Did you know that you can produce electricity by simply moving a magnet? In this lesson, learn more about electromagnetic induction and how electricity is generated.
How Do We Make Electricity?
Flip on the light switch. Did the light come on? Of course it did! It seems like magic, but have you ever stopped to think about exactly how the electricity that powers your lights and other appliances is generated? It's a very interesting process that requires magnets and motion.
About 200 years ago, an English scientist named Michael Faraday first developed a method of producing electricity that utilizes magnets, a process called electromagnetic induction. Today, we still generate electricity in much the same way. Faraday's Law says that the rate of change of the magnetic flux through a conductor will be equal to the induced emf, and Lenz's Law can be used to determine the direction of the induced emf.
Faraday discovered that changing the amount of magnetic field passing through a coil of wire, a quantity called magnetic flux, would induce a current in the wire. So, what exactly is magnetic flux? All magnets create an invisible magnetic field around them, and the amount of this field that passes through the center of a coil of wire is the magnetic flux through the coil. The word 'flux' is derived from a Latin word meaning 'flow,' so think of magnetic flux as the amount of magnetic field that's flowing through the area formed by the coil of wire, just like water might flow in a pipe.
The magnetic flux can change if the magnetic field changes or if the coil rotates, and in both cases, this changing magnetic flux will induce a current in the wire. This equation shows you how to calculate magnetic flux, which is measured in units of Webers, abbreviated Wb.
Faraday's Law describes the relationship between the potential difference between the ends of the wire, also known as the emf (which stands for ElectroMotive Force), and the rate of change of the magnetic flux through the coil. This induced voltage will cause current to flow in the wire, but remember that this will only happen if there is a change in magnetic flux! It's not enough simply to have a wire in a magnetic field, but the field must be changing in some way.
Remember that before you can calculate the induced emf using Faraday's Law, you'll first have to calculate the magnetic flux using our first equation.
How can the magnetic flux change? Well, there are a few ways this could happen. First, you could move the wire. The voltage induced in this case is called motional emf because it is caused by moving a wire through a magnetic field. Second, you could make the loop bigger or smaller, and therefore, change the area. You could also rotate the loop so that the magnetic field that passes through it changes. This is what happens in an electric generator.
Did you notice that there is a negative sign in Faraday's Law? You may have wondered what that meant. Initially, in Faraday's original formulation, there was no negative sign. The negative sign tells you what direction emf will be induced, and therefore, what direction current will flow. Faraday discovered that a changing magnetic flux will induce an emf and current in a coil of wire, but it took another scientist, Heinrich Lenz, to figure out in what direction this would happen.
Lenz's Law says that current will be induced in a coil in a direction to oppose any change in the magnetic flux through the coil. Once again, change is the important element, and because current is induced to oppose a change, the induced emf calculated using Faraday's Law is always in the opposite direction to the change in magnetic flux. This is why the negative sign was added. Change in magnetic flux and induced emf are always in opposite directions.
While Lenz's Law does not have an equation associated with it and just forms a small part of Faraday's Law, it's a powerful tool that allows us to determine in what direction current will be induced. Moving a magnet in or out of a coil of wire will induce a current in the wire, and the direction of the current can be determined from Lenz's Law.
Putting It All Together
Now, let's try to put all this together and see if you really understand how to use Faraday's Law and Lenz's Law.
A circular loop of wire with an area of 0.50 square meters is placed in a 6.2 T magnetic field that points upward through the loop. Suddenly, the magnetic field is removed and the magnetic field through the loop becomes zero over a period of 0.10 seconds. Calculate the emf induced in the loop of wire and determine what direction the current will flow (clockwise or counterclockwise).
First, determine if the magnetic flux is changing and, if it is, calculate the flux at the beginning and end. In this problem, the magnetic field is removed, so the flux is changing. Let's calculate the magnetic flux at the beginning:
When we apply Lenz's Law, we get -31 V. Since we're moving the magnet out of the coil of wire and the magnetic field is pointing upward, the induced current will be counter clockwise.
Magnetic flux quantifies the amount of magnetic field that passes through an area. When the magnetic flux through a loop of wire changes, it induces electric current to flow in the wire, a phenomenon called electromagnetic induction. Current flows in the wire because of an induced potential difference, known as an emf, between the two ends of the wire.
Faraday's Law says that an emf will be induced whenever the magnetic flux changes. This change can come from moving the wire through a magnetic field, and this is called motional emf. It can also be caused by adding or removing a magnetic field. Lenz's Law says that current will be induced in a direction that will oppose any change in the magnetic flux through the loop.