# 15.1: Our Sun, a Giant Nuclear Power Plant

Learning Objectives

• Define energy and distinguish between kinetic and potential enregy.
• Describe photosynthesis
• Perform calculations involving power and energy.

The Sun currently fuses about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result. Sunlight at the top of Earth's atmosphere is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light. The atmosphere in particular filters out over 70% of solar ultraviolet, especially at the shorter wavelengths. Solar ultraviolet radiation ionizes Earth's dayside upper atmosphere, creating the electrically conducting ionosphere.The energy of sunlight supports almost all life[c] on Earth by photosynthesis, and drives Earth's climate and weather.

Energy can be defined as the capacity to supply heat or do work. One type of work (w) is the process of causing matter to move against an opposing force. For example, we do work when we inflate a bicycle tire—we move matter (the air in the pump) against the opposing force of the air surrounding the tire. Energy is measured in SI units, in joules (J). The Joule (= 1 Newton-meter) is a very small unit, however it is equivalent to the amount of work of lifting a small apple (98 grams) vertically up one meter. In terms of heat,

$$\mathrm{1\: cal = 4.184\: J}$$

The rate of work done or energy transfer is called power, and its unit is watt.

$$\mathrm{1\: watt = 1\: J/s}$$

Your calculator may consume some milliwatt, and a computer consumes about 100 watt, as does a 100-W light bulb. The kilowatt (kW) is equal to one thousand (103) watts. This unit is typically used to express the output power of engines and the power of electric motors, tools, machines, and heaters. It is also a common unit used to express the electromagnetic power output of broadcast radio and television transmitters. Different examples of power are given in Table $$\PageIndex{1}$$.

One kilowatt is approximately equal to 1.34 horsepower. A small electric heater with one heating element can use 1.0 kilowatt. The average electric power consumption of a household in the United States is about one kilowatt.

Table $$\PageIndex{1}$$ Power in Kilowatts of Various Objects or Phenomenon.
Object or Phenomenon Power in kilowatts (kW)
Supernova (at peak) $$5 \times 10^{34}$$
Milky Way galaxy $$10^{34}$$
Crab Nebula pulsar $$10^{25}$$
The Sun $$4 \times 10^{23}$$
Volcanic eruption (maximum) $$4 \times 10^{12}$$
Lightning bolt $$2 \times 10^{9}$$
Nuclear power plant (total electric and heat transfer) $$3 \times 10^6$$
Aircraft carrier (total useful and heat transfer) $$10^5$$
Dragster (total useful and heat transfer) $$2 \times 10^3$$
Car (total useful and heat transfer) $$8 \times 10^1$$
Football player (total useful and heat transfer) $$5$$
Clothes dryer $$4$$
Typical incandescent light bulb (total useful and heat transfer) (60W) $$0.06$$
Electric clock $$0.003$$
Pocket calculator $$10^{-6}$$

Example $$\PageIndex{1}$$ Power and Energy Conversion

Calculate the electrical energy, in joules (J) , that is used by a 20W LED bulb that was left "on" for 3.0 hours.

Solution

1. Since 1 W = 1 J/s then 20W = 20 J/s.

2. Convert 3 h to s.

3. Calculate the amount in J for 10,800 s.

Exercise $$\PageIndex{1}$$

Calculate the electrical energy, in joules (J) , that is used by a 100W incandescent bulb that was left "on" for 2 hours.

720,000 J

## Energy and the Life-Support System

Photosynthesis is essential to all life on earth; both plants and animals depend on it. It is the only biological process that can capture energy that originates in outer space (sunlight) and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism. In brief, the energy of sunlight is captured and used to energize electrons, which are then stored in the covalent bonds of sugar molecules. How long lasting and stable are those covalent bonds? The energy extracted today by the burning of coal and petroleum products represents sunlight energy captured and stored by photosynthesis almost 200 million years ago.

Photosynthesis is a multi-step process that requires sunlight, carbon dioxide (which is low in energy), and water as substrates (Figure $$\PageIndex{1}$$). After the process is complete, it releases oxygen and produces simple carbohydrate molecules (which are high in energy) that can subsequently be converted into glucose, sucrose, or any of dozens of other sugar molecules. These sugar molecules contain energy and the energized carbon that all living things need to survive. Figure $$\PageIndex{1}$$ Photosynthesis uses solar energy , carbon dioxide, and water to produce energy-storing carbohydrates. Oxygen is generated as a waste product of photosynthesis.

The chemical equation for photosynthesis is given in Figure $$\PageIndex{2}$$. Although the equation looks simple, the many steps that take place during photosynthesis are actually quite complex. In reality, the process takes place in many steps involving intermediate reactants and products. Figure $$\PageIndex{2}$$ The basic equation for photosynthesis.

## Kinetic and Potential Energy

Like matter, energy comes in different types. One scheme classifies energy into two types: potential energy, the energy an object has because of its relative position, composition, or condition, and kinetic energy, the energy that an object possesses because of its motion. Water at the top of a waterfall or dam has potential energy because of its position; when it flows downward through generators, it has kinetic energy that can be used to do work and produce electricity in a hydroelectric plant (Figure $$\PageIndex{3}$$). A battery has potential energy because the chemicals within it can produce electricity that can do work. Figure $$\PageIndex{3}$$ The (a) Water that is higher in elevation, for example, at the top of Victoria Falls, has a higher potential energy than water at a lower elevation. As the water falls, some of its potential energy is converted into kinetic energy. (b) If the water flows through generators at the bottom of a dam, such as the Hoover Dam shown here, its kinetic energy is converted into electrical energy. (credit a: modification of work by Steve Jurvetson; credit b: modification of work by “curimedia”/Wikimedia commons).

Potential energy is not only associated with the location of matter, but also with the structure of matter. Even a spring on the ground has potential energy if it is compressed; so does a rubber band that is pulled taut. On a molecular level, the bonds that hold the atoms of molecules together exist in a particular structure that has potential energy. The fact that energy can be released by the breakdown of certain chemical bonds implies that those bonds have potential energy. In fact, there is potential energy stored within the bonds of all the food molecules we eat, which is harnessed for use. The type of potential energy that exists within chemical bonds, and is released when those bonds are broken, is called chemical energy. Chemical energy is responsible for providing living cells with energy from food. The release of energy occurs when the molecular bonds within food molecules are broken.

## Summary

• Energy can be defined as the capacity to supply heat or do work and is measured in SI units, in joules (J)
• Photosynthesis is essential to all life on earth and it is the only biological process that can capture energy that originates in outer space (sunlight) and convert it into chemical compounds (carbohydrates) that every organism uses to power its metabolism.
• Energy comes in two fundamentally different forms – kinetic energy and potential energy.
• Kinetic energy is the energy of motion.
• Potential energy is stored energy that depends on the position of an object relative to another object.