By Laura J. Bottomley, NC State University
Many of us are familiar with the generation and distribution of electric power only by the plug we insert from our appliances in order to use it. Working backward, the first piece of the puzzle of power distribution that we encounter in everyday life is the number of prongs in plugs—some have two and some have three. Why is that?
One, Two, and Three Prongs
Let’s start by thinking about plugs with two prongs. We can explain the two prongs of the plugs just by talking about voltage. When you remember that voltage is potential energy, you’ll realize that “potential” only has meaning if you measure it between two things. You need two wires for one to have a potential value relative to the other.
Or, conversely, we can talk about the two prongs in terms of current, which you can think of as moving electric charges, or electrons. Electrons will only move in a current if they have somewhere to go. Electrons will not move as current, on their own, down a single conductor, because there would be no way for them to return.
If you think of one prong as current emerging from the grid, and one prong as current returning, you’ll have an accurate mental model of what’s happening. Since we are talking about alternating current, which prong is carrying current in which direction switches—60 times a second.
When there’s a third prong, that’s also known as neutral. It’s connected to earth ground, and it’s there to be sure that the voltage of the two powered wires doesn’t drift. In theory, this can’t happen, but in reality, the two wires could accumulate a charge that raises the varying current up by a constant amount. This can represent a danger to certain electrical appliances. The neutral wire is connected to the system to keep unwanted charges from building up.
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Conservation of Energy
Voltage and current are related. In fact, what we call electric power is voltage multiplied by current. In order to have electric power available to our plugs, we first have to generate it. And our ability to generate electrical energy really means that we convert it from another form of energy.
This fact arises from a very important physical law, known as the conservation of energy. What this law tells us is that energy can neither be created nor destroyed, only converted from one form to another.
Electrical Power in a Flashlight
In order for electrical energy to move, it has to either be conducted through some kind of substance that allows its movement, or it has to be radiated through air and space as an electromagnetic wave, like light. In order to understand that energy, engineers have developed ways to model its behavior.
We use the models to engineer ways to control energy and make it do what we want it to do. Let’s nail that down. What was just said is actually how a shake flashlight works. The tube is just there to hold the coil of wire. The coil is there to wrap over the changing magnetic field as many times as possible.
Where’s the magnetic field? Inside the tube is a very strong neodymium magnet. Magnets create magnetic fields around them naturally. When you shake the tube, it causes the values of the magnetic field at various points in space to change, which gives us an electric field that changes over time, getting stronger and weaker as the magnetic field goes by it.
The current can flow along the wire, and then that current is flowing through the LED connected to the wire. When we get enough current, the LED lights. It is basically acting as a current detector for us. By changing how fast you shake the magnet through the loop of wire, you can observe the intensity of the light from the LED.
Changing Magnetic Fields in Time
Look at a small electric motor that might be found in a model airplane or a child’s toy. It’s essentially a loop of wire surrounding a magnet. And you can see multiple loops of wire surrounding a set of magnets. If you look at the outside of a motor taken out of a blender, you can see the different loops of wire. They will allow us to multiply the output by the number of loops that we have.
When we run current through the outside loops of wire by attaching a battery to them, it causes magnetic fields that oppose the magnetic fields inside of the magnet. This pushes on the magnet inside and causes the inner piece, or the rotor, to turn, and we get a motor. If, instead of putting current in and getting motion out, we put motion in by physically turning the motor shaft, we will get current out.
We can show this by attaching a multimeter. A multimeter is a device that measures multiple electric quantities. We can attach this multimeter and use it to measure voltage. The meter reads zero when we first attach it. Then, when we begin turning the shaft, you can see a voltage being measured. What you’re seeing is energy being converted from the mechanical energy of my hand to electrical energy in the form of current flowing. Now what we have is a generator.
Common Questions about How to See Electrical Power Producing a Magnetic Field in Action
The third prong in plugs is known as neutral. Its purpose is to make sure the voltage of the two wires with electrical power doesn’t drift. The third prong is connected to the earth ground on one end and connected to the system on the other end, so the system isn’t damaged.
The conservation of energy is a physical law that states energy cannot be created or destroyed. In order to get energy for electrical power or anything else, we have to convert it from other forms of energy.
To get electrical power, we have to convert energy to electrical energy and subsequently move that energy. Electrical energy can only be conducted through a substance that allows its movement or through space or air as an electromagnetic wave such as light.
