![]() ![]() In an energy transformation, energy changes form. Materials on Earth then absorb these waves to be used for energy or reflect them back into space. Nearly all thermal energy on Earth originates from the sun and radiates to the surface of our planet, traveling in the form of electromagnetic waves, such as visible light. With radiation, a heat source does not have to touch the object being heated radiation can transfer heat even through the vacuum of space. The third type of heat transfer-radiation-is critical to life on Earth and is important for heating bodies of water. The process repeats, creating a current of molecules sinking, heating up, rising, cooling down, and sinking again. These molecules then rise to the top of the pot and are replaced at the bottom by cooler, denser water. ![]() They begin to move faster and spread out, creating a lower density of molecules at the bottom of the pot. When water is boiled on a stove, the water molecules at the bottom of the pot are closest to the heat source and gain thermal energy first. Convection only occurs in fluids, such as liquids and gases. Some materials, such as wood and plastic, are not good conductors-heat does not easily travel through these materials-and are instead known as insulators. The vibrations of molecules at the end of the spoon touching the water spread throughout the spoon, until all the molecules are vibrating faster (i.e., the whole spoon gets hot). This happens because metal is an efficient conductor, meaning that heat travels through the material with ease. If a metal spoon is placed in a pot of boiling water, even the end not touching the water gets very hot. When thermal energy is transferred between neighboring molecules that are in contact with one another, this is called conduction. Thermal energy transfers occur in three ways: through conduction, convection, and radiation. Temperature is used as a measurement of the degree of “hotness” or “coldness” of an object, and the term heat is used to refer to thermal energy being transferred from a hotter system to a cooler one. When a substance is heated, its temperature rises because the molecules it is composed of move faster and gain thermal energy through heat transfer. Thermal energy has to do with the internal energy of a system due to its temperature. A kinetic energy transfer is easy to observe and understand, but other important transfers are not as easy to visualize. In an energy transfer such as this one, energy moves from one object to another, but stays in the same form. When a golf club is swung and hits a stationary golf ball, some of the club’s kinetic energy transfers to the ball as the club does “work” on the ball. In physics, work is a measure of energy transfer and refers to the force applied by an object over a distance. A common example of energy transfer that we see in everyday life is the transfer of kinetic energy-the energy associated with motion-from one moving object to a stationary object via work. However, this does not mean that energy is immutable it can change form and even transfer between objects. Energy cannot be created or destroyed, meaning that the total amount of energy in the universe has always been and will always be constant.
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