The Flow of Electricity in a Metal

Knowledge of the existence of electrons permits us to discuss some of the properties of electricity in a simple way. 

In a metal or similar conductor of electricity there are electrons that have considerable freedom of motion and that move along between the atoms of the metal when an electric potential difference is applied. A direct current of electricity passing along a copper wire is a flow of electrons along the wire. 

Let us call to mind the analogy between the flow of electricity along a wire and the flow of water in a pipe. Quantity of water is measured in liters or cubic feet quantity of electricity is usually measured in coulombs (ampere seconds). Rate of flow, or current, of water, the quantity passing a given point of the pipe in unit time, is measured in liters per second, or cubic feet per second current of electricity is measured in amperes (coulombs per second). The rate of flow of water in a pipe depends on the difference in the pressures at the two ends of the pipe, with atmospheres or pounds per square inch as units. The current of electricity in a wire depends on the electric potential difference or voltage drop between its ends, which is usually measured in volts. The definitions of the unit of quantity of electricity (the coulomb) and the unit of electric potential (the volt) have been made by international agreement. 

Example 3-1. An ordinary electric light bulb is operated under conditions such that one ampere is passing through the filament. How many electrons pass through the filament each second (Remember that the charge of the electron is —0.1602 x C). 

Solution. One ampere is one coulomb per second. The number of electrons passing per second is one coulomb divided by the charge of the electron in coulombs  

---