6.2 Thermal Physics Flashcards
What are the main two ways in which energy can transfer from one place to another?
- When work is done on an object
2. If one object is hotter than another and conduction, convection or radiation occur
What are the two types of energy that molecules in a hot substance will have?
Kinetic and potential
What is internal energy?
The sum of the randomly distributed kinetic and potential energies of the particles in a body
What is the symbol for internal energy?
U
What is internal energy measured in?
J
What is 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
What is the first law of thermodynamics linked to?
Conservation of energy
What is a useful outcome of the first law of thermodynamics?
If work is being done on an object and it is not increasing its internal energy, then it must have an output rate identical to the work being done on it
What happens when a sample is heated?
- heat energy supplied increases internal energy
- Ek increases, so mean molecular speed increases
- also mean separation slightly increased so small increase in molecular Ep
What happens when a samples changes state?
- temperature remains constant
* so mean Ek is constant
When a sample is changing state, what is the energy being used for?
To break bonds as the sample melts or boils
What does heat flow result from?
Temperature difference
What happens if two objects at different temperatures are placed in thermal contact?
Heat flows from the higher to the lower temperature until the temperatures equalise
When will two bodies be in thermal equilibrium?
When two objects at different temperatures are placed in thermal contact and heat flows from the higher to the lower temperature until the temperatures equalise
What happens to internal energy when a substance is hotter?
It is increased
What does a temperature scale require?
Two fixed points with fixed degrees between them
What is 0°C in the Celsius scale?
Ice point
What is 100°C in the Celsius scale?
Steam point
What is steam point?
The temperature of pure steam at standard atmospheric pressure
What is the lowest possible temperature on the absolute scale?
0K
What is definition of the triple point of water?
The temperature at which water can exist in all three states
What is the value of the triple point of water?
273.16 K
What is the value of the ice point of water in K?
273.15 K
How do you convert from Kelvin to Celsius?
Add 273.15
What happens when you cool a gas within a fixed volume?
Its pressure drops
If you plot a graph of pressure against temperature, with different gases, where will the lines intercept?
At absolute 0, on the negative x-axis
What unit does the absolute scale use?
Kelvin
What does the rise in temperature for a substance being heated depend on?
- mass of substance
- how much energy is put in
- what the substance is
What is the equation for the energy required to heat a substance?
E = mcΔθ
What is m in E = mcΔθ?
Mass (kg)
What is c in E = mcΔθ?
Specific heat capacity (Jkg⁻¹K⁻¹)
What is Δθ in E = mcΔθ?
Temperature change (°C or K)
What is the specific heat capacity of a substance?
The energy needed to raise the temperature of unit mass of the substance by 1K without change of state
What is the symbol for specific heat capacity?
c
How can adding an exact amount of energy to a system be achieved?
By doing work on the system
In the inversion tube experiment, what happens in terms of energy transfers?
As the contents fall down tube, potential energy is converted into thermal energy
If the inversion tube experiment is repeated n times, what is the equation for total energy change?
E = mgLn
What is measured in the inversion tube experiment?
The temperature change of the balls inside the tube
What equation can help to calculate specific heat capacity in the inversion tube experiment?
c = gLn / ΔT
What experiment can be carried out to find the specific heat capacity of a metal?
- block of metal of known mass in insulated container
- heater and thermometer inserted
- temperature rise measured
- energy supplied = heat current x pd x time
- c = IVt / mΔT
To find the specific heat capacity of a liquid instead of a metal, what changes must be made?
Liquid placed inside container instead and stirred
What equipment is used to find the specific heat capacity of a liquid?
Calorimeter
What must be included in the calculations to find the specific heat capacity of a liquid? Why is this?
The specific heat capacity of the calorimeter, as it absorbs energy
What does the rate of change of temperature in boilers and showers create?
A rate of energy input, which is power
What is the equation that is useful in continuous flow heating?
IV = P = mcΔT / t
What are the units of c?
J kg⁻¹ K⁻¹
What does the rate of energy flow to sustain the heating in a system calculate?
The power in Watts
Properties of solids?
- maintain shape
- constant volume
- cannot be easily compressed
- molecules/atoms close together and vibrate in fixed positions
Properties of liquids?
- flow and take shape of vessel
- constant volume
- cannot be easily compressed
- molecules/atoms are close together and can move around each other
Properties of gases?
- flow and take any shape, fill any space
- take the volume of any vessel or space
- can be easily compressed
- molecules/atoms are far apart and are free to move
What will be proportional to the energy supplied when a pure substance is heated?
Its temperature
Does a material change state at a constant temperature?
Yes
Why is the energy transferred to a substance to melt/boil it not obvious or ‘hidden’?
Temperature does not change during the change of state
What is the energy needed to make a change of state occur?
Latent heat
What is latent heat?
The energy needed to make a change of state occur
What is the latent heat of fusion?
The energy needed to melt a substance
What is the latent heat of vaporisation?
The energy needed to evaporate a substance
What is specific latent heat?
How much energy is needed to make a unit mass of a pure substance change state
What are the units for specific latent heat?
J kg⁻¹
On a temperature-time graph, what does the gradient represent?
The rate of change of temperature
On a temperature-time graph, what will a steeper gradient show?
That state heats more quickly - and has a lower specific heat capacity
On a temperature-time graph, what is the length of time to change state proportional to?
The specific latent heat for each state change
What is the equation for specific latent heat?
E = ml
What does E stand for in E = ml?
Energy (J)
What does m stand for in E = ml?
Mass (kg)
What does l stand for in E = ml?
Specific latent heat (Jkg⁻¹)
What is the specific latent heat of fusion of ice?
3.3. x 10³ Jkg⁻¹
What is the specific latent heat of vaporisation of water?
22.6 x 10³ Jkg⁻¹
What is the pressure of a gas?
The force per unit area that is exerts at right angles to surface
What is pressure affected by?
- temperature
- volume
- mass of gas particles
What are the units of pressure?
pascals (Pa or Nm⁻²)
What type of collisions do gas molecules have with the walls of the container?
Elastic
Why do gas molecules move at the same speed after they have collided with the container wall?
The collisions are elastic
What is the magnitude of pressure proportional to?
The rate of collisions with the container wall
How can the pressure of a gas be increased?
- increasing temperature - particles move faster
- reducing volume of container - increases chance of particles colliding with wall
- adding more gas - increasing the number of particles
What is Boyle’s law in words?
The pressure of a fixed mass of gas at constant temperature is inversely proportional to its volume
What is Boyle’s law in equations?
p ∝ 1/V
pV = constant
What is an isothermal change?
An experiment done at constant temperature
What is an experiment done at constant temperature?
Isothermal change
For a Boyle’s law experiment at constant temperature, what shape will a pressure-volume graph be?
Downwards curve
How could a pressure-volume graph for a Boyle’s law experiment be made into a straight line?
Plot p against 1/V
What is Charles’ law?
Reducing the temperature of a gas but maintaining the same pressure causes the volume to decrease
In Charles’ law, what does volume increase in proportion to?
Absolute temperature
In a graph showing Charles’ law, where will the x-intercept always occur?
At -273.15 °C (absolute zero)
What is the equation for Charles’ law?
V/T = constant
What is an experiment done at constant pressure?
An isobaric change
What is an isobaric change?
An experiment done at constant pressure
In a Charles’ law experiment, what condition must there be for the x-intercept to be at absolute zero?
The gas must be ideal
What must happen if a volume of gas is compressed but the pressure is maintained?
Heat must be transferred
What happens when you reduce the temperature of a gas at a fixed volume?
Pressure is reduced
What equation relates to the pressure law?
p / T = constant
What has the same shape as a Charles’ law (volume-temperature) graph?
Pressure against temperature
What are the three relationships between volume, pressure and temperature?
- Boyle’s law
- Charles’ law
- The pressure law
What is an ideal gas?
One which obeys the gas law exactly
When can real gas behaviour be classed as ideal?
When gases are considered at low pressures and higher temperatures
What can the gas laws be combined to give?
PV/T = constant
or
P₁V₁ / T₁ = P₂V₂ / T₂
What must be true of a gas to be ideal?
- particles themselves can be thought of as taking up no volume
- no significant forces between particles
- motion of particles is random
What is Brownian motion?
The random movement of particles in a fluid
What is responsible for diffusion?
Brownian motion
Why must the same volume of two gases at the same temperature contain the same number of particles?
Because the particles in an ideal gas take up no volume themselves
What is the definition of the Avogadro constant?
The number of carbon atoms in 12g of carbon-12
Why did the definition of Avogadro’s constant change from using hydrogen to carbon?
It is difficult to get a pure enough sample of Hydrogen-1 without isotopes being present
What is the value of the Avogadro constant?
6.023 x 10²³
How particles does one mole of a pure substance contain?
Avogadro’s number
What is the molarity of a sample?
How many moles in contains - unit is mol
What is the molar mass of a sample?
The mass of 1 mol in kgmol⁻¹
Equation for number of molecules in a specific mass?
Ms = NA x Ms / Molar mass
How is the constant calculated in pV/T = constant?
- plot pV against T (in K)
- straight line through origin - find gradient (nR)
- rearrange formula: pV = nRT
What is the Boltzmann constant?
A way of using the ideal gas equation to know the number of particles
What equation is formed when the Boltzmann constant is used in pV/T = constant?
pV = NkT
What does k equal in pV = NkT?
k = R/NA = 1.38 x 10⁻²³ JK⁻¹
How to calculate the root mean square?
Square every value, divide by the number of data points then square root the total
What does the graph of the speeds of molecules in a gas look like?
A bell curve
What is a useful value to compare the speeds of molecules at different temperatures?
Root mean square of their speeds
What are the 6 assumptions that have to be made to create a kinetic theory equation?
- the particles themselves take up no volume
- there is no force of attraction between the particles
- their motion is random
- their collisions are elastic
- the length of time of the collisions against the walls of the container and each other are negligible
- Newton’s laws of motion can be applied
What are the three factors that affect the pressure of a gas in a given volume?
- mass of molecules
- speed of molecules
- how many molecules there are in the container
What is the theoretical equation for an ideal gas?
PV = 1/3 Nmc²
What does P mean in PV = 1/3 Nmc²?
Pressure (Pa)
What does V mean in PV = 1/3 Nmc²?
Volume (m³)
What does N mean in PV = 1/3 Nmc²?
Number of molecules
What does m mean in PV = 1/3 Nmc²?
Mass of one molecule (kg)
What does c mean in PV = 1/3 Nmc²?
Average molecular speed (ms⁻¹)
What happens when you combine the theoretical equation for ideal gas and the ideal gas equation?
1/2mc² = 3RT/2Na = 3/2 kt
How is E = 3/2mc² formed from 1/2mc² = 3RT/2Na = 3/2 kt?
Because 1/2 mc² is the mean kinetic energy of a gas molecule
What is the temperature of a gas a mean of?
The average kinetic energy of the particles
How can you calculate the mean kinetic energy of the molecules?
By dividing total energy by the number of particles
Equation to calculate mean kinetic energy of molecules?
E = 1/2 m(c RMS)²
What are the two useful formulas to find the mean kinetic energy of molecules?
- E = 1/2 m(c RMS)²
* E = 3/2kT
How can the total energy of n moles of a gas at temperature T kelvin be calculated?
3/2 nRT
How can internal energy be calculated?
3/2 nRT
Make sure you practice how to derive the kinetic energy equation.
Ok.
