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Chapter 19 Thermal Physics (19.3 (Latent heat (Latent heat of fusion…
Chapter 19
Thermal Physics
19.1
Energy transfer between 2 objects takes place if:
1)
One object exerts a force on the other and makes it move (does work)
2)
One object is hotter than the other- energy transferred due to temperature difference
Internal Energy
Definition
- Sum of the random distributions of the kinetic and potential energies of an object
Internal energy increases if:
1)
Energy transfer by heating the object
2)
Work done on the object
Internal energy constant if:
1)
No energy transfer by heating and no work done
2)
Energy transfer by heating and work done balance each other out
First law of thermodynamics
Change of internal energy= total energy transfer due to work done and heating
About molecules
Molecule- smallest particle of a pure substance that is characteristic of the substance
Atom- smallest particle of an element that is characteristic of the element
Solid
Atoms and molecules held to each other by forces due to the electrical charges of the protons and electrons in the atoms
Molecules vibrate randomly about a fixed position- higher temp. more vibration
Energy supplied increasing KE- temp raised enough-melts- molecules vibrate so much they break free from each other
Liquid
Molecules move about at random in contact with each other
Forces between molecules not strong enough to hold molecules in fixed positions
Higher temp. faster molecules move- energy supplied increases KE- heat enough they vaporise- enough energy to break free and move away from each other
Gas
Move randomly- heating makes the molecules speed up (more KE)
Temperature and temperature scales
Temperature
- measure of the degree of hotness of an object- hotter= more internal energy
Thermal equilibrium
- When no overall heat transfer occurs between 2 objects at the same temperature
Temperature scale
- defined in terms of fixed points that are standard degrees of hotness
Celsius scale
- ºC- defined in terms of
1)
Ice point (0ºC)
2)
Steam point (100ºC) at standard atmospheric pressure
Absolute scale
- K - defined in terms of
1)
Absolute zero (0K)
2)
Triple point of water (273.16K) where ice, water and water vapour co-exist in thermodynamic equilibrium
ºC-> K = -273.15K
Absolute Zero
- lowest possible temperature- object has minimum internal energy- zero pressure at -273.15ºC
19.2
Heating and Cooling
Temp rise depends on: mass, amount of energy supplied and substance it's made from
Specific heat capacity
: energy required to raise the temperature of 1kg of an object by 1ºK with no change of state
Continuous flow heater
Electric shower
Water hotter at outlet that inlet
For mass m of liquid passing through the heater in time t at a steady flow rate and assuming no heat lost to surroundings-
Electrical energy supplied per second IV= mc(ΔT/t)
Solar heating panel
Energy gained per second by heating the liquid that flows through the panel = mc(ΔT/t)
19.3
Pure substances melt at a well-defined temperature-
melting point
Point substances vaporise-
boiling point
Density of gas less than density liquid or solid- in solid or liquid molecules are packed together in contact- gas molecules are far apart
Liquids and solids can flow gases cannot- solids locked together by strong force bonds- liquid and gases have too much KE and force bonds not strong enough to keep them fixed
Latent heat
Latent heat of fusion
Latent heat released when liquid-> solid
Latent heat supplied when solid-> liquid
Amount of energy required to change the state of 1kg of a substance from solid to liquid with no temperature change
Latent heat of vaporisation
Amount of energy required to change the state of 1kg of a substance from liquid to gas with no temperature change
Latent heat released when gas -> liquid
Latent heat supplied when liquid -> gas
Sublimation
- Solid-> gas
Temperature-time graphs
