Introduction to Energy (College Board AP® Environmental Science): Study Guide

What is energy?

• Energy is the ability to do work or cause change

• Energy cannot be destroyed or created

  • This is the first law of thermodynamics

• Energy can be changed from one form to another, but the energy quality is less

  • This is the second law of thermodynamics

• Energy can take many forms, like heat, light, or motion

• The sun powers most living things on Earth

• At the sun's center, nuclear fusion converts nuclear energy into heat, making it incredibly hot like other stars

• Protons (hydrogen atom building blocks) begin their journey in the sun’s core

• Through fusion, protons combine to form helium, releasing energy

• Fusion in the sun's core maintains its high temperature and sends energy out into the solar system

• Energy is essential for life

  • Herbivores get energy from plants

  • Carnivores get energy from eating other animals

  • Plants absorb sunlight for energy through photosynthesis

• Organisms that eat dead matter break it down to release energy

• Energy is vital for society

  • It powers human activities and technology

  • We get energy from fossil fuels, nuclear power, and renewable sources like wind and solar energy

  • The sun’s energy and geothermal heat influence Earth's climate and environment

Forms of energy

• Energy comes in a number of forms:

  • Chemical energy is stored in a molecule's atomic bonds

  • Electron movement generates electrical energy

  • Electromagnetic energy flows at light speed in waves

  • Potential energy (a book on a table) and kinetic energy (a baseball flying through the air) are mechanical energies

  • Splitting or combining atoms releases nuclear energy

  • The movement and vibration of particles within a substance is thermal energy

Units of energy

Joules

• The joule (J) is the most common unit of energy worldwide

• It is the energy transferred when a force of one newton (N) moves an object one meter

• 1 joule is the energy needed to to keep a 1 W LED light bulb on for 1 second

• An average person sitting still produces about 100 joules of heat per second

British thermal unit (BTU or Btu)

• A BTU is the amount of heat necessary to raise one pound of water by 1 degree Fahrenheit (F)

• 1 BTU = 1,055 joules (J)

• 1 Btu is roughly equal to the energy released when a match is burned

• In 2023, the United States used around 94 quadrillion Btu of energy (source: U.S. Energy Information Administration 2025)

Horsepower (hp)

• 1 horsepower (hp) = 746 watts

• The measurement was based on a horse being able to lift 33,000 pounds a height of one foot in one minute

• James Watt used this to set a standard for how well his steam engines worked as a replacement for horses

Kilowatt (kW)

• A kilowatt (kW) measures how much power a specific appliance consumes while being used

• 1 kW = 1,000 W

• For example, a 1 kW electric heater consumes 1 kW of power when it is being used

• 1 watt (W) = 1 joule per second, so a 1 kW appliance uses 1,000 joules every second

Kilowatt hour (kWh)

• The kilowatt-hour (kWh) measures energy: the energy used by a 1 kW appliance running for 1 hour

• 1 kWh = 3.6 × 10⁶ joules (J)

Power vs energy

• Power is the rate at which work is done or energy is transmitted

  • Power can be generated but not saved

  • It is an immediate output

• Energy is the capacity to do the work

  • Energy can be saved but not generated

  • Energy is how much power is used over time

Power vs energy example

• Consider 100 lumps of coal, which store a fixed amount of energy

• Burning 1 lump at a time gives a low-powered fire that lasts a long time

• Burning 10 lumps at a time gives a fire with 10 times more power, but the coal runs out 10 times faster

• Burning all 100 lumps at once gives a very hot fire that runs out 100 times faster

• The total energy released is the same each time (100 lumps of coal); only the power (how fast it is released) changes

Electricity costs

• The kilowatt-hour (kWh) is the most common unit of electrical energy used in the U.S.

• In the U.S., 1 kilowatt-hour (kWh) of electricity costs about 12 cents

• Example: electric dryers draw 1,800 to 5,000 watts (1.8 to 5 kW) of power, so running one for an hour uses 1.8 to 5 kWh of energy (about 21 to 60 cents)