SPM Phyiscs - Thermal energy


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SPM Phyiscs - Thermal energy

  1. 1. Thermal Energy Form 4 Physics (SPM) – Chapter 4
  2. 2. Thermodynamics  1st Law: Energy is conserved. i.e. It can’t be created or destroyed, only transferred from one form to another
  3. 3. Definitions  Thermal Energy   Temperature   Total mechanical energy contained in a body Degree of hotness or coldness of a body Heat  The transfer of energy from one system to another
  4. 4.    Thermal energy depends on the temperature, number of particles and arrangement of particles in a body Heat on the other hand is thermal energy moving from one place to another Temperature depends on kinetic energy in an object
  5. 5. Heat and Temperature  Similarities    Both are quantitative (measureable) Both are scalar quantities (no direction) Differences   Temperature is measured in Kelvin (SI unit) with a thermometer Heat is measured in Joule (Derived unit) with a joulemeter or calorimeter
  6. 6. Thermal Equilibrium   State where there is no net heat transfer between two or more systems, resulting in constant temperature 0th Law of Thermodynamics
  7. 7.   Heat exchange between System A and System B occurs through thermal conduction Time taken for both systems to reach thermal equilibrium depends on the rate of heat transfer
  8. 8. Thermometer  A good thermometer has     Suitable thermometric liquid Thin bulb to allow quicker response to heat Thin capillary tube to increase sensitivity Thick glass bore to allow magnification of scale for easier reading and for increased durability Capillary tube Glass bore with scale
  9. 9.  Thermometric properties   Properties that change with changing temperature Example    When temperature , object expands (volume ) When temperature , pressure When temperature , electrical resistance
  10. 10. Thermometric fluid  Properties:      Should be easily seen Able to expand and contract uniformly with temperature Does not stick to wall of capillary tube Good heat conductor Types:  Mercury   Opaque and suitable for measuring high temperatures due to high boiling point and non-volatility Alcohol  Volatile and very low melting point makes it suitable for measuring low temperatures
  11. 11. Thermometer - Calibration  Thermometer placed in melting ice has a column length of l0  When placed in boiling water, the length is  Thermometer placed in a solution of unknown temperature has a length of lϴ l100
  12. 12. Based on the recordings, 100˚C = (l100 – l0) and Unknown temperature, ϴ = (lϴ – l0) Proportionally, = ϴ 100 ˚C Hence, ϴ = (lϴ – l0) (l100 – l0) (lϴ – l0) (l100 – l0) X 100 ˚C
  13. 13. Heat Capacity    The amount of heat change required to change the temperature of an object by 1˚C Heat capacity, C = ∆Q/ ∆T , where ∆Q = Heat change and ∆T = Temperature change Unit = J˚C-1
  14. 14. Specific Heat Capacity  Amount of heat change required to change the temperature of a 1kg object by 1˚C Specific means a unit quantity of a physical property (in this case, mass) Specific heat capacity, c = ∆Q/(m∆T) , where m = mass.  Unit = Jkg-1˚C-1  
  15. 15. Observations of SHC  Sea breeze    During the day, temperature of air above land rises quicker than air above sea (land has a lower SHC than the sea) This warmer air moves upwards and toward the sea, creating a convection current The cooler sea acts as a heat sink for this warm air, causing air above the sea level to blow inland to replace risen air
  16. 16.  Land breeze    During the night, the sea is warmer than the land due to accumulated heat gained during the day becomes enough to raise its temperature. Air above the sea is now warmer causing the air above the sea to rise upwards, flow toward and sink at the land. The convection current created causes the air above the land to blow towards the sea
  17. 17. Sea Breeze Ocean is cooler than land (cold source, a.k.a. heat sink)
  18. 18. Land Breeze Ocean is warmer than land (heat source)
  19. 19. This means…  A body with high SHC will heat or cool slower (i.e. poor conductor)  A body with low SHC will heat or cool faster (i.e. good conductor)  Water has a very high SHC value (4200 Jkg-1˚C-1). It’s suitable as a ‘coolant’ in engines and machines to sink heat away from hot components Water is used as coolant in cooling systems, radiators and the mammalian body 
  20. 20. Change in physical state
  21. 21. Heating  At gradients:   Heat absorbed  Kinetic energy (Temperature rises) At plateaus:   Heat absorbed is used to overcome bonds Kinetic energy (and temperature) is constant (melting and boiling point)
  22. 22. Cooling  At gradients:   Kinetic energy  Heat released (Temperature drops) At plateaus:   Rebonding releases heat energy Kinetic energy (and temperature) is constant (condensation and freezing point)
  23. 23. Techniques  Insulation   Prevents heat loss or gain from the surroundings Stirring with the thermometer   To ensure even heating and cooling. If stirring is uneven during cooling, supercooling (liquid state below freezing point) occurs
  24. 24.  At gradients of both curves   The heat change is causing a change in temperature. This heat is the heat capacity At the plateaus of both curves:  The heat change occurs at constant temperature. This is latent heat
  25. 25. Latent Heat    Heat change that occurs when a substance changes its physical state at constant temperature Latent heat, L = ∆H, where ∆H = Heat change Unit = Joule (J)
  26. 26. Specific Latent Heat    Heat change that occurs when 1kg of substance changes its physical state at constant temperature Specific latent heat, L = ∆H/m , where ∆H = Heat change and m = mass Unit = Jkg-1
  27. 27. Two types of specific latent heat  Specific latent heat of fusion (Lf)   Heat change that occurs when 1kg of substance changes between the solid and liquid phases with no change in temperature Specific latent heat of vapourisation (Lv)  Heat change that occurs when 1kg of substance changes between the liquid and gas phases with no change in temperature
  28. 28. Applications of Latent Heat  Steam cooking   Steam has a high latent heat and when it condenses on food, the heat released is used to cook the food. Sweating  Evaporation of sweat makes us feel cold because when water evaporates, the latent heat of vapourisation is absorbed from the surface of the skin, thus cooling it down.
  29. 29. Ideal Gas      An idealistic paradigm of gases in real life The absolute zero is the temperature where all motion of ideal gas particles ceases (Kinetic energy = 0) The absolute zero is -273 ˚C The absolute zero scale is Kelvin (K) 0K = -273 ˚C
  30. 30. Ideal Gas Laws  Boyle’s Law    Charles’ Law    Pressure of a gas is inversely proportional to its volume at constant temperature P1V1 = P2V2 Volume of a gas is directly proportional to its temperature in the absolute zero scale at constant pressure V1/T1 = V2/T2 Pressure Law   Pressure of a gas is directly proportional to its temperature in the absolute zero scale at constant volume P1/T1 = P2/T2
  31. 31. Boyle’s Law Charles’ Law Pressure Law
  32. 32. Universal Gas Law P1V1 / T1 = P2V2 / T2