B.1 Thermal Energy Transfers Flashcards
Temperature
Measure of the average random Ek of the molecules in the object. Scalar quantoty
What does the kelvin scale measure?
Absolute temperature - at 0K, the molecules of a substance have stopped moving. It therefore cannot be a negative value.
Internal Energy
The total of the total intermolecular potential energy of the molecules and the total random kinetic energy.
What is the total intermolecular potential energy?
Energy due to the forces between molecules
If you heat a substance, what happens to the total intermolecular potential energy?
It increases it by moving them further apart
What are the two types of kinetic energy?
The translational (whole molecules moving in a certain direction) and the rotational (molecules rotating about an axis)
If two substances are the same temperature, how could you describe their internal energy?
They will be different due to their different masses (number of molecules) and molecular arrangements
Transfer of energy
Energy moves from hot to cold objects
If two objects are in thermal contact,
Transfer of energy will occur until thermal equilibrium is achieved.
What is the difference between internal and thermal energy?
Internal energy is energy objects have, thermal energy is when the energy is being transferred.
What determines the density of a substance?
Size and arrangement of the molecules
Specific Heat Capacity (c)
The amount of energy required to raise the temperature of the unit mass of a substance by 1K.
Why do substances have different specific heat capacities?
Because different substances have different molecular structures, forces between molecules and/or atoms and densities.
Specific latent heat (L)
Amount of energy per unit mass of a substance absorbed or released during a change in phase without a change in temperature (Jkg-1)
Latent heat of fusion
Solid to liquid
Latent heat of vaporization
Liquid to gas
When a substance changes phase, its temperature does not change. Why is this?
Molecules move further apart, bonds are broken/formed but speed does not change. Molecules gain/lose potential energy but not kinetic. Temperature is dependent on average Ek so doesn’t change
Which is bigger - latent heat of vaporization or fusion
Vaporization because change from liquid to gas requires molecules to be further apart so are seperated into gas. Solid to liquid is less movement
Boiling
Takes place throughout a liquid at the same temp (boiling point)
Evaporating
Takes place on the surface of the liquid and can happen at all temperatures (only some particles)
Absolute zero
When particles have 0 energy
All objects with a temperature over 0
Possess internal energy due to Ek and Ep
During transfer of energy through temperature, what is the energy known as?
Thermal energy
Thermal conduction
Thermal energy is transferred without movement of atoms/molecules. Ek is passed
What are the mechanisms that contribute to thermal conduction?
Free electrons and atomic vibration
Atomic vibration (thermal conduction)
The higher the temperature, speed/amplitude of molecules vibration increases. Particles collide with neighbouring atoms/molecules and transfer internal energy.
When does conduction occur best?
In solids as gases/liquids have weaker interatomic/molecular connections
Free electrons (thermal conduction)
Electrons are in thermal equilibrium with ions in atomic lattice. If metal is heated, electrons gain energy and transfer it to the lattice increasing vibration of ions.
Thermal conductivity (K)
Measure of how good a thermal conductor is at transferring energy through itself (W m-1 k-1)
Larger the thermal conductivity (K)
Better the substance is at transferring energy
Rate of energy transfer depends on what?
- Physical dimensions of the conductor (area and thickness)
- Thermal conductivity
- Temperature gradient across the conductor
Convection
When thermal energy is transferred by bulk movement of the molecules that make up the substance
Explain a convection current
Heated region of fluid expands, particles gain internal energy and move apart, becomes less dense than surroundings and so rises, then cools, becomes more dense and sinks. Current occurs as process repeats.
Thermal radiation
Emission of electromagnetic radiation by an object
How does thermal radiation occur?
No medium is needed for particles, the atoms contain charged particles. When the atoms are in motion due to temperature, they emit photons due to acceleration of charged particles. The photons make up the thermal radiation.
Where is thermal radiation emitted from?
The surface of an object
The amount and type of electromagnetic radiation emitted by an object will depend on?
The temperature of the object
The amount of thermal radiation emitted by an object depends on?
Its surface area and the nature of the surface
The darker/matter the surface, the what the thermal radiation
Bigger emitting of thermal radiation
Black body
Perfect emitter of radiation - absorbs all radiation incident on it and emits dependent on its temp and surface area.
What does the amount of radiation of a black body depend on?
The temperature and surface area
As the temperature of the black body increase,
The peak wavelength decreases (frequency increases), intensity increases, overall intensity (power per m2 emitted) of each wavelength increases
Luminosity (L)
Total power radiated (W)
Luminosity is proportional to…
T^4 (where T is in K)
Apparent brightness (b) units
Wm-2
Stars
Considered to be black bodies, luminosity is given relative to the luminosity of the sun
Specific heat capacity (c) units
J kg^-1 K-^1
What happens to the molecules of a liquid during evaporation?
Their average Ek of molecules decreases and the temperature of the liquid decreases
Apparent brightness (B/Wm-2) as distance from black body increases
As the distance from a black body increases, intensity of radiation recieved will decrease as it spreads from the source
Thermal conductivity (k) units
Wm-1K-1
Specific latent heat (L) units
Jkg-1
