Module 6.2: Thermal Physics
Chapter 19: Thermal Physics
Internal energy and temperature
Energy transfer
● One object exerts force on other object and makes it move
● One object hotter than other object so energy transfer by conduction, convection or radiation
Internal energy
● Sum of randomly distributed kinetic energies and potential energies of particles in body - only
kinetic energy for ideal gas
● Thermal energy: internal energy of object due to its temperature
● Can be increased by heating object or work done on object
● Can stay constant if no energy transfer by heating and work done or energy transfer by
heating and work done balance each other out
● First law of thermodynamics: when work is done on or by object and/or energy transferred by
heating, change of internal energy = total energy transfer by work done and heating
Molecules (simple kinetic model)
● In solid:
○ Held together by forces by electrical charges of protons and electrons
○ Vibrate randomly about fixed positions - higher temperature = more vibrations
○ Energy supplied to raise temperature of solid increases kinetic energy of molecules
○ If temperature increased enough, solid melts - molecules vibrate so much they break
free from each other and substance loses its shape
○ Energy supplied to melt solid raises potential energy of molecules - break free
● In liquid:
○ Forces not strong enough to hold molecules in fixed positions
○ Move about at random in contact with each other - higher temperature = more moving
○ Energy supplied to raise temperature of liquid increases kinetic energy of molecules
○ If temperature increased enough, liquid vaporises - molecules move so much they
break free and move away from each other
● In gas:
○ Move about randomly but much further apart on average relative to liquid
○ Energy supplied to raise temperature of gas increases kinetic energy of molecules
Temperature
● Measure of degree of hotness of object - hotter = more internal energy
● Thermal equilibrium: both objects same temperature - no overall energy transfer by heating
● Temperature scale defined in terms of fixed points - standard degrees of hotness that can be
accurately reproduced
● Celsius scale: ice point 0oC (temperature of pure melting ice), steam point 100oC
(temperature of steam at standard atmospheric pressure)
● Absolute scale: absolute zero 0K (lowest possible temperature), triple point of water 273.16K
(ice, water, water vapour co-exist in thermodynamic equilibrium)
𝑜
● 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 ( 𝐶) + 273. 15 = 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 (𝐾)
Absolute zero
● Object at absolute zero has minimum internal energy, regardless of substances in object
● If pressure measured at ice point and steam point plotted on graph of different substances,
lines always cut temperature axis at -273oC
, Specific heat capacity
● 𝑒𝑛𝑒𝑟𝑔𝑦 = 𝑚𝑎𝑠𝑠 × 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 ℎ𝑒𝑎𝑡 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 × 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 (𝑄 = 𝑚𝑐∆𝑇)
● In temperature-time graph (assuming no heat loss and energy transferred at constant rate P)
∆𝑇 𝑃
SHC ∆𝑡
= 𝑚𝑐
SHC Inversion tube experiment
● GPE of object (tiny lead spheres) falling in tube converted into internal energy when hitting
bottom of tube
● Temperature of lead shot measured initially then tube inverted each time spheres hit bottom
of tube certain times then temperature remeasured
● If m is mass of lead shot, L is length of tube, loss of GPE = mgL
● For n inversions, loss of GPE = nmgL, gain of internal energy = mc∆T - if mc∆T = nmgL then
𝑛𝑔𝐿
𝑐= ∆𝑇
SHC measurement electrically
● Block of metal mass m in insulated container used
● 12V electrical heater inserted into hole in metal and used to heat metal by
supplying measured amount of electrical energy
● Thermometer inserted into another hole in metal to measure ∆T
● Small amount of water or oil in thermometer hole will improve thermal contact
𝐼𝑉𝑡
● 𝐸 = 𝐼𝑉𝑡, 𝑐 = 𝑚∆𝑇
● To find SHC of liquid, replace metal with liquid, use different type of heater and add
calorimeter and stir when measuring temperature
● 𝐼𝑉𝑡 − 𝑚𝑐𝑎𝑙𝑐𝑐𝑎𝑙∆𝑇 = 𝑚1𝑐1∆𝑇
Continuous flow heating
● In electric shower, water passes steadily through copper coils heated by electrical heater
● Water hotter at outlet than inlet - example of continuous flow heating
∆𝑇
● For mass m of liquid passing through heater in time t, assuming no heat loss 𝐼𝑉 = 𝑚𝑐 𝑡
● If outflowing water is at steady temperature, temperature of copper coils doesn’t change so no
mc∆T needed
∆𝑇
● For solar heating panel, power by heating liquid flowing through panel = 𝑚𝑐 𝑡
Change of state
Heating an object
● Melting point: temperature at which substance melts from solid to liquid
● Boiling point: temperature at which substance evaporates from liquid to vapour
● Gas density much less than liquid or solid - molecules separated by relatively large distances
in gas but packed together in solid and liquid
● Solids can’t flow but liquids and gases can - atoms in solid locked together by strong force
bonds but not locked together in solid or liquid (too much kinetic energy or weak bonds)
Latent heat
● Fusion: solid heated to melting point, atoms vibrate so much they break free from each other -
energy supplied at melting point makes solid become liquid
Chapter 19: Thermal Physics
Internal energy and temperature
Energy transfer
● One object exerts force on other object and makes it move
● One object hotter than other object so energy transfer by conduction, convection or radiation
Internal energy
● Sum of randomly distributed kinetic energies and potential energies of particles in body - only
kinetic energy for ideal gas
● Thermal energy: internal energy of object due to its temperature
● Can be increased by heating object or work done on object
● Can stay constant if no energy transfer by heating and work done or energy transfer by
heating and work done balance each other out
● First law of thermodynamics: when work is done on or by object and/or energy transferred by
heating, change of internal energy = total energy transfer by work done and heating
Molecules (simple kinetic model)
● In solid:
○ Held together by forces by electrical charges of protons and electrons
○ Vibrate randomly about fixed positions - higher temperature = more vibrations
○ Energy supplied to raise temperature of solid increases kinetic energy of molecules
○ If temperature increased enough, solid melts - molecules vibrate so much they break
free from each other and substance loses its shape
○ Energy supplied to melt solid raises potential energy of molecules - break free
● In liquid:
○ Forces not strong enough to hold molecules in fixed positions
○ Move about at random in contact with each other - higher temperature = more moving
○ Energy supplied to raise temperature of liquid increases kinetic energy of molecules
○ If temperature increased enough, liquid vaporises - molecules move so much they
break free and move away from each other
● In gas:
○ Move about randomly but much further apart on average relative to liquid
○ Energy supplied to raise temperature of gas increases kinetic energy of molecules
Temperature
● Measure of degree of hotness of object - hotter = more internal energy
● Thermal equilibrium: both objects same temperature - no overall energy transfer by heating
● Temperature scale defined in terms of fixed points - standard degrees of hotness that can be
accurately reproduced
● Celsius scale: ice point 0oC (temperature of pure melting ice), steam point 100oC
(temperature of steam at standard atmospheric pressure)
● Absolute scale: absolute zero 0K (lowest possible temperature), triple point of water 273.16K
(ice, water, water vapour co-exist in thermodynamic equilibrium)
𝑜
● 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 ( 𝐶) + 273. 15 = 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 (𝐾)
Absolute zero
● Object at absolute zero has minimum internal energy, regardless of substances in object
● If pressure measured at ice point and steam point plotted on graph of different substances,
lines always cut temperature axis at -273oC
, Specific heat capacity
● 𝑒𝑛𝑒𝑟𝑔𝑦 = 𝑚𝑎𝑠𝑠 × 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 ℎ𝑒𝑎𝑡 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 × 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 (𝑄 = 𝑚𝑐∆𝑇)
● In temperature-time graph (assuming no heat loss and energy transferred at constant rate P)
∆𝑇 𝑃
SHC ∆𝑡
= 𝑚𝑐
SHC Inversion tube experiment
● GPE of object (tiny lead spheres) falling in tube converted into internal energy when hitting
bottom of tube
● Temperature of lead shot measured initially then tube inverted each time spheres hit bottom
of tube certain times then temperature remeasured
● If m is mass of lead shot, L is length of tube, loss of GPE = mgL
● For n inversions, loss of GPE = nmgL, gain of internal energy = mc∆T - if mc∆T = nmgL then
𝑛𝑔𝐿
𝑐= ∆𝑇
SHC measurement electrically
● Block of metal mass m in insulated container used
● 12V electrical heater inserted into hole in metal and used to heat metal by
supplying measured amount of electrical energy
● Thermometer inserted into another hole in metal to measure ∆T
● Small amount of water or oil in thermometer hole will improve thermal contact
𝐼𝑉𝑡
● 𝐸 = 𝐼𝑉𝑡, 𝑐 = 𝑚∆𝑇
● To find SHC of liquid, replace metal with liquid, use different type of heater and add
calorimeter and stir when measuring temperature
● 𝐼𝑉𝑡 − 𝑚𝑐𝑎𝑙𝑐𝑐𝑎𝑙∆𝑇 = 𝑚1𝑐1∆𝑇
Continuous flow heating
● In electric shower, water passes steadily through copper coils heated by electrical heater
● Water hotter at outlet than inlet - example of continuous flow heating
∆𝑇
● For mass m of liquid passing through heater in time t, assuming no heat loss 𝐼𝑉 = 𝑚𝑐 𝑡
● If outflowing water is at steady temperature, temperature of copper coils doesn’t change so no
mc∆T needed
∆𝑇
● For solar heating panel, power by heating liquid flowing through panel = 𝑚𝑐 𝑡
Change of state
Heating an object
● Melting point: temperature at which substance melts from solid to liquid
● Boiling point: temperature at which substance evaporates from liquid to vapour
● Gas density much less than liquid or solid - molecules separated by relatively large distances
in gas but packed together in solid and liquid
● Solids can’t flow but liquids and gases can - atoms in solid locked together by strong force
bonds but not locked together in solid or liquid (too much kinetic energy or weak bonds)
Latent heat
● Fusion: solid heated to melting point, atoms vibrate so much they break free from each other -
energy supplied at melting point makes solid become liquid
