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Heat a form of Energy

Heat a form of Energy. By Neil Bronks. Temperature. Measure of how hot or cold something is This is the science version of shouting at a waiter in Ibiza (it really does not help but it’s the best we have). Fixed Point Usually the boiling point and melting points of water.

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Heat a form of Energy

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  1. Heat a form of Energy By Neil Bronks

  2. Temperature • Measure of how hot or cold something is • This is the science version of shouting at a waiter in Ibiza (it really does not help but it’s the best we have).

  3. Fixed Point Usually the boiling point and melting points of water ------------------------------ Thermometric Property Something that varies Measurably with temperature Scale Divisions between the fixed points Thermometers Three things that make up a thermometer

  4. Emf R Pressure Temp Temp Temp Different Thermometers Platinum Wire Resistance CVGT Pressure Thermocouple Junction emf The only linear thermometric property is the CVG. All the others must be calibrated to the CVG

  5. Show them the CVGT

  6. Emf R Temp Temp Different Temperatures Thermocouple Junction emf Platinum Wire Resistance Because the thermometric properties are non-linear we may get different readings for the same temperature

  7. Emf R Pressure Temp Temp Temp Different Thermometers Platinum Wire Resistance CVGT Pressure Thermocouple Junction emf CVG is a standard thermometer and is used to calibrate the others

  8. CALIBRATION CURVE OF A THERMOMETER USING THE LABORATORY MERCURY THERMOMETER AS A STANDARD Alcohol thermometer uncalibrated Mercury thermometer Boiling tube Water Glycerol Heat source

  9. Temperature in Celsius 43 23 Length in cm

  10. Fixed Points – Alternative to Calibration Graph • Use BP and MP of water • Divide up gap between into 100 division scale

  11. Kelvin and Celsius • Add 273 to Celsius and you get the temperature in Kelvin • Lowest possible temperature is -273oC • This is zero Kelvin OK

  12. Calibration Movie

  13. H/W • LC Ord 2007 Q 3 • And LC Ord 2005 Q12(a)

  14. Heat Transfer • Convection • Hot air • rising • carrying • the • heat up • with it. Conduction -Transfer by vibrations Radiation -Transfer by Electro-magnetic wave

  15. Conduction In a solid every atom is physically bonded to its neighbours in some way. If heat energy is supplied to one part of a solid, the vibration travels through the solid. Conduction is the transfer of energy through matter from particle to particle. It is the transfer and distribution of heat energy from atom to atom within a substance.

  16. Practical Conduction • A spoon in a cup of hot soup becomes warmer because the heat from the soup is conducted along the spoon. Conduction is most effective in solids • It is also why stone and metals appear cold. They are just good conductors. Chilly

  17. Test Tube of water Metal Gauze ICE HEAT Water as a Poor Conductor The ice does not melt as the water is a terrible conductor and convection only works up.

  18. U-Value Q/t • U- Value (or Heat transmittance) is a measure of how good an insulator something is. A good insulator has a low U-value. • Defined as the rate of heat energy transfer through 1m2 where the temperature difference is 1k (θ+1)0C θ0C 1m2

  19. Convection Most houses have radiators to heat their rooms. This is a bad name for them - as they give off heat mainly by convection! The air expands and is less dense so it rises It cools and falls (So hot fluids rise not heat) CONVECTION CURRENT

  20. Domestic Heating System

  21. Sea Breezes Day – On Shore HOT LAND WARM SEA

  22. Sea Breeze Night Night – Off Shore COLD LAND WARM SEA

  23. Radiation • The transfer of heat in the form of an electro-magnetic wave. • Only form of heat that can travel through a vacuum

  24. A silver or white body holds heat in so to reduce heat loss we use silver or white. Black bodies radiate more heat so we paint things black when we want to lose heat.

  25. Vacuum Flask

  26. Solar Constant • The average amountof solar energy falling on 1 square meter of atmosphere per second • About 1.35kWm-2 At the poles the same amount of energy from the sun is spread over a much larger surface area. Than the equator

  27. H/W • LC Ord 2006 Q 7 • LC Ord 2004 Q7

  28. Melting point Boiling point Heating a solid Temperature Time

  29. Boiling point Heat raises temperature Energy=mcΔθ Boiling Melting point Liquid Melting Latent Heat Only Energy=ml Solid Heating a solid Temperature Gas Time

  30. Liquid boils and takes in Latent Heat from the food Gas turns back into a liquid giving out heat The Refrigerator Liquid Gas Compressor

  31. Heating Up Heat that raises temperature Energy Supplied=Q=mcΔθ Where m = mass of body Δθ=Change in Temperature c = Specific Heat Capacity Amount of heat energy to raise 1kg by 1k

  32. Example How much energy does it take to heat up 2kg of copper by 30 degrees? (Where c=390 j/kg/kelvin) As Q=mc∆ Q= 2 x 390 x 30 = 23400 Joules

  33. Example How much energy does it take to heat up 500ml of water from 20oC to B.P.? (Where c=4200 j/kg/kelvin) As Q=mc∆ Q= 0.5 x 4200 x 80 = 168000 Joules

  34. Power • If this takes 5 mins how much power is needed? Power = Work done/ Time = 168000/300s = 560 Watts

  35. H/W • LC Ord 2008 • Q 7

  36. Latent Heat Heat that changes state without changing temperature Energy Supplied=ml Where m = mass of body l = Specific Latent Heat Amount of heat energy to change state of1kg without changing temp.

  37. Example How much energy does it take to turn 2kg of copper into a liquid? (latent heat of fusion of Copper l=38900000 j/kg) As Q=ml Q= 2 x 38900000 = 77800000 Joules A lot more than heating it up!

  38. Frozen Wine A litre of wine at 20 0C. is left in the freezer by accident. It freezes and reduces to -10 0C. How much energy does this take? 3 stages • Cools to zero • Freezes • Cools to -10 0C.

  39. Stage 1 Wine has c=4000j/kg/kelvin, =1kg/litre Using Q=mc =  V c  =1x1x4000x20 =80000joules

  40. Stage 2 Wine has latent heat of fussion l = 300000j/kg Using Q=ml =  V l =1x1x300000 =300000joules

  41. Stage 3 Frozen Wine has c=3000j/kg/kelvin, =1kg/litre Using Q=mc =  V c  =1x1x3000x10 =30000joules Different from liquid

  42. Total = 80000+300000+30000 =410000 joules How long will this take in a 100Watt fridge? 100w = 100 joules/second Time = 410000/100 = 4100 seconds = 4100/3600 = 1.13 hours

  43. H/W • Higher level • 2005 Q2

  44. MEASUREMENT OF THE SPECIFIC HEAT CAPACITY OF A METAL BY AN ELECTRICAL METHOD Metal block 12 V a.c. Power supply Joule meter 10°C 350 J Heating coil Glycerol Lagging

  45. 1.Find the mass of the metal block m. 2.Set up the apparatus as shown in the diagram. 3.Record the initial temperature θ1 of the metal block. 4.Zero the joule meter and allow current to flow until there is a temperature rise of 10 C. 6.   Switch off the power supply, allow time for the heat energy to spread throughout the metal block and record the highest temperature θ2. 8.   Record the final joule meter reading Q. Energy supplied electrically = Energy gained by metal block Q = mc (θ2 – θ1)

  46. MEASUREMENT OF SPECIFIC HEAT CAPACITY OF WATER BY AN ELECTRICAL METHOD Joule meter 12 V a.c. Power supply Cover Digital thermometer Water Lagging Calorimeter Heating coil 10°C 350 J

  47. 1.Find the mass of the calorimeter mcal. 2.Find the mass of the calorimeter plus the water m1. Hence the mass of the water mw is m1 – mcal. 3.Set up the apparatus as shown. Record the initial temperature θ1. 4.Plug in the joule meter , switch it on and zero it. 5.Switch on the power supply and allow current to flow until a temperature rise of 10 C has been achieved. 6.   Switch off the power supply, stir the water well and record the highest temperature θ2. Hence the rise in temperature is θ2 – θ1. 7.   Record the final joule meter reading Q.

  48. Electrical energy supplied = energy gained by (water +calorimeter) Q = mwcw + mcalccal. Precautions 1/. Lagging 2/. Cool water slightly so final temperature not far from room temperature.

  49. Cotton wool 10°C Boiling tube Water Digital thermometer Copper rivets Water Lagging Calorimeter Heat source MEASUREMENT OF THE SPECIFIC HEAT CAPACITY OF A METAL OR WATER BY A MECHANICAL METHOD

  50. 1.Place some copper rivets in a boiling tube. Fill a beaker with water and place the boiling tube in it. 2.Heat the beaker until the water boils. Allow boiling for a further five minutes to ensure that the copper pieces are 100° C. 3.Find the mass of the copper calorimeter mcal. 4.Fill the calorimeter, one quarter full with cold water. Find the combined mass of the calorimeter and water m1. 5.Record the initial temperature of the calorimeter plus water θ1.Place in lagging 6.Quickly add the hot copper rivets to the calorimeter, without splashing. 7.Stir the water and record the highest temperature θ2. 8.Find the mass of the calorimeter plus water plus copper rivets m2 and hence find the mass of the rivets mco.

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