Science: An Elementary Teacher’s Guide/Energy, work, power, heat
Introduction to Energy edit
What Is Energy? edit
It is usually described as the ability to do work or produce a chemical change. Energy on Earth is usually derived from the Sun, other parts of energy though are stored within the Earth itself, such as heat, gravitational potential energy, and also nuclear energy. In physics, energy is a property of objects which can be transferred to other objects or converted into different forms.The "ability of a system to perform work" is a common description, but it is misleading because energy is not necessarily available to do work. For instance, in SI units, energy is measured in joules, and one joule is defined "mechanically", being the energy transferred to an object by the mechanical work of moving it a distance of 1 meter against a force of 1 newton. However, there are many other definitions of energy, depending on the context, such as thermal energy, radiant energy, electromagnetic, nuclear, etc., where definitions are derived that are the most convenient.
Common energy forms include the kinetic energy of a moving object, the potential energy stored by an object's position in a force field (gravitational, electric or magnetic), the elastic energy stored by stretching solid objects, the chemical energy released when a fuel burns, the radiant energy carried by light, and the thermal energy due to an object's temperature. Nuclear energy is present within every atom, but is unavailable except for certain elements under particular conditions (nuclear reactors, nuclear bombs). Our sun constantly releases nuclear energy from hydrogen-based fusion, and this energy reaches us as photons and electromagnetic waves. The nuclear energy from the sun is what ultimately powers all life on earth, because a small portion of that energy gets captured by plants and photosynthetic plankton (using photosynthesis), converted to chemical energy, and from there travels through food chains as animals eat plants or other animals. Even our cars and coal-fired power plants are ultimately powered by the sun, since gasoline and coal are "fossil fuels" that contain the chemical energy from hundreds of millions of years of ancient algae and plant growth. All of the many forms of energy are convertible to other kinds of energy. In Newtonian physics, there is a universal law of conservation of energy which says that energy can be neither created nor be destroyed; however, it can change from one form to another.
Types of Energy edit
There are two major types of energy considered at this point: Kinetic and Potential. Each is important to know for scientific purposes.
Kinetic Energy edit
There are many forms of kinetic energy, for example the vibrational, rotational, and translational. The amount of translational kinetic energy (from here on, the phrase kinetic energy will refer to translational kinetic energy) that an object has depends upon two variables: the mass (m) of the object and the speed (v) of the object. The following equations represents the kinetic energy of an object: KE = 0.5 • m • v2, where m= mass of object and v= speed (velocity) of object.
Like work and potential energy, the standard metric unit of measurement for kinetic energy is the Joule. As might be implied by the above equation, 1 Joule is equivalent to 1 kg*(m/s)2. vibrational
- Translational kinetic energy is the kinetic energy that an object has due to its motion in a straight line from one from one place to another place.
- Rotational kinetic energy is the kinetic energy an object has due to its rotational motion around an axis. It is also called as angular kinetic energy. The rotating object has kinetic energy associated with rotation, even if its center of mass is at rest.
- Vibrational kinetic energy is the kinetic energy an object has due to its vibrational motion. Cell phone that vibrates when it is ringing and vibration of a drum when it is hit by a hammer are some examples of kinetic energy.
Kinetic energy is also known as the energy of motion. The opposite of kinetic energy is potential energy. The kinetic energy of an object is the energy that the object possesses because it is in motion. In order for something to have kinetic energy, you must "do work" on it--push or pull. Energy of motion
The energy of motion is the energy an object has because it is moving. An object obtains and maintain its kinetic energy unless the speed changes.
Examples of kinetic energy are; a baseball speeding toward a bat, and bat swinging toward the ball, a rock falling from a cliff, a football flying in the air, water rushing down a dam, or a wheel rolling down a hill.
Check your understanding
Potential Energy edit
Potential energy, is the energy that is stored within an object, it is not in motion but it is capable of becoming active. When at rest, every object has rest mass potential energy; it has gravitational potential energy if the object is in a position to be affected by gravity and to fall. When an object is in motion, potential energy is transformed to kinetic energy , which is the energy of motion.
Potential energy can also be thought of as stored energy.
Examples of potential energy; a tree has potential energy, because it will fall if something made it fall by cutting the trunk or destroying the roots that hold it in place, a ball on the edge of a table has stored energy because its potential to fall, pulling back on a rubber band, and a stationary bicycle.
Additional sub-types of Energy edit
Gravitational Potential Energy edit
Is the potential energy associated with a gravitational force, as work is needed to elevate objects against the Earth's gravity. If an object falls from one point to the other point inside a gravitational field, the force of gravity will do positive work on the object, and the gravitational potential energy will decrease by the same amount. The factors that affect an object's gravitational potential energy are its height relative to some reference point, its mass, and the strength of the gravitational field it is in.
Example Consider a book placed on top of a table. As the book is raised from the floor, to the table, some external force works against the gravitational force. If the book falls back to the floor, the "falling" energy the book receives is provided by the gravitational force.
Acceleration of Gravity: 32 feet/second² Therefore: 32 ft/sec after 1 seconds 64 ft/sec after 2 seconds 96 ft/sec after 3 seconds, etc...
Acceleration of Gravity edit
As an object falls toward the Earth, it increases its rate of descent (assuming no air resistance or other interference) by 32 feet per second (usually expressed as 9.8 meters/second2).
Internal Energy edit
Internal energy is defined as energy that is contained within matter itself (not the kinetic energy or gravitational potential energy of an object, but the energy that is inherent in and between the atoms). Most commonly, internal energy is referring to heat, because heat is easily measured and can be easily transferred between masses. Heat is a microscopic kinetic energy due to the motion of the system's particles. If you heat up a pan on the stove, the protons and neutrons of the atoms vibrate faster and the electrons also move faster. If you cool matter down, the atomic particles move slower and slower. Absolute zero is defined as the stopping point subatomic motion and is reached at −273.15 °C, −459.67°F, 0°K (we can experimentally cool things very close to this, but cannot quite reach absolute zero).
Other internal energy includes chemical bonds holding atoms together, and the strong nuclear force holding protons and neutrons together within the atomic nucleus. This strong nuclear force actually accounts for the majority of mass within an atom--remember that mass and energy are related to each other through Einstein's Theory of Relativity where we learn that energy (E) is equal to mass (m) times the square of the speed of light (c2). It is difficult to release the energy found within the strong nuclear force--when we do release it we call it a nuclear reaction. When you pick up a rock, there is potential energy thanks to gravity; there may be kinetic energy (if you are throwing it), but most of the rock's energy is the internal energy, which is almost entirely inaccessible and will just stay within the atoms of the rock. If you had a superpower where you could release all of that energy, then every rock would be a small nuclear bomb! (but, of course, you would use your superpower to generate electricity from the fusion of water molecules! Right??)
Conservation and Transformation of Energy edit
The Law of Conservation of Energy states that energy can neither be created or destroyed. Energy can only be transferred from one place to another and from one form to another but the amount of energy does not change. While the total amount of energy remains the same, it is constantly being changed from one form to another. If such transformation could not happen, plants could not make food from nutrients and sunlight, you could not grow from eating, and you could not transfer energy from gasoline into the movement of your car.
Einstein's famous equation, E=mc², ties together the properties of mass and energy. This is not readily apparent in our everyday world--mass and energy do not seem like two expressions of the same thing. In the everyday world there is constant transfer of energy from one form to another (electric energy into light, chemical energy into motion, etc), but we do not witness mass being converted into energy. This can happen, but only under special circumstances. Creating those circumstances is what allowed for the atomic bomb and for nuclear power plants.
Let's examine this a bit more closely:
Einstein's equation is E=mc² E=energy; m=mass; c=speed of light squared (186,00 miles/sec*186,00 miles/sec)
What you will notice is that c² is a very large number! If we use metric numbers, here are the calculations for converting 1 kg of mass (2.2 pounds) into another form of energy: E/m = c² = (299792458 m/s)² = 89875517873681764 J/kg (≈ 9.0 × 1016 joules per kilogram) This is the energy equivalent of 695 million gallons of gasoline or 25 billion Kilowatt hours of electricity!
When a person talks about work, most often the average person will think about city jobs like a fast food joint, or working as a janitor to clean the local highschool. This is all well and good, but for the up and coming science student, "work" has a different meaning altogether. In this case, work is the exertion of force on an object to alter the state of the object in question. This can be the relocation of the object to another point, painting the object different colors, etc.
Now this is not to say that all exertions of force are considered work. Take for instance a student in a football team. He is at work pressing the dummy bar around the field. This is work, as the student is exerting effort on the bar which in turn is being moved around. Take that same student and place him up against a wall and tell him to push. Though he is exerting himself against the wall, there is no work occuring at this point as the wall is not changing place, nor is he altering it in any noticable fashion.
We often use the word "heat" in our daily lives. We hear and feel it all around us. For, example when we are outside we say "I can feel the heat from outside." The first thing that comes to mind when we hear the word heat may be the feeling of hotness or imagining ourselves sweating a lot. But the topic of heat goes beyond that. The total amount of energy within a material is its thermal energy as it is the movement of molecules. Heat is the transfer of thermal energy from one object to another.
Heat and energy interrelate, as in thermal energy is a form of kinetic energy. It is is the motion of molecules, but we can not see them. But we can feel it by the effect in temperature. Also, whenever energy is converted from one form to another, some of the energy (sometimes most of it, in fact) gets converted to wasteful, disorganized heat (for example, converting electrical energy to light produces heat from the light bulb; converting chemical energy of gasoline into kinetic energy of moving the car produces a great deal of heat, requiring a radiator and cooling system for the car).
"Temperature" is how fast or slow molecules are moving. The faster the molecules move the warmer, the slower they move the colder. "Cold" is actually not a form of energy, but it is only a term people use to describe conditions that are very low in thermal energy. When you touch something cold, you are transferring thermal energy from your body into the object (technically if you hold an ice pack to your head the ice pack is not cooling you--you are heating it! Of course, in the process of heating the ice pack there is thermal energy leaving your body, so you are getting cooler, but it is not that the cold moved in to you, it is that the heat moved out).
Temperature vs. Thermal Energy edit
Temperature and thermal energy are not the same thing. Temperature is a measure of kinetic energy of molecules (how fast are atomic particles vibrating or spinning), and thermal energy is the total heat energy of a substance. An easy way to understand this is if you made a large pot of soup and then scoop a bit of soup into a bowl. If you were to take a thermometer, you would find that the soup in the bowl and the soup in the pot are the same temperature. However, there is a lot more thermal energy in the pot. You could verify this by thinking about the amount of energy that would be required to cool down a bowl of soup vs a pot of soup. If you have 10 times more soup in the pot than in the bowl, your refrigerator is going to have to do 10 times the work to cool the pot of soup down compared to cooling down the bowlful. Thermal energy, therefore, is a combination of temperature and mass.
Anything that affects molecular movement can affect temperature
ex: Rubbing hands together, or mechanical pieces sliding against each other in a machine
ex: gas compression (pump up a bicycle tire and then feel the pump), skate's blades (the pressure from an ice skater melts a thin layer of ice directly beneath the skate, reducing friction and allowing gliding)
ex: hammer on metal (hit a hammer hard against something, then feel how hot it is)
- Chemical Reaction (exothermic)
ex: burning wood, coal, gas, oil, etc...
- From the sun
- Main source of all energy on Earth
- Fusion Reaction
- Comes to earth in the form of light, but also other parts of the electromagnetic spectrum
- Fusion= Combining 2 atoms to form one larger atom
- Fission= Splitting an atom into 2 or smaller atoms
- Conduction is the transfer of thermal energy from one substance to another via physical contact
- Convection is the transfer of thermal energy via moving liquid or gases because it require moving molecules, convection cannot occur in space.
- Radiation is the transfer of thermal energy via electromagnetic waves.
Insulation occurs when materials can slow or stop the conduction of heat.
Some examples of good insulators are wood, glass, rubber, vacuum.
Conduction is defined as the process of molecules bumping into molecules, transferring thermal energy from one to another through a substance. Since molecules are constantly in motion this causes them to bump into each other. As molecules are energized, or consume energy from other molecules, they move faster. Molecules contain thermal energy, and when they bump into other molecules at higher speeds the kinetic energy is passed on to another molecule. The energy creates a chain reaction that goes on and on (in a glass of water at room temperature, the individual molecules are moving at many different speeds-in other words, not every molecule is at the same temperature; some of the fast-moving molecules leave the rest of the water through evaporation). A common example of conduction is putting a pot on the stovetop--the direct contact between the heat of the stove coil and the pot leads to a transfer of heat energy into the pot.
Generally materials that are great heat conductors are also good conductors of electricity. Examples include
Convection is defined as a transfer of energy as energy passes through moving liquids and gases. Heat causes molecules to move faster, causing materials to expand and spread farther apart which results in a reduction of density. If air gets warmer, the heated portion expands; with the molecules further apart from each other, this heated gas is lighter than the surrounding cooler air, so it rises. The movement of fluids caused by temperature differences is referred to as "convection current". Such currents are in a certain way responsible for ocean currents and atmospheric winds. A convection current cannot take place in an empty vacuum, because it involves moving molecules of air or other components (a Yeti mug is made of metal, which is a good conductor of heat, but there is a vacuum of space between two layers, and this vacuum prevents transfer of heat energy between the inside and outside layers of the mug). Perhaps you have thought, while heating soup on the stove, that the convection currents causing the soup to rise in certain places looks like miniature volcanoes. That is a good insight--real volcanoes are part of slow-motion convection currents beneath the earth's crust!
Radiation is defined as the transfer of energy through electromagnetic waves, and is another way to transfer heat. This process does not depend on moving molecules and electromagnetic waves can move within a vacuum (thankfully! Otherwise the energy from the sun could not reach us through the vacuum of space!). When we think of thermal radiation we typically think of the sun. The sun radiates enormous amounts of energy through space. The sun's energy is so strong that even though only one-two-billionths of it enters the Earth's atmosphere this is sufficient to meet our planet's needs in terms of energy.
When radiant energy, such as sunlight, reaches a solid, opaque object the energy is automatically consumed causing the molecules to move a faster rate, so the object gets warmer. A dark object will absorb more radiant energy than a light-colored object. Air is warmed up by the heat that comes from the surfaces of these objects, and on a large scale this creates wind. Another example of radiant energy is when you are standing near a fire on a cold night. If you were to take a thermometer and measure the air right in front of you and the air right behind you, you would find they would be almost the same temperature, so you are getting very little heat from the fire through convection currents (most of the heated air is going straight up, not spreading out). Unless you put your hand right on the fire or on a rock next to the fire then you are not getting heat through conduction. However, the part of you that faces the fire receives a lot of radiant heat (if you are really cold you will keep turning to warm different parts of your body). Some of the heat produces light, and part of it is in other parts of the electromagnetic spectrum, especially infrared lightwaves. The military uses thermal imagery to see into the infrared, allowing "heat sensing" goggles that can make even a well-camouflaged person show up.
Heat Questions edit
- How is energy transferred by conduction?
- What is convection?
- How is heat transferred by radiation?
- What is the difference between temperature and thermal energy?
- Name good conductors of electricity.
Try this quick quiz and test what you have learned by reading this chapter!