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AP Environmental Science · Unit 6 of 9
Energy Resources & Consumption
10–15% of the exam4 lessons · 51 min22 terms
What this unit covers
The topics below follow the published Env. Science course framework for Unit 6. This unit is worth 10–15% of the exam, so budget your time against that rather than against how long the unit takes to teach.
Fossil fuelsRenewablesNuclearEnergy efficiency
Lessons in this unit
- Fossil Fuels13 min · 3 objectivesDescribe how coal, oil, and natural gas form and are extracted · Compare the fossil fuels by their carbon emissions and energy content · Use net energy (EROEI) to compare energy sources
- Nuclear Energy13 min · 3 objectivesExplain how nuclear fission generates electricity in a reactor · Weigh the benefits and risks of nuclear power, including waste and accidents · Calculate how much of a radioactive isotope remains after several half-lives
- Renewable Energy13 min · 3 objectivesCompare the major renewable energy sources and their trade-offs · Distinguish intermittent from continuously available renewables · Calculate the power output of a solar array from area, intensity, and efficiency
- Energy Efficiency & Conservation12 min · 3 objectivesDistinguish energy conservation from energy efficiency · Calculate the energy used and cost of running an electrical appliance · Explain how efficiency measures reduce energy demand and emissions
Formulas in Unit 6
Net energy (EROEI)
EROEI = energy returned / energy invested
The energy return on energy invested measures how much usable energy a source yields per unit of energy spent obtaining it. A higher EROEI means a better net-energy source; a ratio near 1 means the source is barely worth extracting.
Radioactive decay
amount remaining = initial amount × (1/2)ⁿ, where n = time / half-life
n is the number of half-lives that have elapsed. Each half-life halves the remaining amount: after n half-lives, the fraction left is (1/2)ⁿ.
Solar array power output
power output = area × solar intensity × efficiency
Area in m², solar intensity in W/m², efficiency as a decimal. The result is the electrical power in watts. Real panels convert only ~15–22% of incoming sunlight.
Energy use and cost
energy (kWh) = power (kW) × time (h); cost = energy (kWh) × price per kWh
Convert watts to kilowatts by dividing by 1,000 (1,500 W = 1.5 kW). Efficiency (%) = useful energy output / total energy input × 100.
Every term in Unit 6
All 22 terms we publish for Energy Resources & Consumption, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.
- Renewable vs nonrenewable
- Renewables replenish on a human timescale; fossil fuels and nuclear fuel do not. Renewable does not automatically mean low-impact.
- Fossil fuel formation
- Coal from compressed plant matter in swamps; oil and natural gas from marine microorganisms buried under heat and pressure over millions of years.
- Coal types and quality
- Peat, lignite, bituminous, anthracite in order of increasing carbon content and energy density and decreasing impurities.
- Coal combustion pollutants
- CO₂, sulfur dioxide (acid rain), nitrogen oxides, particulates and mercury. Scrubbers remove SO₂ and precipitators remove particulates.
- Natural gas advantages
- Burns cleanest of the fossil fuels, roughly half the CO₂ of coal per unit energy, but methane leakage is a potent greenhouse gas.
- Hydraulic fracturing
- Injecting high-pressure fluid to fracture shale and release gas. Concerns include groundwater contamination, water use and induced seismicity.
- Nuclear fission
- Splitting uranium-235 releases heat that generates steam. No CO₂ during operation, but produces long-lived radioactive waste.
- Nuclear reactor components
- Control rods absorb neutrons to regulate the reaction, a moderator slows neutrons, and coolant carries heat away. Loss of coolant causes meltdown.
- Chernobyl and Fukushima
- Chernobyl was a flawed design plus operator error with no containment; Fukushima was a tsunami disabling backup cooling.
- Solar energy
- Photovoltaic cells convert light directly to electricity; solar thermal concentrates heat. No emissions in operation, but intermittent and land-intensive.
- Wind energy
- No fuel and no emissions in operation, but intermittent, requires specific sites, and raises bird and bat mortality and noise concerns.
- Hydroelectric power
- Reliable and emission-free in operation, but dams flood habitat, block fish migration, trap sediment and displace communities.
- Geothermal energy
- Uses Earth's internal heat. Reliable and low-emission, but limited to tectonically active regions and can release hydrogen sulfide.
- Biomass and biofuels
- Carbon-neutral in principle since growth reabsorbs CO₂, but competes with food production and can drive deforestation.
- Energy conservation
- Reducing demand through insulation, efficient appliances, public transit and CAFE standards — usually cheaper per unit than new generation.
- Energy efficiency of conversion
- Every conversion loses energy as heat, which is why an incandescent bulb wastes about 90% of its input and a power plant is roughly one-third efficient.
- Energy return on investment
- Energy obtained divided by energy spent obtaining it. Conventional oil has historically been high; tar sands and biofuels are much lower.
- Cogeneration
- Capturing waste heat from electricity generation for heating, raising overall efficiency from about a third to as much as 80%.
- CAFE standards
- Corporate Average Fuel Economy requirements for vehicle fleets — a conservation policy rather than a generation one.
- Passive vs active solar
- Passive design uses building orientation, thermal mass and glazing with no moving parts; active systems use pumps, fans and photovoltaic cells.
- Intermittency and storage
- Solar and wind output varies with weather and time of day, so they need storage or backup generation to match demand.
- Hydrogen fuel cells
- Combine hydrogen and oxygen to produce electricity with water as the only emission — but the hydrogen itself is usually made from natural gas.
What examiners penalize here
- Net energy (EROEI) is a favorite comparison tool: higher EROEI = more usable energy per unit invested. Sources with EROEI near 1 (some biofuels, tar sands) barely pay back the energy used to produce them — a strong argument against them even before considering emissions.
- For half-life problems, always compute n = elapsed time ÷ half-life first, then multiply the starting amount by (1/2)ⁿ. Stepping the halving down one period at a time (80 → 40 → 20 → 10) is a reliable check against calculator slips.
- For solar-output problems, keep the three factors straight — area, intensity (W/m²), and efficiency as a decimal — and multiply. A frequent follow-up asks how much energy (kWh) the array makes over several hours: multiply the power (in kW) by the number of hours.
- For energy-cost problems, the killer step is unit conversion: watts → kilowatts by dividing by 1,000. Then kWh = kW × hours, and cost = kWh × price. Lay the units out and cancel them; if your answer is off by a factor of 1,000, you skipped the W-to-kW conversion.
Practice Env. Science
Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.
Questions about this unit
How much of the AP Environmental Science exam is Unit 6?
Unit 6, Energy Resources & Consumption, is worth 10–15% of the Env. Science multiple-choice section according to the published course framework. Across all 9 units that makes it a substantial share — heavier than an even split would give it.
What topics are covered in Env. Science Unit 6?
Energy Resources & Consumption covers Fossil fuels, Renewables, Nuclear and Energy efficiency. We publish 22 terms with definitions for this unit, all of them on this page.
How should I study Env. Science Unit 6?
Read the 4 lessons below first — about 50 minutes — then drill the 22 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.
All 9 units of AP Environmental Science
Unit names, topics and exam weights follow the published College Board course framework for AP Environmental Science. AP® is a trademark registered by the College Board, which does not endorse this site.