8.1.1 Specific Energy

• The amount of energy that a fuel can provide for the amount of fuel used is an important consideration for the modern world
• In order to compare fuels by unit, either mass or volume can be used
  • Comparing by mass is known as specific energy
  • Comparing by volume is known as energy density

• Specific energy (E_S) is a measure of the amount of energy per unit mass of a fuel
  • It is measured in J kg^-1

• Energy density (E_D) is a measure of the amount of energy per unit volume of a fuel
  • It is measured in J m^-3

• The formula used to calculate density from specific energy and energy density is:

• Energy density is related to specific energy by the density of relevant fuel and can be converted between the two quantities this value of density

Worked Example

Step 1: List known values

• • Specific energy: 35 MJ kg^-1
  • energy density: 2 × 10⁴ MJ m^-3

Step 2: Identify relationship needed

• • Density has units of kg m^-3
  • Therefore, energy density must be divided by specific energy

Step 3: Perform the division

ED ÷ SE = (2 × 10⁴) ÷ 35 ≈ 571 kg m^-3

Step 4: State the final answer

• • The approximate density of coal is: 571 kg m^-3

Examples of Common Fuels: Energy Density and Specific Energy Table

Exam Tip