Every Energy concept in the middle school Science Bowl study set (grades 6-8), with a short definition for each. Read through them here, then drill them as flashcards or practise real Energy questions.
First Law of Thermodynamics: energy cannot be created or destroyed — only converted between forms. Total energy of an isolated system is constant. Forms include kinetic, gravitational potential, elastic potential, thermal, chemical, electrical, radiant, and nuclear energy.
Heavy nucleus (U-235, Pu-239) absorbs a neutron and splits into lighter nuclei + 2–3 neutrons + energy. Released neutrons can trigger more fissions (chain reaction). Requires critical mass. Used in nuclear reactors (controlled, moderated by water or graphite) and atomic bombs (uncontrolled).
Light nuclei (deuterium + tritium, hydrogen isotopes) combine to form helium-4 + a neutron + enormous energy. Releases ~3–4× more energy per mass than fission. Powers stars via the proton-proton chain. Requires extreme temperatures (>10⁷ K) to overcome Coulomb repulsion. Tokamak reactors pursue controlled fusion.
Coal (compressed ancient plants, highest carbon content, most CO₂/energy), petroleum (marine organisms; refined to gasoline, diesel, jet fuel), natural gas (mainly CH₄, cleanest-burning). All release CO₂ and other pollutants on combustion, driving the enhanced greenhouse effect. Non-renewable on human timescales.
Photons strike a silicon p-n junction semiconductor, exciting electrons across the band gap and producing direct current (DC). Inverters convert DC to AC. Typical panel efficiency: 15–22%. The photoelectric effect (Einstein, 1905) is the quantum basis. Output scales with light intensity and angle.
Wind turbines convert kinetic energy of wind to rotational energy via blades, then to electricity via generator. Betz limit: maximum theoretical efficiency = 59.3% (can never capture all wind energy). Offshore wind is stronger and more consistent. Power output scales with the cube of wind speed (P ∝ v³).
No heat engine can be 100% efficient (2nd Law). Carnot efficiency (maximum theoretical): η = 1 − Tc/Th (temperatures in Kelvin). Real engines are always less than Carnot. COP (coefficient of performance) for refrigerators: COP = Qc/W. For heat pumps: COP = Qh/W.
Converts gravitational PE of water to electricity via turbines and generators. Power P = ρghQ. Pumped-storage: pump water uphill when energy is cheap; release to generate on demand — acts as a grid-scale battery. Run-of-river vs reservoir (dam) types. Largest renewable electricity source globally.
Entropy of an isolated system always increases (or stays the same). Heat flows spontaneously from hot to cold. No process is 100% efficient — some energy is always lost as waste heat. Clausius statement: heat cannot spontaneously flow from cold to hot. Arrow of time is defined by entropy increase.
Conduction: heat transfer through direct contact; rate depends on thermal conductivity, area, temperature difference, and thickness (Fourier's Law). Convection: heat carried by fluid movement; drives ocean circulation and weather. Radiation: electromagnetic waves (infrared); requires no medium; described by Stefan-Boltzmann Law (P = σεAT⁴).
Heat from Earth's interior (radioactive decay + residual heat from formation). Used directly for heating or to generate electricity (steam drives turbines). Geothermal power plants require high-temperature geothermal reservoirs (e.g., Iceland, Kenya, Geysers in California). Baseload renewable — consistent output unlike solar/wind.
Harnesses energy from tidal changes in sea level. Tidal barrages: dam-like structures capture potential energy of tidal range. Tidal stream generators: underwater turbines in tidal currents (like underwater wind turbines). Predictable unlike solar/wind. Limited to coastal regions with large tidal ranges (e.g., Bay of Fundy).
Biomass: organic material (wood, crop waste, animal waste) burned directly or converted to biogas/biofuels. Ethanol: fermented from sugars/starches (corn, sugarcane). Biodiesel: made from vegetable oils/animal fats via transesterification. Biogas (mostly CH₄) from anaerobic digestion. Carbon-neutral if sustainably sourced.
Electrochemical device that converts H₂ and O₂ directly to electricity + water (2H₂ + O₂ → 2H₂O + electricity). Anode: H₂ oxidized, releases electrons. Cathode: O₂ reduced with electrons + H⁺. Efficiency ~40–60%, higher than combustion. Zero emissions at point of use. Hydrogen must be produced (electrolysis or steam reforming).
Lithium-ion batteries: Li⁺ ions move between anode (graphite) and cathode (LiCoO₂) through electrolyte. High energy density. Pumped hydro: largest grid storage. Flow batteries: electrolyte stored in tanks (scalable). Supercapacitors: fast charge/discharge, lower energy density. Storage critical for intermittent renewables.
Solar radiation (shortwave) passes through atmosphere and warms Earth's surface. Earth re-radiates longwave infrared radiation, which is absorbed and re-emitted by greenhouse gases (CO₂, CH₄, H₂O, N₂O, ozone). Enhanced greenhouse effect: human-emitted CO₂ from fossil fuels increases warming above natural levels.
CCS captures CO₂ from power plants or atmosphere and stores it underground in geological formations, depleted oil/gas fields, or mineralizes it. Direct air capture (DAC): pulls CO₂ directly from ambient air. Bioenergy with CCS (BECCS) could achieve negative emissions. High cost limits widespread deployment.
Unstable nuclei emit particles/energy spontaneously. Alpha (α): emits ²He nucleus (A−4, Z−2); stopped by paper. Beta-minus (β⁻): neutron → proton + electron; stopped by metal foil. Gamma (γ): high-energy photon; requires lead shielding. Half-life (t½): time for half the nuclei to decay; used in radiometric dating.
Joule (J): SI unit of energy. 1 calorie = 4.184 J. 1 kWh = 3.6×10⁶ J. 1 BTU ≈ 1055 J. 1 eV = 1.6×10⁻¹⁹ J. Power: 1 Watt = 1 J/s; 1 horsepower = 746 W. Energy content of fuels measured in MJ/kg (hydrogen: 142 MJ/kg; gasoline: 46 MJ/kg; coal: ~30 MJ/kg).
Power is transmitted at high voltage to minimize resistive losses (P_loss = I²R; high V → low I for same power). Step-up transformers increase voltage for transmission; step-down at substations reduce for consumers. AC (alternating current) used because transformers only work with AC. DC transmission used for some long-distance high-voltage links.
High-level waste (HLW): spent nuclear fuel; highly radioactive, requires deep geological disposal (stable for 10,000+ years). Low-level waste: contaminated tools/clothing. Spent fuel rods cooled in water pools first, then dry cask storage. Reprocessing extracts remaining fissile material (France, Russia) but creates proliferation concerns.
Efficiency = useful energy output / total energy input × 100%. LED bulbs: ~90% efficient (vs ~5% for incandescent). Electric vehicles: ~77% tank-to-wheel vs ~20% for gasoline. Insulation reduces conduction heat losses in buildings. ENERGY STAR ratings. Cogeneration (CHP): captures waste heat for useful heating.
Measure of disorder or randomness in a system (S, units: J/K). 2nd Law: ΔS_universe ≥ 0 for any process. Entropy increases when: ice melts, gas expands, heat flows from hot to cold. Mixing increases entropy. Boltzmann's definition: S = k·ln(W) where W is the number of microstates. Low entropy states are less probable.
Renewables replenish on human timescales: solar, wind, hydro, geothermal, tidal, biomass. Non-renewables: fossil fuels (coal, oil, gas) and nuclear (uranium is finite). Renewables have near-zero marginal fuel cost but high capital cost. Wind and solar have intermittency challenges requiring storage or backup. Global energy still ~80% from fossil fuels.
OTEC exploits temperature differences between warm surface water (~25°C) and cold deep water (~5°C) to run a heat engine. Low Carnot efficiency (~3–5%) but uses a continuous, renewable resource. Three types: closed-cycle (ammonia as working fluid), open-cycle (flash steam), hybrid. Best near tropical coastlines.
Nuclear reactors convert fission energy to electricity. Core components: fuel rods (enriched U-235 or Pu-239 pellets), moderator (slows neutrons to thermal energies; water, heavy water, or graphite), control rods (absorb neutrons; boron or cadmium; adjust reaction rate), coolant (removes heat; water, CO₂, or liquid sodium). Pressurized Water Reactor (PWR): most common; primary loop heats secondary loop via heat exchanger. Boiling Water Reactor (BWR): steam generated directly in core.
EVs use electric motors powered by lithium-ion battery packs (typically 40–100 kWh). Regenerative braking recovers kinetic energy as electricity. Energy efficiency: ~77% tank-to-wheel vs ~20% for gasoline ICE. Well-to-wheel emissions depend on electricity grid mix. Range anxiety: addressed by expanding charging infrastructure (Level 1/2 AC and Level 3 DC fast charging). Battery degradation: ~2% capacity loss per year. Solid-state batteries promise higher energy density.
Smart grid: modernized electricity network using digital communication to improve reliability and efficiency. Two-way communication between utilities and consumers. Advanced metering infrastructure (AMI): real-time usage monitoring and dynamic pricing. Demand response: consumers reduce load during peak periods in exchange for incentives. Enables integration of distributed renewables and EVs. Microgrids: self-sufficient local grids that can disconnect from main grid (island mode) during outages.
Four-stroke cycle (Otto cycle): intake (piston down, fuel-air mixture in), compression (piston up, mixture compressed), power (spark ignites mixture, piston driven down), exhaust (piston up, combustion gases expelled). Diesel cycle: fuel ignites by compression heat (no spark plug); higher compression ratio → higher efficiency (~45% vs ~25% for gasoline). Engine efficiency limited by Carnot theorem. Emissions: CO, NOₓ, hydrocarbons, CO₂, particulates. Catalytic converter reduces CO and NOₓ.
Seebeck effect: when two dissimilar conductors are joined and junction temperatures differ, a voltage is produced. V = S·ΔT where S = Seebeck coefficient. Peltier effect (reverse): current through junction creates temperature difference (used in cooling modules). Figure of merit ZT = S²σT/κ (σ = electrical conductivity; κ = thermal conductivity); high ZT materials are good thermoelectrics. Applications: spacecraft radioisotope thermoelectric generators (RTGs), waste heat recovery, wearable electronics.
Wave energy converters (WECs): oscillating water column (OWC) traps air above water surface; wave compresses air through turbine. Point absorbers: floating buoy drives hydraulic pump. Overtopping devices: wave runs up ramp into reservoir, water falls through turbine. Tidal stream generators: underwater turbines (like wind turbines) in tidal currents; predictable energy output. Tidal barrage: dam across estuary captures tidal range; generates electricity on ebb and flow. Limited by suitable sites.
Carbon footprint: total GHG emissions caused by an individual, organization, event, or product (measured in CO₂-equivalent). Life cycle assessment (LCA): cradle-to-grave analysis of environmental impacts, including manufacturing, use, and disposal. Scope 1: direct emissions. Scope 2: purchased electricity. Scope 3: value chain (supply chain, product use). GWP (Global Warming Potential): compares warming effect of GHGs over 100 years relative to CO₂ (CH₄ = 28 GWP; N₂O = 265 GWP).
LCOE = total lifetime costs / total lifetime energy produced ($/MWh). Accounts for capital costs (CAPEX), operating costs (OPEX), fuel costs, and financing. Allows comparison across technologies. Solar PV and onshore wind now have LCOE competitive with or below fossil fuels in many regions (~$30–50/MWh). Nuclear: high capital costs despite low fuel costs (~$90–130/MWh). Natural gas: low capital, variable fuel costs. Discount rate strongly affects LCOE of capital-intensive technologies.
Net zero: achieving a balance between GHG emissions produced and removed from the atmosphere. Requires: deep decarbonization of energy, transport, and industry; carbon removal (forests, BECCS, direct air capture). Carbon neutrality: offsetting remaining emissions with carbon credits or removals. Paris Agreement (2015): limit warming to 1.5–2°C above pre-industrial levels; 195+ countries. IPCC reports summarize climate science and mitigation pathways. Carbon trading: cap-and-trade systems put a price on emissions.
PEM (proton exchange membrane): uses solid polymer electrolyte; operates at ~80°C; used in vehicles and portable power. SOFC (solid oxide fuel cell): operates at 700–1000°C; high efficiency; used in stationary power; can use various fuels (H₂, methane). Alkaline fuel cell (AFC): used in spacecraft (Apollo); uses KOH electrolyte. Phosphoric acid fuel cell (PAFC): first commercial fuel cell. MCFC (molten carbonate): high temperature; suitable for industrial use. Efficiency typically 40–60%.
Pumped hydro storage (PHS): 90% of global grid storage capacity; pumps water uphill during low-demand periods, releases through turbines when needed. Round-trip efficiency ~70–85%. Flywheel energy storage: kinetic energy stored in rotating mass; charge/discharge in seconds; high power density, low energy density; used for frequency regulation. Compressed air energy storage (CAES): stores air in underground caverns under pressure; efficiency ~70%. All address intermittency of solar and wind.
First-generation PV: monocrystalline silicon (highest efficiency, ~22–26%), polycrystalline silicon (~17–20%). Second-generation thin films: CdTe (cadmium telluride, ~18%), CIGS (copper indium gallium selenide, ~23%). Third-generation: perovskite solar cells (rapidly improving, >29% in lab); multi-junction concentrator cells (>47% in lab). Shockley-Queisser limit: theoretical max efficiency for single-junction solar cell ~33%. Agrivoltaics: combining solar panels with agriculture.
Energy conservation for thermodynamic systems: ΔU = Q − W. ΔU = change in internal energy; Q = heat added to system (positive when absorbed); W = work done by system (positive when system expands). For ideal gas: U depends only on temperature (ΔU = nCᵥΔT). Isothermal process: ΔT = 0, ΔU = 0 → Q = W. Adiabatic: Q = 0 → ΔU = −W. Isochoric (constant volume): W = 0 → ΔU = Q. Isobaric (constant pressure): W = PΔV.
Piezoelectric effect: certain crystals (quartz, PZT) generate voltage when mechanically stressed; used in sensors, lighters, ultrasound, and energy harvesting from vibrations/footsteps. Magnetohydrodynamic (MHD) generators: conducting fluid (plasma or liquid metal) flows through magnetic field → induces EMF (no moving parts). Used experimentally in nuclear fusion containment and propulsion. Triboelectric nanogenerators: harvest energy from friction between surfaces.
Zeroth Law: if system A is in thermal equilibrium with B, and B is in equilibrium with C, then A is in equilibrium with C — establishes temperature as a measurable property. Celsius (°C): 0 = ice point, 100 = steam point at 1 atm. Kelvin (K): absolute scale, 0 K = absolute zero (−273.15°C); no negative values. Fahrenheit (°F): °F = (°C × 9/5) + 32. Rankine (°R): absolute Fahrenheit scale. Triple point of water: 273.16 K = 0.01°C (defines kelvin).
Energy exists in many forms: kinetic (motion), gravitational potential (stored due to height), elastic potential (stored in springs/rubber), thermal (heat, related to temperature), chemical (stored in chemical bonds; released by combustion or metabolism), electrical (moving charges), radiant/light (electromagnetic waves), sound (mechanical vibrations), nuclear (stored in atomic nuclei).
Energy can change from one form to another but is never created or destroyed (Law of Conservation of Energy). Examples: a light bulb converts electrical energy to light and heat; a car engine converts chemical energy to kinetic energy and heat; a solar panel converts radiant energy to electrical energy. Some energy is always lost as heat in any transformation.
Renewable energy sources can be replenished naturally in a short time: solar (sunlight), wind, hydroelectric (flowing water), geothermal (Earth's heat), biomass (organic matter). Non-renewable sources form over millions of years and will eventually run out: coal, oil, natural gas (fossil fuels), and nuclear (uranium). Fossil fuels release CO₂ when burned, contributing to climate change.
Fossil fuels — coal, oil (petroleum), and natural gas — formed from the remains of ancient plants and animals over millions of years under heat and pressure. Coal is burned in power plants to generate electricity. Oil is refined into gasoline, diesel, and other fuels. Natural gas (mostly methane, CH₄) is the cleanest-burning fossil fuel. All release CO₂ when burned.
Solar energy comes from the Sun as light and heat. Solar panels (photovoltaic cells) convert sunlight directly to electricity. Solar water heaters use sunlight to heat water. Wind energy: wind turbines use blades to capture wind's kinetic energy and drive a generator to produce electricity. Both are renewable, produce no direct emissions, and are increasingly cost-competitive.
Conduction: heat transfers through direct contact between molecules (metal rod in a flame). Convection: heat transfers through the movement of fluids (warm air rises, cool air sinks; boiling water). Radiation: heat transfers as electromagnetic waves without needing matter (heat from the Sun, warmth from a campfire). Insulators slow conduction; reflectors reduce radiation absorption.
Most electricity is generated by spinning a turbine connected to a generator. Turbines can be spun by steam (heated by burning fossil fuels, nuclear fission, or geothermal heat) or by moving water (hydroelectric) or wind. Generators work by electromagnetic induction — rotating a coil in a magnetic field induces a current. The electricity is then transmitted over power lines.
The greenhouse effect: sunlight warms Earth's surface; Earth radiates heat (infrared) back upward; greenhouse gases (CO₂, water vapor, methane) absorb and re-radiate this heat, warming the lower atmosphere. The natural greenhouse effect makes Earth habitable. Human activities (burning fossil fuels, deforestation) increase CO₂ levels, enhancing the greenhouse effect and raising global temperatures.
All living things need energy to survive. Producers (plants, algae) capture sunlight energy via photosynthesis and store it as chemical energy in glucose. When organisms eat food, cellular respiration breaks down glucose to release ATP (energy that cells can use). Energy flows through ecosystems in a food chain: sun → producers → primary consumers → secondary consumers → tertiary consumers. Only about 10% of energy transfers between each trophic level (the rest is lost as heat). Energy cannot be recycled — unlike matter, it flows in one direction through ecosystems.
Coal formed from ancient swamp plants (mainly ferns and mosses) that were buried under sediment ~300 million years ago during the Carboniferous period; heat and pressure transformed them over millions of years. Types by carbon content: lignite (lowest, 'brown coal'), sub-bituminous, bituminous (most common), anthracite (highest, hardest). Used mainly for electricity generation and steel production (coke from coal is used in blast furnaces). Burning coal releases CO₂, SO₂ (acid rain), NOₓ, mercury, and particulate matter. World's largest coal reserves: USA, Russia, Australia, China.
Oil (petroleum) and natural gas formed from marine organisms (plankton, algae) buried under ocean sediments millions of years ago; heat and pressure converted organic matter to hydrocarbons. Trapped in porous rock beneath impermeable cap rock (petroleum traps). Extracted by drilling; offshore oil rigs drill under the ocean. Oil refining: crude oil is heated and separated by boiling point (fractional distillation) into gasoline, diesel, jet fuel, kerosene, lubricants, plastics. Natural gas (mainly methane, CH₄) is the cleanest-burning fossil fuel but is a potent greenhouse gas if released unburned (methane leaks).
The Sun releases energy through nuclear fusion (hydrogen → helium). Solar energy reaches Earth as electromagnetic radiation (mainly visible light and infrared). Passive solar: designing buildings to capture sunlight for heating (south-facing windows, thermal mass). Active solar: solar water heaters use the Sun to heat water in panels. Photovoltaic (PV) cells: semiconductor materials convert sunlight directly into electricity; no moving parts; produce DC current (inverter converts to AC). Solar farms: large arrays of panels; increasingly cost-competitive. Concentrated solar power (CSP): mirrors focus sunlight to heat fluid and drive a steam turbine.
Wind is caused by uneven heating of Earth's surface by the Sun → creates pressure differences → air flows from high to low pressure. Wind turbines: three blades (aerodynamic design — similar to airplane wings) capture wind's kinetic energy; blades spin a shaft connected to a generator; produces electricity. Onshore vs offshore wind (stronger, more consistent offshore but more expensive to build). A typical large wind turbine can power ~500 homes. Wind farms: many turbines in an area with consistently strong winds. Wind does not produce any air or water pollution during operation. Intermittent — doesn't blow all the time.
Hydroelectric power uses the energy of falling or flowing water to generate electricity. A dam stores water in a reservoir (creating potential energy). Water flows through intake pipes → spins turbine → drives generator → produces electricity. The height the water falls (head) and the flow rate determine power output. Largest renewable electricity source globally (~16% of world electricity). Advantages: no fuel costs, no air emissions, long-lasting, can store energy (pumped storage). Disadvantages: floods habitats, blocks fish migration, displaces communities, dependent on rainfall. Three Gorges Dam (China) is the world's largest hydroelectric station.
Reducing energy use saves money and reduces environmental impact. Lighting: LED bulbs use ~75% less electricity than incandescent bulbs and last much longer. Insulation: prevents heat loss in winter and heat gain in summer (walls, attic, double-glazed windows). Thermostat: programmable thermostats reduce heating/cooling when not needed. Appliances: ENERGY STAR-rated appliances are more efficient. Phantom load/standby power: electronics use electricity even when 'off' — unplugging reduces waste. Transportation: walking, cycling, carpooling, public transit reduce fuel consumption. Renewable energy: rooftop solar panels can supply home electricity.
Geothermal energy comes from heat inside Earth, generated by radioactive decay of elements and residual heat from Earth's formation. In geothermal hot spots (Iceland, Yellowstone, Kenya), hot water or steam can be accessed by drilling. Geothermal power plants: steam from hot water underground drives turbines to generate electricity. Geothermal heat pumps: even in non-volcanic areas, ground temperature stays ~10–15°C year-round; heat pumps use this to heat buildings in winter and cool in summer. Advantages: reliable (baseload — produces power 24/7), low emissions, small land footprint. Disadvantages: location-specific.
Biomass: organic material from plants and animals that can be burned or converted to fuel. Direct combustion: burning wood, crop waste, or animal dung for heat or electricity (oldest energy source). Biogas: decomposition of organic waste (manure, food scraps) by bacteria without oxygen (anaerobic digestion) produces methane → can be burned for heat/electricity. Ethanol: fermented from sugars/starches (sugarcane, corn) → blended with gasoline (E10, E85). Biodiesel: made from vegetable oils or animal fats → used in diesel engines. Biomass is considered renewable but still releases CO₂ when burned (net neutral if sustainably managed).
A battery converts chemical energy to electrical energy. Consists of one or more electrochemical cells, each with: anode (negative terminal, where oxidation occurs), cathode (positive terminal, where reduction occurs), and electrolyte (allows ions to flow between electrodes). Primary batteries: not rechargeable (alkaline, zinc-carbon — used in TV remotes, flashlights). Secondary batteries: rechargeable — chemical reaction reversed by applying current (lithium-ion used in phones/laptops/EVs; lead-acid used in cars; nickel-metal hydride). Voltage: energy per unit charge; larger batteries don't have higher voltage — they have more capacity (amp-hours).
Energy is measured in Joules (J) in the SI system. 1 calorie (cal) = 4.18 J (amount of heat to raise 1 g of water by 1°C). Food energy uses Calories (Cal) = kilocalories (kcal) = 1000 cal. Electricity is billed in kilowatt-hours (kWh): 1 kWh = 3,600,000 J. Power (rate of energy use) measured in Watts (W): 1 W = 1 J/s. A 60W light bulb uses 60 J every second. 1 kilowatt (kW) = 1,000 W. 1 megawatt (MW) = 1,000,000 W. Example: a typical home uses about 900 kWh per month. Horsepower (hp): 1 hp = 746 W.
Electric motor: converts electrical energy to mechanical (kinetic) energy. Works because current-carrying wire in a magnetic field experiences a force (motor effect). Used in fans, washing machines, electric vehicles, tools. Generator: converts mechanical energy to electrical energy. Works by electromagnetic induction — rotating a coil of wire in a magnetic field induces a current (Faraday's Law). Power plants use generators. Dynamo: simple DC generator. AC generators (alternators) produce alternating current used in homes. Motors and generators are essentially the same device working in reverse.
Transformers change the voltage of alternating current (AC). Step-up transformer: increases voltage, decreases current (used at power plants before long-distance transmission). Step-down transformer: decreases voltage, increases current (used at substations before homes). Electricity is transmitted at high voltage (e.g., 230,000 V) to minimize energy loss in wires (P_loss = I²R; high voltage → low current → less heat loss). Home voltage: typically 120 V (USA) or 230 V (Europe). Transformers only work with AC, not DC. Ratio of coil turns determines voltage change: Vp/Vs = Np/Ns.
Transportation uses about 28% of energy in developed countries. Cars: internal combustion engines (gasoline, diesel) are ~20-25% efficient — most energy lost as heat. Electric vehicles: ~77% efficient; zero direct emissions but depend on electricity grid. Hybrid vehicles: combine gasoline engine and electric motor; use regenerative braking. Fuel efficiency measured in miles per gallon (mpg) or liters per 100 km. Airplanes are very fuel-intensive per trip but similar per passenger-kilometer to driving alone. Trains are most energy-efficient for passengers and freight. Shipping large quantities by sea is very efficient per ton-km.
Thermal conductors: materials that allow heat to transfer easily; high thermal conductivity; metals (copper, aluminum, iron) are good conductors (loosely bound electrons carry heat). Thermal insulators: materials that resist heat transfer; low conductivity; wood, plastic, fiberglass, air gaps, wool, styrofoam. Air is an excellent insulator (low thermal conductivity) — this is why double-paned windows and down jackets work well. Electrical conductors: metals. Electrical insulators: rubber, plastic, glass. Semiconductors: conduct under some conditions (silicon) — used in solar cells and electronics. R-value: measures insulation effectiveness (higher = better).
Photosynthesis is the process by which plants, algae, and cyanobacteria convert light energy into chemical energy stored as glucose. The Sun's energy drives the reaction: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. Glucose stores chemical energy that can be released later by cellular respiration (in the same plant) or by the animals that eat the plant. Fossil fuels are essentially ancient photosynthesis energy — they are the compressed remains of organisms that stored solar energy millions of years ago. This makes fossil fuels a form of stored solar energy.
Carbon cycles through the atmosphere, biosphere, hydrosphere, and lithosphere. In terms of energy: plants absorb CO₂ and store carbon as organic compounds (powered by sunlight). Animals eat plants, releasing some carbon as CO₂ through respiration. Decomposers break down dead matter, releasing CO₂. Combustion of fossil fuels releases carbon stored millions of years ago rapidly into the atmosphere. Oceans absorb about 25% of human CO₂ emissions (becoming more acidic). The carbon cycle is closely linked to energy flow because almost all organic energy-rich molecules contain carbon.
Heating systems: furnaces (burn natural gas, oil, or propane to heat air); heat pumps (move heat from outside to inside, even in cold weather — very efficient); electric resistance heating (least efficient); radiant floor heating (hot water pipes in floor). Cooling systems (air conditioning): use a refrigeration cycle — refrigerant absorbs heat inside the building, compressed and cooled outside, releasing heat outside; fans circulate cooled air. COP (coefficient of performance): heat pumps and AC efficiency measure — higher COP = more heat moved per unit electricity. Proper insulation reduces both heating and cooling energy needs.
Incandescent bulbs: heat tungsten filament until it glows; only ~5% of energy becomes light, 95% is heat — very inefficient. CFL (compact fluorescent lamp): uses mercury vapor and phosphor coating to produce light; ~4× more efficient than incandescent. LED (light-emitting diode): semiconductors emit light when current flows through a p-n junction; ~7–10× more efficient than incandescent, lasts 15–25× longer; contains no mercury. Lumens measure light output; watts measure power consumption. Switching all incandescent bulbs to LEDs could reduce global electricity use for lighting by ~50%.
Nuclear power plants use nuclear fission to generate electricity. Uranium-235 atoms split when struck by neutrons, releasing enormous energy and 2–3 more neutrons (chain reaction). In a reactor, the chain reaction is controlled (moderator and control rods). Heat produced boils water → steam → spins turbine → generates electricity. Nuclear power produces no CO₂ during operation (low-carbon electricity source). Concerns: nuclear waste (radioactive for thousands of years), risk of meltdown (Chernobyl 1986, Fukushima 2011), high cost of construction, uranium mining impacts. Provides about 10% of world's electricity.
Energy production and use have major environmental impacts. Burning fossil fuels: releases CO₂ (climate change), SO₂ (acid rain), NOₓ, mercury, and particulates (health impacts). Oil spills: damage marine ecosystems. Coal mining: strip mining destroys landscapes; mountaintop removal. Natural gas: methane leaks (powerful greenhouse gas). Renewable energy impacts: wind turbines (bird/bat strikes, noise, visual impact), solar panels (land use, manufacturing emissions), hydroelectric dams (habitat flooding, fish migration). Nuclear waste: long-term radioactive storage. Energy efficiency and renewables are key to reducing environmental damage.
A country's electricity generation mix describes the sources used to produce electricity. Globally: coal (~36%), natural gas (~23%), hydroelectric (~16%), nuclear (~10%), wind (~7%), solar (~4%), other renewables and oil. Countries with high renewable shares: Iceland (nearly 100% geothermal + hydro), Norway (nearly 100% hydro), Denmark (>50% wind). USA mix: natural gas (~40%), coal (~20%), nuclear (~19%), renewables (~21%). Transitioning to cleaner sources reduces emissions. Grid reliability requires matching supply and demand at all times; storage and diverse sources help.
The Law of Conservation of Energy states that energy cannot be created or destroyed — it can only be converted from one form to another or transferred between objects. The total energy in a closed system always remains constant. Examples: a falling ball converts PE to KE (total stays constant); burning wood converts chemical energy to heat and light; food energy converts to kinetic energy when you run. In all real processes, some energy is converted to thermal energy (heat) that is hard to use further. This is why perpetual motion machines are impossible — they would violate conservation of energy.
Tides are caused by the gravitational pull of the Moon (and Sun) on Earth's oceans, creating predictable rises and falls in sea level twice a day. Tidal energy: harness this movement using tidal barrages (dam across an estuary; water flows through turbines as tide rises and falls) or tidal stream generators (underwater turbines in fast-moving tidal currents). Wave energy: ocean waves carry kinetic and potential energy from wind; wave energy converters (buoys, oscillating columns) capture this energy. Both are renewable and predictable. Limited by suitable geographic locations. Currently a small fraction of global energy.