Describe how useful energy may be obtained, or electrical power generated, from: (a) chemical energy stored in fossil fuels (b) chemical energy stored in biofuels (c) water, including the energy stored in waves, in tides and in water behind hydroelec

1.7.3 Energy Resources

The Cambridge IGCSE Physics syllabus (0625) requires you to describe how useful energy or electrical power can be obtained from the eight energy resources listed below. For each resource the notes give:

• The primary form of energy stored in the resource.
• A clear, step‑by‑step conversion process that leads to useful heat or electricity.
• Key advantages and disadvantages (summarised in the comparison table).

(a) Chemical energy stored in fossil fuels

Primary energy form: Chemical energy in hydrocarbons (coal, oil, natural gas).

1. Combustion in a boiler: Fuel + O₂ → CO₂ + H₂O + Q (heat)
2. The heat raises the temperature of water, producing high‑pressure steam.
3. Steam expands through a turbine, turning the turbine shaft.
4. The shaft drives an electrical generator, producing electricity ( P = V I ).

(b) Chemical energy stored in biofuels

Primary energy form: Chemical energy in recent biological material (e.g., ethanol, biodiesel).

1. Production – fermentation (ethanol) or trans‑esterification (biodiesel) converts biomass into a fuel.
2. Combustion in a boiler releases heat: CₓHᵧ + O₂ → CO₂ + H₂O + Q.
3. Heat generates high‑pressure steam (same boiler‑turbine‑generator chain as fossil fuels).
4. Steam drives a turbine → generator → electricity ( P = V I ).

Because the carbon was recently fixed from the atmosphere, the net increase in atmospheric CO₂ can be lower than for fossil fuels, provided the feedstock is grown sustainably.

(c) Water – wave, tidal and hydro‑electric power

The syllabus treats “water” as a single resource; the three sub‑types below together satisfy that requirement.

Wave energy

Primary energy form: Kinetic energy of surface water particles.

1. Surface waves carry energy : E = ½ ρ g A² λ (ρ = density, g = gravity, A = amplitude, λ = wavelength).
2. Devices (oscillating water columns, floating buoys, hinged pontoons) convert the up‑and‑down motion into a pressurised‑air or hydraulic flow.
3. The pressurised flow drives a rotary pump or hydraulic motor, which rotates a generator shaft.
4. Electrical power is produced ( P = V I ).

Tidal energy

Primary energy form: Gravitational potential energy of seawater.

1. High and low tides are caused by the Earth‑Moon‑Sun gravitational system and are highly predictable.
2. A tidal barrage traps water at high tide.
3. When the tide falls, the stored water is released through turbines (or drives a hydraulic pump).
4. The rotating turbine shaft drives a generator, delivering electricity ( P = V I ).

Hydroelectric dams

Primary energy form: Gravitational potential energy of water stored at height h.

1. Potential energy: E = m g h (m = mass of water, g = 9.8 m s⁻², h = height).
2. Water released from the reservoir travels down a penstock, converting potential energy into kinetic energy.
3. The high‑velocity water strikes turbine blades, turning the turbine shaft.
4. The shaft drives an electrical generator ( P = V I ).

(d) Geothermal resources

Primary energy form: Thermal energy from the Earth’s interior.

1. Dry‑steam plant: Steam naturally rises from hot rocks and is piped directly to a turbine.
2. Binary‑cycle plant: Hot water transfers heat to a secondary fluid with a lower boiling point; the secondary fluid vaporises and drives the turbine.
3. In both cases the turbine shaft drives a generator, producing electricity ( P = V I ).

(e) Nuclear fuel

Primary energy form: Nuclear binding energy released by fission (originates from the mass‑energy conversion E = mc²).

Typical fission reaction (example):

⁽²³⁵⁾U + n → ⁽²³⁶⁾U* → fission fragments + 2–3 n + ≈200 MeV

1. Heat from fission raises the temperature of water in a primary loop, producing high‑pressure steam.
2. Steam drives a turbine → generator → electricity ( P = V I ).
3. A heat exchanger transfers waste heat from the primary loop to a secondary cooling system (often a cooling tower).

(f) Light from the Sun – photovoltaic (PV) cells

Primary energy form: Electromagnetic energy (photons).

1. When a photon with energy Eₚₕ ≥ φ (work function) strikes a semiconductor, it excites an electron from the valence band to the conduction band, creating an electron–hole pair.
2. The built‑in electric field at the p‑n junction separates the charges.
3. When an external circuit is closed, a current I flows; the cell voltage is typically 0.5–0.6 V.
4. Electrical power is given by P = V I and can be fed directly to the grid or stored.

(g) Infrared and other electromagnetic waves – solar‑thermal panels

Primary energy form: Electromagnetic radiation (visible + infrared).

1. Sunlight is absorbed by a dark‑coated collector (often a glazed metal tube).
2. Heat is transferred to a circulating fluid (water or oil): Q = m c ΔT.
3. The hot fluid passes to a boiler where water is turned into high‑pressure steam (heat‑to‑steam conversion).
4. Steam expands through a turbine, turning a generator to produce electricity ( P = V I ).

Comparison of Energy Resources

Key Points to Remember

• All conversion processes obey the conservation of energy; real systems lose some energy as heat, friction or electrical resistance, limiting efficiency.
• Electrical power is given by P = W / t = V I (where W is energy and t is time).
• The boiler–steam–turbine–generator chain is the common “thermal‑power” route for fossil fuels, biofuels, nuclear, geothermal and solar‑thermal schemes.
• Renewable resources (water, geothermal, solar, biofuels) are preferred for long‑term sustainability, but each has site‑specific constraints and environmental considerations.
• Understanding each stage of the conversion process helps compare efficiencies, costs and environmental impacts across the eight resources.