Thermal Physics Flashcards
(22 cards)
internal energy
The internal energy of a body is equal to the sum of all of the kinetic energies and potential
energies of all its particles. The kinetic and potential energies of a body are randomly distributed.
The internal energy of a system can be increased in two ways:
- Do work on the system to transfer energy to it, (e.g moving its particles/changing its
shape). - Increase the temperature of the system.
When the state of a substance is changed, what happens to internal energy?
Internal energy = kinetic energy + potential energy of the particles.
During a change of state:
- Temperature stays the same, so kinetic energy stays the same.
- Potential energy changes, because particles move further apart (like when melting or boiling), or get closer together (like when freezing or condensing).
So:
Melting or boiling → particles break bonds → potential energy increases → internal energy increases
Freezing or condensing → particles form bonds → potential energy decreases → internal energy decreases
How can you measure the amount of energy required to change the temperature of a substance?
Q = mcΔθ
Where Q is energy required, m is the mass, c is the specific heat capacity, and is the change in temperature.
SHC
The specific heat capacity of a substance is the amount of energy required to increase the temperature of 1 kg of a substance by 1 °C/1 K, without changing its state.
You can measure the amount of energy required to change the state of a substance using…
Q= ml
Where Q is energy required and l is the specific latent heat.
SLH
The specific latent heat of a substance is the amount of energy required to change the state of 1 kg of material, without changing its temperature. There are two types of specific latent heat:
- the specific latent heat of fusion (when solid changes to liquid) and
- specific latent heat of vaporisation (when liquid changes to gas).
What are the ideal gas laws?
- Boyle’s Law -When temperature is constant, pressure and volume are inversely proportional
- Charles’ Law -When pressure is constant, volume is directly proportional to absolute temperature
- The Pressure Law -When volume is constant, pressure is directly proportional to absolute temperature
What is Boyles law graph?
What is the Charles law graph and the pressure law graph?
Kelvin scale
The absolute scale of temperature is the kelvin scale. All equations in thermal physics will use temperature measured in kelvin (K).
A change of 1 K is equal to a change of 1°C , and to convert between the two you can use the formula:
K = C + 273
Where K is the temperature in kelvin and C is the temperature in Celsius
What is absolute zero?
Absolute zero (- 273°C ), also known as 0 K, is the lowest possible temperature, and is the temperature at which particles have no kinetic energy and the volume and pressure of a gas are zero.
What is the ideal gas equations?
PV = nRT
P = pressure (Pa)
V = volume (m³)
n = amount of gas (mol)
R = ideal gas constant (8.314 J·mol⁻¹·K⁻¹)
T = temperature (K)
pV = NkT
p = pressure (Pa)
V = volume (m³)
N = number of particles (not moles)
k = Boltzmann constant (1.38 × 10⁻²³ J·K⁻¹)
T = temperature (K)
P₁V₁ / T₁ = P₂V₂ / T₂
This is used when a gas undergoes changes in state, combining Boyle’s, Charles’s, and Gay-Lussac’s laws. (not given)
What is 1 mole
1 mole of a substance is equal to atoms/molecules, so you can convert between the 6.02 × 10^ 23 number of moles (n) and the number of molecules (N) by multiplying the number of moles by
, which is defined as the Avogadro constant
Molar mass
Molar mass is the mass (in grams) of one mole of a substance and can be found by finding the relative molecular mass, which is (approximately) equal to the sum of the nucleons in a molecule
of the substance.
Work done
Work is done on a gas to change its volume when it is at constant pressure, (this is usually done through the transfer of thermal energy) the value of work done can be calculated using the formula:
Work done = pΔV
Where p is the pressure and V Δ is the change in volume.
Brownian motion
Brownian motion is the random motion of larger particles in a fluid caused by collisions with surrounding particles, and can be observed through looking at smoke particles under a microscope. Brownian motion contributed to the evidence for the existence of atoms and molecules.
You can use a simple molecular model to explain each of the gas laws:
● Boyle’s law - Pressure is inversely proportional to volume at constant temperature
E.g If you increase the volume of a fixed mass of gas, its molecules will move further apart so collisions will be less frequent therefore pressure decreases.
● Charles’s law - Volume is directly proportional to temperature at constant pressure. When the temperature of a gas is increased, its molecules gain kinetic energy meaning they will move more quickly and because pressure is kept constant (therefore frequency of collisions is constant) the molecules move further apart and volume is increased.
● Pressure Law - Pressure is directly proportional to temperature at constant volume. When the temperature of a gas is increased, its molecules gain kinetic energy meaning they will move more quickly, as volume is constant the frequency of collisions between molecules and their container increases and they collide at higher speeds therefore pressure is increased.
The kinetic theory model assumptions
- Gas particles are point masses
Particles have negligible volume compared to the volume of the container. - No intermolecular forces
There are no attractive or repulsive forces between particles, except during collisions. - Random motion
Particles move in random, straight-line motion until they collide with something. - Elastic collisions
All collisions (particle-particle and particle-wall) are perfectly elastic, meaning no kinetic energy is lost (though it may be redistributed). - Kinetic energy and temperature are related. The average kinetic energy of the particles is proportional to the absolute temperature (T) of the gas:
Kinetic energy of a single gas molecule equations
C rms
stands for the root mean square speed of gas particles. It’s a measure of the average speed of particles in a gas, accounting for the fact that not all particles move at the same speed.
Gas laws scientific acceptance
Knowledge and understanding of gases has changed greatly over time; the gas laws were discovered by a number of scientists and later explained by the development of the kinetic theory model, however this model wasn’t accepted at first. Knowledge and understanding of any scientific
concept changes over time in accordance to the experimental evidence gathered by the scientific community.
