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This post explains the concepts of and relationship between current, voltage, and resistance in a way that is easy to understand for people who don't have an electrical background or specialization.
The aim of this post is tor provide fundamental knowledge about electricity for controls and automation engineers. It provides an introduction to electricity for people who do not specialize in electrical engineering. In the post, you will learn;
Let's start at the beginning by answering a very basic question - what is electricity?
Electricity is a form of energy that is all around us and that we use every day. To most people, electricity is a mysterious force. They know that electricity provides the means to use their devices, that if they come into contact with electricity they may be hurt, and that's about all they know.
At its most fundamental level, electricity can be described as moving charged particles. These charged particles are called electrons and they are usually found vibrating around the core of an atom. In certain circumstances, electrons can be forced to move from one atom to the next to create an electrical current.
So, how what forces electrons to move?
It turns out that there is an electrical pressure that can be applied to electrons to force them to move. This pressure is called voltage. Voltage can come from many different places including the magnetic fields in a generator or a chemical reaction in a battery. In fact, you can generate voltage by scuffing your feet across a carpeted floor. What you create by scuffing your feet is called static electricity and its not very useful.
When voltage is applied to a material, the electrons in the material move in response to that voltage. The flow of electrons from one atom to the next is called current. The current in a circuit is what does the work - it is the current that creates light, heat, and the magnetic fields that drive motors.
Most people find the concept of voltage and current abstract and hard to picture. To make these concepts more digestible, we'll use an analogy to explain the relationship between voltage, current, and resistance.
Before we do that though, let's discuss some important properties of electricity.
Current likes to flow along the easiest path that it can find. In an electrical circuit, the easiest path is usually a copper wire. Copper, like most metals, is a good conductor of electricity. We use the term "good conductor" to describe a material where the electrons are free to move around.
In contrast, a bad conductor is a material where every atom holds tightly to its own electrons and the electrons cannot move from atom to atom easily. We call a really bad conductor an insulator. Insulators are important because they help us to manage the flow of electricity. This is why electric wires are always covered with an insulator. In general, plastic and rubber make very good insulators.
Most of the time, current will only flow when there is a continuous conductive path from the voltage source, through the load, and back to the opposite pole of the voltage source. This is called a closed circuit.
A circuit that does not have a continuous conductive path from one side of the voltage source to the other is called an open circuit. In an open circuit, there is no current to do work.
In the example below, you can see an open and a closed circuit. In the open circuit, there is no currently flowing through the circuit so the light is not illuminated.
The open circuit concept works well at normal voltage levels, but breaks down when we are dealing with extremely high voltage. When voltage gets high enough, there may be enough pressure on the electrons in a circuit to make them jump through the air to reach the next easiest conductive path. This is called a spark.
Lightning is a very impressive example of a spark. There is no conductive path from the closed to the earth during a thunderstorm but there is enough voltage present, in the millions of volts, to force the electrons to jump a mile or more through the air to reach the ground.
While working in industry, you will come across two types of current, which are created by two types of voltage. They are Direct Current, DC, and Alternating Current, AC.
DC is most often associated with batteries and power supplies. In a DC system, the pressure on electrons is constant and in one direction. The popular (although, incorrect) convention is that electrons flow from the positive side of the voltage source to the negative side.
AC is what powers most of the devices in your home and industry. In an AC system, the voltage oscillates from a positive value to a negative value and back. The voltage typically oscillates 50 to 60 times per second. We measure the number of oscillations using the units hertz, where 1 hertz is 1 cycle per second.
AC systems may be harder to understand than DC but AC systems are more common since it is easier to generate AC power and AC motors are much simpler to design than DC motors.
When we talk about electricity, we inevitably end up talking about abstract, invisible concepts like current, voltage, and electrons. These things are very real but cannot be visualized easily.
Personally, I've always found it easier to understand electricity by learning though an analogy. One of my favorite analogies for electricity is one that compares an electrical circuit to the flow of water through a piping system. This analogy works particularly well to explain a DC system.
On the left hand side of the diagram above, you see a loop of pipe with water flowing through it. The force that drives the water is created by the pump. This force creates a flow of water through the system. At a certain point in the loop of pipe, sand creates resistance to the flow of water.
Intuitively, we can understand that in the water system the pump pressure, sand resistance, water flow are all related to each other.
We know that if we increase the pump pressure, then the flow of water will increase and that if we decrease the pump pressure, then the flow of water will decrease. We can summarize this idea by saying that there is a direct relationship between the pump pressure and the flow of water.
We also know that if we increase the amount of sand in the system (which provides resistance to the flow of water), then the flow of water will decrease and if we decrease the amount of sand in the system, then the flow of water will increase. We can summarize this idea by saying that there is an inverse relationship between the flow of water and resistance.
It turns out that these relationships are true in electrical circuits as well. On the right of the image above, you can see a simple electrical circuit which has the same characteristics as the loop of pipe on the left. In this circuit, a battery applies pressure on the electrons to move, a loop of wire provides a complete conductive path for the electrons to travel along, a light bulb provides resistance to the flow of electrons.
If we increase the pressure on electrons in a circuit (that is, the voltage measured in Volts), then the flow of electrons (the current, measured in Amperes) will increase. If we decrease the voltage, then the current will decrease.
If we increase the resistance to the flow of electrons (that is, the resistance, measured in Ohms) then the flow of electrons will decrease. If we decrease the resistance, then the current will increase.
The relationship between voltage, current, and resistance in an electrical system is described by Ohm's Law which says that the current in a circuit is equal to the voltage divided by the resistance. The formula for Ohm's Law is commonly written as I = V / R, where I is the current of a circuit, V is the voltage of a circuit and R is the resistance of a circuit.
In this formula, you can see the direct relationship between current and voltage, where I increases as V increases. You can also see the inverse relationship between I and R, where I decreases as R increases.
When you start looking at the details of AC systems, you find that the resistance of a device may not be constant.
The resistance of devices powered by AC electricity like motors, electromagnets, relays, and other devices is not constant. Instead, it varies with the frequency of the voltage applied to it.
This concept of varying resistance is too advanced to discuss in this post but I would like to make you aware that AC devices may have different resistance ratings for thermal and inductive loads.
In this post, we learned the fundamental concepts related to electricity. After reading this post, you should understand what current, voltage, and resistance are and be able to explain the relationship between them in an electrical circuit. You should also understand that current only flows in a closed circuit in normal circumstances and the difference between AC and DC systems.
In a future post, we'll expand on these fundamental concepts to learn how we can harness the power of electricity to do useful work in industry.
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