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Electromagnetic Induction: Definition & Variables that Affect Induction

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  • 0:05 Electromagnetic Induction
  • 3:10 Applications of…
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Lesson Transcript
Instructor: Jim Heald

Jim has taught undergraduate engineering courses and has a master's degree in mechanical engineering.

We use electromagnetic induction every day, but do you have any idea what it is? In this lesson, we'll discuss this everyday phenomenon, the factors that affect its strength and its most common applications.

Electromagnetic Induction

Just the other day, I was at the store buying groceries. I tried to pay with my credit card, but when I swiped the card through the pay terminal, a message popped up that it was unable to read my card. After trying a couple more times, the cashier finally offered to help me out. She took my card and ran it through the card reader really fast and…voila! It worked! It may seem like she just got lucky, but it turns out there is a very scientific reason why this worked. However, we'll need to learn about electromagnetic induction before it all makes sense.

In the early 19th century, a scientist by the name of Michael Faraday published several papers on electromagnetic induction, which is the ability of a changing magnetic field to induce a voltage in a conductor. To better understand this phenomenon, Faraday conducted a number of experiments. One of these experiments used a coil of wire, a permanent magnet and a device to detect voltage in the wire. When the magnet was passed through the coil of wire, a voltage was induced in the wire, but it disappeared when the magnet stopped moving. Faraday found that there were two factors that affected how much voltage was induced in the coil.

Faraday conducted many experiments involving magnetic fields and conductors.
Michael Faraday Picture

The first factor was the number of turns of wire in the coil, which increased the amount of wire exposed to the magnetic field. The results of Faraday's experiments showed that the induced voltage increased in direct proportion to the number of turns in the electrical coil. In other words, doubling the number of turns resulted in a doubling of the induced voltage.

The second factor was how quickly the magnetic field was changing. There are a couple of different ways that we can make a magnetic field change. One way is to change the strength of the field produced by the magnet. If we use an electromagnet to create the magnetic field, we can turn the magnet on and off or simply vary the current to change the strength of the field. The second way is to move the field relative to the conductor. We could do this by moving the coil around in the field or by moving the magnet around the coil - it doesn't matter which, as long as there is relative motion.

Faraday's Law came about as a result of his experiments. It simply states that the magnitude of the induced voltage is proportional to both the number of turns of wire and to the rate at which the magnetic field changes. One of the most important things to take away from this statement is that the induced voltage is the result of a changing magnetic field. In other words, simply holding a magnet stationary near a wire will not induce a voltage. The field must be changing in some way.

Voltage strength is proportional to the number of wire turns and magnetic field changes.
Faradays Law Experiments

Applications of Electromagnetic Induction

Electromagnetic induction has many practical applications, including data storage - for example, the magnetic strip on the back of a credit card like the one I was having trouble with at the grocery store. The strip stores data in the form of many magnetized regions, each surrounded by a small magnetic field. Have you ever wondered why you have to swipe a credit card through a reader as opposed to simply putting it in the machine? Based on what we've learned about electromagnetic induction, the answer should be pretty clear. By moving the card through the reader, the magnetic domains induce a voltage in the circuit of the machine, which allows it to read the data. If you don't move the card, then there won't be any voltage.

We've also learned that moving the card faster causes a greater voltage to be induced, which is why the card worked better when the cashier swiped it for me. The idea of storing data in the form of magnetic media has been around for a long time. For example, audio and video cassettes, floppy disks, and even conventional hard drives in your computer use magnetic regions to store information.

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