In this short post I hope to make the concept of voltage, current, and resistance easy to understand. I will answer the following questions. What is current? What is voltage? What is the difference between voltage and current? How are voltage, current and resistance related? I will explain these concepts by comparing electricity flowing through a wire to water flowing through a water hose.
Current, Voltage, and Resistance
Let’s start with an example of water flowing through a water hose since we are all familiar with this. You start the water flowing by opening a spigot (technically called a hose bib). Almost immediately, water begins to flow out of the end of the hose. As long as nothing is changed, that same amount of water will continue to flow out of the hose indefinitely. The amount of water that is flowing out of the hose is called the flowrate and is commonly measured in gallons per minute (gpm). If you were to run the water into a 5-gallon bucket and time how long it takes the bucket to fill up, you could measure the flow rate of the water. Let’s assume that you do this and that the flowrate that you measure is 8 gpm.
Now, you turn the spigot to partially close it and slow the flow of the water. Now, if you run water into that bucket, you might measure the flowrate to be only 5 gpm. This is because you have created a resistance to flow by partially closing the spigot. If you connect the pressure gauge and measure the pressure, it will still measure 60 psi, indicating that the pressure has not changed. Closing the spigot only changed the flowrate, it does not change the pressure.
After measuring the pressure, you remove the gauge and turn the water back on. As you are running the water, there is a small break in the city water line that brings water to your neighborhood. This break causes a drop in the water pressure at your home. When this happens, you notice that the water is only shooting out of the end of your hose and going about half as far. Now, when you hook up the pressure gauge, the gauge may only read 30 – 35 psi. After checking the pressure, you again run water into the bucket and now you only get about 4 gallons per minute – about half of what it was at the beginning of this hypothetical experiment.
In this example, we had three variables/measurements that changed: flowrate, pressure, and how much the spigot was open. Either lowering the pressure to the house or partially closing the spigot to increase the resistance would lower the flowrate, and vice versa. Now let’s relate this to electricity.
- Voltage – Voltage can be compared to the water pressure in our example. Voltage is what causes electricity to flow. If nothing else changes, raising the voltage will cause more electricity to flow. Our homes normally run on 120 volts of electricity. (There are some things that run on 240 volts, but it is not necessary to discuss that here.) For the most part, the voltage on our home’s electrical system remains constant at 120.
- Current – Current can be compared to the flow rate of the water in the example above. Just like flowing water is called a current, flowing electricity is called current. Current is measured in amps and tells us how much electricity is actually moving through the wire. It is similar to how much water is moving through the water hose.
- Resistance – Resistance can be compared to how much the spigot is opened in our example. It measures how much resistance there is to the flow of electricity. Resistance is measured in ohms. Everything that electricity flows through has some resistance; some things have much more resistance than others. Even the copper wiring in our homes has some resistance to the flow of electricity. Light bulbs, toasters, TVs, all “resist” the flow of electricity to some extent. A light switch can offer a lot of resistance or essentially no resistance to the flow of electricity. When the switch is open (turned OFF), it has essentially infinite resistance to the flow of electricity and stops the flow altogether. When the switch is closed (turned ON), it offers almost no resistance and allows electricity to flow freely.
To understand how current, voltage and resistance are related, it is necessary to have at least a basic understanding of Ohm’s law.
What is Ohm’s Law?
Ohm’s Law describes how current, voltage, and resistance are related to each other. It says that voltage equals current times resistance. It is written E=IR, where “E” stands for voltage, “I” stands for current, and “R” stands for resistance. (At least one of those letters (R for resistance) makes sense.)
Let’s take a look at a couple of examples related to our homes. Since the voltage in our homes is always 120, I’ll rewrite the equation as 120 = current x resistance. This can be rearranged as 120/resistance = current. If you have a load that has one ohm of resistance the current flowing through that load will be 120 amps (120 volts divided by 1 ohm). If there is a load that has 60 ohms of resistance, then there will be two amps of current flowing (120/60=2).
Here is a real-life example using Ohm’s law. A 60-watt light bulb will have about 0.5 amps flowing through it. (This can be calculated using some other formulas.) Rearranging Ohm’s Law to E/I=R or 120/current = resistance, we can calculate that that 60-watt bulb has about 240 ohms of resistance (120/0.5=240).
I hope that these examples and the comparing of the flow of electricity to water flowing have helped you to understand the concept of and relationship between current, voltage, and resistance.