Energy Changes and Energy Conservation
Let’s pursue a bit further the relationship between potential energy and kinetic energy. According to the conservation of energy law, energy can be neither created nor destroyed; it can only be converted from one form into another.
CONSERVATION OF ENERGY LAW: Energy cannot be created or destroyed; it can only be converted from one form into another.
To take the example of falling water again, the water in a reservoir has potential energy because of its height above the outlet stream but has no kinetic energy because it isn’t moving. As the water starts to fall over the dam, though, its height and potential energy decrease while its velocity and kinetic energy increase. The total of potential energy plus kinetic energy always remains constant. When the water reaches the bottom and dashes against the rocks or drives the turbine of a generator, its kinetic energy is converted to other forms of energy perhaps into heat that raises the temperature of the water or into electrical energy.
The conversion of kinetic energy into heat when water falls over a dam and strikes the rocks at the bottom illustrates several other important points about energy. One point is that energy has many forms. Thermal energy, for example, seems different from the kinetic energy of falling water, yet it’s really quite similar. Thermal energy is simply the kinetic energy of molecular motion, which we measure by finding the temperature of an object. An object has a low temperature and we perceive it as “cold” if its atoms or molecules are moving slowly. Conversely, an object has a high temperature and we perceive it as “hot” if its atoms or molecules are moving rapidly and are colliding forcefully with a thermometer or other measuring device.
Heat, in turn, is the amount of kinetic energy transferred from one object to another as the result of a temperature difference between them. Rapidly moving molecules in the hotter object collide with more slowly moving molecules in the colder object, transferring kinetic energy and causing the slower-moving molecules to speed up.
(to be continued)
pursue: theo đuổi, tìm hiểu
reservoir: bể chứa nước
outlet stream: dòng nước thoát ra
dam: đập (ngăn nước)
velocity: vận tốc
reach: đến, xuống
dash = strike = collide: va chạm, va đập
against: đập vào, dựa và
generator: máy phát điện
illustrate: minh họa
motion: sự chuyển động, sự di chuyển
perceive: hiểu, nhận thức, thấy
forcefully: mạnh mẽ
thermometer: nhiệt kế