Ohm's Law

Water flow is often used to describe electricity. In the water analogy, water is the medium to transfer force. In an electrical circuit, the medium is the “free” electrons of conductive metals like copper or aluminum. Water transmits force through the water in a pipe; in an electrical circuit, the force is transmitted over the electrons of a conductor. In our moving picture, we use water to turn a waterwheel. The weight of the water in the tower exerts pressure on the outflow pipe. As the water flows out and under the paddles of the waterwheel, most of the water’s kinetic energy (objects in motion) has been converted into rotational force (torque) at the axle of our illustrative contraption. Pressure is said to be dropped across the load. To maintain pressure at the source, new energy must be added—somebody has to scoop up the water, climb the tower and keep the water level constant. In an electrical system, it is the utility that must maintain this pressure, known as voltage.
The force exerted on the axle depends on three things: The pressure (volts) in the pipe, the gallons per minute of water flow (amps), and the restrictive effect of the pipe and valve diameter (ohms). There is a simple formula to express this relationship of pressure, flow rate, and resistance. The formula is called Ohm’s law after the German scientist George Simon Ohm who first described it. It reads:

E = IR, or in lay terms,
Volts = Amps x Ohms

E = electromotive force = V = voltage
I = intensity = A = current (measured in amperes)
R = resistance in ohms


We can use the energy of the spinning axle to do work (convert energy). The job in this example is to lift 33,000 pounds 1 ft. in 1 minute. One healthy draft horse can do it, so we’ll call this motor a 1-horsepower motor (1 hp = 33,000ft. lb./min.). Power is the rate at which work is done. More power means more energy is being transferred from one form to another. The rate at which electricity is converted to another form of energy (heat, light, motion) is measured in Watts. The power formula is the most commonly used formula for electrical work. It is essential to understanding its application. That formula is expressed as follows:

P (power in watts) = I (amps) x E (volts)

This formula is also useful to utility companies. They can move large amounts of power through small low-current wires by distributing it at high voltage, then lowering the voltage with transformers near the point of use by the customer.