IB Physics Topic 3 and Topic 10 Thermal Physics
Terms in this set (40)
State that temperature determines the direction of thermal energy transfer between two objects.
•Thermal energy naturally flows from hot to cold
•Reach the same temperature, when no more transfer of thermal energy, THERMAL EQUILIBRIUM
State the relation between the Kelvin and Celsius scales of temperature.
T(K) = t(°C) + 273 is sufficient.
State that the internal energy of a substance is the total potential energy and random kinetic energy of the molecules of the substance.
→from random motion of molecules
→translational KE from moving in certain direction
→rotational KE from rotating about one or more axes
→due to intermolecular forces
Distinguish between microscopic and macroscopic.
MICROSCOPIC: inside the system to see how its component parts interact
MACROSCOPIC: consider the system as a whole to see how it interacts with its surrounding
Distinguish between the macroscopic and microscopic concept of temperature.
MACROSCOPIC: Hotness or coldness of an object as measured by a thermometer
MICROSCOPIC: average kinetic energy per molecule of the molecules in the substance
Macroscopic concept of internal energy.
Total energy related to the thermal motion of the molecules in a substance, includes both vibrational and translational motion, and comprised of both the kinetic and potentials energies of the molecules
Define thermal energy (heat).
•Refers to the non-mechanical transfer of energy between a system and its surroundings
•Thermal energy in a body is the work in a body
Define the mole and molar mass
•SI unit for 'amount of substance'
•one mole of any substance is equal to the amount of that substance that contains as many atoms (or molecules) as there are in 12g of Carbon-12.
•mass per amount of substance
•mass of one mole of substance
•mass of 6.02 ⨉10²³ molecules of a substance
Define the Avogadro constant.
A mole of any material contains 6.02 x 10²³ atoms or molecules
Define specific heat capacity and thermal capacity
SPECIFIC HEAT CAPACITY:
Energy required to raise the temperature of 1kg of the material by 1℃
Energy required to raise the temperature of a body by 1℃
Problems involved when measuring specific heat capacities and thermal capacities.
•If an object is raised above room temperature, it starts to lose energy.
•The hotter it becomes the greater the rate at which it loses energy.
Explain the physical differences between the solid, liquid and gaseous phases in terms of molecular structure and particle motion.
•fixed shape and volume
•molecules held in position by strong force (bonds) between atoms
•vibrate around a mean (average) position
•lowest internal energy
•no fixed shape but fixed volume
•weaker forces (some bonds broken), this keeps molecules close
•vibrating but not in completely fixed positing, free to move around each other
•very weak forces (all bonds broken), molecules essentially independent
•atoms completely free to move at high speed, occasionally collide
•gas fills container
•No fixed shape or volume, no force between molecules
Describe and explain melting in terms of molecular behaviour.
•particles gain enough energy to overcome intermolecular bonds
•break away from their lattice structure
•able to move freely through the substance, becoming a liquid
Describe and explain freezing in terms of molecular behaviour.
•decrease in energy allowing intermolecular bonds to grow stronger
•molecules are slowed by the removal of energy
•pull them into a lattice, creating a solid.
Describe and explain evaporation in terms of molecular behaviour.
•fast moving molecules on surface gain enough energy to break intermolecular bonds and escape
•average kinetic energy of the remaining particles is lower resulting in a drop in temperature
Describe and explain boiling (vaporisation) in terms of molecular behaviour.
•particles gain enough energy to break intermolecular bonds
•as temp increases, faster molecules can escape, and eventually all do, forming a gas
Describe and explain condensation in terms of molecular behaviour.
•particles lose energy that overcame intermolecular bonds between them
•as energy is removed, molecules slow down, eventually the intermolecular forces become dominant enough to hold the molecules together in a liquid
Explain in terms of molecular behaviour why temperature does not change during a phase change.
•energy given does not increase KE as it increases there PE
•Intermolecular bonds being broken require energy
•when substance forms bonds, releases energy
Distinguish between evaporation and boiling.
BOILING: takes place throughout the liquid and always at the same temperature.
EVAPORATION: takes place only at the surface of the liquid and can happen at all temperatures, faster moving molecules escape liquid, slower ones left behind causing the temperature to fall
RATE OF EVAPORATION DEPENDS ON:
Define specific latent heat
Amount of energy required per unit mass absorbed or released during a chinage of phase
It is the force per unit area acting on a surface.
State the assumptions of the kinetic model of an ideal gas.
•Molecule collisions are perfectly elastic
•The molecules are perfect spheres
•The molecules are identical
•No intermolecular forces between molecules
•Molecules are in constant random motion (constant velocity)
•No time spent in collisions
•Molecules very small, that is, their total volume is much smaller than the volume of the gas
•Newtons laws apply to molecular behaviour
State that temperature is a measure of the average random kinetic energy of the molecules of an ideal gas.
Temperature is a measure of the average kinetic energy of the molecules of an ideal gas.
Explain the macroscopic behaviour of an ideal gas in terms of a molecular model.
PRESSURE AND VOLUME:
•if volume is reduced, particles hit the walls more often since the walls are closer together.
•force exerted by the molecules equal to rate of change of the momentum
•this increaseS if collides more frequent, resulting in an incease pressure
PRESSURE AND TEMPERATURE:
•increase in temperature increases the speed of the molecules
•When the molecules hit the walls, their change of momentum will be greater and they will hit the walls more often
•result in greater rate of change of momentum and hence a larger force
•results in an increase in pressure
DOING WORK ON A GAS:
•When push piston of a pump it collides with the molecules giving them energy.
•doing work on the gas
•increase in kinetic energy results in an increase in temperature and pressure
GAS DOES WORK:
•when a gas expands pushes on surrounding air
•in pushing, gas does work which requires energy
•this energy comes form the kinetic energy of the molecules, resulting in a reduction in temperature
State the gas laws.
PRESSURE LAW (GAY LUSSAC'S LAW):
State the equation of state for an ideal gas.
PV = nRT
•P is pressure
•V is volume
•n is moles
•R is molar gas constant (8.31Jmol-1K⁻¹)
•T is temperature
Describe the difference between an ideal gas and a real gas.
•assume gas particles are discrete point particles
•follows gas laws for all values of p, V and T
•cannot be liquefied
•cannot change phase due to extremely high or low pressures
•no intermolecular forces
•can approximate to ideal behaviour if their intermolecular forces are small enough to be ignored
•for this to apply pressure/density of the gas must be low and temperature must be reasonably high
Describe the concept of the absolute zero of temperature and the Kelvin scale of temperature.
Temperature at which molecules have zero KE, OK=-273°C
Temperature at which water exists as solid, liquid and gas in equilibrium, 0.01℃, almost the same as the freezing point.
Deduce an expression for the work involved in a volume change of a gas at constant pressure.
State the first law of thermodynamics.
•principle of conservation
ΔQ = ΔW + ΔU
•ΔQ if positive, thermal energy is going into the system
•ΔU if positive, internal energy of system is increasing
•ΔW if positive, then system is doing work on the surroundings (gas is expanding)
Identify the first law of thermodynamics as a statement of the principle of energy conservation.
The first law of thermodynamics is a statement of the principle of energy conservation.
Describe the isochoric (isovolumetric), isobaric, isothermal and adiabatic changes of state of an ideal gas.
-expansion (pressure decrease): W=0, ΔU<0, Q<0
-expansion (volume increase): W>0, Q increases, T increases
-expansion (no change in internal energy): W>0, Q>0
-no heat transfer
-change in internal energy = negative work
-expansion (work is done by gas): V decreases, T increases
NOTE: compression is merely the opposite of expansion
Calculate from a P-V diagram the work done in a thermodynamic cycle
area enclosed between the graph and the x axis
State that the second law of thermodynamics implies that thermal energy cannot spontaneously transfer from a region of low temperature to a region of high temperature.
•continuous conversion of thermal energy into work requires a cyclical process- a heat engine
•no heat engine, operating in a cycle, can take in heat from its surroundings and totally convert it into work (kelvin plank)
•no heat pump can transfer thermal energy from a low temperature reservoir to a high temperature reservoir without work being done on it (Clausius)
•the entropy of the universe can never decrease.
A system property that expresses the degree of disorder in the system.
State the second law of thermodynamics in terms of entropy changes.
The total entropy of the universe is constantly increasing.
ΔS = Q/T
Discuss examples of natural processes in terms of entropy changes.
•A refrigerator is an example of a heat pump
•ice melting; solid turns into liquid (no distinct shape = more disorder = more entropy)
•rock breaking into particles (more disorder = more entropy)
Describe a heat engine.
•Device that uses a source of thermal energy in order to do work
•converts heat into work
•thermal energy can be taken from the hot reservoir without causing the temperature of the hot reservoir to change
•thermal energy can be given to the cold reservoir without increasing its temperature
•PROCESS: Isobaric expansion, isovolumetric decrease in pressure, isobaric compression, isovolumetric increase in pressure
•work done by gas is area enclosed by cycle
Describe a heat pump.
•causes thermal energy to be moved from a cold reservoir to a hot reservoir
•in order for this to be achieved, mechanical work must be done
•PROCESS: isovolumetric decrease in pressure, isobaric expansion, isovolumetric increase in pressure, isobaric compression
•work done on gas is area enclosed by cycle
Describe the Carnot cycle and the Carnot theorem.
•Carnot cycle represents the cycle of processes for a theoretical heat engine with maximum possible efficiency
•PROCESS: isothermal expansion, adiabatic expansion, isothermal compression, adiabatic compression
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