Review AP Environmental Science Unit 6 to understand how humans produce and consume energy, why fossil fuels handle global use, and what environmental trade-offs come with every energy source from coal to hydrogen fuel cells. This unit covers nonrenewable and renewable sources, extraction methods, power generation mechanics, and conservation strategies.
Use the topic guides, practice questions, and FRQ practice available for this unit to work through energy trade-offs and calculation problems before exam day.
Energy Resources and Consumption asks you to think about energy from three angles: where it comes from, how it is converted into usable electricity or heat, and what environmental damage results. Every energy source in this unit has a specific mechanism, a set of environmental impacts, and a place in the global energy mix.
Unit 6 covers nonrenewable sources (fossil fuels, nuclear) and renewable sources (solar, wind, hydro, geothermal, biomass, hydrogen fuel cells), global consumption trends, and energy conservation. The central skill is comparing energy sources by how they work, what pollutants or waste they produce, and what limits their use.
Nonrenewable sources exist in fixed amounts and cannot be replaced on a human timescale: coal, crude oil, natural gas, and uranium. Renewable sources are naturally replenished at or near the rate of consumption: solar, wind, hydroelectric, geothermal, and biomass. The key distinction is replenishment rate, not whether the source is clean or dirty.
Most power plants share the same basic sequence: a fuel source produces heat, heat converts water to steam, steam spins a turbine, and the turbine drives a generator. Fossil fuels, nuclear fission, geothermal, and biomass all follow this path. Solar PV and wind turbines skip the steam step entirely, converting energy directly into electricity through the photovoltaic effect or kinetic energy conversion.
Every energy source carries costs. Fossil fuels emit CO2, SO2, NOx, and particulates. Nuclear produces radioactive waste and thermal pollution. Hydroelectric dams alter habitats and block fish migration. Wind turbines kill birds and bats. Solar farms can disrupt desert ecosystems. Biomass burning releases the same pollutants as fossil fuels. No source is impact-free.
The global reliance on fossil fuels drives climate change, air pollution, and ocean acidification covered in Units 7 and 9. As countries industrialize, energy demand rises and fossil fuel use typically increases before renewables scale up. Understanding the mechanisms, trade-offs, and distribution of energy resources explains why the energy transition is both necessary and difficult.
Nonrenewable sources exist in fixed amounts and cannot be replaced on a human timescale. Renewable sources are replenished naturally at or near the rate of consumption. Replenishment rate is the key distinction.
Fossil fuels handle global energy use. Developed countries consume more energy per capita than developing countries. As nations industrialize, fossil fuel demand rises. Availability, price, and regulations shape energy choices.
Wood, peat, lignite, bituminous coal, anthracite, natural gas, crude oil, and tar sands each have distinct formation conditions, energy content, and uses. Cogeneration produces both heat and electricity from a single fuel source.
Coal, oil, gas, and uranium deposits are unevenly distributed based on regional geologic history. Unequal distribution creates unequal energy access and drives international energy trade.
Combustion of fossil fuels produces CO2 and water while releasing energy used to generate steam and electricity. Extraction methods like fracking can contaminate groundwater and release VOCs. Combustion also emits SO2, NOx, and particulates.
Nuclear fission splits U-235 atoms to release heat, generate steam, and produce electricity. No air pollutants are emitted, but radioactive waste persists for thousands of years. Three Mile Island, Chernobyl, and Fukushima are key accident case studies.
Burning biomass releases CO2, CO, NOx, particulates, and VOCs. Overharvesting for fuelwood causes deforestation. Ethanol substitutes for gasoline but has a low energy return on energy investment.
Photovoltaic cells convert sunlight directly into electricity. Active solar systems use equipment to collect and store heat. Passive solar systems absorb heat with no equipment and cannot store energy. Large solar farms can disrupt desert ecosystems.
Dams store water in reservoirs; flowing water spins turbines to generate electricity. Tidal energy uses tidal flows to spin turbines. Hydroelectric power produces no air pollution but dams alter habitats and block fish migration.
Earth's internal heat converts water to steam, which drives a generator. Geothermal is location-dependent, expensive to access, and can release hydrogen sulfide gas during operation.
Fuel cells combine hydrogen and oxygen to produce electricity; water is the only emission. However, producing hydrogen requires energy input, and the technology is expensive. Carbon-free only if hydrogen is produced from water using clean energy.
Wind turbines convert the kinetic energy of moving air into electricity. Wind is renewable and produces no air pollution, but turbines kill birds and bats and output is intermittent depending on wind conditions.
Household strategies include efficient appliances, thermostat adjustments, and xeriscaping. Large-scale strategies include BEVs, hybrid vehicles, public transportation, fuel economy standards, and green building design.
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Review Hydrogen Fuel Cell with attention to how the concept appears in AP-style source and evidence questions.
Review Fuel Types and Uses with attention to how the concept appears in AP-style source and evidence questions.
Review Energy from Biomass with attention to how the concept appears in AP-style source and evidence questions.
Review Distribution of Natural Energy Resources with attention to how the concept appears in AP-style source and evidence questions.
The core distinction in 6.1 is replenishment rate. Nonrenewable sources exist in a fixed amount; once used, they cannot be replaced on a human timescale. Renewable sources are naturally replenished at or near the rate of consumption. Topic 6.2 adds the consumption layer: fossil fuels handle the global energy mix, developed countries consume far more energy per capita than developing countries, and as nations industrialize, their fossil fuel demand rises. Availability, price, and government regulations all shape which sources a country actually uses.
Can you explain why natural gas is renewable or nonrenewable, and why a developing country's energy mix shifts as it industrializes?
| Category | Examples | Replenishment | Key concern |
|---|---|---|---|
| Nonrenewable | Coal, oil, natural gas, uranium | Cannot be replaced on human timescale | Depletion and pollution |
| Renewable | Solar, wind, hydro, geothermal, biomass | Replenished naturally at or near rate of use | Intermittency and land use |
Topic 6.3 covers the specific fuels humans burn and why. Wood and charcoal are common in developing countries because they are accessible and cheap. Peat is partially decomposed organic material burned for heat. Coal comes in three grades: lignite has the lowest energy content, bituminous is the most widely used, and anthracite has the highest carbon content and energy value. Heat, pressure, and depth of burial determine coal grade. Natural gas is mostly methane and is the cleanest fossil fuel. Crude oil is refined into gasoline, diesel, and jet fuel. Tar sands contain bitumen mixed with clay, sand, and water. Cogeneration uses a single fuel source to produce both heat and electricity simultaneously. Topic 6.4 explains why these resources are unevenly distributed: geologic history determines where sedimentary basins, coal seams, and oil reservoirs formed, so access to energy resources varies dramatically by region.
Can you rank the three coal types by energy content and explain what geologic conditions produce anthracite versus lignite?
| Fuel | Type | Primary use | Key environmental concern |
|---|---|---|---|
| Wood/charcoal | Biomass (renewable) | Heating and cooking in developing countries | Deforestation, indoor air pollution |
| Lignite | Coal (nonrenewable) | Electricity generation | High CO2 and SO2 emissions |
| Bituminous coal | Coal (nonrenewable) | Electricity and steel production | SO2, NOx, particulates |
| Anthracite | Coal (nonrenewable) | Heating | High CO2 per unit burned |
| Natural gas | Fossil fuel (nonrenewable) | Electricity, heating, cooking | Methane leakage during extraction |
Fossil fuel combustion is a chemical reaction between a hydrocarbon fuel and oxygen that produces carbon dioxide and water while releasing energy. That energy heats water into steam, which spins a turbine connected to a generator. This steam-turbine sequence is the foundation of most fossil fuel power plants. Extraction methods include surface mining, mountaintop removal, and hydraulic fracturing. Fracking injects high-pressure fluid into shale formations to release trapped natural gas or oil, but it can contaminate groundwater and release volatile organic compounds (VOCs). Additional combustion pollutants include sulfur dioxide, nitrogen oxides, particulate matter, and mercury.
Can you describe the full sequence from burning coal to generating electricity, and name two environmental impacts of hydraulic fracturing?
Nuclear power plants split Uranium-235 atoms in a process called nuclear fission. When a neutron strikes a U-235 nucleus, the atom splits and releases a large amount of heat. That heat follows the same steam-turbine-generator sequence used in fossil fuel plants. Nuclear power produces no air pollutants during operation, but it does release thermal pollution from condenser discharge and generates long-lasting radioactive solid waste. Spent fuel rods remain radioactive for thousands of years, making disposal a major challenge. Half-life calculations are used to determine how long a radioactive material remains hazardous. Three accidents illustrate nuclear risk: Three Mile Island (1979, partial meltdown, minimal radiation release), Chernobyl (1986, reactor explosion, widespread contamination), and Fukushima (2011, tsunami-triggered meltdown, radiation release into ocean and atmosphere).
Can you explain why nuclear power is classified as nonrenewable and describe two environmental impacts that distinguish it from fossil fuel plants?
Biomass energy comes from burning organic material such as wood, crop waste, or dedicated energy crops. It is relatively low cost and accessible, especially in developing countries, but burning biomass releases CO2, carbon monoxide, nitrogen oxides, particulates, and VOCs. Overharvesting trees for fuelwood accelerates deforestation. Ethanol is a liquid biofuel made from crops like corn or sugarcane and can substitute for gasoline. Burning ethanol does not introduce new carbon into the atmosphere because the carbon was recently absorbed by the plant during growth. However, the energy return on energy investment (EROI) for ethanol is low, meaning a significant amount of energy is consumed to grow, harvest, and process the feedstock.
Why is ethanol considered carbon-neutral in combustion but still criticized for low energy efficiency?
These three renewable sources each use a distinct mechanism. Solar PV cells convert sunlight directly into electricity through the photovoltaic effect; output is limited by sunlight availability and intermittency. Active solar systems use mechanical equipment to collect and store solar heat in a liquid. Passive solar systems absorb heat directly from the sun with no equipment and cannot store energy. Large solar farms can disrupt desert ecosystems. Hydroelectric power uses moving water to spin a turbine; dams create reservoirs but block fish migration, alter habitats, and can cause sediment buildup. Tidal energy uses tidal flow to spin turbines. Geothermal energy taps heat stored in Earth's interior to produce steam that drives a generator. It is location-dependent, expensive to access, and can release hydrogen sulfide gas.
Can you compare active and passive solar systems and name one environmental impact specific to each of the three energy sources in this group?
| Source | Mechanism | Key advantage | Key environmental concern |
|---|---|---|---|
| Solar PV | Photovoltaic effect converts sunlight to electricity | No emissions during operation | Intermittency; land use in desert ecosystems |
| Hydroelectric | Moving water spins turbine via dam or run-of-river | Reliable, no air pollution | Habitat loss, fish migration blocked, sediment trapping |
| Geothermal | Earth's internal heat converts water to steam | Low emissions, continuous output | Location-limited; hydrogen sulfide release; high cost |
Hydrogen fuel cells combine hydrogen with oxygen from the air to produce electricity; the only emission is water. This makes them a clean alternative to fossil fuels. However, producing hydrogen gas requires energy, and if that energy comes from fossil fuels, the overall process is not carbon-free. The technology is also expensive. Wind turbines capture the kinetic energy of moving air to spin rotor blades, which turn a generator and produce electricity. Wind is renewable and produces no air pollution, but turbines kill birds and bats through blade collisions. Wind output is intermittent and depends on wind speed and location.
Why is a hydrogen fuel cell not automatically carbon-free, and what is the main wildlife concern associated with wind turbines?
Energy conservation reduces demand rather than changing supply. At the household scale, methods include adjusting thermostats, using energy-efficient appliances, conserving water, and using conservation landscaping (xeriscaping) to reduce irrigation needs. At the large scale, key strategies include improving vehicle fuel economy standards, adopting battery electric vehicles (BEVs) and hybrid vehicles, expanding public transportation, and implementing green building design features such as improved insulation, efficient HVAC systems, and passive solar heating. Conservation reduces fossil fuel consumption, lowers greenhouse gas emissions, and decreases pollution without requiring new energy infrastructure.
Name two household and two large-scale energy conservation strategies and explain how each reduces fossil fuel demand.
Try stimulus-based AP practice questions and written prompts after you review the notes.
A timed, AP-style test built only from unit 6 questions — multiple choice plus free response, scored with AI feedback. Use the standard format or build your own.
| Term | Definition |
|---|---|
| Renewable energy | Energy from sources replenished naturally at or near the rate of consumption, including solar, wind, hydro, geothermal, and biomass. |
| Fossil Fuel Combustion | Burning coal, oil, or natural gas in the presence of oxygen to release energy; produces CO2, water, and pollutants including SO2, NOx, and particulates. |
| Hydraulic Fracturing | High-pressure fluid injection into shale formations to release trapped oil or gas; risks include groundwater contamination and VOC release. |
| Nuclear Fission | A neutron strikes a U-235 nucleus, splitting it and releasing large amounts of heat used to generate steam and electricity. |
| Radioactive Waste | Spent nuclear fuel rods that remain hazardous for thousands of years due to ongoing radioactive decay; long-term disposal is a major challenge. |
| half-life | The time required for a radioactive element to decay to half its original activity; used to calculate how long nuclear waste remains dangerous. |
| Photovoltaic Cells | Devices that convert sunlight directly into electricity through the photovoltaic effect; output is limited by sunlight availability and intermittency. |
| passive solar energy systems | Systems that absorb heat directly from the sun without mechanical or electrical equipment; energy cannot be collected or stored. |
| Ethanol | A liquid biofuel made from corn or sugarcane that substitutes for gasoline; does not add new atmospheric carbon when burned but has a low energy return on energy investment. |
| energy return on energy investment | The ratio of energy output from a fuel to the energy required to produce it; ethanol has a low ratio, meaning production consumes nearly as much energy as the fuel yields. |
| intermittency | The variability of renewable energy output due to weather, time of day, or season; a key limitation of solar and wind power. |
| tar sands | A mixture of clay, sand, water, and bitumen from which crude oil can be recovered; extraction is energy-intensive and environmentally damaging. |
| battery electric vehicles (BEVs) | Vehicles powered entirely by rechargeable batteries and electric motors; produce zero direct emissions and reduce fossil fuel consumption. |
| green building design | Construction practices that minimize energy use through improved insulation, efficient HVAC systems, passive solar heating, and renewable energy integration. |
| Turbine | A device that converts the kinetic energy of moving fluid, such as steam, water, or wind, into mechanical energy that drives a generator to produce electricity. |
Burning biomass does release CO2 and other pollutants. The carbon-neutral argument applies only to the combustion step because the carbon was recently absorbed by the plant. Biomass burning still produces CO, NOx, particulates, and VOCs, and overharvesting causes deforestation.
Active solar systems use mechanical and electrical equipment to collect and store heat in a liquid. Passive solar systems absorb heat directly from the sun with no equipment and cannot store energy. PV cells are a third distinct category that converts light to electricity, not heat.
Nuclear power is nonrenewable because it relies on uranium ore, which exists in a fixed amount and cannot be replenished. The fact that it produces no air pollutants does not make it renewable.
A hydrogen fuel cell emits only water during operation, but producing the hydrogen gas requires energy. If that energy comes from fossil fuels, the overall process generates CO2 upstream. The fuel cell is only truly carbon-free when hydrogen is produced using clean energy.
Fossil fuel environmental impacts are not limited to combustion. Hydraulic fracturing can contaminate groundwater and release VOCs. Surface mining and mountaintop removal destroy habitats. Always consider both extraction and combustion impacts when evaluating fossil fuels.
A common task in Unit 6 is describing how a specific energy source generates electricity and then identifying its environmental trade-offs. Practice writing responses that name the mechanism (for example, kinetic energy to turbine to generator for wind), the key advantage (no air pollution), and the key impact (bird and bat mortality). This describe-and-evaluate pattern applies to every source in the unit.
Nuclear power questions may require you to calculate how much radioactive material remains after a given number of half-lives. Practice the pattern: after one half-life, 50% remains; after two, 25%; after three, 12.5%. Be ready to apply this to a specific isotope and explain why the result matters for waste storage decisions.
Questions may present a scenario, such as a household or a city, and ask you to identify or justify specific conservation strategies. Be ready to distinguish household-scale actions like thermostat adjustment and efficient appliances from large-scale strategies like BEVs, fuel economy standards, and green building design, and to explain how each reduces fossil fuel demand or greenhouse gas emissions.
For each source in 6.1-6.12, state whether it is renewable or nonrenewable and explain why using the replenishment rate definition. Do not rely on whether a source is clean as a proxy for renewable.
Explain how fossil fuels, nuclear fission, geothermal, and biomass each produce heat that becomes steam, spins a turbine, and drives a generator. Identify which sources skip this sequence entirely.
For each source, name at least one specific environmental impact: combustion pollutants for fossil fuels and biomass, radioactive waste and thermal pollution for nuclear, habitat loss for hydro, hydrogen sulfide for geothermal, bird and bat mortality for wind, and land use for solar.
Practice calculating how much radioactive material remains after one, two, or three half-lives. Understand why long half-lives make nuclear waste disposal a long-term environmental challenge.
Describe why developed countries consume more energy per capita, why fossil fuel use rises with industrialization, and how availability, price, and regulation shape a country's energy mix.
Active solar uses equipment to collect and store heat. Passive solar absorbs heat directly with no equipment and cannot store energy. PV cells convert light directly to electricity. Know which can store energy and which cannot.
Be ready to give concrete examples: thermostat adjustment and xeriscaping at home; BEVs, fuel economy standards, public transit, and green building design at the societal scale.
Start with topics 6.1 and 6.2. Make a two-column list sorting every energy source in the unit into renewable or nonrenewable, then add a note on why each belongs there using the replenishment rate definition. Add a second layer noting which sources handle global consumption and why fossil fuel use rises with development.
Work through topics 6.3 and 6.4. Create a table comparing wood, peat, lignite, bituminous coal, anthracite, natural gas, crude oil, and tar sands by formation conditions, energy content, and primary use. Add a map note on why geologic history determines where these resources are found.
Focus on topics 6.5 and 6.6. Write out the combustion equation for fossil fuels and the steam-turbine-generator sequence. Then do the same for nuclear fission. Practice a half-life calculation for radioactive waste. Review the Three Mile Island, Chernobyl, and Fukushima cases and their environmental impacts.
Work through topics 6.7 through 6.12 using a comparison table. For each source, record the mechanism, key advantage, and key environmental impact. Pay special attention to the active vs. passive solar distinction, the EROI problem with ethanol, and why hydrogen fuel cells are not automatically carbon-free.
Finish with topic 6.13. List at least three household and three large-scale conservation strategies with a brief explanation of how each reduces fossil fuel demand. Then do a full unit review using the comparison table from Step 4 to practice explaining trade-offs across all energy sources.
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get printablesAPES Unit 6: Energy Resources and Consumption covers 13 topics: Renewable and Nonrenewable Resources, Global Energy Consumption, Fuel Types and Uses, Distribution of Natural Energy Resources, Fossil Fuels, Nuclear Power, Energy from Biomass, Solar Energy, Hydroelectric Power, Geothermal Energy, Hydrogen Fuel Cells, Wind Energy, and Energy Conservation. The unit traces how humans produce and use energy and the environmental consequences of each source. See all 13 topics at /ap-enviro/unit-6.
APES Unit 6 makes up 10-15% of the AP exam, making it one of the more heavily tested units. It covers energy sources ranging from fossil fuels and nuclear power to renewable options like geothermal energy, wind energy, solar, and hydroelectric power, plus the environmental trade-offs and energy conservation strategies tied to each.
The APES Unit 6 progress check in AP Classroom includes both MCQ and FRQ parts drawn from all 13 topics in the unit. MCQ questions typically test your ability to compare renewable and nonrenewable resources, interpret global energy consumption data, and evaluate the environmental impacts of fossil fuels, nuclear power, and sources like geothermal energy and wind energy. The FRQ portion often asks you to analyze trade-offs between energy sources or propose energy conservation strategies, so you need to know the pros, cons, and environmental effects of each source cold. Practice with questions matched to every progress check topic at /ap-enviro/unit-6.
APES Unit 6 FRQs most often pull from Fossil Fuels, Geothermal Energy, Wind Energy, Nuclear Power, and Energy Conservation, asking you to calculate energy trade-offs, describe environmental impacts, or justify a policy recommendation. To practice effectively, write out full responses to past prompts, use the College Board scoring guidelines to check your work, and make sure every claim is backed by a specific environmental mechanism, not just a general statement. Find Unit 6 FRQ practice sets at /ap-enviro/unit-6.
The best place to find APES Unit 6 practice questions, including multiple-choice and practice test sets, is /ap-enviro/unit-6. You'll find MCQs covering every topic from Renewable and Nonrenewable Resources through Energy Conservation, plus FRQ prompts on high-frequency topics like geothermal energy, wind energy, and fossil fuels. Working through topic-by-topic MCQs before taking a full practice test helps you spot exactly which energy sources you still need to review.
Start APES Unit 6 by building a comparison chart of every energy source, covering how it works, its environmental impacts, and whether it's renewable or nonrenewable. That single chart will carry you through most MCQs and FRQs. Then focus extra time on geothermal energy, wind energy, and fossil fuels since those show up most on exams. After that, layer in energy conservation strategies and global energy consumption patterns. A solid study sequence looks like this: 1. Read and annotate each of the 13 topics, starting with 6.1 Renewable and Nonrenewable Resources. 2. Build your energy-source comparison chart as you go. 3. Do topic-level MCQs right after each topic to catch gaps early. 4. Write at least two timed FRQ responses on trade-offs between energy sources. 5. Finish with a full unit practice test at /ap-enviro/unit-6 to simulate exam conditions.