Specific heat capacity and change of phase IB DP Physics Study Notes - 2025 Syllabus

Specific heat capacity and change of phase IB DP Physics Study Notes

Specific heat capacity and change of phase 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

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
that a phase change represents a change in particle behaviour arising from a change in energy at constant temperature.
quantitative analysis of thermal energy transfers Q with the use of specific heat capacity c and specific latent heat of fusion and vaporization of substances L as given by Q = mcΔT and Q = mL

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

Internal energy

All substances are composed of individual molecules that are in vibration.
As we heat up a substance its vibrations become more energetic. This is an increase in the kinetic energy of the molecules.
Simultaneously, as heat energy is being added the molecules are also moving farther apart. This is an increase in the potential energy of the substance.
The two energies together are called the internal energy of the substance. Thus $E_{INT}=E_K+E_P$.
When thermal energy (heat) is added to a substance it is stored as internal energy.

Sketching and interpreting phase change graphs

Suppose a thermometer is frozen in ice, and the ice is further cooled to a temperature of -20°C. We slowly add heat, and plot the temperature vs. the heat Q added:
Since the thermometer measures kinetic energy, and since the temperature doesn’t change during the phase change, the heat must be getting stored ONLY as potential energy during phase change
As a model to help explain phase change consider a molecule in an open box which can move left and right but must remain “captured” in the box.
As more heat is stored as potential energy, the particle in our model gains height.
Finally, the potential energy is great enough to break the intermolecular bonds and change the phase of the substance.
The molecule is free!

Specific heat capacity

Different materials absorb heat energy in different ways.
This means that if we have two different substances having the same mass m, and each absorbs the same amount of heat Q, their increase in temperature ΔT may be different.
We define the specific heat capacity c of a substance as the amount of thermal energy needed per unit mass to change the temperature by 1 degree.
Each material has its own unique value for c.

Here are some specific heats for various materials.

FYI

Note that specific heat units for c are (J kg-1 C°-1).

EXAMPLE:

Air has a density of about $\rho=1.2 \mathrm{~kg} \mathrm{~m}^{-3}$. How much heat, in joules, is needed to raise the temperature of the air in a 3.0 m by 4.0 m by 5.0 m room by $5^{\circ} \mathrm{C}$ ?

▶️Answer/Explanation

SOLUTION:

From the previous table we see that $c=1050$.
The change in temperature is given: $\Delta T=5^{\circ} \mathrm{C}$.
We get the mass from $\rho=\frac{m}{V}$ or

$$ \begin{gathered} m=\rho V=(1.2)[(3)(4)(5)]=72 \mathrm{~kg} . \\ Q=m c \Delta T=(72)(1050)(5)=378000 \mathrm{~J} \text { or } 380 \mathrm{~kJ} . \end{gathered} $$

Specific latent heat

Latent heat means hidden heat,by which we mean that there is no temperature indication that heat is being lost or gained by a substance.
The specific latent heat L is defined in this formula:
Note that since there is no temperature change during a phase change, there is no ΔT in our formula. The units for L are (J kg-1).

FYI

Use Q = mL during phase change (when ΔT = 0).
Use Q = mcΔT otherwise (when ΔT ≠ 0).
Since there are two phase changes (two plateaus), each substance has two latent heats.

Lf is the latent heat of fusion.
Lv is the latent heat of vaporization.
The temperatures associated with the phase changes are also given.

EXAMPLE:

Bob has designed a 525-kg ice chair. How much heat must he remove from water at 0°C to make the ice chair (also at 0°C)?

▶️Answer/Explanation

SOLUTION:

In a phase change ΔT = 0 so we use Q = mL.

Since the phase change is freezing, we use Lf.

For the water-to-ice phase change Lf = 3.33×105 J kg-1.

Thus Q = mL = (525)(3.33×105) = 175×106 J.

Specific heat capacity and change of phase IB DP Physics Study Notes

Specific heat capacity and change of phase IB DP Physics Study Notes - 2025 Syllabus