In 1819 Hans Christian Oersted performed an experiment to show that a wire carrying an electrical current could influence a nearby compass needle.
Remember two things from our last chapter; 1) magnets can only affect other magnets, and 2) a compass needle is a magnet. As the current flowed through the wire, the compass needle moved. What did this tell Oersted? That's right, an electric current can produce magnetism. Not long after Oersted's discovery Andre' Ampere invented a device known as an electromagnet. 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.
Look at the picture to the right, an electromagnet needs a power source, a wire that connects the + and - ends, and some type of magnetic material like an iron nail. By wrapping the wire around the nail, and closing the circuit, the nail becomes a magnet. The more times you wrap the wire around the nail, the greater the magnetism it shows.
To understand how an electric motor works, the key is to understand how the electromagnet works. An electromagnet is the basis of an electric motor. You can understand how things work in the motor by imagining the following scenario. Say that you created a simple electromagnet by wrapping 100 loops of wire around a nail and connecting it to a battery. The nail would become a magnet and have a north and south pole while the battery is connected.
Now say that you take your nail electromagnet, run an axle through the middle of it and suspend it in the middle of a horseshoe magnet as shown in the figure below. If you were to attach a battery to the electromagnet so that the north end of the nail appeared as shown, the basic law of magnetism tells you what would happen: The north end of the electromagnet would be repelled from the north end of the horseshoe magnet and attracted to the south end of the horseshoe magnet. The south end of the electromagnet would be repelled in a similar way. The nail would move about half a turn and then stop in the position shown.
Electromagnet in a horseshoe magnet
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You can see that this half-turn of motion is simple and obvious because of the way magnets naturally attract and repel one another. The key to an electric motor is to then go one step further so that, at the moment that this half-turn of motion completes, the field of the electromagnet flips. The flip causes the electromagnet to complete another half-turn of motion. You flip the magnetic field simply by changing the direction of the electrons flowing in the wire (you do that by flipping the battery over). If the field of the electromagnet flipped at just the right moment at the end of each half-turn of motion, the electric motor would spin freely.
The armature takes the place of the nail in an electric motor. The armature is an electromagnet made by coiling thin wire around two or more poles of a metal core.
The armature has an axle, and the commutator is attached to the axle. In the diagram to the right you can see three different views of the same armature: front, side and end-on. In the end-on view the winding is eliminated to make the commutator more obvious. You can see that the commutator is simply a pair of plates attached to the axle. These plates provide the two connections for the coil of the electromagnet.
The "flipping the electric field" part of an electric motor is accomplished by two parts: the commutator and the brushes.
The diagram at the right shows how the commutator and brushes work together to let current flow to the electromagnet, and also to flip the direction that the electrons are flowing at just the right moment. The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the magnet. The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator.
When you put all of these parts together, what you have is a complete electric motor:
In this figure, the key thing to notice is that as the armature passes through the horizontal position, the poles of the electromagnet flip. Because of the flip, the north pole of the electromagnet is always above the axle so it can repel the field magnet's north pole and attract the field magnet's south pole.
In 1831, English scientist Michael Faraday discovered that electricity could be made by moving a magnet inside a wire coil. From Oersted's discovery, Faraday knew an electric current could produce magnetism, but could magnetism produce electricity? The answer is yes. Through several experiments,
Faraday was able to show that a changing magnetic field will induce a current in a wire.
Look at the animation to the right, notice how the electrons in the circuit change their direction of flow as the magnet moves. As the magnet moves the magnetic field is changed. According to Faraday, a changing magnetic field induces an electric current in a metal wire. An important aspect of this process is shown by an electric generator. A simple generator consists of a loop of wire mounted on a rod, or axle, that can rotate. The loop of wire, which is attached to a power source, is placed between the poles of a magnet. When the loop of wire is rotated by the power source, it moves through the field of the magnet. Thus it experiences a changing magnetic field. The result is an electric current. The large generators in power plants have many loops of wire rotating inside large electromagnets. The speed of the generators is controlled very carefully. The current is also controlled so that it reverses direction 120 times per second. Because two reversals make one complete cycle we describe this type of current as alternating current. Alternating current (AC) in the United States has a frequency of 60 Hertz.
Can you increase or decrease the voltage of the current? Yes, a device known as a transformer operates on the principle that a current in one coil induces a current in another coil. A transformer consists of two coils of insulated wire wrapped around the same iron core.
One coil is called the primary coil and the other coil is called the secondary coil. When an alternating current passes through the primary coil, a magnetic field is created. The magnetic field varies as a result of the alternating current. Electromagnetic induction causes a current to flow in the secondary coil. If the number of loops in the primary and secondary coils are the same, an equal amount of voltage is transferred. If the secondary coil has more loops than the primary coil, voltage is increased. What happens if the secondary coil has fewer loops than the primary coil? That's right, the voltage is decreased. Look at the picture to the right, can you identify the difference in the number of loops in each coil? As you can tell, a transformer can either increase or decrease voltage. The terms step-up transformer and step-down transformer are used to describe whether a transformer is increasing or decreasing voltage.
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If a magnet is allowed to float (a compass is a floating magnet), one end will always point North and one end will always align South. Because of this the two ends of a magnet are referred to as the North and South poles. When two magnets are brought near each other, they follow the Law of Charge - like poles repel; opposite poles attract. Although magnetic forces are strongest at the poles of the magnet, they are not limited to the poles alone. Magnetic forces are felt around the rest of the magnet as well. The region in which mangetic forces can act is called a magnetic field. Why does one pole of a bar magnet always point north and the other pole south? In the 1600s, William Gilbert proposed the idea that the Earth itself was a magnet. He correctly predicted that since the Earth was a magnet, it must have magnetic poles.
Today, scientist know that the Earth behaves as if it has a huge bar magnet buried deep within it. Since the Earth is a magnet, there must be a magnetic field that surrounds the Earth (amagnetosphere>.
Scientist have been able to learn a great deal about the Earth's magnetic field and how it changes over time. Tiny magnets floated in liquid molten rock thousand of years ago. As the liquid rock cooled, the magnets were frozen in position thus a permant record of Earth's magnetism is recorded. The pattern of the magnets reveals that the magnetic poles of the Earth have reversed themselves many times. The north pole of a compass needle points to the North Pole of the Earth. But to exactly which north pole? As you have learned, like poles repel and opposites attract.
So the magnetic pole of the Earth to which the north pole of a compass points must actually be a magnetic south pole. In other words, the north pole of a compasss needle points toward the geographic North Pole, which is actually the magnetic south pole. Confused? Think about it for a moment and it will make sense. The location of Earth's south magnetic pole currently is in northern Canada about 1,500 km from the geographic north pole. Remember there are two poles of the Earth - the geographic and the magnetic. Are all things magnetic? No, only certain materials show magnetic properties. The most highly magnetic materials are called ferromagnetic materials. The name comes from the Latin name for iron, ferrum. So metals such as iron along with cobalt, neodymium , and nickel are just a few substances that show magnetic properties. Why do some materials show stronger magnetism than other materials? Do you recall the term "magnetic field" from earlier in this chapter?The region in which mangetic forces can act is called a magnetic field. These magnetic fields for magnetic domains. When you break a magnet into what happens? That's right you have two magnets instead of one. A magnet is actually made of several tiny magnets, each with its own magnetic field. If the magnetic fields of all the tiny magnets align in the same direction (meaning all north poles point north and all south poles point south) you have a strong magnet. A permanent magnet has all of its magnetic fields (domains) lined up in fixed positions. If the fields do not line up together, the magnet is weak. A temporary magnet has domains that line up properly only when a permant magnet is brought near. Once the permanent magnet is removed, the temporary magnet's domains return to their original "unaligned position."
