Electronics Engineering

Magnetic Field Lines Show Magnitude and Direction:

When the lines of the magnetic field are close together the magnetic field is strong. In a region where the line density is high, one says that it has great intensity or strength. Technically, the density of lines is proportional to the magnitude of the field.

The direction that magnetic field lines are oriented provides the information on the direction of the field, with the convention that the lines have arrows drawn so the lines exit from the North pole of a permanent magnet, or equivalently, the arrows point in the direction that the North end of a compass needle would point if placed on the line.

More Information:

Magnetic field lines do not really exist any more than electric field lines exist, but both are extremely valuable representations of the strength and direction of magnet (or electric) fields. (Pictures of field lines are human creations to help understand fields, but the pictures of field lines that are correctly drawn do provide a faithful representation of the mathematical description of the field.)

When one says a magnetic field is "strong" one does not mean that it exerts a great force on another magnetic object. Strength of a magnetic field is defined in terms of torque, not force. Here is how.

Magnetic Vector Field Definition:

If a magnetic field exists at a certain point, then one characterizes it with a strength and direction. Indeed, one can do this at all points and know the magnitude and direction of the field at all points. Because vectors have magnitude and direction, we can associate a vector with the magnetic field at any point and that is why one says that a magnetic field is a vector field.

Magnetic Field Direction:

We define strength or magnitude of the field using the classic behavior that we see when we use a compass. The field exerts a torque on the compass needle to orient it. One end of the compass is marked "North" because, in the absence of any other field, the compass needle points to the "North" geographic pole of the Earth. Using this definition, when a compass is brought near a permanent magnet, the North end of the compass is said to point towards the South end of the magnet. (North poles attract South poles.) This pointing direction can be mapped out in the whole space around the bar magnet, orienting the compass not just flat or horizontal, but in all directions so as to determine which way the magnetic field points for all positions in the space around the permanent magnet. (This works equally well for electromagnets, but we describe it for the most simple situation.)

Magnetic Field Strength:

The field direction of at each point, as described above, is the direction the North end of the compass needle points when there is zero torque on the needle. If one turns the needle so it is perpendicular to the field direction, then the needle experiences maximum torque. That torque measures the strength or magnitude of the field. Thus, once you set the strength scale by assigning one value of the field strength for one particular compass, you can map out the field strength at all points with the same compass and you get the magnitude and direction of the magnetic field everywhere.

Magnetic Field Lines:

The above process produces a vector field meaning that at every point in space one could draw an small arrow with a magnitude and direction indicating the field. Such a drawing produces a picture a bunch of arrows in the space around the magnet representing the magnetic field. That is a perfectly good representation of a vector field.

One can also create a line representation of a vector field, magnetic, electric or other. For the magnet above, if you start at a point on the North end of the magnet and draw a line in the direction of the arrow, you immediately encounter another arrow. At the point of the new arrow you continue drawing the line in the direction of that arrow, until you get to the next arrow which gives the next direction and on and on. Basically, if you start at the North end of the magnet and draw a line in a direction that follows the directions of the arrows, that line ends up at the south end of the magnet. One can the pick a new starting position on the surface of the magnet and repeat the process. Doing this over and over produces a whole set of lines which eventually fills the entire space around the magnet.

The set of lines drawn are not the magnetic field lines. But, each of the lines drawn does follow a magnetic field line. That sounds odd, but to fairly represent the strength of the field, one has to have one more condition applied that determines how many lines are drawn. That condition is that the number of lines in any region of space is proportional to the strength of the field in that region. That requires some additional rule because the number or density of lines won't communicate the field strength if one just draws lines wherever one likes.

Density of Magnetic Field Lines:

When using field lines to represent a vector field (electric or magnetic or whatever) one needs the information of both magnitude and direction and the individual lines only give direction. So, convention is that the number of lines that you draw in any particular region of space is proportional to the strength (magnitude) of the field in that region. That is hard to do. Fortunately, magnetic field lines have a special property that makes this easier.

Magnetic field lines never begin or end.

The lines in a picture of a bar magnet makes it look like lines exit the North end and enter the South end and they do, but they keep going through the interior and come out the opposite end forming closed loops. Usually the part of the loops inside the magnet are not drawn, but they exist. It is clearer for electromagnets where the inside is visible in the drawing.

Drawing Lines:

The technical aspects of drawing the lines is too tricky to explain in detail, but here is a simple and correct approach.

First, decide how many lines are to be drawn. More is better, but let us suppose 100 for now. That means that all 100 must come out from the parts of the magnet deemed to be North and into the parts identified as South. That includes the ends and the sides and all regions of the surface where lines may go in or out. Draw more lines in the region where strength of the field at the surface is high and fewer out of regions where the strength is low. You have arranged the lines with the right density when putting your imaginary compass into a region with some density of lines will produce a maximum torque corresponding to the field strength. This all sounds a lot easier than it is.

They form complete loops.

Analog data is carried by an alternating current. If we were to graphically represent alternating current, it would appear as a wave, with voltage bouncing above and below the zero level. There are three factors to consider: frequency, amplitude, and phase.

Frequency is the rate at which the current alternates above and below the zero current level. When the current rises above zero, dips below zero and then returns to zero, we say the current has completed one "cycle". The name applied to the number of cycles per second is Hertz (Hz). Therefore, if there are 500 cycles per second for an analog signal, we say the frequency is 500 Hertz (500 Hz).

Amplitude would be viewed as the height (peak) and the depth (trough) of the graphic wave. As analog data travels over distance, the amplitude of the wave decreases. This characteristic is called "attenuation". The amplitude of analog waves is measured in watts, amps or volts. The measurement decibel is often used to describe the power of a signal.

Finally, phase describes the difference in the start of the cycle of one signal to the start of the cycle of another. One signal acts as a reference signal, the other signal is the phased signal. The level of non-synchronization is measured in degrees. If a signal is 180º out of phase, it means that as the reference signal reaches zero voltage following a peak, the phased signal begins. Thus as the reference signal is peaking, the phased signal is (for lack of a better term) troughing. The figure below illustrates this more clearly.

Phasing is the result of creating a signal out of sync with a reference signal.Altering the frequency, amplitude or phase of a signal is called modulation

.ASK:AM radio is produced by taking a basic signal (radio wave) and modulating its amplitude according to another signal (i.e. voice and music). AM stands for Amplitude Modulation. We may use the same technology for carrying computer data as well. For digital data, it's called Amplitude-Shift Keying (ASK).

Figure 5.2: Amplitude Modulation can be used to encode data in analog signals.

FSK: FM radio is produced by taking a basic signal (radio wave) and modulating its frequency according to another signal (i.e. music and voice). In this case, FM is an acronym for Frequency Modulation. In the digital data realm, the same technology can be applied using Frequency-Shift Keying(FSK). Fig illustrates a couple of examples.

Figure 5.3: Frequency Modulation may be used to encode data into an analog signal as well.

PSK:

Figure 5.4: Phase Modulation can be used to encode data in an analog signal. The amplitude is varied in some technologies.

In analyzing methods for carrying digital data, Amplitude-Shift Keying is fairly easy to accomplish. On the other hand, any kind of amplitude modulated signal is very susceptible to outside interference. Therefore, ASK is not really suitable for transmission over long distances.

Just as FM radio is not generally affected by weather, neither are FSK transmissions. In spite of this, Frequency-Shift Modulation is seldom used for transmission over high-speed lines as the technology does not allow as many bits per second throughput as PSK does.

Phase-Shift Keying technology is what is utilized by most high speed modern modems. It allows four different phases (in degrees) to encode data. The result is a potential for 600 phase shifts per second. Each phase shift represents a certain combination of 2 bits (i.e. 00 01 10 or 11). It then logically follows that since two bits are transmitted per phase shift and there are 600 phase shifts per second.

PSK is very resistant to external interference as it enjoys most of the same characteristics that FM or FSK devices do. The signals encoded using PSK may be used for synchronization purposes as well for the sender and receiver.