Define internal energy .

The sum of the kinetic and potential energies of all the molecules within a given mass of a substance.

Which molecular property gives a substance its temperature ?

The kinetic energy of its molecules, which is due to their speed.

The potential energy of the molecules is due to what?

The separation between the molecules and their position within the structure.

.......... have the highest internal energy, while .......... have the lowest.

Gases have the highest internal energy, while solids have the lowest.

State two ways the internal energy of a system can be increased .

Doing work on it Adding heat to it

State two ways the internal energy of a system can be decreased .

Losing heat to its surroundings Changing state from a gas to a liquid, or a liquid to a solid

True or False? The internal energy of a substance includes the kinetic energy of the object's overall motion through space.

False. Internal energy is the energy of the molecules on the atomic scale — their random motion and their separation. It is separate from the kinetic energy of the object's overall movement, e.g. a thrown egg has kinetic energy because the whole egg moves, not because of its internal energy.

Define absolute zero .

The lowest temperature possible , equal to 0 K or −273.15 °C — the temperature at which the molecules in a substance have zero kinetic energy .

How do you convert a temperature in °C to K ?

Add 273.15: T / K = θ / °C + 273.15

Why can a temperature in kelvin never be negative?

Because 0 K (absolute zero) is the lowest temperature possible, so nothing can be colder.

The divisions on the Kelvin and Celsius scales are .......... in size, so a temperature change of 1 K is the same as a change of 1 °C.

The divisions on the Kelvin and Celsius scales are equal in size, so a temperature change of 1 K is the same as a change of 1 °C.

According to kinetic theory , what happens to the molecules of an object when energy is supplied to it?

They receive the energy as kinetic energy and move faster — in solids as vibrations , and in gases the molecules move faster around their container.

On the molecular scale, what determines the temperature of a substance?

The kinetic energy of its molecules.

True or False? At absolute zero, molecules still have some kinetic energy.

False. At absolute zero ( 0 K ) the molecules have zero kinetic energy — it is the point at which no more energy can be removed from the substance.

State the assumptions of the kinetic theory of gases.

Molecules behave as identical, hard, perfectly elastic spheres The volume of the molecules is negligible compared to the volume of the container The time of a collision is negligible compared to the time between collisions There are no forces of attraction or repulsion between molecules The molecules are in continuous random motion

Why must the molecular velocities be squared to find the mean square speed?

Molecules travel in all directions, so positive and negative velocities cancel to zero overall. Squaring makes every value positive , giving a meaningful non-zero average.

The kinetic theory links the .......... properties of particles (mass and speed) to the .......... properties of a gas (pressure and volume).

The kinetic theory links the microscopic properties of particles (mass and speed) to the macroscopic properties of a gas (pressure and volume).

True or False? In an ideal gas, the molecules exert forces of attraction and repulsion on one another.

False. A key assumption of the kinetic theory is that there are no forces of attraction or repulsion between the molecules.

State Charles's law .

At constant pressure , the volume of a fixed mass of gas is proportional to its thermodynamic temperature (V ∝ T).

State the pressure law .

At constant volume , the pressure of a fixed mass of gas is proportional to its thermodynamic temperature (p ∝ T).

For each of the gas laws, the .......... and the .......... of the gas are assumed to be constant.

For each of the gas laws, the mass and the number of molecules of the gas are assumed to be constant.

True or False? When applying the gas laws, the temperature may be used in degrees Celsius.

False. The temperature must be the thermodynamic temperature in kelvin (K) , because the laws depend on absolute temperature.

Which relationship is the key focus of Core Practical 14: Investigating Gas Pressure & Volume?

The relationship between the pressure and volume of a gas at constant temperature (Boyle's law, pV = constant).

In this experiment, state the independent , dependent and control variables.

Independent: mass added (kg) Dependent: volume (m 3 ) Control: temperature and cross-sectional area of the syringe

What graph is plotted to confirm Boyle's law ?

A graph of p against 1/V — a straight line through the origin confirms p ∝ 1/V (pV = constant).

Pressure of the gas = .......... − exerted pressure from the masses.

Pressure of the gas = atmospheric pressure − exerted pressure from the masses.

Why should you wait a few seconds after adding each mass before recording the volume?

To allow the temperature to equilibrate after work is done as the volume changes, keeping temperature constant.

Identify a systematic error in this experiment and how to reduce it.

Friction in the syringe. Use a low-friction or lubricated syringe so the only force acting is from the weights.

True or False? A straight-line graph of pressure against volume confirms Boyle's law.

False. Boyle's law is confirmed by a straight line through the origin on a graph of p against 1/V . A graph of p against V is a curve .

The mean kinetic energy of an ideal gas molecule is proportional to what?

Its thermodynamic (absolute) temperature : E k ∝ T.

Equating pV = NkT with the kinetic theory equation gives the result m(c rms ) 2 = ..........

Equating pV = NkT with the kinetic theory equation gives the result m(c rms ) 2 = 3kT

In the average kinetic energy equation, what does (c rms ) 2 represent?

The mean square speed of the molecules (units m 2 s -2 ) — the average of the squared speeds of all the molecules.

True or False? If the thermodynamic temperature of a gas is doubled, the r.m.s. speed of its molecules also doubles.

False. The average kinetic energy is proportional to T, and since E k ∝ (c rms ) 2 , the r.m.s. speed is proportional to √T . Doubling the temperature increases the r.m.s. speed by a factor of √2 , not 2.

A LevelPhysicsEdexcelFlashcards9. ThermodynamicsKinetic Theory & Ideal Gases

Kinetic Theory & Ideal Gases (Edexcel A Level Physics): Flashcards

Exam code: 9PH0

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FrontInternal Energy
Define internal energy.

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Cards in this collection (42)

Define internal energy.
The sum of the kinetic and potential energies of all the molecules within a given mass of a substance.
Which molecular property gives a substance its temperature?
The kinetic energy of its molecules, which is due to their speed.
The potential energy of the molecules is due to what?
The separation between the molecules and their position within the structure.
.......... have the highest internal energy, while .......... have the lowest.
Gases have the highest internal energy, while solids have the lowest.
State two ways the internal energy of a system can be increased.
- Doing work on it
- Adding heat to it
State two ways the internal energy of a system can be decreased.
- Losing heat to its surroundings
- Changing state from a gas to a liquid, or a liquid to a solid
True or False?
The internal energy of a substance includes the kinetic energy of the object's overall motion through space.
False.
Internal energy is the energy of the molecules on the atomic scale — their random motion and their separation. It is separate from the kinetic energy of the object's overall movement, e.g. a thrown egg has kinetic energy because the whole egg moves, not because of its internal energy.
Define absolute zero.
The lowest temperature possible, equal to 0 K or −273.15 °C — the temperature at which the molecules in a substance have zero kinetic energy.
How do you convert a temperature in °C to K?
Add 273.15: T / K = θ / °C + 273.15
Why can a temperature in kelvin never be negative?
Because 0 K (absolute zero) is the lowest temperature possible, so nothing can be colder.
The divisions on the Kelvin and Celsius scales are .......... in size, so a temperature change of 1 K is the same as a change of 1 °C.
The divisions on the Kelvin and Celsius scales are equal in size, so a temperature change of 1 K is the same as a change of 1 °C.
According to kinetic theory, what happens to the molecules of an object when energy is supplied to it?
They receive the energy as kinetic energy and move faster — in solids as vibrations, and in gases the molecules move faster around their container.
On the molecular scale, what determines the temperature of a substance?
The kinetic energy of its molecules.
True or False?
At absolute zero, molecules still have some kinetic energy.
False.
At absolute zero (0 K) the molecules have zero kinetic energy — it is the point at which no more energy can be removed from the substance.
State the assumptions of the kinetic theory of gases.
- Molecules behave as identical, hard, perfectly elastic spheres
- The volume of the molecules is negligible compared to the volume of the container
- The time of a collision is negligible compared to the time between collisions
- There are no forces of attraction or repulsion between molecules
- The molecules are in continuous random motion
Define root-mean-square (r.m.s.) speed.
The square root of the mean square speed of the gas molecules, $c_{r m s} = \sqrt{< c^{2} >}$ with units m s^-1.
Why must the molecular velocities be squared to find the mean square speed?
Molecules travel in all directions, so positive and negative velocities cancel to zero overall. Squaring makes every value positive, giving a meaningful non-zero average.
The kinetic theory links the .......... properties of particles (mass and speed) to the .......... properties of a gas (pressure and volume).
The kinetic theory links the microscopic properties of particles (mass and speed) to the macroscopic properties of a gas (pressure and volume).
State the kinetic theory of gases equation.
$p V = \frac{1}{3} N m < c^{2} >$
Using density ρ it can also be written as $p = \frac{1}{3} ρ < c^{2} >$
In the derivation, what is the change in momentum when one molecule hits a wall perpendicularly and rebounds?
$Δ p = - 2 m c$
Its momentum changes from +mc to −mc.
True or False?
In an ideal gas, the molecules exert forces of attraction and repulsion on one another.
False.
A key assumption of the kinetic theory is that there are no forces of attraction or repulsion between the molecules.
State the ideal gas equation in terms of the number of molecules.
$p V = N k T$
where N = number of molecules, k = Boltzmann constant and T = thermodynamic temperature (K).
Define the Boltzmann constant.
$k = \frac{R}{N_{A}}$
where R = molar gas constant and N_A = Avogadro's constant, giving a value of 1.38 × 10^-23 J K^-1. It relates the microscopic properties of particles (e.g. molecular kinetic energy) to macroscopic properties (e.g. temperature).
State Boyle's law.
At constant temperature, the pressure of a fixed mass of gas is inversely proportional to its volume ( $p \propto \frac{1}{V}$ , or p₁V₁ = p₂V₂).
State Charles's law.
At constant pressure, the volume of a fixed mass of gas is proportional to its thermodynamic temperature (V ∝ T).
State the pressure law.
At constant volume, the pressure of a fixed mass of gas is proportional to its thermodynamic temperature (p ∝ T).
For each of the gas laws, the .......... and the .......... of the gas are assumed to be constant.
For each of the gas laws, the mass and the number of molecules of the gas are assumed to be constant.
True or False?
When applying the gas laws, the temperature may be used in degrees Celsius.
False.
The temperature must be the thermodynamic temperature in kelvin (K), because the laws depend on absolute temperature.
Which relationship is the key focus of Core Practical 14: Investigating Gas Pressure & Volume?
The relationship between the pressure and volume of a gas at constant temperature (Boyle's law, pV = constant).
In this experiment, state the independent, dependent and control variables.
- Independent: mass added (kg)
- Dependent: volume (m³)
- Control: temperature and cross-sectional area of the syringe
What graph is plotted to confirm Boyle's law?
A graph of p against 1/V — a straight line through the origin confirms p ∝ 1/V (pV = constant).
Pressure of the gas = .......... − exerted pressure from the masses.
Pressure of the gas = atmospheric pressure − exerted pressure from the masses.
How is the exerted pressure from the masses calculated?
$p = \frac{F}{A}$
where F = weight of the masses (mg) and A = cross-sectional area of the syringe ( $A = \frac{π d^{2}}{4}$ ).
Why should you wait a few seconds after adding each mass before recording the volume?
To allow the temperature to equilibrate after work is done as the volume changes, keeping temperature constant.
Identify a systematic error in this experiment and how to reduce it.
Friction in the syringe. Use a low-friction or lubricated syringe so the only force acting is from the weights.
True or False?
A straight-line graph of pressure against volume confirms Boyle's law.
False.
Boyle's law is confirmed by a straight line through the origin on a graph of p against 1/V. A graph of p against V is a curve.
State the equation for the average kinetic energy of a single molecule of an ideal gas.
$E_{k} = \frac{1}{2} m (c_{r m s})^{2} = \frac{3}{2} k T$
where k = Boltzmann constant and T = thermodynamic temperature (K).
The mean kinetic energy of an ideal gas molecule is proportional to what?
Its thermodynamic (absolute) temperature: E_k ∝ T.
Equating pV = NkT with the kinetic theory equation gives the result m(c_rms)² = ..........
Equating pV = NkT with the kinetic theory equation gives the result m(c_rms)² = 3kT
In the average kinetic energy equation, what does (c_rms)² represent?
The mean square speed of the molecules (units m² s^-2) — the average of the squared speeds of all the molecules.
How do you find the total average kinetic energy of N molecules of a gas?
Multiply the single-molecule equation by N:
$E_{k} = \frac{3}{2} N k T$
True or False?
If the thermodynamic temperature of a gas is doubled, the r.m.s. speed of its molecules also doubles.
False.
The average kinetic energy is proportional to T, and since E_k ∝ (c_rms)², the r.m.s. speed is proportional to √T. Doubling the temperature increases the r.m.s. speed by a factor of √2, not 2.

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