Thermal Physics Flashcards
What is the SI unit of temperature?
K
What is the absolute zero temperature?
The lowest possible temperature an object can theoretically have
What is the value of absolute zero temperature?
0K (-273°C)
Describe the kinetic energy of molecules in an object at absolute zero temperature
They have no kinetic energy - so everything would stop
What do molecules have more of at higher temperatures?
They have more energy
In Kelvin scale, what 2 things are proportional to each other?
A molecule’s energy is proportional to the temperature
What is a change of 1K equal to in °C?
A change of 1°C
How do you convert from Kelvin to °C?
Minus 273°C
What is 0°C equal to in Kelvin?
273K
What is 100°C equal to in Kelvin?
373K
What can be said about particles in a gas, and what this depends on
All particles travel at different speeds, which is dependent on temperature (most travel at average speed)
What 3 things happen when the temperature of a gas is increased?
The average particle speed increases
The average KE of the particles increases
The distribution of speeds of particles becomes more spread out
What is the internal energy?
Internal energy of a body is the sum of the randomly distributed kinetic and potential energies of all its particles
What is a closed system?
A system which doesn’t allow any transfer of matter in or out
Describe the total internal energy for a closed system
It is constant, as long as it’s at a constant temperature and no energy is transferred in or from the system
What is transferred in a system when particles collide?
Energy is transferred between colliding particles almost constantly, but the total combined energy of all the particles doesn’t change
The average speed of particles in a system will stay the same provided that…?
The temperature of the closed system stays the same and no work is done on the system
How can the internal energy of a system be increased?
By doing work on the system by either heating or changing the systems shape
What happens to the average speed of the particles if work is done on the system?
The average speed will increase
How can the internal energy of a system be decreased?
By doing work to remove energy or by cooling
Describe what happens to the particles in a system if the internal energy decreases
The average kinetic or potential energy of the particles will decrease as a result of energy being transferred out of the system
Describe how heat is transferred
Heat is transferred from hot to cold
What is the difference between heat and temperature?
Heat is the total energy of molecules in a substance while temperature is a measure of the average energy of molecules in a substance
Describe how heat is transferred, in terms of particles
The particle with more energy transfers energy to the particle with less energy
The higher the difference in temperature between 2 substances…?
The faster heat will transfer
Describe another way, apart from particles, that heat is transferred
By radiation, a hotter substance will radiate heat quicker than cooler substances
What is the specific heat capacity?
Amount of energy needed to raise the temperature of 1kg of the substance by 1K (or by 1°C)
What is the equation that involves energy change and specific heat capacity?
Q = mcΔT
Q: Energy change
m: Mass of substance
c: Specific heat capacity
ΔT: Change in temperature
What is continuous-flow heating?
When a fluid flows continuously over a heating element
What are the 7 steps for the investigation to find the specific heat capacity using a continuous-flow calorimeter?
- Set up the apparatus
- Record the flow of water and the duration of the experiment
- Measure the temperature difference between when the water flows in and where the water flows out
- Record the current and potential difference of the heating element
- Work out the energy supplied to the water by using the equation Q=mcΔT+H (H is heat lost to surroundings)
- Repeat the experiment changing the potential difference and the flow rate so that ΔT remains constant
- Form equations for each experiment and then solve for c
Why, in the investigation to find the specific heat capacity using a continuous-flow calorimeter, do you record the flow of water and the duration of the experiment?
To work out the mass of water used
What is a change in phase?
A change in state
What changes and what remains the same when a substance changes state?
The internal energy changes, but the kinetic energy and temperature stays the same
Why does the kinetic energy stay the same when a substance changes state?
Because the potential energy of the particles changes, but the kinetic energy remains constant
What is energy used for when boiling water?
Energy is used to convert the water into steam
As a liquid turns into a gas, its potential energy…?
… increases even though the water molecules in both states are at 100°C
What do you need to do to melt a solid or boil a liquid?
You need energy to break the bonds that holds the particles in place
What is the latent heat of a substance?
The energy needed to break the bonds that holds particles in place
What is the relationship between mass and the amount of energy needed to break the bonds for a change in state?
The larger the mass of the substance, the more energy is needed to break the bonds for a change in state
What is the specific latent heat?
The quantity of thermal energy needed to be gained or lost to change the state of 1kg of a substance
When is the specific latent heat of fusion used?
When a substance is melting or freezing
When is the specific latent heat of vaporisation used?
When a substance is boiling or condensing
What is the equation that links specific latent heat to energy change?
Q = ml
Q: Energy change
m: Mass of substance
l: Specific latent heat
What is an ideal gas?
A theoretical gas that obeys the 3 gas laws
What is Boyle’s Law?
At a constant temperature, the pressure and volume of a gas are inversely proportional
For a gas at constant temperature, what happens to the pressure if the volume is increased?
The pressure decreases (Boyle’s Law)
What is the equation for a gas obeying Boyle’s Law?
pV = constant
therefore p1V1 = p2V2
For a pressure-volume graph, what happens to the curve of an ideal gas following Boyle’s Law if the temperature is increased?
The curve gets further away from the origin
For a pressure-volume graph, what happens to the curve of an ideal gas following Boyle’s Law if the temperature is decreased?
The curve gets closer to the origin
What is Charles’ Law?
At constant pressure, the volume of a gas is directly proportional to the absolute temperature
For a gas at constant pressure following Charles’ Law, what happens if the absolute temperature is increased?
The volume increases at the same rate
What is the equation for Charles’ Law?
V/T = constant
therefore V1/T1 = V2/T2
What does the volume-temperature (measured in K) graph look like for a gas obeying Charles’ Law?
A straight line through the origin
What does the volume-temperature (measured in °C) graph look like for a gas obeying Charles’ Law?
A straight line crossing the X-axis at -273.15°C
Why does the volume of gas, at constant pressure, increase when the absolute temperature is increased?
The particles gain more kinetic energy and move more quickly. At a constant pressure, this means they move apart so the volume increases
What is the pressure law?
At constant volume, the pressure of an ideal gas is directly proportional to the absolute temperature
What are the 3 gas laws that an ideal gas has to obey?
Boyle’s Law
Charles’ Law
pressure law
Why does the pressure of an ideal gas, at constant volume, increase when the absolute temperature is increased?
The particles gain more kinetic energy so they move more quickly. Because the volume is constant, the particles will collide with each other and the sides of the container more and at higher speeds, causing an increase in pressure
What is the equation for a gas that obeys the pressure law?
P/T = constant
therefore P1/T1 = P2/T2
What does a pressure-temperature (measured in K) graph look like for a gas obeying the pressure law?
A straight line through the origin
What does a pressure-temperature (measured in °C) graph look like for a gas obeying the pressure law?
A straight line that goes through the X-axis at -273.15
What is the ideal gas equation?
pV = nRT
p: pressure V: volume n: number of moles of gas R: molar gas constant = 8.31 T: temperature
What is molecular mass?
The sum of the masses of all the atoms that make up a single molecule
What is the relative atomic mass?
The weighted mean mass of an atom of an element compared to the mass of 1/12th of an atom of carbon-12
What is the relative molecular mass?
The average mass of a molecule compared to 1/12th the mass of an atom of carbon-12
What is the Avogadro Constant?
The number of atoms in exactly 12g of carbon-12
What is the value of the Avogadro Constant?
6.02x10^23
What is a substance containing 6.02x10^23 atoms defined as?
1 mole of that substance
What is the molar mass?
The mass that 1 mole of a substance would have (measured in g)
What is the molar mass of a substance equal to?
Relative atomic mass or relative molecular mass
What is the molar mass of Helium (Mr = 4.0)?
4.0g
Why is the mass of gases that only contain one element normally doubles their relative atomic mass?
Because many gases (e.g. Oxygen) exist as diatomic molecules
What equation links the number of moles, number of molecules and Avogadro’s Constant?
Number of molecules = number of moles x Avogadro’s Constant
What do you get if you combine the 3 gas laws?
(pV)/T = constant
therefore (p1V1)/T1 = (p2V2)/T2
What is the Boltzmann constant equal to?
molar gas constant / Avogadro constant
What is the value of the Boltzmann constant?
1.38x10^-23
What is the Boltzmann constant?
The gas constant for 1 mole of gas
What is the ideal gas equation, for when you know the number of molecules?
pV = NkT
p: pressure V: volume N: number of molecules of gas k: Boltzmann constant T: temperature
How many litres are there in 1m^3?
1000 litres
What is the equation for the work done in changing the volume of a gas at constant pressure?
work done = pΔV
What is the area under a pressure-volume graph equal to?
The energy (work done) transferred to change the volume of a gas
What is the change in momentum equal to for a gas molecule colliding elastically with a wall of a container?
mu - (-mu) = 2mu
What is kinetic theory?
A term given to explaining an object’s properties by considering the motion of its particles
What equation links pressure, volume, number of molecules of gas, mass of a gas molecule and the mean square speed?
pV = 1/3 Nm c̅ ^2 = 1/3 Nm (c rms)^2
p: Pressure
v: Volume
N: Number of molecules of gas
m: Mass of a gas molecule
c̅ ^2: Mean square speed
c rms: Root mean square speed
What are the units of mean square speed, c̅ ^2?
m^2 s^-2
What is the root mean square speed (r.m.s. speed)?
The square root of the mean square speed, which gives you the typical speed
Explain why the pressure in a fixed volume increases when the temperature increases, using kinetic theory?
As temperature increases, the average speed of the molecules increases. This mean the rate of change of momentum when molecules collide against the walls of the container increases, and so the force exerted on the wall increases
What are the 2 reasons why if the volume is fixed, the pressure will increase when the temperature increases?
- There will be more collisions between the wall and the molecules per second
- On average, a collision will result in a larger change of momentum, and so exert a large force on the walls
What are the 2 reasons why if the pressure is fixed, the volume will increase when the temperature increases?
- If the volume is larger, there will be a longer time between molecule-wall collisions, therefore the rate of change of momentum, and so the force, is reduced
- As the volume increases, the surface area also increases. Pressure is defined as force per unit area, so increasing area stops the pressure from increasing
What are the 8 assumptions made for the kinetic theory?
- All molecules of the gas are identical
- The gas contains a large number of molecules
- The molecules have a negligible volume compared to the volume of the container
- The molecules continually move about randomly
- The motion of molecules follow Newton’s law
- All collisions are perfectly elastic
- Molecules move in straight lines between collisions
- The force that acts during collisions lasts for much less time than the time between collisions
What are the 3 equations for the average molecular kinetic energy of a gas?
(given in formula book)
1/2m(c rms)^2 = 1.5 (nRT)/(N) = 1.5kT = 1.5 (RT)/Na
1/2m(c rms)^2: Average molecular kinetic energy of a gas n: Number of moles of gas R: Molar gas constant T: Temperature of gas in Kelvin N: Number of molecules of gas k: Boltzmann constant Na: Avogadro's constant
How do you find the total kinetic energy of molecules in a gas, if you know the average kinetic energy?
Multiply the average kinetic energy by the total number of molecules present
What empirical laws?
A rule based on observations and evidence that predicts what will happen, but doesn’t explain why it happens
What is a theory?
A rule based on assumption and derivations from knowledge and theories we already had, and will both predict and explain why something happens
What is validation?
The process of repeating an experiment done by somebody else, using the theory to make new predictions, and then testing them with new experiments in order to support or refute the theory
What is Brownian motion?
The zigzag, random motion of particles suspended in a fluid
How did Brownian motion help the kinetic theory gain acceptance in the scientific equation?
The random motion of Brownian motion is a result of the collisions between fast, randomly-moving particles in a fluid, showing that the air is made up of lots of tiny particles
