The Sun: Our Energy Source

Solar Radiation, Photosynthesis, and Earth's Energy Balance

The Sun is the primary source of energy for almost every process at Earth's surface. Sunlight warms land, oceans, and air, making weather, climate, and the water cycle possible. Plants capture solar energy to make food, which supports most ecosystems. Even many energy resources humans use today, such as wind, hydropower, and fossil fuels, are linked to solar input.

Solar energy reaches Earth as electromagnetic radiation, mainly visible light, infrared, and some ultraviolet. When this energy is absorbed, it can heat matter, drive evaporation, and power photosynthesis. Uneven heating of Earth's curved surface creates pressure differences that move air and water around the planet. Some incoming sunlight is reflected back to space, while the rest is absorbed and later re-emitted as infrared radiation.

Key Facts

Solar energy reaches Earth by radiation, so it does not need matter to travel through space.
Average solar energy at the top of Earth's atmosphere is about 1361 W/m^2.
Absorbed sunlight can be transformed into heat, chemical energy, and motion in Earth systems.
Photosynthesis stores solar energy: 6CO2 + 6H2O + light -> C6H12O6 + 6O2.
Uneven solar heating helps create winds because warm air rises and cool air sinks.
Earth's energy balance can be summarized as incoming solar energy = reflected energy + emitted infrared energy.

Vocabulary

Radiation: The transfer of energy by electromagnetic waves through space or matter.
Photosynthesis: The process by which plants, algae, and some bacteria use sunlight to make sugars from carbon dioxide and water.
Albedo: Albedo is the fraction of incoming sunlight that a surface reflects.
Infrared radiation: Infrared radiation is energy emitted as heat by Earth and other objects.
Energy transfer: Energy transfer is the movement of energy from one object, place, or system to another.

Common Mistakes to Avoid

Thinking the Sun heats Earth mainly by conduction or convection, which is wrong because space is mostly empty and solar energy travels to Earth by radiation.
Assuming all solar energy is absorbed by Earth, which is wrong because clouds, ice, and other bright surfaces reflect part of the incoming sunlight back to space.
Believing fossil fuels are unrelated to the Sun, which is wrong because they formed from ancient organisms that originally stored solar energy by photosynthesis.
Saying the Sun drives only temperature, which is wrong because solar energy also powers winds, ocean circulation, evaporation, and food production in ecosystems.

Practice Questions

1 A solar panel receives 800 W/m^2 of sunlight over an area of 2.5 m^2. If the panel converts 20% of that energy into electricity, how much electrical power does it produce?
2 A plant stores 4500 J of chemical energy from sunlight in one day. If it captured 3% of the incoming solar energy, how much solar energy reached the plant that day?
3 Explain why uneven heating by the Sun can produce wind and affect weather patterns on Earth.