Transcript Physics Bell
Thermodynamics
Why do you burn the roof of your mouth when you eat pizza?
Why is sand at the beach so hot?
Specific Heat
Amt. of energy needed to raise temp. of one Kg of a substance 1 K
Unit J/kg•K
Specific Heat Equation Q = m c
T
For Example
How much energy does a 350g sample of water gain when its temperature is raised from 25 o C to 80 o C?
Caution!
Specific heat equation only works within a phase!
Phase Change Graph
Note…
A substance stays at its melting point until all solid becomes liquid…
–No change in temp.
During phase changes…
Energy added diminishes the forces holding molecules together
Heat of Fusion
Energy needed to melt a substance Q = m H f
Heat of Vaporization
Energy needed to vaporize a liquid Q = m H v
Heat Formulas Q = m c ΔT Q = m H f Q = m H v
Practice #1
Suppose you cool a hot pan by placing it in a sink of water… Q lost by pan = Q gained by water - Q = Q
Practice #2
Why do you place a “stuck” jar lid in warm water to loosen it?
Expansion Joint in a Bridge
Linear and Volumetric Expansion
Solids, liquids, gases
Prop. to
T and orig. length
Linear Expansion
L =
L 1
T
Volumetric Expansion
V =
V 1
T
Day 2
Temperature
Average KE of particles in a substance
Absolute Zero
No thermal energy present
0 Kelvin
Is it reachable?
Energy Transfer
Conduction – objects in contact
Convection – mvt. of fluids
Radiation – electromagnetic waves
Example
Thermal Energy
Total energy of particles (Cool swimming pool has greater thermal energy than hot cup of coffee.)
1 st Law of Thermodynamics
Thermal energy increased by adding heat or doing work
Magic Paper Clip!
Why does this obey the 1 st of Thermodynamics?
Law
Entropy
Degree of disorder
Increased temp. = increased entropy
Decreased temp. = decreased entropy
Entropy pic
Cartoon
As your car burns gas, is energy being lost?
No, converted to heat, ME, etc.
Entropy increases
2 nd Law of Thermodynamics Total entropy of the universe is increasing