Topic 3: Thermal physics Flashcards
Define: temperature (2)
- A measure of the average kinetic energy of the particles within a substance.
- If two substances have the same temperature, then their molecules have the same average kinetic energy.
Define: thermal contact
Two objects are in thermal contact if it is possible for thermal energy to be transferred directly from one object to the other as a result of the temperature difference.
Define: heat/thermal energy
- The energy transfer that results when two objects are in thermal contact with each other
- Thermal energy always flows from hot to cold
- The temperature difference between the two objects will determine the direction of the natural transfer of thermal energy
- Heat is the non-mechanical transfer of energy between a system and its surroundings.
Explain the concept of thermal equilibrium.
- The net transfer of thermal energy is always from the object with the highest temperature to the object with the lowest temperature
- Therefore, the hottest object will cool down and the coolest object will warm up until they both reach the same temperature
- At this point the two objects are said to be in thermal equilibrium
- If two objects are in thermal contact and are both at the same temperature, it does not mean that no thermal energy is being transferred from one object to the other, but that there is no overall transfer of energy
- For this to be the situation, the objects must be in thermal equilibrium.
State the relationship between the Kelvin and Celsius scales of temperature
temperature in Kelvin = temperature in ºC +273 temperature in ºC = temperature in Kelvin -273
Define: internal energy
The total potential energy and kinetic energy of the molecules/atoms/particles OR the amount of energy stored in a substance.
Macroscopic concept of temperature
Ttemperature is the measure of the average kinetic energy of molecules in a substance.
Macroscopic concept of internal energy.
Sum of its molecular kinetic and potential energies. Molecules have kinetic energy due to their motion, both vibrational and translational. Molecules have potential energy due to the intermolecular forces.
Macroscopic concept of heat/thermal energy.
The transfer or change in thermal energy that is due only to a difference of temperature
What can adding energy to a substance result in?
- increase in temperature (EK increases)
- change in state (EP increases)
- energy loss by the substance at an equal rate (thermal equilibrium)
- chemical change (burn, decompose, etc.)
What does the rate of temperature change depend on? (3)
- the rate at which net energy is added
- the mass of a substance
- the specific heat capacity of a substance
What does the rate at which state can change depend on? (2)
- the rate at which net energy is added
- the specific latent heat of fusion/vaporisation of the substance
Define: kinetic theory
Molecules are arranged in different ways depending on the phase of the substance (solid/liquid/gas)
Define: conduction (4)
- Thermal energy is passed on by molecules colliding with their neighbouring molecules – neighbouring molecules therefore move more, increasing their thermal energy and so on
- Is the only type of heat transfer that can occur in solids and also occurs in fluids
- Requires matter
- Metals are good conductors of heat as they contain free electrons that assist the passage of heat through the substance.
Define: convection (3)
- Thermal energy is moved by molecules containing thermal energy moving
- Requires matter
- Is usually in the upwards direction, since hotter particles move more, take up more space and make that part of the fluid less dense.
Define: radiation (3)
- Thermal energy is transferred from a hot body by infrared radiation
- This radiation can then be absorbed by another body, whose internal energy would then increase. Is most effective in a vacuum
- All objects radiate heat.
Define: mole
The amount of a substance that contains as many elementary entities as the number of atoms in 12 g of the isotope carbon-12.
Define: molar mass
The mass of one mole of a substance. If an element has a certain mass number, A, then the molar mass will be A grams.
Define: Avogadro constant
The number of atoms in 0.012 kg of carbon-12 (12C).
6.02 X 1023 mol-1
Define: specific heat capacity
The energy required to increase 1 kg of a substance by 1 K.
What is the defining equation for specific heat capacity?
c = E/(mΔT)
Why do different substances have different specific heat capacities?
- They contain different numbers of molecules per kilogram
- The chemical (bonding) properties are different for different substances
Define: thermal capacity
Energy required to raise an object’s temperature by 1 K.
Symbol for thermal capacity
mc
Standard index measurement for thermal capacity
J K-1
Standard index measurement for specific heat capacity
J kg-1 K-1
Explain the physical characteristics of a solid. (2)
- Fixed volume and fixed shape because molecules are held in position by bonds. However, these bonds are not absolutely rigid
- The molecules vibrate around a mean (average) position; the higher the temperature, the greater the vibration.
Explain the physical characteristics of a liquid.
- Fixed volume but not a fixed shape because molecules are vibrating but are not completely fixed in position
- There are strong forces between the molecules, which keeps them close to one another; however, they are free to move around each other.
Explain the physical characteristics of a gas.
- will expand to fill its container because the molecules are not fixed in position
- Forces between the molecules are very weak.
Describe and explain the process of phase changes in terms of molecular behaviour
Explain what happens when a substance is being heated whilst in a solid state.
- energy needed = m c Δt
- Increased energy results in an increase in the kinetic energy of the particles
- The greater average separation of the particles means that there is also a slight increase in their potential energy.
Explain what happens when a substance changes phase from a solid to a liquid
- energy required = mL
- Results in an increase in the potential energy of the particles to a point where the attractive forces are weak enough to allow translational movement of the particles
Explain what happens when a substance is being heated whilst in a liquid state.
- energy needed = m c Δ t
- Increased energy results in an increase in the kinetic energy of the particles
- The greater average separation of the particles means that there is also a slight increase in their potential energy.
Explain what happens when a substance changes phase form a liquid to a gas.
- energy needed = m L
- Results in an increase in the potential energy of the particles to a point where there is no longer attractive forces between them. The potential energy of the particles is now zero. If the gas is heated further, the particles move faster, their kinetic energies are increased and the temperature continues to increase.