Voltage

Author: Joe H., Inflow Engineer

In today’s post, we’re going to cover voltage, give an exact explanation of what an ohm is, and learn about Ohm’s law, which is the foundation of all electrical engineering. But first, we’re going to take one more side trip into the realms of basic physics, since we need one more concept before we can define a volt. Specifically, we need energy.

Energy, generally speaking, is the ability of something to do work. It can be stored up, or used to accomplish something. For example, if you pick up a bucket of water and raise it 1 meter into the air, you have used energy to move your hands and the bucket. However, that energy doesn’t just disappear or get burned up; in fact, the energy you used has mostly been transferred to the bucket. You could tip to bucket over and pour the water over a water wheel and the wheel would turn. When the water is moving and pushing things, we say it has kinetic energy, and when it’s sitting in the bucket waiting to be tipped over, it has potential energy. If you used the water wheel to lift up buckets full of water, you’d get a device that converts kinetic energy into potential energy and back again, at least for a little while. Eventually, your water wheel wouldn’t work anymore. The reason is that energy can take more forms than just kinetic and potential, and one of those forms is heat. As the water wheel spins, the shaft rubs against the mounting brackets, causing them to heat up. Eventually, all the usable energy is transformed into heat and dissipated into the air.

If you want to do a quick experiment at home, you can rub your hands together and feel them heat up. That heat is a form of energy and as your hands cool off the heat is transferred to the air around you. With just a simple bit of back and forth, you’ve converted the stored energy in your body into kinetic energy then into heat energy, which has then been transferred to the air around you. If you really wanted to, you could calculate how many calories you burned rubbing your hands together, since calories are a measure of potential energy stored in food.

So, what does all this have to do with electricity? Well, just like rubbing your hands together converts kinetic energy into heat, when electrons flow through a wire they emit a certain amount of heat. We measure this heat in joules (J), which is a unit of energy equal to about 0.00024 calories. A volt (V) is a measurement of potential energy, specially, it’s the amount of electrical potential energy needed to generate 1 J of heat in one second by moving 1 amp (A) worth of current [1]. In plain English, a volt is a measurement of how hard electrons are pushed through a wire by a given power source. The higher the voltage, the harder electrons can be pushed through a wire.

Now that we know what a volt is we can give an exact definition of an ohm (Ω). Remember, the atoms in a wire try to hold onto the electrons as they pass, limiting the amount of electrons that can flow through a wire at any given time (resistance). With a higher voltage, you can push more electrons through the wire. The Ω is the unit we measure resistance in, and it’s defined at the amount of resistance which allows 1A of electrons to flow through a wire when 1V of power is applied. We can write this relationship as 1Ω=1V/1A.

The really cool thing about this definition is that because an Ω is a relationship between voltage and current, we can always find the resistance in a wire if we know the voltage and current, or find the current if we know the voltage and resistance, or find the voltage if we know the resistance and current. If we use the letter R to represent any resistance, the letter I to represent and current, and the letter V to represent any DC voltage, we get the following general relationship: V=IR. This relationship is known as Ohms law, and as I said in the first paragraph, it’s the foundation of all electrical engineering. With Ohms law, we can go back to our lightbulb example from last week, and instead of just guessing at how much resistance we need to make sure that the bulb doesn’t burn out, we can calculate an exact value. For instance, if our bulb needs exactly 0.5A of current to function, and our power source is 9V, then we need 18 Ω of resistance in our device.

Now that we have most of the basics covered, we’re going to start looking at some simple electrical components and circuits. We already touched on resistors briefly in the last post, but they’ll be coming up again in the next post, along with some of the basic laws for working with circuits. One more important note, you’ll notice that when I gave you Ohms law, I specified that the voltage is DC, or direct current. This is because alternating currents create some interesting effects when they flow through wires. Current and voltage are the same, but instead of resistance, we get something called impedance. It’s a little more complicated and so in order to give you some time to get familiar with the basics, we’ll be limiting ourselves to working with DC based circuits for several posts. That’s it for this post, be sure to check back soon for the next one!

[1] For those of you who like math, the relationship can be given as 1 V= 1J/(1A*1s)

 

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