![]() The batteries repel electrons from their negative terminals (A) through whatever circuitry is involved and attract them to their positive terminals (B) as shown in Figure 19.3. These batteries, like many electrical systems, actually move negative charge-electrons in particular. The change in potential energy for the battery is negative, since it loses energy. Note that the energies calculated in the previous example are absolute values. The energy supplied by the battery is still calculated as in this example, but not all of the energy is available for external use. Note also that as a battery is discharged, some of its energy is used internally and its terminal voltage drops, such as when headlights dim because of a low car battery. The voltages of the batteries are identical, but the energy supplied by each is quite different. While voltage and energy are related, they are not the same thing. To have a physical quantity that is independent of test charge, we define electric potential V V (or simply potential, since electric is understood) to be the potential energy per unit charge: But we do know that, since F = qE F = qE, the work, and hence ΔPE ΔPE, is proportional to the test charge q. It is much more common, for example, to use the concept of voltage (related to electric potential energy) than to deal with the Coulomb force directly.Ĭalculating the work directly is generally difficult, since W = Fd cos θ W = Fd cos θ and the direction and magnitude of F F can be complex for multiple charges, for odd-shaped objects, and along arbitrary paths. Potential energy accounts for work done by a conservative force and gives added insight regarding energy and energy transformation without the necessity of dealing with the force directly. Gravitational potential energy and electric potential energy are quite analogous. PE can be found at any point by taking one point as a reference and calculating the work needed to move a charge to the other point. There must be a minus sign in front of ΔPE ΔPE to make W W positive. For example, work W W done to accelerate a positive charge from rest is positive and results from a loss in PE, or a negative ΔPE. There must be a minus sign in front of Δ PE Δ PE to make W W positive. For example, work W W done to accelerate a positive charge from rest is positive and results from a loss in PE, or a negative Δ PE Δ PE. The change in potential energy, Δ PE Δ PE, is crucial, since the work done by a conservative force is the negative of the change in potential energy that is, W = –Δ PE W = –Δ PE. ![]() We use the letters PE to denote electric potential energy, which has units of joules (J). When a force is conservative, it is possible to define a potential energy associated with the force, and it is usually easier to deal with the potential energy (because it depends only on position) than to calculate the work directly. ![]() This is exactly analogous to the gravitational force in the absence of dissipative forces such as friction. The electrostatic or Coulomb force is conservative, which means that the work done on q q is independent of the path taken. Work is done by a force, but since this force is conservative, we can write W = –Δ PE W = –Δ PE. In both cases potential energy is converted to another form. Figure 19.2 A charge accelerated by an electric field is analogous to a mass going down a hill.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |