5.1 Thermal physics Flashcards
What does it mean for a systems of objects to be in thermal equilibrium?
There is no net flow of energy between the objects, they are all at the same temperature.
If two objects or materials are in thermal contact, what will happen?
Thermal energy will transfew from the hotter to the cooler, until they are the same temperature.
How do you convert from Celsius to Kelvin?
Add 273.
Eg. 10 degrees C = 10 + 273 = 283 K.
Why is the absolute scale of temperature (also known as the thermodynamic scale) used to measure temperature?
Because it does not depend on the properties of any particular substance?
Describe the properties of solids.
Particles vibrate about fixed positions in a regular lattice. They’re held in position by strong forces of attraction.
Describe the properties of liquids.
Particles are constantly moving around and are free to move past one other, but are attracted to one another.
Describe the properties of gases.
Particles are free to move around with constant random motion. There are no forces of attraction between particles in an ideal gas.
What is Brownian motion?
The random movement of small particles suspended in a fluid.
How does Brownian motion give evidence for the particle model of matter?
Smoke particles suspended in air can be seen (with a microscope) to move randomly in all directions. This must be as a result of random collisions with particles making up the air.
How can the internal energy of a system be calculated?
Internal energy is the sum of the potential and kinetic energies of a system.
What is the temperature of absolute zero in Kelvin?
0 Kelvin.
What is the temperature at which a
substance has minimum internal energy known as?
Absolute zero.
As the internal energy of a body rises, what happens to its temperature?
It rises too.
When a substance changes state, what happens to its temperature and why?
The temperature of the substance will also stay the same whilst it changes phase, because the thermal energy is being used to overcome electrostatic bonds between molecules.
When a substance changes state, what happens to its potential energy?
It will increase.
When a substance changes state, what happens to its kinetic energy and why?
The kinetic energy of the substance remains the same, meaning that the temperature will remain the same, even though the substance is still being heated.
How does the internal energy of a substance change during a state change?
When a substance changes state (from solid -> liquid -> gas) the potential energy increases, but the kinetic energy remains the same.
What is the specific heat capacity of substance?
The energy required to raise the temperature of 1kg of a substance by 1K.
What equation can be used to determine the energy required to change the temperature of a substance, and what do the symbols in it mean?
E = mcΔ𝜭
Where E = energy, m = mass, c = specific heat capacity, Δ𝜭 = temperature change.
What is the specific latent heat of fusion?
The amount of energy needed to change the state of 1 kg of a substance without changing its temperature.
What does Avogadro’s constant represent?
The number of atoms there are in one mole of a substance.
How can the number of moles in a substance be calculated?
Number of moles = mass / relative formula mass
What is the kinetic theory of gases?
A model which can be used to describe how particles within in ideal gas behave.
What is an ideal gas?
A hypothetical gas which follows all of the gas laws exactly.
What assumptions are made for ideal gases?
- There are a large number of molecules in rapid, random motion.
- Particles (atoms or molecules) occupy negligible volume compared to the volume of gas.
- All collisions are perfectly elastic and the time of the collisions is negligible compared to the time
between collisions. - The molecules exert negligible force on each other, except during interactions (there are no intermolecular forces).
Why do gases exert a pressure on the container they’re in?
- Gas particles collide with the surfaces of the container.
- The container exerts a force on the particles to change their direction. The particles exert an equal and opposite force on the container.
- Pressure is force applied (in total, by all particles) per unit area.
What is the change in momentum of an atom as it collides with and rebounds from the wall of a container?
The collisions between atoms and the wall of the container are perfectly elastic, so the atoms rebound from the wall at the same speed they travel in at. This makes their change in momentum equal to 2mv.
How can the average force excerpted by an atom on a container wall be calculated?
As the change in momentum is equal to force multiplied by time, the average force exerted on the atom by the wall is given by F = 2mv / t
What is Boyle’s law?
Pressure is inversely proportional to volume, providing temperature is constant.
i.e. pV = constant.
What does Charles’ law state?
For a fixed mass of gas at a constant pressure, the volume is directly proportional to temperature.
What is the ideal gas equation, and what do the symbols in it represent?
pV = nRT
where p is the pressure of the gas (Pa), V is the volume the gas is contained in (m^3), n is the number of moles of gas (mol), R is the molar gas constant (8.31 J mol^-1 K^-1), and T is the temperature of the gas (Kelvin).
Assuming constant volume, how are the pressure and temperature of a gas related?
They’re directly proportional.
ie. P/T = constant.
Use the kinetic theory of gases to explain why a temperature increase leads to an increase in pressure.
- A temperature increase means the particles have more kinetic energy.
- More kinetic energy means a greater change in momentum during collisions with the container. There are also more frequent collisions.
- Change in momentum is proportional to force applied, and therefore to pressure as well.
What is meant by the root mean square speed?
The square root of the mean of the squares of the speeds of the molecules.
What does the Maxwell-Boltzmann distribution show?
the number of molecules with each speed, against speed c.
What does the area beneath a Maxwell-Boltzmann distribution show?
The total number of molecules.
As the temperature of a gas increases, how does its Maxwell-Boltzmann distribution change?
The peak of the graph shifts to a higher speed, and the distribution becomes more spread out.
What is the Boltzmann constant?
True or false: ‘The internal energy of an ideal gas is proportional to absolute temperature’
True.
In an ideal gas there is no ‘potential energy’ component in the internal energy. This means the internal energy is proportional to the kinetic energy (which is, in turn, dependent on temperature).
