Electrical Units and Ohm's Law

Author: J.B. Hoag

Nature's unit of electricity, the electron, is too small for everyday use. In fact, long before the electron was discovered, man had chosen a unit of electricity, the coulomb, of practical size. There is another, more theoretical unit called the electro-static unit or "e.s.u." One coulomb is equal to three billion electrostatic units. The relationship of these units to that of the electron was given in the preceding chapter.

When we speak of a current of electricity, we refer to the rate at which it flows in a wire. Thus, I = Q/t, where I = current in units called amperes, Q = quantity in coulombs and t = time in seconds. Amperes of electricity flowing in a wire are analogous to gallons of water per second flowing down a pipe.

A battery or a generator separates positive and negative charges from each other. These charges try to get back together again to set up the neutral state. The greater this tendency, the greater the electro-motive force ("e.m.f."). The volt is a unit of e.m.f. A single dry cell produces approximately 1.5 volts; a storage battery, about 2 volts per cell; our lighting circuits use 110 to 117 volts. We think of voltage as a driving force, although technically it is a measure of work done upon a unit charge. After expending work in moving a pail of water to the top of a hill against the pull of gravity, it is in a position to give back the energy put into it. It is said to be at a higher potential than before.

The higher we lift the water, the more force it will exert when it falls. Similarly, the higher the voltage of an electrical circuit, the more readily will it drive an electric current through a wire. Doubling the voltage will double the current; trebling the voltage will treble the current.

The free electrons of a metal do not drift down the wire under an impressed e.m.f. without some opposition; they collide with atoms and other electrons; there are no perfect conductors. The measure of this lack of perfection is called resistance; and its unit is called the ohm. Thus we have Ohm's law, E = IR; or, volts equal amperes times ohms. The product IR is often referred to as the potential drop in the wire.

According to convention, electricity flows from the positive to the negative terminal of a battery, from plus to minus. Actually the free electrons move from negative to positive. In this book, unless we speak of the "electron flow" specifically, we shall mean the conventional positive to negative direction.