Magnetism

Undoubtedly you have some familiarity with magnetism. It holds notes to the fridge and makes compasses point north. You probably come across magnetism everyday without knowing it. There are large magnets in your stereo speakers and magnetism is involved with your phone, car, and any motor.

Magnetism can be perplexing. A simple bar magnet has two "poles" label north and south. Similar to electrical charge, like poles repel one another and unlike poles attract. The earth itself exhibits magnetism. There is a magnetic south pole near the earth's geographic north pole. (And there is a magnetic north pole near the earth's geographic south pole.) A compass needle is a simple suspended magnet. The north pole of the compass needle (shown as blue) is attracted to the earth's magnetic south pole ... i.e. it points to the geographic North.

But there is a very significant difference. You cannot isolate magnetic north and south poles ... they always come in pairs. You can separate electric poles (postive and negative charge), but not magnetic poles. Why is this? Because magnetism is not caused by magnetic charges. It is caused by current! We introduce the concept of the magnetic field to help visualize how current creates magnetism and why poles always come in N - S pairs. (Michael Faraday was responsible for much of our understanding of magnetism. He was not very good at math, so he described his findings with picture-oriented fields.)

We did not need to introduce the idea of electric fields in the prior sections, but we do so now to help understand magnetic fields. Electric charges exert forces on each other without touching. (Gravity works this way as well.) The field is a scheme of picturing how this can happen. Every charge fills the space around it with its own electric field. It exerts forces on all other charges via this field. The field is represented by field lines or lines of force. The force on one charge is always tangent to the lines of force created by the other charge(s). (A battery creates an electric field along the wires and circuit elements of a circuit. The electric field exerts the force on the electrons that pushes them through the circuit.) The units for the electric field are N/C. The strength of an electric field is measured by the number of Newtons of force it exerts on a charge, divided by the magnitude of that charge.

By convention, electric field lines point away from positive charges and towards negative charges. Look at the electric field lines created by the dipole (dipoles are composed of equal but opposite charges) shown. A third positive "test charge" is shown at various places. (Yes, the test charge also creates it own electric field, but for simplicity we have left it out.) Note how the direction of the net force on this test charge is consistent with it's attraction to the negative charge and repulsion from the positive charge. The positive test charged placed at any point in space would be pushed along the field lines until crashed into the negative charge. Can you guess what the electric field of two positive charges would look like?

Analogous to the electric field, the magnetic field is a way of visualizing how magnets effect each other. Look at the magnetic field of the bar magnet (note how similar it looks to the electric dipole!). By convention, the magnetic field lines "flow" away from the N-pole and into the S-pole. A "test magnet" or compass placed anywhere will feel a force along the field lines of the bar magnet. But don't forget the compass always has both a north (blue) and south (red) pole. So it will experience opposite forces creating a twist or torque that lines it up with the magnetic field lines. (The compass will also experience a small net force as well, if the force on its N-pole is different from that on its S-pole. Only if the field is perfectly uniform, will the net force be exactly zero. But the twist is the most important aspect to remember.)

The magnetic field is not created by north and south magnetic "charges". It is created by current ... moving electric charges! And just where is the current in a bar magnet? It is created by the electrons orbiting about the nucleus. The orbiting electron constitutes a current loop that creates its own magnetic field. Most atoms have a magnetic field. But typically, the atoms are randomly oriented and the fields created by the billions upon billions of atoms tend to cancel one another. However, in some substances like iron, nickle, and niobium, a small percentage can aligned to create a net magnetic field.

Magnetic field lines always form circles around the current creating them. Look at the diagrams below for three important cases ... the long straight wire, a simple loop, and a solenoid (which consists for many loops wrapped around a cylinder). Try imagining how the field lines move with a straight wire as it is wrapped into a circle. Can you see how the magnetic field for the loop is consistent with that of the straight wire? It takes some time and 3-D visualization to see it. If you are artistically inclined, you'll probably find it easier to see. The solenoid is just the combination of many closely space loops. The field inside the solenoid can be very uniform if the winding of the wire is tight.

An important thing to notice is that the magnetic fields lines are continuous. Unlike electric fields, they don't start at one pole and end at another. This is why you can't separate the N and S poles of a bar magnet. If you break it in two, you have two magnets! How can this be?

Just like the solenoid, there are field lines inside the bar magnet ... completing the loop! Inside the magnet, the lines flow South to North. Notice how the solenoid field looks very similar to that of the bar magnet? That's no accident. A bar magnet has a small percentage of its "atom" current loops align. The net effect is very similar to the solenoid. When the bar magnet is broken in half, you simply have two magnets. You could theoretical continue to divide the bar magnet until you got down to the individual atoms, where the field looks like of the single current loop. The unit for the magnetic field is the Tesla (T), named after a truly bizarre scientist from the late 1800's and early 1900's. Similar to how the electric field is defined, the tesla is defined by the force a magnetic field exerts on a current. The units are T = N / A^.m. The tesla is a large unit. The earth's magnetic field is about 10^-5 T and the largest magnetic fields produced by man in a laboratory is about 50 T.

Do magnetic fields created with currents have N and S poles? It would be hard to label N or S anywhere for the straight wire. But for the loop and solenoid, we use the same convention as for the bar magnet. Look at the diagrams for the bar magnet and solenoid and see how the definitions are consistent. At this point, you should visit NASA's Earth's Magnetosphere site. Although centered about the earth's magnetic field, it provides a good overview of magnetism in general.

We are now in a position to understand motors and generators. We will start with a very simple DC motor. A coil of wire, through which current can flow, is supported such that the coil can rotate. The coil is immersed in a magnetic field. In the diagram, we have two permanent magnets with opposite poles facing. If we connect the two ends of the coil to a battery, current will flow and create a magnetic field. Note the direction of the current and the resulting magnetic field. The top of the loop looks just like the N-pole of a magnet and the bottom like a S-pole. The attraction to the opposite poles of the permanent magnets nearby cause the coil to rotate. (The repulsion of like poles does the same thing!) We've started the motion, but we have to be crafty to keep it going.

The coil would rotate 90^o such that opposite poles faced and then stop. But we use a clever device called the DC commutator that reverses the direction of the current at just the right point. Look at the diagram of the DC commutator. Note that as soon as the coil rotates 90^o from the postion shown, the commutator contacts (called brushes) switch the ends to which the battery is connected, reversing the current flowing through the coils and reversing the magnetic field. So the coil rotates 180^o, at which point, the contacts switch again. And so the rotation continues. The starter motor in your car is a DC motor.

AC motors are essentially the same, but the AC commutator is even simpler. Since AC current already switches direction, the AC commutator does not have to switch contacts. Although the construction of a good motor will be considerably more complex than that shown, it operates on this very simple principle.

Believe it or not, you have now seen a generator as well! An AC generator is constructed, in principle, exactly the same as the motor. An outside agent rotates the coil and an AC potential is induced in the coil. It's the electromagnetic analog to Newton's Third Law .... if object A pushes on object B, then object B pushes back on object A. Similarly, If an AC current will make a coil spin, then spinning the coil will create an AC current. Faraday looked at it in the following way. When a wire is moved in a way such that it "cuts across" magnetic field lines, a force is exerted on the charge in the wire that tries to move it along the wire. Hence, a current can be created by the magnetic force on the electrons in the wire. (Of course, you need a complete circuit for current to flow, but the force or potential is still there.) Check out this great animated Generator Java Applet and watch exactly how it happens.

Giant coils (called armatures) spin in the magnetic fields in WAPA's generators and produce AC potential for your home. The picture shown is one of the first hand-cranked generators built and is called the Ampere-Pixii generator. In this version, it is the magnets that spin! It was Werner Siemens who invented the modern commercial generator. The magnetic field is provided not by magnets, but by field coils that use part of the current generated current. For more information on the history of power generation, check out the Dranetz/BMI Early History of Electric Power.

The effect of magnetic induction can happen in just about any way you can imagine. What is required is a change. A wire can be moved relative a magnet, a magnet relative to a wire, the shape or orientation of a wire loop can be changed, or the current creating a magnetic field can change. It is this last variation that can create problems for the unwary do-it-yourselfer. The National Electric Code (NEC) specifies that Electrical Metallic Tubing (EMT) conduit is to be used for running wires and groups of wires in all but a few cases. (Plastic conduit may be used in dry, exposed areas.) The NEC further specifies that all wiring must be done in pairs. That is, the return or ground wire and the hot wire must be within the same conduit. This guarantees that at any instant, there is equal current runiing in opposite directions within the conduit. Effectively, the magnetic fields from each cancel one another. If the hot wire runs along one section of the conduit and the return runs elsewhere, then the AC current in the "unpaired" wires will create an alternating magnetic field. This changing magnetic field will produce an induced current in the EMT conduit. If the wire current is high enough, the induced current may produce dangerous heating of the EMT.

Since most household wiring is manufactured in pairs, this problem does not usually arise in the initial construction. But when lights or outlets are added to a home, care must be taken not to introduce this problem by using single wires or by being careless with ground connections within breakout boxes. A breakout box is simply a junction (contained within an NEC approved protective box) where two or more circuit "branches" are connected together with the main line. If the hot and ground wires of the new outlet are connected to different branches, then the returned current may not always run along the same path as the current.

There is something about an electric motor that you may have wondered about. How does it know when it's supposed to be a motor and when it is supposed to be a generator. The short answer is ... it doesn't! It is always both. And that's why the lights dim when the compresssor motor in the fridge comes on ... that, and perhaps inadequate wiring. A basic property of motors is that they use a large initial current to get started. The actual resistance in the coils of the motor is quite small, so the initial current is large. If your wiring is not really designed to handle this much current and the lights are on the same circuit, you'll notice the potential drop across the wiring. It's the same as the car example discussed earlier.

But as the motor starts to spin, its generator nature starts to produce a back potential. The back potential grows as the motor picks up speed. And this back potential always opposes the applied potential that started it moving. It's Newton's Third Law for electricity again. Eventually, equilibrium is reached when the motor reaches its final speed. The current is at a minimum at this point. Because of this property of motors, circuit breakers are usually designed to handle excess loads for a few seconds. The lights dimming in the house may not be a serious matter if it is small and short-lived. But proper wiring provides a separate circuit for large appliance such as the fridge, washer and dryer. They all have motors and draw a large initial current.

Incidently, this why there is often a dangerous fire hazard with large motors "freezing" up. If the motor bearings fail or something stops the rotation, the coils may heat up and ignite the insulation within the motor or other nearby flammable materials.

The magnitude of the potential you get from a generator depends on many factors. The most important factors are the size and number of turns of the coil, the strength of the magnetic field, and the frequency of rotation. (The frequency of rotation is fixed at the industry standard of 60 Hz.) The AC potential output is built into the design of the generator. Most commercial generators are designed to produce about 600 VAC. But you are well aware that there is not 600 VAC at your electrical outlets.

Probably the most important factor in allowing AC power to be efficiently transported from the power plant to your home is the ability to easily transform from one AC voltage to another. An AC current in one coil can induce an AC potential in another without physically touching it. It does this in a fashion very similar to the generator. When an AC current flows in one coil, which is called the primary coil, it produces a magnetic field that increases and decreases and reverses direction just as does the current. If the magnetic field lines are made to pass through the second coil, called the secondary coil, then it will induce an AC potential. (The iron "doughnut" shown in the picture efficiently directs the field lines created by the primary coil through the secondary coil.) It's as if the secondary coil was moved back and forth through a magnetic field ... but without actually having to move. There is a very simple and symmetric relationship between the potential across the primary coil and the potential induced in the secondary coil. The potential per loop is the same for both coils. Hence, the ratio of the primary potential to the secondary potential is the ratio of the number of turns in each. In equation form, we can express the relationship as
N_p / N_s = V_p / V_s or

A step-up transformer increases the voltage (N_s > N_p) and a step-down transformer decreases the voltage (N_s < N_p). Recall the problem from the previous section where you calculated the power loss in a high tension wire. The higher the potential, the lower the current and the smaller the power loss through the resistance in the powerlines. WAPA uses a step-up transformer to increase the 600 VAC from the generators to about 14, 000 VAC. Of course, this would be far too dangerous for home use. So a step-down transformer (the cylinders you see on the poles outside your house) reduces the 14,000 VAC to a more reasonable 120 or 240 VAC.

Why 240 VAC rather than 120 VAC was discussed in a previous section. Some devices, such as clothes dryers and electric ranges, have large power requirements. Such devices are designed to use 240 VAC and use only half the current they would with 120 VAC. (Recall that power = IV.) Hence, 240 VAC reduces heating losses in the house wiring. So are there two transformers outside your house? Most likely not.

Most residential buildings are supplied with what is called three wire - one phase power service. The secondary coil of the transformer outside your home produces 240 VAC. There are three "taps" on the secondary, two at either end and one at the center. The two wires coming from the ends have a potential difference of 240 VAC. But each have a potential difference of 120 VAC relative to the center tap. The center tap is the white wire referred to as the ground or return. The end taps are the hot wires, usually color coded as black and red in the house wiring. (These two hot wires are usually referred to as the different "phases" of the power service.) For 120 VAC devices, you may connect them to either the black and white lines or to the red and white lines. The load should be equally divided between the two. All 240 VAC devices are connected to the black and red lines. The white line is usually near local ground potential, but that is not always the case. When working with household wiring, it is best not to assume that you can safely touch a white wire. Always turn off the circuit breaker to the area where you are working.

What is shown in the diagram is the transformer, the meter box (which includes fuses and a main cutoff switch), and a simplified schematic of the service panel box that is usually located inside the house. There will be two main breakers that control current flow from the two hot wires and many smaller breakers that control the different branches or circuits in the house. The main breakers are typically 100 to 150 A while the smaller breakers range from 15 to 60 A, depending upon the load expected for each branch. One of the special projects for this module is to outline the details for the service panel box.

The service panel box contains more than can be shown in this simple diagram. Check out a more realistic drawing. of the service panel and an animated circuit breaker in action.

It is worth mentioning that there are other power configurations that are common for commercial and industrial needs. There might be three or four line service in which the VAC potentials actually do have different phases. That means the sine wave forms are shifted relative to one another, making potentials such as 208 VAC possible.

Magnetism and the AC Generator