The energy density of a fuel has been included in the IB Physics curriculum since the first assessment in 2009. However, the definition has been changed in the following guide, and a distinction between specific energy and energy density of a fuel was made. This is an excellent example of how mistakes can be made and then corrected in curricula. This is a crucial case as it reflects the self-correcting tradition in science. I will be referring to three documents: IB Physics Guide for the first assessment in 2009 (FA2009), the first assessment (2016), and the first assessment in 2025 (FA2025).
Definition of Energy Density in FA2009
Energy density is defined in “Topic 8: Energy, power and climate change” under the subtopic “8.2 World energy sources” in the assessment statement “8.2.3 Define the energy density of a fuel”. The teacher notes accompanying this assessment statement is “Energy density is measured in $Jkg^{–1}$.” In Paper 1, about five questions were asked that were directly related to this definition. Either we are defining the energy density as energy per mass, or the SI unit of energy density as $Jkg^{–1}$. If we were to answer these questions with our knowledge after FA2016, although our answer would be correct, we would not get credit for the question. Why? Because energy density is not energy per mass.
Distinction between Specific Energy and Energy Density in FA2016
In FA2016, energy density is distinguished from specific energy. What the FA2009 called energy density became specific energy: energy per mass for a fuel. The energy density became what it should be in the first place: energy per volume for a fuel. The SI unit for specific energy is $Jkg^{-1}$ and for energy density is $Jm^{-3}$.
In physics, whenever we hear a term involving the word specific we know we are talking about a ratio concerning mass. For example, specific heat capacity c. Whenever we hear density in a term, we know we are talking about a ratio concerning volume—for example, mass density.
The hallmark question for relating the to is, what is the ratio of:
$$ \frac{energy \space density}{specific \space energy} = ? $$
The answer is the mass density of the fuel. Could you find it? Let me know in the comments.
Including Energy Density but not mentioning Specific Energy in FA2025
In FA2025, energy density is included within the understanding of the topic “A.3 Work, energy and power”. Students are expected to understand the “energy density of the fuel sources.” However, neither the definition of energy density is given nor specific energy is mentioned. It was a good idea to include both and provide definitions of both.
Conclusion
When mistakes are made and corrected, the trust in self-correction processes increases. However, when curricular documents involve ambiguities, much is left to the textbook writers to not only interpret but, at times, read the minds of the curriculum authors. I think energy density will be interpreted as energy per volume in FA2025. Finally, did you know energy density can also refer to pressure?