Heat and Temperature
PHYS 1090 Unit 4
Put Your Hands Together!
(Activity 1)
• Doing work on your hands made them
warmer.
• Adding energy raised the temperature.
• Work had the same effect as heat.
Mechanical Equivalent of Heat
James Joule’s life-long obsession
No difference between
adding heat to a
system and doing work
on it.
Source: Griffith, The Physics of Everyday Phenomena
Heat Units
• Joule
• Calorie (cal): amount of heat needed to
raise 1 gram of water 1 degree C (or K) =
4.184 J.
• British Thermal Unit (BTU): amount of
heat needed to raise 1 pound of water 1
degree F = 1054.35 J
Specific Heat
• The amount of heat required to change the
temperature of a unit mass of substance.
Cp =
q
mDT
– C = specific heat
– q = heat added
– m = mass of sample
– DT = temperature change
Another Heat Unit
• U.S. Food Calorie: Cal = 1000 cal
• Food energy values are often presented in
kJ in other countries
First Law of Thermodynamics
DE = q + w
where
DE = change in internal energy
q = heat input
w = work input
Kinetic-Molecular Theory
• Everything is made of molecules.
• The molecules are constantly moving in
random directions.
• (Absolute) temperature is proportional to
molecular translational kinetic energy.
• Molecules colliding with objects they
contact causes pressure.
Absolute Zero
•
•
•
•
•
When molecules have zero kinetic energy
Absolute zero = −273.15 °C
Kelvin temperature = °C + 273.15
Absolute zero = 0 K
0 °C = 273.15 K
Liquid N2 Boil
(Activity 2)
• More boiled away with cold water
• More heat transferred from cold water
• Because there was more cold than hot
water
Warm and Cool Colors
(Activity 3)
• Dye dispersed faster in warm water.
• Why?
• Water molecules were moving faster in the
warm water.
Phase Change
(Activity 4)
• Melting ice temperature was constant
even though heat was being added
– Specific heat is infinite?
• Boiling water temperature was also
constant
– Boiling temperature  100°C?
Sensing Latent Heats
(Activity 5)
• Evaporation is a cooling process.
• Condensation is a warming process.
Phase Changes
• Potential energies:
Gas
evaporate
sublime
Liquid
melt
condense
deposit
freeze
Solid
• During a phase change, potential energy,
not kinetic energy (temperature) changes.
• Heating or cooling a changing phase does
not change its temperature!
Phase Changes
• Melting, boiling, freezing, condensing…
• Added or removed heat changes the
substance’s potential rather than kinetic
energy
• Water freezes at 0 °C, boils 100 °C
• Not all heat transfer is expressed as a
temperature change.
Heats of Phase Changes
• To melt 1 kg water at 0 °C: 335 kJ
• To boil 1 kg water at 100 °C: 2,255 kJ
Heating Curve for Water
Water temperature with heating
temperature (C)
200
Water boils
150
100
50 Ice melts
0
2,255 kJ/kg
335 kJ/kg
-50
-100
0.0E+00
1.0E+06
2.0E+06
heat input (J/kg)
steam
Liquid
water
ice
3.0E+06
4.0E+06
Heat Transfer Mechanisms
• Heat transfers between objects by:
• Conduction: collisional transfer of kinetic
energy
• Convection: buoyancy-driven fluid
circulation
• Radiation and absorption of
electromagnetic waves
Conduction
(Activity 6)
• Thermal energy moves through different
materials at different speeds.
• Conductivity varies with material;
solids > liquids > gases
Convection
• Hot water stayed on top, cold stayed on
the bottom
• Hot water moved to the top, cold to the
bottom (with mixing)
Radiation
(Activity 8)
• Coil heated your hand from afar.
• Heating less intense farther away.
• Aluminum blocked heat.
Radiation
• Power output increases as T4.
• Objects are heated by absorbing radiation.
• Objects are cooled by emitting radiation.
Entropy
(Activity 9)
• Dice moved in either direction with equal
probability
• Began highly localized
Localized  Dispersed
• Inevitable and irreversible
Model of Diffusion
• Particles randomly move from high
concentration to low concentration
– Individual flow either way
– Net flow high → low
– Equilibrium: uniformity
• Energy flows in the same way
– Transfer in collisions
– Heat flow: high temp → low temp
Entropy
• A measure of “disorder”
• Related to the number of equivalent ways to
arrange a system
Low entropy
High entropy
Overall Summary
• Particles and energy tend to become
spread out uniformly.
• Total entropy increases in all processes
that actually occur.
Drinking Bird
• Evaporation from the bird’s head cooled
the vapor inside.
• This reduced the head pressure to less
than in the bottom bulb.
• The higher pressure in the bottom pushed
the fluid uphill, making the bird top-heavy.