Simple Nature - Light and Matter

Force depends only on position. Since the charge distributionis not changing, the total electrical force on a chargedparticle depends only on its own charge and on its location.If another charged particle of the same type visits the samelocation later on, it will feel exactly the same force.The second observation tells us that there is nothing all thatdifferent about the experience of one charged particle as comparedto another’s. If we single out one particle to pay attention to, andfigure out the amount of work done on it by electrical forces as itgoes from point A to point B along a certain path, then this isthe same amount of work that will be done on any other chargedparticles of the same type as it follows the same path. For the sake ofvisualization, let’s think about the path that starts at one terminalof the battery, goes through the light bulb’s filament, and ends atthe other terminal. When an object experiences a force that dependsonly on its position (and when certain other, technical conditionsare satisfied), we can define an electrical energy associated with theposition of that object. The amount of work done on the particleby electrical forces as it moves from A to B equals the drop inelectrical energy between A and B. This electrical energy is what isbeing converted into other forms of energy such as heat and light.We therefore define voltage in general as electrical energy per unitcharge:asThe difference in voltage between two points in space is defined∆V = ∆U elec /q ,where ∆U elec is the change in the electrical energy of a particle withcharge q as it moves from the initial point to the final point.The amount of power dissipated (i.e., rate at which energy istransformed by the flow of electricity) is then given by the equationP = I∆V .d / Example 4.Energy stored in a battery example 4⊲ The 1.2 V rechargeable battery in figure d is labeled 1800 milliamphours.What is the maximum amount of energy the battery canstore?⊲ An ampere-hour is a unit of current multiplied by a unit of time.Current is charge per unit time, so an ampere-hour is in fact afunny unit of charge:(1 A)(1 hour) = (1 C/s)(3600 s)= 3600 C1800 milliamp-hours is therefore 1800 × 10 −3 × 3600 C = 6.5 ×10 3 C. That’s a huge number of charged particles, but the total516 Chapter 9 Circuits