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Energy, Work, and Power

GED Science Practice Test: Energy

Energy is defined as the capacity to perform, or act of performing, work.  We measure energy in Joules, just like work.  Energy exists in several forms. All forms or energy fall into one of two categories: kinetic energy and potential energy. Kinetic energy is any kind of energy that is in motion. Moving water and wind are good examples of things with kinetic energy that are easy to see. Other types of kinetic energy include heat and electricity, which include motion at a molecular level, which cannot be seen by the naked eye. Potential energy is stored energy that is not in motion, but can be used to do work. Examples of potential energy are oil sitting in a barrel, or water in a lake in the mountains. This energy is referred to as potential energy, because if it were released, it would do work.

There are different forms of kinetic and potential energy, including heat, light, electrical, chemical, nuclear, mechanical, and radiant energy. The diagram below summarizes some of the main forms of energy.

Now we will examine some behaviors of a few common types of energy.

Transformation:  Energy can change from one form to another. A classic example of this is a ball falling through the air.  If you hold a ball, motionless, in the air, it has gravitational potential energy.  Due to its position above the ground, it has the capacity to do work on objects below it.  When you drop the ball, the gravitational energy (a type of kinetic energy) within it transforms into motion energy (a type of kinetic energy).

This diagram shows that (gravitational) potential energy is at its highest the farther from the ground the ball is.  The kinetic (motion) energy is at its highest the faster the ball is going (right before it hits the ground).  However, a ball can have more than one type of energy at one time—midway through the fall (maybe at 45m), approximately half of the gravitational potential energy in the ball has transformed into motion kinetic energy.  At this point, the ball might have 300J of potential (gravitational) energy and 300J of (motion) kinetic energy.

Some other common transformations are shown in the diagram below:

Transfers: The energy within a ball does not simply transform and change types.  The energy can also transfer, which means that it move from one object to the next.  Energy gets transferred from one object to another via a force.  (To review forces, see the previous lesson).  Continuing our example for the ball, the ball transfers energy to the ground when it hits.  The motion energy in the ball transfers to the floor.  When this happens, you can hear it (sound energy), and maybe feel it (floor vibrating).  Though you might not be able to tell, some of the motion energy that got transferred to the floor ends up as thermal energy through friction and the vibration of the floor.

The following diagram shows some transfers of energy involved in the operation of a lamp:

While energy can be transferred from one place to another, and transformed from one form to another, the total amount of energy in a closed system must remain constant.  A closed system is one in which energy cannot enter or leave.  This principle is known as the law of conservation of energy. The law of conservation of energy states that the total energy of a closed system cannot change—it is said to be conserved over time. Energy can be neither created nor destroyed.

The law of conservation of energy can be difficult to believe for a number of reasons.  Let us examine the ball bouncing idea again for a moment.  If you release a ball and watch it bounce, it will eventually come to a stop.  This seems to imply that the energy disappeared, which would contradict the law of conservation of energy.  However, we must remember that energy does not simple change back and forth from gravitational to motion energy.  Some energy leaves the ball and becomes thermal energy (both when it hits the floor, and a bit as it encounters air resistance); some energy leaves the ball and becomes sound energy; and some energy leaves the ball and enters the floor.  Each time the ball bounces, a bit of energy becomes less “useful” to the ball as it transforms and transfers.  However, at no point does the energy in the ball disappear.

The law of conservation is also difficult to believe because of how much we hear about conserving energy in the media and in educational settings.  We are told to turn off the lights to conserve energy.  If energy is always conserved, why do we need to worry about turning off the lights?  The answer is the same as for the ball; while the energy does not disappear, it changes to forms that are less useful to us.  Those forms can be less useful because they are dispersed, or spread out (like when thermal energy is generated through friction, etc.), or because they are difficult to obtain (fossil fuels for political and geological reasons).

This idea of the utility of different forms of energy can be represented with a Sankey diagram, which shows the relative amounts of different types of energy as it transfers and transforms in a system.  Sankey diagrams can also be said to show efficiency, which is measure of how well energy gets changed into useful forms.  The following is a Sankey diagram for our lamp example, above:

For a particular lamp, with, for example, an incandescent light bulb, the Sankey diagram above could be an accurate representation of its energy story.  If 100J of electrical energy come into the lamp, only 10J may transfer and transform into light energy.  The other 90J of energy may get transferred to other locations in the lamp, as well as to the surroundings, as heat (thermal) energy.  A fluorescent or LED bulb’s Sankey diagram might look different, with more energy changing to light, and less energy transferring and transforming to heat.

Regardless, a Sankey diagram does show the law of conservation of energy.  The energy you start with in the system (100J of electrical energy), is all there after the transfers and transformations (90J of heat energy + 10J of light energy=100J total).

Because thermal energy plays such an important role in systems, we will examine it in more detail below.

Atoms are always in different types of motion (translation, rotational, vibrational), and this motion of atoms and molecules creates thermal energy. All matter has this thermal energy. The more motion the atoms or molecules have the more heat or thermal energy they will have.

Many people think that heat and temperature are the same thing, but that is not the case. Heat is energy from atomic and molecular motion that is transferred from one body to another as the result of a difference in temperature. In contrast, temperature is a measure of hotness or coldness expressed in terms of any of several arbitrary scales like Celsius and Fahrenheit. The following diagram shows the difference between heat and temperature:

In this diagram, the two beakers of water have the same temperature, but the beaker on the left has more heat; it has more heat available to transfer to something.  Heat is always transferred from a hotter area to a cooler area, but there are different ways that it can be transferred. The three main ways heat can be transferred are described below:

Conduction: In conduction, heat is transferred between two substances that are in direct contact with each other. Conduction occurs when the particles in a substance being heated gain energy, and begin to vibrate more. These molecules then bump into nearby particles and transfer some of their energy to them. This then continues and passes the energy from the hot end down to the colder end of the substance. It is mostly solids that conduct heat, as the particles in solids can do the vibrating necessary to pass on the heat without moving, themselves. The better the conductor, the more rapidly heat will be transferred. Metal is a good conductor of heat, while wood and Styrofoam are not. The following diagram shows the increasing vibration of particles during conduction:

Convection: In convection, occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. Cooler liquid or gas then takes the place of the warmer areas. This results in a continuous circulation pattern of warmer liquid or gas rising, and cooler liquid or gas sinking.  This pattern of rising and sinking occurs because of density, and is called a convection current.  Warmer substances are less dense than cooler substances.  Less dense substances float on substances that are more dense. Another good example of convection is in the atmosphere. The earth’s surface is warmed by the sun, the warm air rises and cool air moves in.  The following diagram shows convection and convection currents:

Radiation: Radiation transfers heat without any medium.  A medium is a substance through which waves or particles can travel.  For example, in conduction, the medium is a solid; the vibrations from the particles must pass through other particles in the solid in order for conduction to occur.  Likewise, in convection, the medium is a liquid or a gas; the warmer particles move up through the liquid or gas, while the cooler particles sink.  In radiation, waves carry heat through empty space.

Heat that is transferred through radiation is also known as infrared radiation, which travels in waves. Examples of infrared radiation include the heat from the sun, and heat that is released from the filament of a light bulb. Infrared radiation is one of many types of radiation in the electromagnetic spectrum.  Types of electromagnetic radiation differ in the length of their waves, and include radio waves, light waves, and microwaves. A diagram of different types of electromagnetic radiation is found below:

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