Thermal Physics
- Created by: CPev3
- Created on: 15-07-20 09:12
What is the triple point of a substance?
One specific temperature and pressure at which all three phases of the substance can exist in thermodynamic equilibrium
What is thermodynamic equilibrium?
There is no net transfer of thermal energy between the phases
What is the triple point of water?
- 0.01 oC
- 0.61 kPa
When one object is hotter than another:
Net flow of thermal energy from hotter object into colder object
.
......Temperature of colder object increases
.
............Temperature of hotter object decreases
When two objects are in thermodynamic equilibrium:
- No net flow of thermal energy between them
.
- Objects must be at the same temperature
What is the Celsius scale?
For pure water
- Freezing point = x oC
- Boiling point = x + 100 oC
.
For pure water when the atmospheric pressure is 1.01 x105 Pa
- Freezing point = 0 oC
- Boiling point = 100 oC
.Any object at 100 oC must be in thermal equilibrium with boiling water
What is the absolute temperature scale?
- Uses the triple point of water (273 K) and absolute zero (0 K) as its fixed points
.
- A temeprature increase of 1 K = a temeprature increase of 1 oC
.
- T (K) ≈ θ (oC) + 273
What is the kinetic model?
A model that describes all substances as made of atoms, ions or molecules, arranged differently depending on the phase of the substance
Particles in solids:
Spacing
- Strong electrostatic forces of attraction
- Packed closely together
Ordering
- Regularly arranged
Motion
- Kinetic energy
- Vibrate around their fixed positions
Particles in liquids:
Spacing
- Weaker electrostatic forces of attraction
- Very close together
Ordering
- Irregularly arranged
Motion
- More kinetic energy
- Flow past each other
- No fixed shape
Particles in gases:
Spacing
- Almost no electrostatic forces of attraction
- Far apart
Ordering
- Irregularly arranged
Motion
- Much kinetic energy
- Move randomly with different (high) speeds in different directions
- No fixed shape or volume
Particles in a heated solid:
Gain kinetic energy
Vibrate more vigorously
Break away from solid structure
Flow past each other
Change in phase to liquid
Particles in a heated liquid:
Gain kinetic energy
Enough for some to break away from the other particles
Particles which escape body of liquid become a gas
Density:
Affected by spacing between particles
Generally highest in solids and lowest in gases
.
Solid water less dense than liquid water
Water freezes into regular crystalline pattern
Held together by strong electrostatic forces of attraction between particles
Particles held slightly further apart than in their random arrangement in liquid water
Slightly less dense
What is Brownian motion?
- The continuous random motion of small particles suspended in a fluid
.
- Visible under a microscope
Pollen grains floating on water:
- Collisions betwen pollen grains and water molecules are elastic
.
- Result in transfer of momentum from molecules to grains
.
- Causes grains to move in haphazard ways
.
- Proves that matter is made up of particles and they have kinetic energy
Smoke cell:
- Smoke particles in random motion
.
- Caused by collisions with air molecules
.
- Air molecules in random motiion
.
- Mean kinetic energy of particles = that of molecules
.
- HOWEVER, particles more massive and thus slower than molecules
What is specific heat capacity?
The energy required per unit mass to change the temperature by 1 K (or 1 oC)
c = E / (m x Δθ)
What is the specific heat capacity of water?
4200 Jkg-1K-1
Experiment for determining c:
Find mass of block using mass balance
Connect heater in series with ammeter, power supply and variable resistor
Connect voltmeter in parallel around heater
Put heater and thermometer in block
Turn on power supply and start stopwatch
Record values for current and voltage (use variable resistor to keep current constant)
Record temperature at regular intervals for 10 mins
Turn off power pupply
Continue to record temperature for 5 mins
Plot results on θ-t graph
c = P / (m x gradient)
Method of mixtures:
mblock x cblock x Δθblock = mwater x cwater x Δθwater = E
What is specific latent heat?
The energy required to change the phase per unit mass while at a constant temperature
L = E / m
What is specific latent heat of fusion?
The energy required to change unit mass of a substance from solid to liquid while at a constant temperature
Lf = E / m
What is specific latent heat of vaporisation?
The energy required to change unit mass of a substance from liquid to gas while at a constant temperature
Lf = E / m
Experiment for determining L of f:
Connect heater in series with ammeter, power supply and variable resistor
Connect voltmeter in parallel around heater
Put heater and thermometer in funnel filled with ice cubes over beaker
Wait for rate of melting of ice cubes to be steady, then wait 10 mins (stopwatch necessary)
Find mass of water using mass balance
Turn on power supply
Record values for current and voltage (keep current constant using variable resistor)
Wait for rate of melting of ice cubes to be steady, then wait 10 mins (stopwatch necessary)
Find mass of water using mass balance
Lf = electrical energy supplied / difference between two recorded masses of water
Experiment for determining L of v:
An electrical heater can be used with a condenser to collect and then measure the mass of liquid that changes phase
Lf = IVt / m where m is the mass of liquid that changed phase during heating
θ-t graph- energy transferred calculations:
1. Heating the solid to its melting point: E = mcsolidΔθ
2. Melting the solid at a constant temperature: E = mLf
3. Heating the liquid to its boiling point: E = mcliquidΔθ
4. Boiling the liquid at a constant temperature: E = mLv
What is absolute zero?
- The lowest possible temperature
.
- The temperature at which substances have minumum internal energy
What is internal energy?
The sum of the randomly distributed kinetic and potential energies of the atoms/ ions/ molecules within the substance
Equation for final temperature:
T = (MCTh + mcTc) / (MC + mc)
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