Definitions
| Term | Definition | Units |
|---|---|---|
| Specific energy | Energy released per unit mass of fuel | kJ g⁻¹ or MJ kg⁻¹ |
| Energy density | Energy released per unit volume of fuel | kJ cm⁻³ or MJ dm⁻³ |
🔑 Hydrogen Paradox
H₂ has the highest specific energy of any fuel (~142 MJ kg⁻¹) because it is so light, but an extremely low energy density because it is a gas at room temperature. Storing enough H₂ requires high-pressure tanks or cryogenic liquefaction.
The Greenhouse Effect. Molecular Mechanism
- Sun emits short-wavelength radiation (UV/visible) → passes through atmosphere
- Earth's surface absorbs it, warms up, re-emits long-wavelength infrared radiation
- Greenhouse gases (CO₂, CH₄, H₂O) absorb the IR because their bond vibrations (asymmetric stretch, bending) cause a temporary dipole change
- Absorbed energy is re-radiated in all directions. Some back to Earth → warming
⚠️ Examiner Trap
N₂ and O₂ are not greenhouse gases. They are non-polar and cannot change their dipole moment when they vibrate, so they do not absorb IR.