Energy Resources & Consumption

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.

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
• Comparing Energy Sources on a Common Set of Criteria17 min · 3 objectivesCompare energy sources on emissions, land use, reliability, and waste · Explain baseload versus intermittent generation and the storage problem · Evaluate nuclear power against both fossil and renewable alternatives
• Energy Efficiency, Cogeneration, and Where the Energy Goes16 min · 3 objectivesTrace energy losses from primary source to end use · Explain cogeneration and why it raises overall efficiency · Evaluate efficiency measures against generation as a policy choice

Formulas in Unit 6

Every term in Unit 6

All 37 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.

Why no conversion is 100% efficient

The second law requires that some energy in any conversion leave as low-grade heat. A heat engine is limited further by the temperature difference it works across, which is why coal plants stall near 35% no matter how well built.

Primary vs secondary energy

Primary energy is the resource as extracted — coal, crude oil, sunlight. Secondary energy is what it is converted into for use, chiefly electricity and refined fuel. Every conversion between them costs energy, which is why electricity is expensive per joule.

Peak oil and proven reserves

Proven reserves are what is known and economically recoverable at current prices and technology, so the figure rises when prices rise. Peak oil is the maximum production rate, not the exhaustion point — production declines long before the last barrel.

Tar sands and oil shale

Unconventional deposits requiring energy-intensive extraction — strip mining or steam injection, then upgrading. Net energy is far lower than conventional crude, and land disturbance and water use per barrel are far higher.

Coal ash and mercury

Burning coal concentrates its trace mercury and heavy metals into fly ash and stack emissions. Mercury deposits into waterways, is methylated by bacteria, and biomagnifies — which is how a power plant becomes a fish advisory.

The methane caveat on natural gas

Burning natural gas emits about half the CO₂ of coal per unit energy, but methane leaking from wells and pipelines is a far more powerful greenhouse gas per molecule. A few percent of leakage erases much of the climate advantage.

Uranium enrichment

Natural uranium is about 0.7% U-235; reactors need roughly 3–5%. Enrichment raises that fraction — and because weapons need far higher enrichment, the same equipment creates the proliferation concern attached to civilian nuclear power.

Nuclear waste storage

Spent fuel is intensely radioactive for thousands of years. Current practice is on-site cooling pools then dry casks, because no country has opened a permanent deep geologic repository at scale — a political problem more than a technical one.

Control rods and moderators

Control rods absorb neutrons to slow or stop the chain reaction; a moderator (usually water) slows neutrons so they are more likely to cause fission. Losing coolant means losing both cooling and moderation, which is the core of reactor accident scenarios.

Photovoltaic vs concentrated solar

Photovoltaic cells convert sunlight directly to electricity and work at any scale, including rooftops. Concentrated solar uses mirrors to make heat that drives a turbine, needs strong direct sun and large land area, but can store heat to generate after sunset.

Capacity factor

Actual output over a year divided by output if the plant ran at full power continuously. Nuclear runs above 90%, wind roughly 35%, solar roughly 25% — which is why nameplate capacity comparisons between sources are misleading.

Run-of-river vs reservoir hydropower

Run-of-river diverts part of the flow without a large reservoir, so it has far less habitat and displacement impact but no storage and variable output. A reservoir dam stores energy and controls floods at much higher ecological cost.

Ethanol and net energy

Corn ethanol requires fertilizer, tractor fuel, irrigation and distillation heat, so its energy return is only modestly above one. It also competes with food cropland — the reason sugarcane and cellulosic feedstocks score better.

Phantom load

Power drawn by devices that are switched off but plugged in — chargers, set-top boxes, anything with a standby light. Individually trivial, collectively several percent of household electricity, and eliminable with a power strip.

Kilowatt vs kilowatt-hour

A kilowatt is a rate of energy use; a kilowatt-hour is an amount. A 100 W bulb burning 10 hours uses 1 kWh. Utility bills and every APES energy calculation depend on keeping the two apart.

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.
• Multiply efficiencies through a chain rather than averaging them, and identify the **weakest link**. A chain's overall efficiency is dominated by its worst step, which is why replacing a 5% bulb matters more than a marginal improvement at a 35% power plant.

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

All 9 units of AP Environmental Science

1. Unit 1 · The Living World: Ecosystems
2. Unit 2 · The Living World: Biodiversity
3. Unit 3 · Populations
4. Unit 4 · Earth Systems & Resources
5. Unit 5 · Land & Water Use
6. Unit 6 · Energy Resources & Consumption
7. Unit 7 · Atmospheric Pollution
8. Unit 8 · Aquatic & Terrestrial Pollution
9. Unit 9 · Global Change