# Physics - Recall for Chapter 15: Ideal Gases

> Source: https://ollybritton.com/notes/a-level/physics/recall-questions/chapter-15-thermal-physics/ · Updated: 2022-03-22 · Tags: physics, school, ideal-gases, recall-questions

## Recall Questions
##### What is an ideal gas??
A theoretical gas that obeys the gas laws at all pressures and temperatures.

##### What is the kinetic theory assumption concerning volume??
The volume of the particles in a gas is negligible compared to the volume of the container.

##### What is the kinetic theory assumption concerning forces??
There are no electrostatic forces between particles except during collisions.

##### What is the kinetic theory assumption concerning motion??
Large number of atoms or molecules that are in moving continuously in random directions with random speeds.

##### What is the kinetic theory assumption concerning collisions??
The particles collide elastically with the walls of each other and the walls of the container.

##### What is the kinetic theory assumption concerning time??
The duration of collisions is negligible compared to the time between collisions.

##### What is the physical basis of Avagadro's constant??
A number of particles equal to the number of atoms in 12 grams of carbon-12.

##### What is one mole of a substance??
An amount in which the number of atoms/molecules is equal to Avogadro's constant.

##### What is the molar mass of a substance??
The mass of one mole of a substance.

##### How do you find the mass of one atom from its nucleon number??
Multiply nucleon number by the atomic mass unit (u) in kg.

##### How is the atomic mass unit defined??
It is $1/12$th the mass of a Carbon-12 atom.

##### How do you calculate the number of particles from the number of moles of a substance??
$$\text{number of particles} = \text{number of moles} × \text{Avagadro's constant}$$

##### How do you find the molar mass of a substance from the molecular mass??
$$\text{molar mass} = \text{molecular mass} \times \text{Avagadro's number}$$

##### How do you find the molar mass of an element from its nucleon number??
Molar mass is equal to nucleon number in grams.

##### How do you find the total mass of a substance from the number of moles??
$$
\text{number of moles} \times \text{molar mass}
$$

##### How do you calculate the number of moles of a substance from the number of particles??
$$
\text{number of particles} \div \text{Avagadro's number}
$$

##### According to Newton's second law if a particle experiences a change in momentum, then...??
There must be a force acting on the particle.

##### According to Newton's third law if a container exerts a force on a particle, then...??
The particle must exert a force on the container.

##### What are the SI units of pressure??
$$
Pascal ($Pa$ or $Nm^{-2}$)
$$
##### What is the definition of pressure??
The force per unit area exerted normally to a surface.

##### What is Boyle's law: Pressure is inversely proportional to...??
Volume for a fixed mass of gas at constant temperature.

##### What is Charles' law: Volume is proportional to...??
Absolute temperature for a fixed mass of gas at constant pressure.

##### What is Gay Lussac's Law: Pressure is proportional to...??
Absolute temperature for a fixed mass of gas at constant volume.

##### What is Avogadro's law: Volume occupied by a gas is proportional to...??
Number of particles/moles if pressure and temperature are constant.

##### What is the equation of state for an ideal gas??
$$pV = NkT$$ or $$pV = nRT$$

##### What is $R$ in $pV = nRT$??
Molar Gas Constant.

##### What is $N$ in $pV = NkT$??
Number of particles.

##### What is $n$ in $pV = nRT$??
Number of moles.

##### What is $k$ in $pV = NkT$??
Boltzmann constant.

##### What is the Boltzmann constant??
The molar gas constant, $R$, divided by Avogadro’s constant, $N_A$.

##### What units of temperature should be used in the ideal gas equation??
Kelvin.

##### How do you find absolute zero from a pressure temperature graph??
Extrapolate the line to find the x-intercept.

##### Why does increasing the temperature of an ideal gas increase pressure??
Particles have greater kinetic energy so there is a greater rate of collisions and greater change in momentum per collision.

##### Why does increasing the volume of an ideal gas decrease pressure??
$P=F/A$. Force exerted decreases as the rate of collisions with the container decreases and the surface area increases.

##### How do you calculate root mean square speed of a gas??
Square the magnitude of the velocity of each individual gas molecule, take the average of these values and then take the square root.

##### Why is root mean square speed used instead of velocity??
Average velocity is zero.

##### What is the relationship between root mean square speed of particles in a gas and absolute temperature??
$$
\text{rms speed} \propto \sqrt{absolute temperature}
$$

##### What is $N$ in $pV = \frac{1}{3}N m c^2$??
Number of particles.

##### What is $c^2$ in $pV = \frac{1}{3} N m c²$??
Mean square speed.

##### What is $\frac{3}{2}kT$ equal to??
The mean kinetic energy of a gas particle.

##### What form is the internal energy of an ideal gas??
Purely kinetic (i.e. no potential energy).

##### Why is there no potential energy in an ideal gas??
Because there are no force between particles (except during collisions).

##### What is $\frac{3}{2}NkT$ equal to??
Total internal energy of an ideal gas.

##### What is the relationship between absolute temperature and the average KE of particles in a substance??
Absolute temperature is proportional to average kinetic energy of the particles in a substance.

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