Sec 5.6 - 154 The Silicon Web: Physics for the Internet Age...

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154 The Silicon Web: Physics for the Internet Age 5.6 ELECTRICAL ENERGY AND VOLTAGE We learned in Chapter 3 that accelerating an object through a distance by applying a force to the object increases its energy. The increased energy equals the work done on the object, and can be in the form of kinetic energy (energy of motion) or potential energy (energy of position). Electric potential energy is the potential energy gained by a charged object if it is accelerated through an opposing electric force. If an object has positive electric potential energy, this means it is located at a position where it has the potential to be pushed to a new position by the existing electric F eld. It is measured in energy units, joules (J). Consider two parallel metal plates, which are oppositely charged, as in Figure 5.15 . If a single proton, which has charge + e , is placed between the plates, it will “feel” a force directed toward the negative plate. Let us say that the proton begins at the nega- tive plate, and you use your hand to overcome the electric force and push the proton to the positive plate. You did physical work on the proton, although this is a very small amount of work, 1 because a proton has a very small charge. Now the proton, being at the plus plate, has the potential to be pushed by the electric F eld back to the minus plate if you release it; that is, the proton has electrical potential energy by virtue of its loca- tion. We specify a charge’s potential energy using the concept of voltage . Voltage is a measure of a charged object’s change in electric potential energy that is associated with moving from one location to another. 1 The work equals 0.16 × 10 18 J, or 0.16aJ, which is sometimes called one electron-volt. Silicon wire, 1 mm Electron fow Electron current Silver wire, 100,000 mm FIGURE 5.13 A silicon wire that is suddenly connected between two oppositely charged objects will allow electric current to ± ow between them, eventually making both neutral. A silver wire of the same diameter and 100,000 times longer will allow the same amount of current to ± ow as does the shorter silicon wire. Material with high conductivity Material with low conductivity FIGURE 5.14 The number of obstructions in a material determines its electrical conductivity. TAF-K10173-08-1107-005.indd 154 TAF-K10173-08-1107-005.indd 154 4/25/09 12:51:05 AM 4/25/09 12:51:05 AM
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Electricity and Magnetism 155 Voltage is not specif ed in terms oF the amount oF charge on an object, but in terms oF the electrical environment around the object. We say that the positive plate on the leFt is at higher voltage than is the negative plate on the right. The voltage at an object depends only on its position in an electric f eld. Consider a situation in which you have a small object—call it the “test object”—having 1 C (6 21 0 18 . × e ) oF positive charge on it, and you move it by hand From the negative plate to the positive plate in ±igure 5.15. IF it requires doing 1 J oF work to move the test object From one plate to the other, then we say that the voltage between the plates equals one volt . The symbol For volt is V.
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Sec 5.6 - 154 The Silicon Web: Physics for the Internet Age...

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