Thermal physics Flashcards
To revise thermal physics
Define internal energy
Sum of the individual potential and kinetic energies of all molecules within the body
State the first law of thermodynamics
The change of internal energy of the object is equal to the total energy transfer due to work done and heating
Define specific heat capacity
The energy required to change the temperature of 1kg of the substance by 1K without changing state
Describe the experiment to measure the specific heat capacity of a metal
Metal block of known mass in an insulated container. Electrical heater inserted into a hole in the metal, thermometer into another.
Measure the temperature change over a given time
Which measurements are used to measure specific heat capacity of a metal, and how?
Heater’s current (I)
Heater’s pd (V)
Time of heating (t)
Temperature change (ΔT)
mcΔT=IVt (rearrange for c)
State Boyle’s law
When a fixed mass of gas is reduced in volume by a compressing piston, the pressure inside the piston increases
Explain why Boyle’s law is true
Gas particles all in motion
Pressure exists from surface collisions exerting a force
Volume decreases
Collisions occur more frequently
Pressure increases
State Charles’ law
For a fixed mass of an ideal gas at constant pressure, its volume is directly proportional to its absolute temperature
State the pressure law
For a fixed mass of an ideal gas at constant volume, its pressure is directly proportional to its absolute temperature
Define Avogadro constant
The number of atoms in exactly 12g of carbon-12
Define molar mass
The mass of 1 mol of the substance
What are the five assumptions that are made for the kinetic theory equation?
Molecules are point molecules
No attractive forces between molecules
Molecules move in continual random motion
All collisions are elastic
Time of collisions is negligible
Define molar heat capacity
The energy required to change 1 mol of gas by 1K
Define heat capacity
The energy required by an entire object to change temperature by 1K
How is work done on a gas calculated from a pressure against volume graph?
Area under the graph
Explain what is meant by specific latent heat of fusion (2)
The energy required to change one kg of an object from liquid to solid without changing temperature
Water is pumped througha heater in a hot tub, why is the temperature rise greater when the pump works at a higher speed? (2)
The pump is doing work on the water. Work can raise the temperature of the body
Use the kinetic theory of gases to explain why the pressure inside a football increases when the temperature of the air inside it rises. Assuming volume remains constant (3)
Molecules have a greater rms velocity. Number of collisions between molecules and inside surface of ball increases per second. pressure = F/A and F = rate of change of momentum
What is the equation for Work Done in a Piston with a set mass of gas in?
A: W = p ΔV W = Work Done p = Pressure ΔV = Change in Volume
What is the Second Law of Thermodynamics?
It is not possible to convert heat continuously into work, without at the same time, transferring some heat from a warm body into a cold body. : the total entropy of a system can never decrease over time (temperature of an object always wants to be in equilibrium with surroundings).
What is the equation for Force?
F = p x A F = Force p = Pressure A = Cross-Sectional Area
What is the equation for internal energy for constant pressure processes?
ΔU = ΔQ – pΔV ΔU = Change in Internal Energy ΔQ = Change in amount of heat supplied to the system p = Pressure ΔV = Change in Volume
What does the p-V graph look like for constant pressure (isobaric process)?
Horizontal line
What does the area below a p-V graph show?
Work Done
What are the rules for a cycle process?
1) For a complete cycle, ΔU = 0 (No change in internal energy)
2) the net work done = area enclosed by the cycle
How do you change from Celsius into Kelvin?
Celsius Temperature + 273
What is specific latent heat?
The amount of thermal energy required to change the state of 1 kg of a substance at constant temperature
What are the three equations for an ideal gas?
pV = NkT pV = nRT pV = (1/3)Nmc²
What are the equations for average kinetic energy?
KE = (1/2)m c² = (3/2)kT = (3/2)(RT/Nₐ)
What is the main property of a gas at absolute zero?
It has minimum internal energy
Define temperature
a measure of the average kinetic energy of the particles in a system
Define heat
the flow of thermal energy
Define thermal equilibrium
when two objects are at the same temperature so there is no flow of heat.
What causes pressure?
The collisions between gas particles and the inside walls of the container
What are the properties of an ideal gas
- Molecules move with random motion, 2. Molecules obey Newton’s laws, 2. Molecules have same average kinetic energy, 3. There are no attractive forces between molecules, 4. Collisions between molecules are elastic, 5. Molecules are point masses (have zero volume), 6. Time taken between collisions is much greater than during collisions.
Describe Brownian motion
particles moving randomly due to collisions with many other small, fast particles.
State the third law of thermodynamics
the entropy of an ideal substance will be zero at absolute zero (0K).
What is Avogadro’s constant?
The number of atoms in exactly 12g of carbon-12 (6.02x1023)
What is the specific latent heat of fusion?
The energy needed to change the state a substance from solid to liquid without a change of temperature.
What is the specific latent heat of vaporisation?
The energy needed to change the state a substance from liquid to gas without a change of temperature.
Define empirical
Based on observation or experience rather than theory or pure fact.
What is meant by the term elastic collisions?
Both KE and momentum are conserved and total E is constant before and after the collision
Define absolute scale
Measured in K starting at absolute zero
Particles exert pressure on the wall of a container because…
Molecules have momentum
Momentum change at wall
Momentum change at wall leads to force
Why is average velocity in a container zero?
Velocity is a vector
random direction
velocities cancel
Sublimation
a chemical process where a solid becomes a gas without going through a liquid stage
What is the difference between heat and temperature?
Heat is the flow of thermal energy as it flows from a high temperature to a low whereas temperature is a measure of the kinetic energies of the particles in a selected substance.
