Flip a switch, start a car, heat a home — nearly everything you do runs on energy, and roughly 80% of the world's energy still comes from fossil fuels formed hundreds of millions of years ago. That single fact drives climate change, air pollution, and geopolitics all at once. Unit 6 is where environmental science meets the power grid: which sources we use, how much, and at what cost. It's also a math-heavy unit — energy calculations with dimensional analysis are guaranteed FRQ material. This lesson maps the global energy landscape and the renewable/nonrenewable divide before the next three lessons dig into each source.
Some sources are called "clean" or low-carbon (solar, wind, hydro, nuclear, geothermal) because they emit little or no CO₂ during operation — a separate axis from renewable/nonrenewable. Nuclear is nonrenewable but low-carbon; biomass is renewable but not carbon-free.
Globally, fossil fuels dominate (roughly 80% of primary energy), split among oil, coal, and natural gas, with the remainder from nuclear and renewables (renewables are the fastest-growing segment). The exact split shifts, but for the exam: - Oil — dominant for transportation. - Coal — historically dominant for electricity generation (declining in many countries; still major in others). - Natural gas — growing share of electricity and heating; lower CO₂ than coal. - Nuclear — significant low-carbon electricity in some nations. - Renewables — rising rapidly, led by wind, solar, and hydro.
[DIAGRAM: Global primary energy mix bar/pie — Oil ~31%, Coal ~27%, Natural gas ~24% (fossil total ~80%), Nuclear ~5%, Hydro ~6%, other renewables ~5% (approximate; label as representative). Arrow noting renewables are the fastest-growing.]
Developed vs. developing nations: Developed nations consume far more energy per capita but have slowing growth; developing nations have lower per-capita use but rapidly rising total demand as they industrialize. This mirrors the ecological-footprint pattern (Lesson 17).
Common units: joule (J), kilowatt-hour (kWh), BTU, and rates in watts (W) or kilowatts (kW). Key relationships:
- Energy = Power × Time (e.g., kWh = kW × hours)
- 1 kW = 1,000 W; 1 kWh = energy of 1 kW running for 1 hour
APES energy math is pure dimensional analysis — carry units and cancel. (You'll drill this heavily in Lesson 22.)
Understanding the renewable/nonrenewable and low-carbon distinctions, reading an energy-mix chart, and doing energy calculations are all high-frequency exam skills. This lesson frames the trade-offs that Lessons 20–22 explore source by source.
Classify each as renewable or nonrenewable: (i) natural gas, (ii) wind, (iii) uranium/nuclear, (iv) geothermal.
Solution: (i) natural gas = nonrenewable (fossil fuel); (ii) wind = renewable; (iii) nuclear = nonrenewable (finite uranium); (iv) geothermal = renewable.
Interpretation: Fossil fuels and nuclear are nonrenewable; sun/wind/water/geothermal/biomass are renewable.
Explain why nuclear is low-carbon but nonrenewable, while biomass is renewable but not carbon-free.
Solution: Nuclear emits essentially no CO₂ during operation (fission, not combustion), so it's low-carbon, but its uranium fuel is finite, so it's nonrenewable. Biomass regrows on human timescales (renewable), but burning it releases CO₂, so it is not carbon-free.
Interpretation: "Renewable" and "low-carbon" are two different axes — a source can be one without the other.
A chart shows fossil fuels supplying ~80% of a country's energy, with oil the largest slice. Identify the sector most dependent on oil and one implication of this energy mix.
Solution: Transportation is most dependent on oil (liquid fuels for vehicles). Implication: heavy fossil-fuel reliance means high CO₂ emissions (climate change) and air pollution, plus vulnerability to oil price/supply shocks.
Interpretation: Oil ↔ transport; an 80% fossil mix means high emissions and supply vulnerability.
A 60-watt bulb runs 5 hours a day. How many kWh does it use in 30 days?
Strategy: Convert to kW, multiply by hours, use dimensional analysis.
Solution:
60 W = 0.060 kW
0.060 kW × 5 h/day × 30 days = 9 kWh
Answer: 9 kWh.
Interpretation: Energy = power × time; convert watts to kilowatts first, and track units.
kWh = kW × h.0.1 kW × 10 h = 1 kWh.Energy = ______ × time.(A) Power.
Renewable = naturally replenished on human timescales; low-carbon = emits little/no CO₂. Renewable but not carbon-free: biomass (regrows but burning emits CO₂). Low-carbon but not renewable: nuclear (no CO₂ in operation, but finite uranium).
(a) Per bulb: 0.015 kW × 6 h = 0.09 kWh; ten bulbs: 0.09 × 10 = 0.9 kWh/day. (b) 0.9 × 30 = 27 kWh/month.
FRQ rubric (10 pts):
- (a) 1 pt each for two problems (CO₂/climate change, air pollution/particulates/SO₂/NOₓ, acid rain, mining impacts, thermal pollution). (2)
- (b) 1 pt setup 500 MW × 8,000 h; 1 pt = 4,000,000 MWh (4 million MWh). (2)
- (c) 1 pt defines the two axes (renewable = replenished; low-carbon = little CO₂); 1 pt classifies: nuclear = nonrenewable + low-carbon; wind = renewable + low-carbon. (2)
- (d) For each of two strategies: 1 pt name + 1 pt justification (shift to wind/solar/nuclear to cut CO₂; improve efficiency to cut demand; natural gas as lower-carbon bridge; carbon pricing; grid modernization). (4)
A country generates 80% of its electricity from coal and wants to reduce CO₂ emissions and diversify its energy mix.
(a) Explain two environmental problems caused by heavy coal dependence. (2 pts) (b) A coal plant produces 500 MW. If it runs 8,000 hours per year, calculate the annual energy output in MWh. Show work. (2 pts) (c) Explain the difference between renewable and low-carbon, and classify nuclear and wind on both axes. (2 pts) (d) Propose two strategies to reduce the country's energy-related CO₂ emissions, and justify each. (4 pts)
MC:
1. (C) Coal.
2. (C) ~80%.
3. (B) Transportation.
4. (B) Nonrenewable but low-carbon during operation.
5. (B) Renewable but releases CO₂ when burned.
6. (A) 0.1 kW × 10 h = 1 kWh.
7. (B) Energy obtained ÷ energy spent.
8. (B) Lower per-capita but rapidly rising total demand.
9. (C) Renewables.
10. (A) Power.
Renewable = naturally replenished on human timescales; low-carbon = emits little/no CO₂. Renewable but not carbon-free: biomass (regrows but burning emits CO₂). Low-carbon but not renewable: nuclear (no CO₂ in operation, but finite uranium).
(a) Per bulb: 0.015 kW × 6 h = 0.09 kWh; ten bulbs: 0.09 × 10 = 0.9 kWh/day. (b) 0.9 × 30 = 27 kWh/month.
FRQ rubric (10 pts):
- (a) 1 pt each for two problems (CO₂/climate change, air pollution/particulates/SO₂/NOₓ, acid rain, mining impacts, thermal pollution). (2)
- (b) 1 pt setup 500 MW × 8,000 h; 1 pt = 4,000,000 MWh (4 million MWh). (2)
- (c) 1 pt defines the two axes (renewable = replenished; low-carbon = little CO₂); 1 pt classifies: nuclear = nonrenewable + low-carbon; wind = renewable + low-carbon. (2)
- (d) For each of two strategies: 1 pt name + 1 pt justification (shift to wind/solar/nuclear to cut CO₂; improve efficiency to cut demand; natural gas as lower-carbon bridge; carbon pricing; grid modernization). (4)
⭐ Exam strategy: Keep two separate axes in mind — renewable vs. nonrenewable AND low-carbon vs. high-carbon. Nuclear (nonrenewable, low-carbon) and biomass (renewable, not carbon-free) are the classic "gotcha" cases. And for energy math, always write kWh = kW × h and cancel units.
Content pending external review.