Energy Resources and Consumption
Unit 6 of AP Environmental Science, worth 10–15% of the exam. 14 questions below, each with the working. Every answer was checked by a second pass before it was published.
Renewable and non-renewable resources, global energy use, fossil fuels, nuclear power, biomass, solar, hydroelectric, geothermal, wind, energy conservation.
How this unit is tested
Start by sorting every energy source into renewable or nonrenewable and knowing why that distinction matters (formation timescale, not just pollution level — a renewable source can still be dirty, like biomass, and a nonrenewable source can be relatively clean in one respect but not another). For each source, learn three things cold: how it generates usable energy, its main environmental and economic tradeoffs, and where geographically or economically it makes sense to use it.
Expect quantitative reasoning questions on this unit. The AP exam likes EROI (energy returned on investment) calculations, capacity factor problems, and percentage/proportion questions about global energy mix or per-capita consumption between developed and developing nations. Practice plugging numbers into the EROI ratio and interpreting what a high versus low ratio means for a resource's long-term viability.
When you compare energy sources, always organize your answer around the same four axes examiners use: energy density and reliability (baseload vs. intermittent), environmental impact (air, water, land, wildlife), economic/infrastructure cost, and geographic or political constraints. This is also how free-response questions are scored — a generic "solar is good for the environment" answer earns fewer points than one that names a specific tradeoff, like habitat loss from utility-scale solar farms sited in desert ecosystems.
Finally, treat energy conservation and efficiency as their own tested category, not an afterthought. Know the difference between conservation (using less) and efficiency (using less to do the same amount of work), and be ready to identify specific technologies (cogeneration, LED lighting, smart grids, insulation) and behaviors that reduce demand.
What you have to know
EROI (Energy Returned on Investment)
$EROI = \dfrac{E_{output}}{E_{input}}$, the ratio of usable energy obtained from a resource to the energy spent extracting, processing, and delivering it. Higher ratios mean a more energetically profitable resource; an EROI near or below 1:1 means the resource consumes as much or more energy than it yields.
Capacity factor
$Capacity\ factor = \dfrac{Actual\ energy\ output}{Maximum\ possible\ output\ if\ running\ at\ full\ capacity\ 100\%\ of\ the\ time}$. It measures how often an intermittent source (wind, solar) actually produces power relative to its rated maximum, distinct from efficiency.
Renewable vs. nonrenewable resource
A renewable resource is replenished by natural processes on a human timescale (sunlight, wind, biomass regrowth, geothermal heat, the water cycle). A nonrenewable resource forms over geologic timescales far longer than it is consumed (coal, oil, natural gas, uranium ore), so current reserves are essentially fixed.
Nuclear fission
The splitting of a heavy, unstable nucleus (typically uranium-235) by a neutron, releasing energy, additional neutrons, and radioactive fission products; the released neutrons can sustain a chain reaction, which nuclear reactors control using control rods and moderators.
Rule of 70
$Doubling\ time\ (years) = \dfrac{70}{annual\ percentage\ growth\ rate}$, used to estimate how quickly global or national energy consumption will double given a constant percentage growth rate.
14 practice questions
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A country's energy consumption is growing at 3.5% per year. Using the Rule of 70, approximately how many years will it take for consumption to double?
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Answer. About 20 years
Doubling time = 70 divided by the annual growth rate percentage: 70 / 3.5 = 20 years. This estimate assumes the growth rate stays constant, which is the key assumption behind the Rule of 70. -
A wind turbine rated at a maximum output of 2,000 kWh per day actually produces an average of 700 kWh per day over a year. What is its capacity factor?
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Answer. 35%
Capacity factor = actual output / maximum possible output = 700/2000 = 0.35, or 35%. This reflects wind's intermittency; turbines rarely run at full rated capacity because wind speed varies. -
Which best explains why an EROI of 2:1 for a resource is concerning for long-term energy strategy?
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Answer. Nearly half the energy gained must be reinvested just to obtain more of the resource, leaving little net energy for other uses
EROI of 2:1 means for every 2 units of energy produced, 1 unit was spent extracting and processing it, so only half the gross energy is actually net available energy — a low margin that becomes uneconomical as it approaches 1:1. -
Coal forms through a sequence of increasing carbon content and energy density. Place these in the correct order from least to most energy-dense: bituminous, peat, anthracite, lignite.
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Answer. Peat, lignite, bituminous, anthracite
Coal formation is a progressive compaction and heating process: plant matter first becomes peat, then lignite (soft, low carbon), then bituminous (higher carbon, most commonly burned), and finally anthracite (hardest, highest carbon and energy content, least abundant). -
What is the primary environmental concern associated with hydraulic fracturing (fracking) for natural gas that is NOT typically associated with conventional natural gas drilling?
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Answer. Contamination of groundwater and increased risk of induced seismicity from high-pressure fluid injection
Fracking injects water, sand, and chemicals at high pressure to fracture shale rock, which can allow fluids and methane to migrate into aquifers and has been linked to increased small earthquakes near injection sites — risks not present in traditional vertical well drilling. -
A student argues that because nuclear power plants emit almost no carbon dioxide during operation, nuclear power has no significant environmental drawbacks. Explain why this reasoning is flawed.
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Answer. It ignores major impacts beyond operational CO2 emissions, especially long-lived radioactive waste storage, thermal water pollution, uranium mining impacts, and catastrophic accident risk
While reactor operation itself is low-carbon, the full picture includes mining and enriching uranium ore, managing high-level radioactive waste for thousands of years, using large volumes of water for cooling (raising local water temperatures), and the low-probability but severe risk of meltdowns like Chernobyl or Fukushima. -
Why is biomass energy sometimes described as 'carbon neutral' but also criticized as not truly carbon neutral in practice?
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Answer. In theory, CO2 released by burning biomass equals CO2 absorbed during the plants' regrowth, but in practice harvesting, transport, and land-use changes add extra emissions, and regrowth may not fully offset combustion in the near term
The theoretical carbon-neutral claim assumes new plants are grown to reabsorb exactly the CO2 released, but real-world logging, transportation fuel use, and delayed regrowth mean net emissions are often positive over relevant timescales. -
Which factor makes geothermal energy most practical in countries like Iceland and the Philippines but far less feasible in most of the central United States?
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Answer. Proximity to tectonic plate boundaries, where magma is close enough to the surface to heat groundwater economically
Geothermal power requires accessible underground heat, which is far more concentrated near tectonic plate boundaries and volcanic regions; most of the interior United States sits on stable continental crust far from such boundaries, making geothermal there far less economical. -
A utility company wants to add a large hydroelectric dam to its power supply. Identify one major ecological tradeoff of this decision.
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Answer. Blocking fish migration routes (e.g., salmon spawning) and altering downstream sediment transport and water temperature
Dams physically block anadromous fish from reaching spawning grounds, trap sediment behind the dam (starving downstream ecosystems and deltas of nutrients), and change natural flow and temperature regimes, all while flooding upstream terrestrial habitat to create the reservoir. -
Which best distinguishes passive solar design from active solar power generation?
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Answer. Passive solar uses building orientation and materials to capture and distribute heat without mechanical devices, while active solar uses equipment like photovoltaic cells or pumped fluid systems to convert or move solar energy
Passive design relies on architecture alone — south-facing windows, thermal mass, insulation — to trap solar heat, while active systems require additional technology such as PV panels or solar water heating pumps to actively capture and convert or move the energy. -
A country generates most of its electricity from coal-fired base load plants and wants to add significant wind capacity. What technical challenge must it address, and why?
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Answer. Wind is intermittent, so the grid needs backup base load power or storage to cover periods of low wind, since wind alone cannot reliably meet constant demand
Because wind speed fluctuates and cannot be controlled on demand, a grid relying heavily on wind needs either dispatchable backup sources (like natural gas plants) or large-scale energy storage (like batteries or pumped hydro) to maintain a stable base load supply during calm periods. -
Explain why cogeneration (combined heat and power) systems achieve higher overall energy efficiency than conventional power plants.
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Answer. Cogeneration captures and uses waste heat produced during electricity generation for heating or industrial processes instead of releasing it, so a much larger share of the input energy is put to productive use
A conventional power plant converts only about 33-40% of fuel energy into usable electricity and vents the rest as waste heat; cogeneration systems capture that waste heat for space heating or industrial steam, pushing total useful energy efficiency up to 70-80%. -
Per capita, energy consumption in the United States is roughly how many times greater than the global average?
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Answer. Roughly 4 to 5 times the global average
The United States and other highly developed nations consume a disproportionately large share of world energy relative to their population; U.S. per capita energy use is commonly cited as several times the global average, driven by transportation, industry, and household consumption. -
Name one nonrenewable energy source and one renewable energy source, and identify a pollutant or environmental impact common to combustion-based sources from each category.
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Answer. Nonrenewable example: coal, which releases CO2, sulfur dioxide (causing acid rain), and particulates when burned. Renewable example: biomass, which also releases CO2 and particulates when burned despite being renewable
This question tests the key misconception that renewable automatically means clean: both coal (nonrenewable) and biomass (renewable) release combustion byproducts like CO2 and particulate matter, showing that renewability and pollution are independent properties of an energy source.
What people get wrong
- Assuming 'renewable' automatically means 'environmentally harmless.' Biomass combustion releases CO₂ and particulates, and large hydroelectric dams flood habitats and block fish migration — always evaluate impact separately from renewability.
- Confusing capacity factor with efficiency. Efficiency compares energy output to energy input at the device level; capacity factor compares actual output over time to theoretical maximum output, which matters for intermittent sources like wind and solar.
- Treating nuclear power as a major direct greenhouse gas source. Reactor operation emits very little CO₂; the real environmental concerns are high-level radioactive waste storage, thermal pollution of water sources, and catastrophic accident risk, not routine air pollution.
- Forgetting that fossil fuels power more than electricity generation. Oil is used heavily for transportation and natural gas for heating and industrial processes, so 'switching the grid to renewables' does not eliminate fossil fuel demand by itself.
- Assuming global per-capita energy consumption is uniform. Developed nations consume disproportionately large shares of world energy per person compared to developing nations, a pattern tested directly on exam questions about global energy use.
- Mixing up fission and fusion. Fission (splitting heavy nuclei) is the process used in all current commercial nuclear power plants; fusion (combining light nuclei, as in the sun) is not yet commercially viable for power generation.
Drill this unit until it sticks
These questions come back on a schedule built from what you get wrong, alongside the rest of AP Environmental Science. Free, and no account needed to start.