Magnetism and Its Fields

If you have a refrigerator, you probably have had experience with magnets. Magnets are not new technology, but have been around since ancient times. In fact our Earth is one Enormous Magnet. Magnets are an important part of current electricity, and they are used in a great many devices, from generators to motors to compasses to cassette tapes (Phil doesn’t have a CD player) to video tapes.

If you’ve ever seen a compass, you will remember that it has the interesting property of always pointing to the north. This is because there is a magnet on the end of the compass needle. If you bring a bar magnet near to a compass, you will notice that the needle repels from the bar magnet’s north pole. This means that the compass needle is also a north pole. Like charges repel. So if the magnet is north, it should point AWAY from the north pole of the earth, since like charges repel. The answer to this little riddle is simple- the north pole of the earth is actually a magnetic south pole. Ah ha! Another mystery explained by Physics.

The properties of magnets should not need much explanation. There is a north and a south end. The north attracts south, and vice versa. They are usually made out of metal, mostly Iron, Neodymium or ALNICO (ALNICO is an alloy of ALuminum, NIckle and CObalt).

Much like an electric field around an electric charge, there is a magnetic field around a magnet. Anything that enters this force (and is magnetically inclined) will experience a force.

A man named Oerstead once discovered that wires that conduct electric charge also creat a magnetic field. (This is called electromagnetism.) If you want to find the direction of this field, it is much easier than the electric field method. All you need to do is grasp the wire in your left hand, with your thumb in the direction of the flow of charge (negative). Your curved fingers will be pointing in the direction of the field. This is called the Left Hand Rule. Magnetic Fields have a circular direction around a wire. This only deals with straight wires. However, sometimes we make what is called a coil. If you want to make a coil, wrap some wire around and around a pencil, but don’t let the wire cross itself. Then remove the pencil. Ta-da, you have a simple coil!

There are actually three left hand rules. The second left hand rule states that if your wrap your left hand around a coil of wire, with your fingers in the direction of the flow of charge, your thumb points to the end of the coil that is the north pole. That’s right- coils of current conducting wire are also magnets!

Since a wire creates a magnetic field of it’s own, if we place a wire into a magnetic field, the two fields will interact. There will be a force on the wire. According to the famous scientist Michael Faraday: “The force on a wire is at right angles to the direction of the magnetic field.” This statement leads us to the third and final left hand rule. Flatten your hand. Point your thumb in the direction of the flow of negative charge. Point your fingers in the direction of the magnetic field. You palm will be pointing in the direction of the force on the wire! It’s a miracle! Well, maybe not a miracle. Still, it is an amazingly easy way to find the direction of a force.

Now that we can find the direction of the force, we need to calculate it’s magnitude. No, don’t groan. It’s just another formula, and all you need to do is plug numbers into it. After you understand the formula, it’s a no-brainer. Here it is:

F=BIL

That’s right, the fore equals a guy named Bill. No wait, that’s not it. B is actually the strength of the magnetic field. I us the current, and L is the length of wire that is inside of the field.

What do we mean, “strength” of the field? Well, you can probably use algebra to turn that equation you just learned into an equation for field strength.

B= F / IL

The field strength is actually called “Magnetic Induction.” B is the magnetic induction, sometimes called the “B field.” The unit of magnetic induction is the telsa (T). One telsa equals one Newton per Ampere times Meter. It’s easier to just remember telsa.

Current carrying wires experience force in a magnetic field. We said that. We also said that current carrying wires have charged particles flowing through them. So if a single charged particle was in a field… all by itself… yes, it would get lonely, but it would also experience force. The equation to find the force on a single particle is:

F= Bqv

F is force (as usual), B is the magnetic induction, q is charge, and v is velocity. Do NOT confuse the velocity v with a Voltage V. That could be bad come test time.

Since a wire in a magnetic field experiences a force, we should be able to use that force. Physics is useful. We have said that a hundred times. Long ago, people figured out how to put a bend of wire into a magnetic field. Then they gave it a nudge. The force took over, and since it was at a right angle to the magnetic field- it turned the loop. A little bit. However, soon the wire was turned so that the force was in the other direction. So what did we brilliant scientists do? Reverse the current. The motor was born. Every half turn, the current in a motor is reversed. This makes the loop inside of the field spin. This makes anything attached to the loop spin. An electric motor- there are literally millions of uses for an electric motor, as we are sure you already know.

Electromagnetic Induction isn’t a complex concept. Faraday, one of the principal founders of electricity and physics, discovered that if he moved a wire through an electric field- it produced a current. This is electromagnetic induction. The current direction can be found using the third left hand rule, which you already know.

When a wire moves in a magnetic field, a force acts on the charges in the wire. Work is done on the charges. Their potential energy is increased, thus the potential difference is increased. This is called inducing an EMF. EMF stands for electromotive force, but EMF isn’t really force at all. EMF is measured in Volts, just like potential difference. The term EMF is misleading, but there is nothing you can do about it- it has been used since before electricity was fully understood.

To calculate EMF, you use a simple formula:

EMF = BLv

This should be easy for an experienced physicist like you to understand. EMF in volts equals the magnetic induction times the length of wire times the velocity of the wire.

We hope you still remember what vectors are. When a wire moves in a magnetic field, only the resolved velocity vector which is perpendicular to the field produces a current. You need to use only the perpendicular velocity, so be careful, sometimes questions can be confusing.

Faraday also invented the generator. Remember the electric motor? Well, the generator is the same. Exactly. Only instead of electricity turning the loop of wire, we ue a force to turn the loop, which produces electricity. Technically, if you spin a motor you are generating electricity.

If you are particularly intelligent, you may have noticed a bit of a paradox in what we have taught. Current induces a magnetic field. Motion in a field induces current. So if a wire moves through a field, it will have current. This current will create a field. The fields will interact. The force on the moving wire will be in the opposite direction of the motion of the wire. The more you move the wire, the greater the force in the opposite direction. This is what a scientist named H. F. E. Lenz figured out. “The direction of the induced current is such that the magnetic field resulting from the induced current opposes the change in the field that caused the induced current.” It is called Lenz’s Law. Lenz’s Law applies to motors. Once a motor gets going, it will start to produce it’s own magnetic field. This will create a current which opposes the current to the motor. There is a current conflict. Luckily, the opposing current is never strong enough to counteract the original current. The occurrence of a counter-current in a motor is called Back-EMF.

You have reached the final lesson in electricity. If you have made it this far, you should go back and read it all again. Then you’ll be a REAL expert. We’ve covered an entire year of physics (in some schools) in only a few paragraphs. But this is intense stuff, it is the stuff that makes you into a physics superman.

A while ago you read that the power company transmits electricity at an extremely high voltage, and a very low current. This is to minimize heat and energy loss. It makes sense. But say you have 200 Volts, 10 Amps. How do you make the current (I) smaller and the Voltage (V) greater?

First of all, you know that power is V times I. This means that 2 Amps times 4 Volts has the same power as 4 Amps and 2 Volts. Right? Right. So:

Power In = Power Out

This is the concept of a Transformer. Transformers are not those robotic toys. But they really ARE “More than meets the eye”. A transformer transforms electricity, increasing voltage and decreasing current. If you know anyone who has a large electrical box on their lawn or street corner, there is probably a transformer inside.

A transformer is a looped conductor, with 2 coils, one on each side. Just look at the diagram and it should be pretty easy to understand. Electricity comes in one side, and loops around the conductor. This coil is the primary coil. The primary coil induces a current in the conductor, which travels over to the other coil, the secondary coil. This induces a current in the coil, which travels out the other side. If the primary coil has more loops, voltage is reduced. If the secondary coil has more loops, voltage is increased.

The formula for figuring out the voltage output of a transformer is easy.

V out = Loops out * V in / Loops in

Now you know all about electricity. Please don’t ever stick a screwdriver into an electrical circuit. It hurts. Lots. Trust us. Did we mention that it hurts? It does. Lots.http://library.thinkquest.org/13526/c3c.htm

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