14. Thermal physics Flashcards
What is the Triple point?
For a given substance, once specific temperature and pressure at which all three phases of that substance can exist in thermal equilibrium
What is Thermal equilibrium?
A state in which there is no net flow of thermal energy between the objects involved (the objects in thermal equilibrium must be at the same temperature)
What is the Absolute scale of temperature?
A scale for measuring temperature based on absolute zero and the triple point of pure water
unit = Kelvin, K
What is Absolute zero?
The lowest possible temperature, the temperature at which substances have minimum internal energy
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
What is Internal energy?
The sum of the random distribution of kinetic and potential energies associated with the molecules of a system
What is Specific heat capacity?
The energy required per unit mass to change the temperature of 1K or (1˚C)
unit = JKg^-1K^-1
What is Specific latent heat?
The energy required to change the phase per unit mass while at constant temperature
symbol = L
What is Specific latent heat of fusion?
The energy require to change unit mass of a substance from solid to liquid while at constant temperature
symbol = Lf
What is Specific latent heat of vaporisation?
The energy required to change unit mass of a substance from liquid to gas while at constant temperature
symbol = Lv
What equations do i need to know for this topic?
T(K) ≈ ø(˚C) + 273
Energy (J) = mass (kg) x specific heat capacity (Jkg^-1K^-1) x change in temp (K or ˚C)
E = mc∆ø
E = IVt
c = IVt/∆ø
P = E/∆t
P = mc∆ø/∆t
Specific latent heat (Jkg^-1) = Energy (J) / mass (kg)
L = E/m
Lf = E/m or Lf = IVt/m
Lv = E/m or Lv = IVt/m
What is the triple point?
One specific temperature and pressure (for a given substance) at which all three phases of that substance can exist in thermal equilibrium
-> There is no net transfer of thermal energy between the phases
What is temperature and what does it have to do with thermal equilibrium
Temperature is a measure of the hotness of an object on a chosen scale. The hotter an object is, the higher its temperature
If one object is hotter than another there is a net flow of thermal energy from the hotter object to the colder one
-> This increases the temperature of the colder object and lowers the temperature of the hotter one
When two objects are in thermal equilibrium there is no net flow of thermal energy between them. This means that any objects in thermal equilibrium must be at the same temperature
What are the different ways of measuring temperature?
CELCIUS SCALE
-> Freezing and boiling points of pure water are the two fixed points
-> There are 100 increments between 0˚C and 100˚C
-> Not perfect as the two points vary depending on the surrounding atmospheric pressure
ABSOLUTE/THERMODYNAMIC TEMPERATURE SCALE
-> Uses the triple point of pure water and absolute zero as its fixed point
-> SI base unit = kelvin K
-> Incriments on both scales are the same (for simplicity)
T(K) = ø(˚C) + 273
From the kinetic model describe solids
- Atoms/molecules are regularly arranged and packed closely together
- There are strong electrostatic forces of attraction between the particles which holds them in fixed positions
- The particles have small amounts of kinetic energy so they can vibrate but cannot move out of their positions in the structure
What happens to the particles when a solid is heated?
- As the temperature of the solid rises the particles gain kinetic energy and vibrate more and more vigorously
- Eventually they may break away from the solid structure and become free to move around
-> When this happens, the solid has turned into liquid; it has melted
From the kinetic model describe Liquids
- Atoms/moleucles are very close together but further spread out than in solids
- Electrostatic forces of attraction are weaker than in solids and kinetic energy is greater so the particles can change position and flow past each other
What happens when a liquid is heated?
- As the temperature of the liquid increases the particles gain kinetic energy
- Once they have gained enough energy they break away from the other particles and escape from the body of the liquid
-> The liquid has become a gas
From the kinetic model describe Gases
- Electrostatic forces are negligible (unless particles collide with each other or container walls) so particles are far apart and free to move past each
- Particles have more kinetic energy than in solids and liquids so the particles move at high speeds all around and collide with the walls of their container
-> These collisions are responsible for the pressure which a gas exerts on its container
Why is solid water less dense than liquid water?
The spacing between the particles in a substance in different phases affects the density of the substance
Water freezes into a regular crystalline pattern held together by strong electrostatic forces between molecules
-> In this structure the molecules are held slightly further apart than in their random arrangement in liquid water, so ice is slightly less dense
What is internal energy?
The sum of the randomly distributed kinetic and potential energies of atoms or molecules within the substance
(The potential energies are due to the electrostatic intermolecular forces between the molecules)
Why will internal energy never be zero?
Absolute zero is the lowest possible temperature, the temperature at which a substance has minimum internal energy.
Decreasing the temperature of a substance decreases its kinetic energy.
At absolute zero the kinetic energy of the atoms and molecules is zero - stopped moving.
However, the substance still has electrostatic intermolecular forces between the molecules and therefore potential energy stored between the particles.
How can you increase the internal energy of a body?
- Increase temperature
-> increasing temperature increases the average kinetic energy of atoms/molecles inside the body
(therefore internal energy increases) - Changing phase
-> When a substance changes phases, the temperature does not change therefore neither does the kinetic energy of the particles.
However, the particles electrostatic potential energy increases significantly
What happens to the energy of the particles when a substance changes phase?
When a substance reaches its melting/boiling point, whilst changing phase the electrostatic potential energy of the substance increases as the electrical forces between the atoms/molecules change.
Once the phase change is complete, the kinetic energy of the atoms/molecules increase further, and is the temperature rises again.
What is the electrostatic potential energy like in the different phases?
GAS:
Electrostatic potential energy = 0
-> because there are negligible electrical forces between atoms/molecules
LIQUID:
The electrostatics forces between atoms/molecules give the electrostatic potential energy a negative value
The negative simply means that energy must be supplied to break the atomic or molecular bonds
SOLID:
The electrostatic forces between atoms/molecules are very large, so the electrostatic potential energy has a large negative value
The electrostatic potential energy is lowest in solids, higher in liquids, and at its highest (0J) in gases.
What is specific heat capacity and the equation?
The energy required per unit mass to change the temperature by 1K (or ˚C)
units = Jkg^-1K^-1
c = E/m∆ø or E = mc∆ø
How do you determine the specific heat capacity of a solid/liquid (in laboratory)?
Draw labelled diagrams for both solid and liquid, and a circuit diagram
Insulation is important to minimise the energy transferred from the substance to the surroundings
Liquid must be carefully stirred to ensure it has uniform temperature throughout
Plot a graph of temp against time for a more accurate determination of the specific heat capacity.
For a time; E=mc∆ø -> E/∆t = mc ∆ø/∆t -> P = mc ∆ø/∆t where ∆ø/∆t is the gradient,
therefore; c = P/m x gradient
What is method of mixtures?
Another way to determine specific heat capacity;
- Known masses of two substances at different temperatures are mixed together
- Recording their final temperature at thermal equilibrium allows the specific heat capacity of one of the substances to be determined if the specific heat capacity of the other is known
What is specific latent heat?
The energy required to change the phase per unit mass while at 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 constant temperature, Lf
How do you determine Lf (in laboratory)?
Draw and label diagram
Thermometer - to ensure the ice is at its melting point, not at a lower temperature
Ice should be seen to be just starting to melt before the heater is switched on
By measure if the p.d. across the heater, the current in the heater, and the time during which the heater is used, the energy transferred to the ice can be determined by E = IVt -> Lf = IVt/m
(important for mass of the substance to be accurately measured)
What is specific latent heat of vaporisation?
The energy required to change unit mass of a substance form liquid to gas while at constant temperature, Lv
How do we determine specific latent heat of vaporisation (in laboratory)?
Draw and label diagram
By measure the p.d. across the heater, the current in the heater, and the time during which the heater is used, the energy transferred to the liquid can be determined using: E = IVt -> Lv = IVt/m
(it is important for the mass of the liquid to be accurately measured)
