Particles, temperature and energy IB DP Physics Study Notes - 2025 Syllabus

Particles, temperature and energy IB DP Physics Study Notes

Particles, temperature and energy IB DP Physics Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Physics syllabus with Students should understand

Molecular theory in solids, liquids and gases
density $\rho$ as given by $\rho = \frac{m}{V}$
that Kelvin and Celsius scales are used to express temperature
that the change in temperature of a system is the same when expressed with the Kelvin or Celsius
scales.
that Kelvin temperature is a measure of the average kinetic energy of particles as given by $\bar{E}_{k} =\frac{3}{2}k_BT$
that the internal energy of a system is the total intermolecular potential energy arising from the forces between the molecules plus the total random kinetic energy of the molecules arising from their random motion.
that temperature difference determines the direction of the resultant thermal energy transfer between
bodies

Standard level and higher level: 6 hours
Additional higher level: There is no additional higher level content

IB DP Physics 2025 -Study Notes -All Topics

Molecular theory of solids, liquids and gases

The three phases of matter are solid, liquid, and gas.

In a solid the molecules can only vibrate. They cannot translate. ∙
In a liquid the molecules can vibrate and move about freely in a fixed volume. ∙

In going from a solid to a liquid, some of the intermolecular bonds are broken, giving the molecules more freedom of motion.
In going from a liquid to a gas, most of the intermolecular bonds are broken.

Phase change

The process of going from a solid to a liquid is called melting.
The process of going from a liquid to a gas is called boiling.
Each process can be reversed.

Density is Given by $\rho = \frac{m}{V}$

Temperature and absolute temperature

Because absorption of thermal energy (heat) causes materials to expand, the fluid in a thermometer can be used to indirectly measure temperature.
Since water is a readily-available substance that can be frozen, and boiled within a narrow range of temperatures, many thermometers are calibrated using these temperatures.

We will be using the Celsius scale in physics because it is a simpler scale.
Temperature only reveals the internal kinetic energy.
Expansion reveals internal potential energy.
When gas is heated in an enclosed space its pressure p increases.
The following experiment plots pressure p vs. temperature T in Celsius.
We can extrapolate the graph.
Now we repeat using different gases.
The lowest pressure p that can exist is zero.
Surprisingly, the temperature at which any gas attains a pressure of zero is the same, regardless of the gas.
The Celsius temperature at which the pressure is zero (for all gases) is -273 °C.

Because the lowest pressure that can exist is zero, this temperature is the lowest temperature that can exist, and it is called absolute zero.
A new temperature scale that has absolute zero as its lowest value is called the Kelvin temperature scale.

Converting between Kelvin and Celsius temperatures

∙The simple relationship between the Kelvin and Celsius scales is given here:

$T(\mathrm{~K})=T\left({ }^{\circ} \mathrm{C}\right)+273 \quad$ Kelvin and Celsius relationship

FYI :Note that there is no degree symbol on Kelvin temperatures.

EXAMPLE: Convert $100^{\circ} \mathrm{C}$ to Kelvin, and 100 K to $\mathrm{C}^{\circ}$.

SOLUTION:

$ \begin{gathered} T(\mathrm{~K})=T\left({ }^{\circ} \mathrm{C}\right)+273 \\ T=100+273=373 \mathrm{~K} . \\ 100=T\left({ }^{\circ} \mathrm{C}\right)+273 \\ T=-173^{\circ} \mathrm{C} . \end{gathered} $

Average kinetic energy and temperature

Since ideal gases have no intermolecular forces, their internal energy is stored completely as kinetic energy.
The individual molecules making up an ideal gas all travel at different speeds:
Without proof, the average kinetic energy EK of each ideal gas molecule has the following form:
$\overline{E_K}=\frac{3}{2} k_b T$ kB is called the Boltzmann constant

EXAMPLE: 2.50 moles of hydrogen gas is contained in a fixed volume of $1.25 \mathrm{~m}^3$ at a temperature of $175^{\circ} \mathrm{C}$.
What is the average kinetic energy of each atom?

▶️Answer/Explanation

$T(\mathrm{~K})=175+273=448 \mathrm{~K}$.

$$
\begin{aligned}
\overline{E_K} & =\left(\frac{3}{2}\right) k_B T \\
& =\left(\frac{3}{2}\right)\left(1.38 \times 10^{-23}\right)(448) \\
& =9.27 \times 10^{-21} \mathrm{~J}
\end{aligned}
$$

Particles, temperature and energy IB DP Physics Study Notes

Particles, temperature and energy IB DP Physics Study Notes - 2025 Syllabus