How Electromagnets Work

Introduction to How Electromagnets Work


­Th­e basic idea behind an electromagnet is extremely simple: By running electric current through a wire, you can create a magnetic field.
By using this simple principle, you can create all sorts of things, including motors, solenoids, read/write heads for hard disks and tape drives, speakers, and so on. In this article, you will learn exactly how electromagnets work. You will also have the chance to try several experiments with an electromagnet that you create yourself!

A Regular Magnet

Before talking about electromagnets, let's talk about normal "permanent" magnets like the ones you have on your refrigerator and that you probably played with as a kid.
You likely know that all magnets have two ends, usually marked "north" and "south," and that magnets attract things made of steel or iron. And you probably know the fundamental law of all magnets: Opposites attract and likes repel. So, if you have two bar magnets with their ends marked "north" and "south," the north end of one magnet will attract the south end of the other. On the other hand, the north end of one magnet will repel the north end of the other (and similarly, south will repel south).
An electromagnet is the same way, except it is "temporary" -- the magnetic field only exists when electric current is flowing.

An Electromagnet

An electromagnet starts with a battery (or some other source of power) and a wire. What a battery produces is electrons.
If you look at a battery, say at a normal D-cell from a flashlight, you can see that there are two ends, one marked plus (+) and the other marked minus (-). Electrons collect at the negative end of the battery, and, if you let them, they will gladly flow to the positive end. The way you "let them" flow is with a wire. If you attach a wire directly between the positive and negative terminals of a D-cell, three things will happen:
Electrons will flow from the negative side of the battery to the positive side as fast as they can.
The battery will drain fairly quickly (in a matter of several minutes). For that reason, it is generally not a good idea to connect the two terminals of a battery to one another directly. Normally, you connect some kind of load in the middle of the wire so the electrons can do useful work. The load might be a motor, a light bulb, a radio or whatever.
A small magnetic field is generated in the wire. It is this small magnetic field that is the basis of an electromagnet.

Magnetic Field


The part about the magnetic field might be a surprise to you, yet this definitely happens in all wires carrying electricity. You can prove it to yourself with the following experiment. You will need:
One AA, C or D-cell battery
A piece of wire (If you have no wire around the house, go buy a spool of insulated thin copper wire down at the local electronics or hardware store. Four-strand telephone wire is perfect -- cut the outer plastic sheath and you will find four perfect wires within.)


Put the compass on the table and, with the wire near the compass, connect the wire between the positive and negative ends of the battery for a few seconds. What you will notice is that the compass needle swings. Initially, the compass will be pointing toward the Earth's north pole (whatever direction that is for you), as shown in the figure on the right. When you connect the wire to the battery, the compass needle swings because the needle is itself a small magnet with a north and south end. Being small, it is sensitive to small magnetic fields. Therefore, the compass is affected by the magnetic field created in the wire by the flow of electrons.


The Coil

The figure below shows the shape of the magnetic field around the wire. In this figure, imagine that you have cut the wire and are looking at it end-on. The green circle in the figure is the cross-section of the wire itself. A circular magnetic field develops around the wire, as shown by the circular lines in the illustration below. The field weakens as you move away from the wire (so the lines are farther apart as they get farther from the wire). You can see that the field is perpendicular to the wire and that the field's direction depends on which direction the current is flowing in the wire. The compass needle aligns itself with this field (perpendicular to the wire). Using the contraption you created in the previous section, if you flip the battery around and repeat the experiment, you will see that the compass needle aligns itself in the opposite direction.

Because the magnetic field around a wire is circular and perpendicular to the wire, an easy way to amplify the wire's magnetic field is to coil the wire, as shown below:

However, the magnet exists only when the current is flowing from the battery. What you have created is an electromagnet! You will find that this magnet is able to pick up small steel things like paper clips, staples and thumb tacks.

Experiments to Try!
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What is the magnetic power of a single coil wrapped around a nail? Of 10 turns of wire? Of 100 turns? Experiment with different numbers of turns and see what happens. One way to measure and compare a magnet's "strength" is to see how many staples it can pick up.

What difference does voltage make in the strength of an electromagnet? If you hook two batteries in series to get 3 volts, what does that do to the strength of the magnet? (Please do not try any more than 6 volts, and please do not use anything other than flashlight batteries. Please do not try house current coming from the wall in your house, as it can kill you. Please do not try a car battery, as its current can kill you as well.)

What is the difference between an iron and an aluminum core for the magnet? For example, roll up some aluminum foil tightly and use it as the core for your magnet in place of the nail. What happens? What if you use a plastic core, like a pen?
What about solenoids? A solenoid is another form of electromagnet. It is an electromagnetic tube generally used to move a piece of metal linearly. Find a drinking straw or an old pen (remove the ink tube). Also find a small nail (or a straightened paperclip) that will slide inside the tube easily. Wrap 100 turns of wire around the tube. Place the nail or paperclip at one end of the coil and then connect the coil to the battery. Notice how the nail moves? Solenoids are used in all sorts of places, especially locks. If your car has power locks, they may operate using a solenoid. Another common thing to do with a solenoid is to replace the nail with a thin, cylindrical permanent magnet. Then you can move the magnet in and out by changing the direction of the magnetic field in the solenoid. (Please be careful if you try placing a magnet in your solenoid, as the magnet can shoot out.)
How do I know there's really a magnetic field? You can look at a wire's magnetic field using iron filings. Buy some iron filings, or find your own iron filings by running a magnet through playground or beach sand. Put a light dusting of filings on a sheet of paper and place the paper over a magnet. Tap the paper lightly and the filings will align with the magnetic field, letting you see its shape!

How Inductors Work

Introduction to How Inductors Work

An inductor is about as simple as an electronic component can get -- it is simply a coil of wire. It turns out, however, that a coil of wire can do some very interesting things because of the magnetic properties of a coil.

Inductor Basics

In a circuit diagram, an inductor is shown like this:

To understand how an inductor can work in a circuit, this figure is helpful:

What you see here is a battery, a light bulb, a coil of wire around a piece of iron (yellow) and a switch. The coil of wire is an inductor. If you know How Electromagnets Work, you might recognize that the inductor is an electromagnet.
If you were to take the inductor out of this circuit, what you would have is a normal flashlight. You close the switch and the bulb lights up. With the inductor in the circuit as shown, the behavior is completely different.

­The light bulb is a resistor (the resistance creates heat to make the filament in the bulb glow -- see How Light Bulbs Work for details). The wire in the coil has much lower resistance (it's just wire), so what you would expect when you turn on the switch is for the bulb to glow very dimly. Most of the current should follow the low-resistance path through the loop. What happens instead is that when you close the switch, the bulb burns brightly and then gets dimmer. When you open the switch, the bulb burns very brightly and then quickly goes out.
The reason for this strange behavior is the inductor. When current first starts flowing in the coil, the coil wants to build up a magnetic field. While the field is building, the coil inhibits the flow of current. Once the field is built, current can flow normally through the wire. When the switch gets opened, the magnetic field around the coil keeps current flowing in the coil until the field collapses. This current keeps the bulb lit for a period of time even though the switch is open. In other words, an inductor can store energy in its magnetic field, and an inductor tends to resist any change in the amount of current flowing through it.

Henries
The capacity of an inductor is controlled by four factors:
The number of coils - More coils means more inductance.
The material that the coils are wrapped around (the core)
The cross-sectional area of the coil - More area means more inductance.
The length of the coil - A short coil means narrower (or overlapping) coils, which means more inductance.
Putting iron in the core of an inductor gives it much more inductance than air or any non-magnetic core would.
The standard unit of inductance is the henry. The equation for calculating the number of henries in an inductor is:
H = (4 * Pi * #Turns * #Turns * coil Area * mu) / (coil Length * 10,000,000)
­ The area and length of the coil are in meters. The term mu is the permeability of the core. Air has a permeability of 1, while steel might have a permeability of 2,000.

Inductor Application: Traffic Light Sensors

Let's say you take a coil of wire perhaps 6 feet (2 meters) in diameter, containing five or six loops of wire. You cut some grooves in a road and place the coil in the grooves. You attach an inductance meter to the coil and see what the inductance of the coil is.

Now you park a car over the coil and check the inductance again. The inductance will be much larger because of the large steel object positioned in the loop's magnetic field. The car parked over the coil is acting like the core of the inductor, and its presence changes the inductance of the coil. Most traffic light sensors use the loop in this way. The sensor constantly tests the inductance of the loop in the road, and when the inductance rises it knows there is a car waiting!
Usually you use a much smaller coil. One big use of inductors is to team them up with capacitors to create oscillators.