AP Environmental Science Unit 6 ReviewEnergy Resources & Consumption

APES Unit 6, Energy Resources and Consumption, is about where the world's energy comes from, how each source actually generates electricity, and what each one costs the environment. The single biggest idea is the tradeoff. Every energy source, from coal to solar, has benefits and drawbacks, and the exam wants you to weigh them. Unit 6 makes up 10-15% of the AP exam, and almost every power source you'll study works the same basic way, by spinning a turbine.

What this unit covers

The big picture of global energy use

• Energy use is not evenly distributed. Developed countries use far more energy per person, and as developing countries industrialize, their fossil fuel use climbs.
• Fossil fuels are the most widely used energy sources on Earth, and demand keeps rising as the world industrializes.
• What people and countries actually use depends on availability, price, and government regulations, not just what's cleanest.
• Where natural energy resources occur depends on geologic history. Coal, crude oil, and natural gas deposits formed under specific conditions over millions of years, so access varies drastically by region.

Fossil fuels and how they make electricity

• Combustion is a chemical reaction between fuel and oxygen that produces carbon dioxide and water and releases energy. That heat boils water into steam, the steam spins a turbine, and the turbine generates electricity. Memorize this chain; it shows up everywhere.
• Coal comes in grades. Lignite, bituminous, and anthracite form as heat, pressure, and depth of burial increase, with anthracite being the highest quality.
• Natural gas (mostly methane) is the cleanest-burning fossil fuel, but it still releases CO2.
• Wood and charcoal are common fuels in developing countries because they're cheap and accessible. Peat is partially decomposed organic material that can also be burned.
• Hydraulic fracturing (fracking) injects high-pressure fluid into rock to release natural gas and oil. It can contaminate groundwater and release volatile organic compounds.

Nuclear power and radioactivity

• Nuclear fission splits Uranium-235 atoms in fuel rods when a neutron strikes them. The fission releases huge amounts of heat, which makes steam, which spins a turbine. No combustion, so no CO2 from generation.
• Radioactivity happens when an unstable nucleus loses energy by emitting radiation. U-235 stays radioactive for a very long time, which creates a long-term waste storage problem.
• Three named accidents matter for the exam. Three Mile Island, Chernobyl, and Fukushima all released radiation, with short- and long-term environmental impacts.
• Half-life calculations let you figure out how much of a radioactive sample remains after a given time. Expect to do this math.

Renewable sources, one by one

• Solar comes in three flavors. Photovoltaic cells convert sunlight directly into electricity. Active solar uses equipment to heat and store a liquid. Passive solar absorbs heat directly with no machinery (think south-facing windows). Solar is clean but expensive, limited by sunlight, and large solar farms can harm desert ecosystems.
• Wind turbines convert the kinetic energy of moving air into electricity. Clean and renewable, but spinning blades kill birds and bats.
• Hydroelectric power uses moving water to spin turbines, either behind dams with reservoirs, in small run-of-river setups, or with tidal flows. No air pollution or waste, but dams are expensive and flood or alter habitats.
• Geothermal energy taps heat from Earth's interior to make steam that drives a generator. It's reliable but only accessible in certain regions, costly to develop, and can release hydrogen sulfide.
• Biomass burning is cheap but dirty. It releases CO2, carbon monoxide, nitrogen oxides, particulates, and VOCs, and overharvesting wood causes deforestation. Ethanol can substitute for gasoline and doesn't add new carbon to the atmosphere, but its energy return on energy investment is low.
• Hydrogen fuel cells combine hydrogen and oxygen to release electricity, with water as the only emission. The catch is cost and the energy needed to produce the hydrogen in the first place.

Energy conservation

• At home, conservation means adjusting the thermostat, conserving water, using energy-efficient appliances, and conservation landscaping (planting to reduce heating and cooling needs).
• At scale, it means better vehicle fuel economy, battery electric vehicles (BEVs) and hybrids, public transportation, and green building design.
• Conservation is the cheapest "energy source" of all. The cleanest kilowatt-hour is the one you never use.

Unit 6, Energy Resources & Consumption at a glance

Why Unit 6, Energy Resources & Consumption matters in APES

Unit 6 is where APES becomes a course about solutions, not just problems. Energy choices drive most of the pollution and climate change you'll study in the rest of the course, so understanding the tradeoffs here is what lets you evaluate proposals later.

• Energy transfer is a core theme of the course, and this unit is its most practical application. Combustion, fission, and kinetic energy all get converted into electricity through the same turbine logic.
• The course constantly asks "how do humans affect the environment, and what can we do about it?" Unit 6 is the "what can we do" unit, since every alternative source and conservation method is a potential solution.
• The developed-versus-developing-country pattern of energy use ties human population and economics directly to environmental impact.

How this unit connects across the course

• The geology behind coal, oil, and gas deposits builds on plate tectonics, soil, and Earth's geologic history from Earth Systems and Resources (Unit 4). Where resources sit is a result of regional geologic history.
• Mining, fracking, and dam construction extend the resource extraction and land use tradeoffs you analyzed in Land and Water Use (Unit 5). Dams changing river habitats is the same logic as channelization and irrigation impacts.
• Fossil fuel combustion is the direct setup for Atmospheric Pollution (Unit 7). The CO, NOx, particulates, and VOCs released here become the smog, acid rain, and air quality problems there.
• The CO2 from fossil fuels is the main driver of climate change and ocean acidification in Global Change (Unit 9). If you understand combustion now, the carbon story in Unit 9 clicks immediately. Energy waste and contamination also feed into Aquatic and Terrestrial Pollution (Unit 8).

Key equations and processes

• Combustion of fossil fuels: fuel + O2 → CO2 + H2O + energy. Use this to explain both how power plants work and where greenhouse gases come from.
• The turbine chain: heat source → steam → turbine spins → generator makes electricity. This applies to coal, natural gas, nuclear, geothermal, and biomass; only the heat source changes.
• Fuel cell reaction: 2H2 + O2 → 2H2O + electricity. Water is the only emission, which is why fuel cells are considered low impact.
• Half-life decay: amount remaining = initial amount × (1/2)^(t / half-life). Use it to calculate how much of a radioactive isotope is left after t years, or how long until a sample reaches a safe level.
• Energy efficiency: efficiency = (useful energy out / total energy in) × 100. Use it for conservation problems and comparing technologies.
• Energy unit conversions: power (watts) × time = energy (e.g., kWh). Expect math problems converting between joules, watts, and kilowatt-hours.

Unit 6, Energy Resources & Consumption on the AP exam

Unit 6 is 10-15% of the exam, making it one of the heavier units. Here's what you actually do with this content:

• Multiple choice loves stimulus questions here. You'll read graphs of global energy consumption by source or by country, interpret data tables comparing energy sources, and identify trends like rising fossil fuel use in industrializing nations.
• Quantitative work is a sure bet. Half-life calculations, efficiency percentages, and kWh or cost conversions show up in both multiple choice and the math-focused free response question. Show every step and include units.
• Free response questions frequently ask you to describe how a source generates electricity (the turbine chain), then identify one environmental advantage and one drawback. The verbs matter. "Identify" wants a short answer; "describe" and "explain" need a mechanism.
• Solution proposing is a recurring FRQ format. Given an environmental problem, you propose a realistic energy alternative or conservation method and justify it with a tradeoff. Knowing two pros and two cons for every source in the table above covers you.

Essential questions

• Why does global energy consumption rise with development, and why is access to energy resources so uneven?
• How do humans convert different forms of energy into electricity, and what does each conversion cost the environment?
• Can renewable sources realistically replace fossil fuels, given their limits on cost, location, and reliability?
• How much can conservation reduce energy demand compared to switching sources?

Key terms to know

• Nonrenewable energy: an energy source that exists in a fixed amount and can't be easily replaced once used (fossil fuels, uranium).
• Renewable energy: an energy source replenished naturally at or near the rate it's consumed (solar, wind, hydro).
• Combustion: the chemical reaction between a fuel and oxygen that yields CO2 and water and releases energy.
• Lignite, bituminous, anthracite: the three coal grades, formed by increasing heat, pressure, and burial depth.
• Hydraulic fracturing (fracking): injecting high-pressure fluid into rock to release oil and gas, risking groundwater contamination and VOC release.
• Nuclear fission: splitting U-235 atoms with a neutron to release heat for steam-driven electricity.
• Half-life: the time it takes for half of a radioactive sample to decay, used to calculate decay rates and radioactivity over time.
• Photovoltaic cell: a device that converts sunlight directly into electrical energy.
• Passive solar: absorbing the sun's heat directly through design (windows, materials) with no mechanical equipment.
• Tidal energy: using the energy of tidal flows to turn a turbine.
• EROEI (energy return on energy investment): the energy you get out relative to the energy you put in; ethanol's is low.
• Hydrogen fuel cell: a device that combines hydrogen and oxygen to produce electricity with water as the only emission.
• Conservation landscaping: planting and yard design that reduces a home's heating, cooling, and water needs.
• BEV (battery electric vehicle): a fully electric vehicle, one of the large-scale energy conservation strategies.

Common mix-ups

• Nuclear is not renewable. It produces no CO2 during generation, but U-235 exists in a fixed amount, so nuclear counts as nonrenewable. Don't let "no emissions" trick you into calling it renewable.
• Active versus passive solar. Active systems use mechanical and electrical equipment to heat and store a liquid. Passive systems use design alone, like sunlight through a south-facing window. PV cells are a third thing entirely, converting light straight to electricity.
• "Cleanest fossil fuel" still pollutes. Natural gas burns cleaner than coal or oil, but it absolutely emits CO2 and methane leaks make it worse. Clean-er, not clean.
• Ethanol's carbon math. Burning ethanol doesn't add new carbon to the atmosphere (the crops absorbed it recently), but that doesn't make it efficient. Its energy return on energy investment is low, which is its real drawback.