机电专业英语 教学课件 作者 杨寿智 刘水平 机电专业英语.Lesson 21.pptVIP

Lesson 21 VOLTAGE Text As you have seen, there is a force of attraction between a positive and a negative charge. A certain amount of energy must be exerted in the form of work to overcome the force and move the charges a given distance apart. All opposite charges possess a certain potential energy because of the separation between them. The difference in potential energy of the charges is the potential difference. For example, consider a water tank that is supported several feet above the ground. A given amount of energy must be exerted in the form of work to pump water up to fill the tank. Once the water is stored in the tank, it has a certain potential energy which, if released, can be used to perform work①. For example, the water can be allowed to fall down a chute to turn a water wheel. Potential difference in electrical terms is more commonly called voltage (V) and is expressed as energy or work (W) per unit charge (Q)②: V= W/Q （21-1） Where W is expressed in joules (J) and Q is in coulombs(C). The unit of voltage is the volt, symbolized by V .One volt is the potential difference (voltage) between two points when one joule of energy is used to move one coulomb of charge from one point to the other③. EXAMPLE 21-1 If 50 joules of energy are available for every 10 coulombs of charge, what is the voltage? Solution: V=W/Q=50J/10C=5V (21-2) SOURCES OF VOLTAGE The Battery A voltage source is a source of potential energy that is also called electromotive force (emf). The battery is one type of voltage source that converts chemical energy into electrical energy③ .A voltage exists between the electrode (terminals) of a battery , as shown by a voltaic cell in Figure 21.1.One electrode is positive and the other negative as a result of the separation of charges caused by the chemical action when two different conducting materials are dissolved in the electrolyte④ . Batteries are generally classified as primary cells, which cannot be recharged,