What Is Energy?
A car races by. A light is switched on. Sound filters out from speakers. These are all forms of energy-motion, light, and sound. Energy is the ability to make things happen. Every action is connected to energy in one form or another. Objects can have energy due to their movement or their position.
Scientists measure energy in joules (J). It takes about one joule for a person to lift an apple one meter off the ground. Eating the apple provides the human body with about 250,000 J. Every form of energy, including movement, stored energy, heat, and light, can be measured in joules.
Kinetic energy is the energy of an object’s or particle’s motion. The amount of kinetic energy depends on two things: the object’s mass and how fast the object is moving. The amount of kinetic energy (KE) in joules that an object has is determined by the equation , here m equals the mass of the object in kilograms and v equals its velocity in meters per second. Energy can be expressed in units of joules, where 1 joule is 1 (kg•m2)/2.
If two objects have equal mass, the object that is moving faster has more kinetic energy. The diagram at the top of the next page shows calculations for the kinetic energy for three vehicles. Notice that Car Band Car C have equal mass, but Car C has more kinetic energy than Car B because it is moving faster. If two objects are moving at the same speed, the object with more mass has more kinetic energy than the object with less mass. As shown in the diagram, Truck A and Car B are traveling at the same speed. However, Truck A has more kinetic energy than Car B because Truck A has more mass. Notice that a change in speed affects energy more than a change in mass. If a car doubles its speed, its kinetic energy increases by a factor of four. A truck with four times the mass of a car has four times as much energy as the car when they travel at the same speed.
Potential energy is energy an object has due to its position. Potential energy does not involve motion, it is dependent on the interaction between two objects and the forces involved. This is considered stored energy.
Consider a book on a table. Together, the book and Earth have potential energy. Gravitational potential energy is the energy resulting from the gravitational forces between two objects. Raising an object above the ground increases the gravitational potential energy because work has been done on the object against the force of gravity. Gravity is a force that is described as a field, meaning there is a region in space that has this force at every point.
Gravitational potential energy (GPE) is related to the mass and height of the object, and acceleration due to the gravitational field. This can be expressed as GPE = mgh, where m is the mass of the object in kilograms, g is the acceleration due to gravity in meters per second squared (9.80 m/s2 near Earth’s surface), and his the height the object is raised in meters.
If you lift a 2.00 kilogram book to a shelf 1.20 meters above the floor, what is the change in potential energy in joules? The gravitational field of the book is not considered because the field of an object only affects other objects.
The gravitational potential energy will increase by 23.5 J.
Potential energy can also be affected by other field forces between objects. For example, a magnetic field exerts a force on a paper clip that pulls the paper clip toward a magnet. The field forces around charged particles are called electrical fields. When a charged particle or object is moved a distance against the force of this field, its electric potential increases.
Think about Science
Directions: Answer the following questions.
- A leaf falls from a tree to the ground. At what point is the gravitational potential energy the greatest? A. while the leaf is still attached to the branch B. after the leaf has fallen a short distance C. when the leaf is about half way to the ground D. when the leaf hits the ground
Types of Energy
You constantly use energy in your daily activities. When you turn on the lights or heat food in a microwave, you know you are using some type of energy. In other cases, your interactions with energy are less obvious. When you are sleeping, your body is using energy to maintain your internal temperature, breathe, digest food, and repair injured cells. There are many types of energy that constantly do work and cause changes around you.
The mechanical energy of an object is the sum of its kinetic energy and its potential energy. As shown below, when the roller coaster is at the top of the hill, all of its energy is stored as gravitational potential energy. When the cars travel down the hill, their kinetic energy increases and the gravitational potential energy decreases by an equal amount. Not counting friction, the mechanical energy remains the same throughout the entire ride as the cars move up and down the hills of the roller coaster.
If you throw a ball in the air, the kinetic energy of its upward motion will decrease as gravitational potential energy increases. When the ball has reached its highest height, it has no kinetic energy at all. As the ball falls back to the ground, kinetic energy again increases and gravitational potential energy decreases by an equal amount. Overall the total amount of mechanical energy does not change as the ball moves from one position to another.
While it may not be obvious, every piece of matter around you is full of energy. The atoms and molecules that make up matter are always interacting. Liquid or gas particles flow from place to place, and even the molecules of a solid constantly vibrate. Thermal energy is the sum of the kinetic energy and the potential energy of the particles that make up matter.
Thermal energy can be detected when it flows from one object to another as heat. Faster-moving particles have more kinetic energy than slower moving particles. When the particles collide, energy transfers from the faster particles to the slower particles. When you touch a hot pan, some of the energy of the rapidly vibrating metal atoms is transferred to your hand, in which atoms are moving more slowly. Wind is generated when heat is transferred from areas of the atmosphere with greater thermal energy to other regions with less thermal energy.
When a match is struck, it emits light, sound, and thermal energy. All this energy had been stored in the match in the form of chemical energy. Chemical energy is the potential energy stored in the bonds between the atoms of a substance. The sources of this stored energy are the electromagnetic force fields of the charged particles that make up the atoms. Interactions among these fields provide energy that can be released during a chemical reaction.
Chemical energy is the source of most of the energy humans need to function. Plants store chemical energy in carbohydrates formed during photosynthesis. Humans release this chemical energy during digestion and use it to power systems inside the body.
The nuclei of atoms contain a tremendous amount of potential energy. Energy stored in the nucleus of an atom is called nuclear energy. Nuclear energy holds the particles of the nucleus together. It can be released when nuclei are combined, which occurs in reactions in the Sun. Nuclear energy can also be released when nuclei are split apart, which occurs in nuclear reactors on Earth. One kilogram of uranium used as fuel in a nuclear power plant produces the same amount of energy as 14,000 kilograms of coal burned in a coal-fired power plant.
Radiant energy is emitted from a source as waves. These waves carry energy from the Sun through the vacuum of space to Earth. Radiant energy is a form of kinetic energy. In addition to the light we can see, radiant energy includes radio waves, microwaves, infrared radiation, ultraviolet radiation, gamma rays, and X-rays.
Think about Science
Directions: Fill in the blank.
- Turning on a flashlight releases [ blank ] energy.
- Thermal energy is the sum of the [ blank ] and potential energy of the particles in an object.
- Energy stored in the nucleus of an atom is [ blank ] energy.
- Chemical energy is a type of potential energy stored in [ blank ].
Electrical Energy and Magnetic Energy
Some objects can affect other objects from a distance due to a force field that exists around the them. A force field is a push or pull exerted in a region around the object producing it. Electrical energy and magnetic energy are both the result of fields. These forms of energy are related to one another.
You may have experienced a shock after walking across a carpet and then touching a metal object, such as a doorknob. The shock comes from a transfer of electric charge. There are two types of electric charge: positive and negative. Two charges that are alike repel one another, and two charges that are different attract one another. Electrons are negatively charged atomic particles that naturally repel each other through the interaction of the electric fields that surround each electron. Electrical potential energy is the result of the positions of the charged particles within the electric fields. The friction of shuffling feet on a carpet rubs electrons from the carpet onto the feet. This buildup of charge generates electrical potential energy. Potential energy becomes kinetic energy when a static shock carries the charges toward positive charges located on the doorknob. It would be reasonable to expect, or anticipate, a static shock when you shuffle your feet on a carpet.
Electrons in a circuit have both kinetic and potential energy. When the electrons travel through a closed path, or electric circuit, some of its kinetic and potential energy can be changed to other forms of energy, such as light, thermal energy, or sound. Electrical energy powers many appliances and machines at home and at work.
A magnet produces a force that can attract or repel other magnets and can attract certain other substances. You can feel this by holding two magnets near each other. Depending on how you hold the magnets, you can feel them push or pull on each other. This push or pull is due to the force of a magnetic field. The field is produced by moving electrons as in atoms. The magnetic field is exerted in a region surrounding the magnet, and it is strongest close to the magnet. The magnetic field stretches between two magnetic poles, which are regions where the magnetic field exerted by a magnet is the strongest. The north and south poles are at opposite ends of a bar magnet.
When two magnets are brought close together, their magnetic fields interact with each other. As shown here, the north pole of one magnet will repel the north pole of another magnet. South poles also repel each other. The north pole of one magnet and the south pole of another magnet, however, attract each other and stick together.
Think about Science
Directions: Answer the following question.
- Earth has a magnetic field that extends from its North and South Magnetic Poles. When a compass is allowed to line up with Earth’s magnetic field, the end labeled with an N points toward the magnetic North Pole. Based on this observation, if Earth’s North Pole were a labeled magnet, should it be labeled as N or as S?
Conservation of Energy
People often talk about energy as if it were used up or lost during an activity. However, the energy still exists, just in a form that may not be obvious. The law of conservation of energy states that energy can be changed in form but it cannot be created or destroyed.
It can take some detective work to follow the path of energy as it changes forms. For example, it takes a lot of energy to run a race. After the race, the runner’s body has less energy than it had before. The chemical energy used to power muscles has been converted into kinetic energy and thermal energy. The total amount of energy in the universe is the same after the race as it was before.
We observe changes in energy all the time. For example, when an object moves against gravity, some of its kinetic energy is transformed to potential energy. An electric circuit that includes a light bulb transforms electrical energy into radiant energy and thermal energy. The change of one form of energy to another form of energy is called energy transformation.
Energy can change forms in many different ways. Energy conversions occur continually in living things. The human body provides many examples of energy transformations. The body takes in chemical potential energy in the form of food. The food is transformed into other chemicals in the digestive system. Sugars provide chemical energy for bodily functions, and fats store potential energy for future use. The heart and other muscles convert chemical energy to kinetic energy as blood circulates and the body moves. Some of the body’s energy is transformed to sound. The body releases thermal energy in the form of heat. Nerves use electrical energy to communicate within the body.
Plants transform radiant energy into chemical energy. Electric eels transform chemical energy into electrical energy. Running deer transform chemical energy into kinetic energy. The table includes more examples of energy transformations.
|Initial Energy Type||Final Energy Type||Example|
|Chemical||Radiant and thermal||Combustion|
|Radiant||Thermal||Absorption of sunlight|
|Thermal energy||Kinetic energy||Steam turbine|
Think about Science
Directions: Answer the following question.
- What energy transformation occurs in a toaster? A. kinetic energy to electrical energy B. electrical energy to thermal energy C. magnetic energy to electrical energy D. potential energy to kinetic energy