Electricity is the force that energizes all that we do. One concept that must be understood as a lighting designer is power. All lamps are rated in watts, a measure of power. We will begin by defining a few terms related to electricity. The four terms we need to understand are Voltage, Amperage, Resistance, and Wattage.
We will use a water model to help define and understand these terms. If we look at a water pipe as an example of an electrical wire, and in the example, the water pipe is hooked up to a water wheel, then we could call the friction that the wheel has while turning “the resistance”. All light bulbs present a certain resistance to the flow of electricity.
If we turn on or open a faucet, water starts to flow. This flow is just like bringing the level up on a dimmer. The pressure of the water in the pipe is directly comparable to the voltage in an electrical circuit. As you bring up a dimmer you are increasing the voltage. In our water model we could say we have increased the water from a trickle to a torrent.
The next term to consider is amperage. The size of the pipe carrying the water is the corollary for amperage. The larger the diameter of a pipe, the more water can flow. The larger the gauge, or diameter of a wire, the more current can flow. Fuses and circuit breakers are rated in amperage. They provide protection in an electrical circuit. When you try to draw more amperage through the circuit than the fuse or circuit breaker is rated for, the fuse or circuit breaker will “blow”. The larger the fuse or circuit breaker, the more lighting fixtures you could use.
In our water model we could measure the amount of water flowing past the wheel as it “spins” the wheel. The amount of water flowing as well as the pressure and rate of flow controls the speed of the spinning wheel. In the case of our lighting instrument, we control the power with our dimmer. The measurement of the amount of power flowing past the light bulb, our spinning wheel in the water model, is called wattage. In electrical terms we measure wattage, the amount of amperage or current flowing past a point at a certain voltage or pressure. We can define “Power” as the measurement of how much work is being done. In the case of the water model example, we would multiply the amount of water that flows by the pressure, or how much we opened the tap.
If the friction or resistance is a small number and we open the tap all the way then the amount of water will be large. If the friction is large then the amount of water flowing will drop down. However, the resistance provided by the wheel is not a factor we need to be concerned about in lamps. Most of the time we are concerned with how much amperage is being used by a lamp, not how much resistance it places in the circuit. Long ago a man named Ohms figured out these relationships and set them down. We know them as Ohms Law. He set them down using the following variables in 2 basic formulas.
Here are the 2 formulas that you will use most. See Ohms Law Equation Chart for all variations of the formula.