In Earlier times, Greeks and Etruscans found that a locally found rock, a lodestone, would always point in the same direction. This is the beginning of the study of magnetism. Only a few naturally occurring objects exhibit magnetic properties, but electrical currents are also affected by magnetic fields, which makes their study much more important.
The lodestone, now known as magnetite, is predominantly an iron alloy. Other elements such as nickel and cobalt can also be magnetized. Recently, many of the rare earth elements have been made into magnets using a high temperature ceramic process. The strongest of these are the ones made of neodymium.
Some magnets are permanent magnets since they do not lose their magnetic ability quickly, but rather stay magnetized for a long time, often for years. Other magnets are temporary magnets since they lose their magnetic ability very quickly. A refrigerator magnet would be a permanent, while an electromagnet would be a temporary one. Materials which are easy to magnetize (such as iron) also lose their ability soon, while those which are hard to magnetize (like nickel and cobalt) tend to hold on to it much longer.
Magnetic materials possess a polarity; two distinct regions of opposite affect. When suspended, a magnetic object will rotate until one end will point toward the north pole of the earth.. If displaced, it will return to this position. This would be the north pole of the magnet and the opposite end would be the south pole.
Poles of a magnet are similar to opposite electrical charges in their interactions: like poles repel and unlike poles attract. Unlike electrical charges, magnetic poles are never found alone. They always exist in pairs.
Since magnetic forces are detected at a distance from the magnetic object their must be a magnetic field. Similar to electric fields in shape and effect, magnetic fields can be drawn to show the intensity and direction at any point around the source of the field.
or infinity, they do not cross, and their density indicates
the strength of the field. The direction of the field lines shows
the direction of the magnetic force on a north mono pole
(if one existed).
In the late 1700"s it was observed that magnetic compasses (small free floating magnets) placed near current carrying wires would be effected when the current was on, but returned to normal when the current was off. It wasn't until the mid 1800's that the connection between the two was formalized into the science of electromagnetism. It is now well understood that moving charges create magnetic fields and that changing magnetic fields cause forces on electrical charges
A moving charge creates a magnetic field which moves radially away from the path of the charge. As in electrical definitions, the direction of the magnetic field is defined by conventional current. The shape of the conductor has a great effect on the shape of the resulting magnetic field.
For a single straight wire, the resulting field lines for a current in the wire look like concentric rings around the wire. There are no poles in this field, since the lines are circles and do not touch the wire. Since the field gets weaker with distance from the wire, the lines should get farther apart.
The direction of the field can be determined by applying a right hand rule; the thumb of the right hand points in the direction of conventional current, the fingers curl around the wire in the direction of the magnetic field.
If the current conductor is wound into a coil, the individual magnetic fields of each of the loops of the coil superimpose to create a stronger, more direct field. Such a current carrying coil is called a solenoid. The field for a solenoid has polarity, one end of the coil acts a a north pole while the other end becomes a south pole. If a soft iron core is placed in the coil, the field becomes more concentrated and stronger in the region of the iron. Notice the similarity of the field for a solenoid and for that of a bar magnet.
The polarity of the coil can be determined through the application of a second right hand rule. If the fingers of the right hand curl around the coil in the same direction as conventional current, the thumb points to the north pole of the coil.
Magnetic Force
If a charged particle sits in a magnetic field, nothing special happens. If the particle is in motion, however, it gets interesting. When a charge moves through a magnetic field, as a single particle or as a current, there is a magnetic force on the particle perpendicular to its motion and also to the magnetic field.
The magnitude of the force depends on three factors; the size of the charge (q), the speed of the particle (v), and the strength of the magnetic field (B).
This equation can be used to experimentally define the strength of a magnetic field. If a one coulomb charge moves through a magnetic field at one m/s and experiences a force of one Newton, then the strength of the field is one Tesla. A Tesla is a large unit of field strength, with even the largest of laboratory magnets reaching a few tens of Tesla. Most everyday magnetic fields, like the earth's or a refrigerator magnet, are in the micro to nanoTesla range.
The direction of the magnetic force on a charged particle can be determined using another right hand rule; the fingers of the right hand point in the direction of the magnetic field, the thumb points in the direction of the moving charge (current), and the force on the particle is perpendicularly away from the palm.
Since the force is always perpendicular to the motion of the charge, a single charge will experience a centripetal force and travel in a circular path, or at least part of one. A device known as a mass spectrometer uses this principle to identify the size and charge of unknown particles.
If the moving charge is part of a current in a conductor then the concuctor will experience a force perpendicular to itself and the field. The rules for the strength of the force and its direction on the conductor are the same as for a single charge.
When two parallel current carrying conductors are close to one another, their respective magnetic fields will superimpose and create a force between the conductors. If the currents are in the same direction, the field between the conductors is weaker than the field outside which causes an unbalanced force pulling the two together (attraction). If the currents are in the opposite direction, then the inner field is stronger and the conductors will be pushed apart (repulsion).
The use of electromagnetic factors in mass spectrometers and power lines has already been mentioned. A use closer to home would be their use in audio speakers.
The guts of a speaker includes a large permanent magnet with a voice coil wrapped around its core. The cone of the speaker (the part we see) is attached to the coil. As the current in the coil changes in response to the sound originally recorded, the force on the coil changes size and direction. This makes the coil move forward and backward in response to the force. As the coil moves, so does the cone, which pushes the air in accordance with the sound and electrical signals involved.
Induced current
You have already seen that a moving charge in a magnetic field can experience a force. The key elements are; a moving charge (current), a magnetic field, and a force. A current created the force.
Now, just turn that around: a wire forced through a magnetic field may have a current created in it. No source required, just move a wire through the magnetic field. Of course, as before, certain factors determine the amount and direction of the current.
The speed, direction, and orientation of the movement of the conductor through the field determines the aspects of the induced current. The maximum amount of current will be induced if the conductor moves perpendicular to the field, is moved at a faster speed, and if there is more conductor present (more wire).
The direction of the induced current is a little more complicated because once the charges are forced to move (current) by the magnetic field they possess a magnetic field of their own. These two fields (the one creating the force and the one of the current) will superimpose to create a separate, new force on the conductor.
If you think about it, the direction of this additional force can not be in the same direction as the force on the conductor in the first place. If it was then a small nudge of the wire would create the current, which would create a force on the wire in the same direction, which would make the wire move faster, which would create more current, more force, etc. It would be a run away situation. Somebody along time ago would have started a wire, and it would continue to move faster ever since.
This situation compels the new force to be in the opposite direction of the force which creates the motion. Think of it as induced electrical friction. The larger the induced current, the larger this opposing force. Many times it is referred to as back emf, or back current.
The formal statement of this phenomenon is Lenz's law; The magnetic field of the induced current opposes the change in the applied magnetic field. In other words, the direction of the induced current is opposite to the current which would create the motion in the first place.
We usually think of wires moving in a stationary magnetic field, but the wire could be stationary and the field move past it. The results would be the same, but sometimes the direction of the currents and fields can be a little challenging to determine.
Self inductance is a term used to describe the creation of a back current in a conductor when the current in a wire changes. If the current is increasing then the induced current tend to keep the current value from increasing as fast. If the current is decreasing, then the induced current would tend to keep it from decreasing as fast. Since solenoids have a larger magnetic effect due to the interaction of one section's magnetic field on another section, the self inductance of a solenoid is more pronounced than for a single, straight wire.
Applications of induction
There are many applications of induced currents and their magnetic fields. One of the more common ones is the recording of sound on magnetic tape (cassettes) or recording computer data on disks. Both of these systems are essentially the same.
A wire carrying a current which changes its magnitude and direction according to the information it carries is wrapped around a piece of iron. The changing current induces a changing magnetic field in the iron. The recording medium is a thin film of iron particles embedded in plastic. As they pass by the "head" of the iron magnet the regions of the iron particles align themselves with the magnetic field at that point in time. As the pattern of current changes, the pattern of the iron particles in the plastic changes accordingly. You can see why bringing a strong magnet close to these materials could ruin the data stored on them.
A device which utilizes the concept of induction is the transformer. A transformer consists of two separate coils of wire wrapped around the same piece of iron. Any changes in current in one of the coils creates a changing magnetic field in the iron. Since the other coil is also wrapped around the iron, any change in the iron's magnetic field causes a change in the current in that coil. Notice that a transformer only transfers electrical energy between the two coils when the current changes. If the current doesn't change in the first coil, there is no current in the second coil. Transformers are usually only used in alternating current circuits, where the current is continually oscillating back and forth. Transformers provide a way of isolating one electrical circuit from another since there is no electrical connection between the two coils, but a magnetic link which connects the two. The amount of energy (voltage and current) which is transferred depends on the sizes of each of the coils as well as their orientation on the iron core.
Generators
A generator is a device which converts mechanical energy into electrical energy. By moving a wire through a magnetic field, you have seen that a current can be produced. If this process can be done repeatedly the current can be maintained. In simplest terms a generator consists of a coil of wire which rotates in a magnetic field. Permanent magnets are usually used to create the magnetic field, and the coil is wrapped around an iron core to concentrate its field. In the diagram below only a single loop of the coil is shown for simplicity
As you can see from the diagram the magnitude and direction of the current changes as the loop turns through the magnetic field. The only parts of the loop which are involved in the generation of current are the sides which are perpendicular to the field (colored in yellow and blue). The other sides are parallel to the field and do not create current as they rotate.
In positions A and C the loop is creating the maximum current since the wires are moving perpendicular to the field. In positions B and D there is no current generated since the wires are moving parallel to the field. Notice that the current, or EMF, follow a sinusoidal curve. Since the current changes its magnitude and direction in a cyclic pattern it is referred to as an alternating current.
Alternating Current
An alternating current follows the same concepts as the waves you have studied in the past; they have an amplitude and frequency. The amplitude of an alternating current is referred to as its maximum value. This is not that useful of a number since the maximum value only exists for a fraction of each rotation.
A more useful value for EMF or current is the effective value. Consider a light bulb connected to both a battery and a generator. The bulb receives energy when the charges move through its filament, no matter which direction they are traveling. The power of the bulb at any point in time is the product of the current and voltage at that point in time (Watt's law).
Since the battery has a constant voltage and current, this calculation is simple to make and the result remains constant. For the generator, these values are changing so the calculation is not constant. This is where the effective value of AC comes in.
If you want both bulbs to have the same brightness (power) then the effective value of the current, or EMF, is 0.707 times the maximum value. If the maximum voltage of a generator is 100 V, then its effective voltage would be 70.0 V. This means that it would create the same brightness in a light bulb as one connected to a 70.7 V DC source.
It is the effective voltage that is usually expressed for AC. American household voltage is around 110 Vac. It is the effective voltage which is 110 V, the maximum voltage would be around 164 V, and the voltage swing during one cycle of the AC would be close to 328 V. That's one reason being shocked by household current is so dangerous.
Magnets
The easiest way to consider motors are that they are generators run in reverse. Since a generator converts mechanical energy into electrical energy, a motor changes electrical energy into mechanical energy. The parts of a motor can be identical to the parts of a generator, but instead of turning the loop to induce a current, current is sent through the loop to make it turn.
A motor involves a turning coil in a magnetic field. This was the condition that created current in the generator. That means that a motor also generates current while it is running. Remember, according to Lenz's law, this generated current is opposite to the direction which is making the motor turn. Because of friction and lack of involvement of the entire magnetic field, the back current is only a fraction of the motor's current.
When a motor first starts to turn there is almost no back current since the coil is barely turning. This means that the motor is drawing the maximum current from the source. As the motor increases the speed of the coil, the back current increases until it reaches a steady value. At this point the motor draws the least current from the source.
You may have noticed this when a large motor is started in your home. For a brief time as the motor is starting, the lights in the house dime, but come back to normal when the motor is running at full speed.
This situation is also why it is more economically responsible to leave a motor running rather than turn it on and off. Also, when a load is attached to the shaft of the motor, the speed of the coil is reduced, this reduction in speed reduces the back current, which increases the current in the coil, and gives the motor more power to maintain its speed as the load is applied. Pretty neat, huh?
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