinetic energy of the particles is increased until they have enough KE to weaken the interparticle bonds to allow melting. With further heating above the melting point, the particles at the surface with the highest KE can escape the surface (evaporate) or vapourise in the bulk liquid at the boiling point.

AQA GCSE Science PHYSICS Unit P1.1.3 Energy transfer by heating

• a) The transfer of energy by conduction, convection, evaporation and condensation involves particles, and how this transfer takes place.

• You should understand in simple terms how the arrangement and movement of particles determine whether a material is a conductor or an insulator.

• You should understand the role of free electrons in conduction through a metal.

• You should be able to use the idea of particles moving apart to make a fluid less dense and use this to explain simple applications of convection - already discussed in P1.1 introduction.

• b) The factors that affect the rate of evaporation and condensation.

• Condensation occurs when a gas/vapour is cooled sufficiently to a low enough temperature to allow the attractive forces to be strong enough to attract the particles together as a liquid. This can only happen if the kinetic energy of the particles is low enough (the lower the temperature the smaller the kinetic energy).

• Water vapour in the air condenses out on cold surfaces in the winter eg window condensation, invisible steam from a boiling kettle condenses out into clouds of tiny droplets of water, which technically isn't steam! and rain drops form in the higher cooler regions of the atmosphere.

• Factors affecting the rate of condensation

• The cooler the gas, the faster it condenses - more lower KE particles can be attracted together.

• The lower the temperature of the surface the gas is in contact with.

• The lower the airflow over the surface, this keeps the concentration of the condensing gas as high as possible.

• Evaporation is when the highest kinetic energy particles of a liquid escape from the surface ie can overcome the attractive forces of the bulk of particles. The greater the KE of a liquid surface particle, the greater the chance to escape and become a gas particle. Evaporation can take place at any temperature between a substance's melting point and boiling point. As the highest KE particles escape, leaving the slower lower KE particles, the bulk of the liquid will cool, so a cooling effect accompanies the evaporation of a liquid. The cooling effect of sweating is due to evaporation of water from your skin.

• Factors affecting the rate of evaporation

• The higher the liquid temperature, the faster the rate of evaporation - more particles with enough kinetic energy to escape from the surface.

• The greater the surface area, the faster the evaporation - more area, more chance of evaporation.

• The greater the airflow over the surface of the faster the evaporation rate - the air can become saturated with the vapour of the liquid, so it is more readily replaced if the already evaporated liquid is swept away by air flowing over the surface.

• Efficient drying of washing is a good example of these three factors - you need a warm sunny day, the washing well spread out on the line and a nice breeze!

• You need to be able to explain evaporation and the cooling effect this causes using the kinetic theory.

• Candidates should be able to explain the design of devices in terms of energy transfer, for example, cooling fins.

• c) Know that the rate at which an object transfers energy by heating depends on ...

• Surface area and volume

• The larger the surface area the greater the rate of heat transfer eg

• radiators (heating room) and fins on engines (cooling effect) are designed to maximise the transfer of infrared radiation (and the same applies to heat transfer by conduction or convection).

• The material from which the object is made.

• Generally speaking metals are the best conductors of heat ie these metallic materials give the highest rate of heat transfer. Plastics are poor heat conductors, ie good insulators, and are no good for heat sinks.

• The nature of the surface with which the object is in contact.

• Heat sinks are devices designed to transfer heat away to keep another device cool eg metal plates with fins are fitted in computers to stop working electronic components overheating.

• The design of a vacuum flask exemplifies many of the aspects of how to reduce heat transfer

• There is a vacuum between the double walls of the glass vessel which minimises heat transfer by conduction and convection.

• The walls are silvered to minimise heat transfer by infrared heat radiation.

• The glass vessel is supported by blocks of poorly conducting insulating material eg porous plastic foam.

• The top is also made of insulating material like cork or an air-filled plastic cap, again this reduces heat losses (org gain) by conduction..

• You should be able to explain animal adaptations in terms of energy transfer, for example, relative ear size of animals in cold and warm climates.

• The hair or fur on mammals can trap insulating air minimising heat loss by conduction or convection in cold climates - conservation of energy is essential to survive in such a cold harsh climate.

• Generally speaking animals in hot climates have bigger ears than those in cold climates. eg arctic animals like foxes have small ears to minimise heat loss by radiation (and convection too?) but desert foxes have large ears allowing much greater infrared radiation emission to keep them cool.

• The way that your body controls heat transfer when you get too hot is to make more blood flow through the surface blood vessels to allow excess heat to escape - not surprisingly you can look a bit pink in the process! It can be accompanied by sweating, discussed already!

• d) The bigger the temperature difference between an object and its surroundings, the faster the rate at which energy is transferred by heating or cooling.

• To state the obvious - heat energy will always move from a higher temperature material to a lower temperature material UNLESS you have some kind of heat pump i.e. some work is done in the energy transfer process.

AQA GCSE Science PHYSICS Unit P1.1.4 Heating and insulating buildings

• a) Know that U-values measure how effective a material is as an insulator.

• Knowledge of the U-values of specific materials is not required, nor is the equation that defines a U-value.

• What is the U-value of a material? What does the U-value mean?

• The U-value of a material gives a numerical value of how efficient heat is transferred through a material.

• Materials with high U-values are relatively good conductors of heat (poorer insulators) and those with low U-values are relatively good insulators (poorer conductors)

• b) The lower the U-value, the better the material is as an insulator.

• c) Solar panels may contain water that is heated by radiation from the Sun.

• This water may then be used to heat buildings or provide domestic hot water.

• Water has a high heat capacity and can store a lot of heat energy.

• d) The specific heat capacity of a substance is the amount of energy required to change the temperature of one kilogram of the substance by one degree Celsius.

• The formula for expressing the amount of heat transferred in a given situation is ...

• E = m x c x θ

• E = energy transferred in Joules, m = mass of material in kilograms kg

• c = specific heat capacity J/kgoC, θ = temperature change in Celsius oC

• The specific heat capacity of water is 4180 J/kgoC, this means it takes 4180 J of heat energy to raise the temperature of 1 kg of water by 1oC.

• The greater the heat capacity of a material, the more heat energy it can hold for a given mass of material.

• This means that high heat capacity materials can store lots of energy when heated and can then release a lot if cooled down. In other words, materials with a high specific heat capacity are good for storing heat energy.

• Materials used in heaters/heating systems, usually have a high specific heat capacity eg water (SHC H2O = 4180 J/kgoC) in central heating systems (easily pumped around house to distribute heat where needed), concrete/bricks (SHC 900 J/kgoC) in night storage heaters (using cheap night-time electricity), oil-filled heaters for a small scale heat storage (SHC oil = 900 J/kgoC, not as good as water or concrete, but will convect in the oil radiator).

• Conserving energy in the home

• What is effective? What is cost effective? Not always the same! Payback time?

• Loft insulation - cheap and effective - a thick layer of fibreglass wool laid all over the loft floor and reduces conduction and convection of heat lost through the roof - payback time a few years.

• Cavity wall insulation - insulating foam injected between brick walls - reduces convection, conduction? and radiation across the walls - quite costly, payback time a few years.

• Hot water tank jacket - cheap and effective - a jacket of lagging of a foam filled plastic cover reduces conduction and radiation heat losses - quick payback time.

• Double glazing - expensive, longer payback time - insulating air is trapped between the glass panes reducing heat losses by conduction and convection.

• Draught-proofing - cheap, effective, a few years payback time - strips of foam or plastic around door frames, thick curtains across the windows - all of these measures reduce heat loss from the house by convection.

• The most effective methods of insulation give you the biggest annual savings, but the most cost-effective methods tend to be the cheapest. For double glazing and cavity wall insulation you need to think long-term to get your money back.

• Check out your practical work you did or teacher demonstrations you observed in Unit P1.1, all of this is part of good revision for your module examination context questions and helps with 'how science works'.

• Passing white light through a prism and detecting the infrared radiation with a thermometer.

• Demonstration using balls in a tray to show the behaviour of particles in substances in different states i.e. gas, liquid and solid.

• Measuring the cooling effect produced by evaporation by putting wet cotton wool over the bulb of a thermometer or temperature probe.

• Plan and carry out an investigation into factors that affect the rate of cooling of a can of water, eg shape, volume, and colour of can using Leslie’s cube to demonstrate the effect on radiation of altering the nature of the surface.

• Plan and carry out an investigation using immersion heaters in a metal block to measure specific heat capacity.

• Investigating thermal conduction using rods of different materials.

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