Energy Transfer
Energy can be transferred from one location to another, as in the sun's energy travels through space to Earth. The two ways that energy can be transferred are by doing work and heat transfer.
Doing Work
Energy can be transferred from one object to another by doing work. When work is done on an object, it results in a change in the object's motion (more specifically, a change in the object's kinetic energy).
Energy is often defined as the ability to do work. Work equals force multiplied by distance. To learn more about work please visit What is Energy? - Section C. Measuring and Quantifying Energy if you have not already.
An illustration of how doing work is an example of energy transfer
Suppose that a person exerts a force on the wheelbarrow that is initially at rest, causing it to move over a certain distance. Recall that the work done on the wheelbarrow by the person is equal to the product of the person's force multiplied by the distance traveled by the wheelbarrow. Notice that when the force is exerted on the wheelbarrow, there's a change in its motion. Its kinetic energy increases. But where did the wheelbarrow get its kinetic energy? It came from the person exerting the force, who used chemical energy stored in the food they ate to move the wheelbarrow. In other words, when the person did work on the wheelbarrow, they transferred a certain amount of chemical energy stored in the person was transferred to the wheelbarrow, causing its kinetic energy to increase. As a result, the person's store of chemical energy decreases and the wheelbarrow's kinetic energy increases.
Wherever you look, you can see examples of energy transfers. When you turn on a light, you see result of energy being transferred from the sun to the plants to the coal to electricity and finally to light you see. During each of these transfers, energy changes form. There are two main forms of energy, kinetic energy (motion) and potential energy (position). To further classify energy, these forms are sometimes further described as thermal (heat), elastic, electromagnetic (light, electrical, magnetic), gravitational, chemical (food), and nuclear energy. See the What is Energy? - Section B. Two Main Forms of Energy for more information on kinetic and potential energy.
Heat Transfer
Heat is given off when an object's thermal energy is transferred. Thermal energy (see below) can be transferred in three ways: by conduction, by convection, and by radiation.
1. Conduction
Conduction is the transfer of energy from one molecule to another. This transfer occurs when molecules hit against each other, similar to a game of pool where one moving ball strikes another, causing the second to move. Conduction takes place in solids, liquids, and gases, but works best in materials that have simple molecules that are located close to each other. For example, metal is a better conductor than wood or plastic.
2. Convection
Convection is the movement of heat by a liquid such as water or a gas such as air. The liquid or gas moves from one location to another, carrying heat along with it. This movement of a mass of heated water or air is called a current.
3. Radiation
Heat travels from the sun by a process called radiation. Radiation is the transfer of heat by electromagnetic waves. When infrared rays strike a material, the molecules in that material move faster. In addition to the sun, light bulbs, irons, and toasters radiate heat. When we feel heat around these items, however, we are feeling convection heat (warmed air molecules) rather than radiated heat since the heat waves strike and energize surrounding air molecules.
More about Thermal (Heat) Energy
Heat is given off whenever energy is being used. You can tell if a television has been on by feeling if it is warm. When you run up a flight of stairs you feel warm because you are burning food energy.
What exactly is heat? Heat is the transfer or flow of energy from a hot object to one that is cooler. When you feel a warm object, you are actually feeling thermal energy, which is the movement of molecules that make up the object. An object has more thermal energy when it is warm than when it is cool.
The more thermal energy an object has, the faster its molecules move. These faster moving molecules bump into each other more frequently and spread out as they require more space (decreasing the density of the molecules). Think of people standing in an
elevator. If they started moving around, they would start bumping into each other and need more space. This is essentially what happens when molecules get more energy and start moving around; they spread out.
For the most part, the volume of an object increases as the amount of thermal energy it receives increases. In other words, the molecules in warmer objects are less densely packed than the molecules in cooler objects. (NOTE: Temperature is a measurement of how fast