Define energy density and name two high-density energy sources.

• A. Energy density is energy per unit mass or volume; high-density examples: gasoline and diesel.
• B. Energy density is energy per unit area; high-density examples: wind and solar.
• C. Energy density is energy per unit time; high-density examples: coal and wood.
• D. Energy density is energy stored per unit volume at sea level; high-density examples: water and air.

Answer: (A)

Energy density is the amount of energy stored in a material or system per unit of mass or per unit of volume. This matters because fuels with high energy density deliver a lot of energy without needing a lot of fuel, keeping weight and tank space manageable. Gasoline and diesel are classic high-density energy sources because they pack a large amount of chemical energy into each kilogram and each liter. For example, gasoline stores roughly 45 MJ per kilogram and about 34 MJ per liter, while diesel is around 43 MJ/kg and 38 MJ/L. By comparison, wind and solar are about capturing energy over larger areas and times, not storing a dense chemical energy form; they aren’t high-density energy sources in the fuel sense. Coal and wood do contain energy, but their energy per volume is lower than liquid fuels, and water or air don’t store much chemical energy to begin with.