WHAT IS ELECTROMAGNETIC INDUCTION – DEFINITION BASICS AND TUTORIALS

Electric current creates a magnetic field, the reverse effect also exists: magnetic fields, in turn, can influence electric charges and cause electric currents to flow. However, there is an important twist: the magnetic field must be changing in order to have any effect.

A static magnetic field, such as a bar magnet, will not cause any motion of nearby charge. Yet if there is any relative motion between the charge and the magnetic field—for example, because either the magnet or the wire is being moved, or because the strength of the magnet itself is changing— then a force will be exerted on the charge, causing it to move.

This force is called an electromotive force (emf) which, just like an ordinary electric field, is distinguished by its property of accelerating electric charges. The most elementary case of the electromotive force involves a single charged particle traveling through a magnetic field, at a right angle to the field lines (the direction along which iron filings would line up).

This charge experiences a force again at right angles to both the field and its velocity, the direction of which (up or down) depends on the sign of the charge (positive or negative) and can be specified in terms of another right-hand rule, as illustrated in Figure 1.3.

This effect can be expressed concisely in mathematical terms of a cross product of vector quantities (i.e., quantities with a directionality in space, represented in boldface), in what is known as the Lorentz equation, F = ¼ qv X B where F denotes the force, q the particle’s charge, v its velocity, and B the magnetic field.

In the case where the angle between v and B is 908 (i.e., the charge travels at right angles to the direction of the field) the magnitude or numerical result for F is simply the arithmetic product of the three quantities. This is the maximum force possible: as the term cross product suggests, the charge has to move across the field in order to experience the effect.

The more v and B are at right angles to each other, the greater the force; the more closely aligned v and B are, the smaller the force. If v and B are parallel—that is, the charge is traveling along the magnetic field lines rather than across them—the force on the charge is zero. Figure 1.3 illustrates a typical application of this relationship.

The charges q reside inside a wire, being moved as a whole so that each of the microscopic charges inside has a velocity v in the direction of the wire’s motion. If we align our right hand with that direction v and then curl our fingers in the direction of the magnetic field B (shown in the illustration as pointing straight back into the page), our thumb will point in the direction of the force F on a positive test charge.

Because in practice the positive charges in a metal cannot move but the negatively charged electrons can, we observe a flow of electrons in the negative or opposite direction of F. 

Because only the relative motion between the charge and the magnetic field matters, the same effect results if the charge is stationary in space and the magnetic field is moved (e.g., by physically moving a bar magnet), or even if both the magnet and the wire are stationary but the magnetic field is somehow made to become stronger or weaker over time.

The phenomenon of electromagnetic induction occurs when this electromagnetic force acts on the electrons inside a wire, accelerating them in one direction along the wire and thus causing a current to flow. The current resulting from such a changing magnetic field is referred to as an induced current.

This is the fundamental process by which electricity is generated, which will be applied over and over within the many elaborate geometric arrangements of wires and magnetic fields inside actual generators.

WHAT IS AN ELECTRIC FIELD - DEFINITION BASICS AND TUTORIALS

We characterized the electric potential as a property of the location at which a charge might find itself. A map of the electric potential would indicate how much potential energy would be possessed by a charge located at any given point.

The electric field is a similar map, but rather of the electric force (such as attraction or repulsion) that would be experienced by that charge at any location.

This force is the result of potential differences between locations: the more dramatically the potential varies from one point to the next, the greater the force would be on an electric charge in between these points. In formal terms, the electric field represents the potential gradient.

Consider the electric field created by a single positive charge, just sitting in space. Another positive charge in its vicinity would experience a repulsive force. This repulsive force would increase as the two charges were positioned closer together, or decrease as they moved father apart; specifically, the electric force drops off at a rate proportional to the square of the distance.

This situation can be represented graphically by drawing straight arrows radially outward from the first charge, as in Figure 1.1a. Such arrows are referred to as field lines. Their direction indicates the direction that a “test charge,” such as the hypothetical second charge that was introduced, would be pushed or pulled (in this case, straight away).

The strength of the force is indicated by the proximity of field lines: the force is stronger where the lines are closer together. This field also indicates what would happen to a negative charge: At any point, it would experience a force of equal strength (assuming equal magnitude of charge), but opposite direction as the positive test charge, since it would be attracted rather than repelled.

Thus, a negative test charge would also move along the field lines, only backwards. By convention, the direction of the electric field lines is drawn so as to represent the movement of a positive test charge. For a slightly more complex situation, consider the electric field created by a positive and a negative charge, sitting at a fixed distance from each other.

We can map the field conceptually by asking, for any location, “What force would be acting on a (positive) test charge if it were placed here?” Each time, the net force on the test charge would be a combination of one attractive force and one repulsive force, in different directions and at different strengths depending on the distance from the respective fixed charges.

Graphically, we can construct an image of the field by drawing an arrow in the direction that the charge would be pulled. The arrows for points along the charge’s hypothetical path then combine into continuous field lines. Again, these field lines will be spaced more closely where the force is stronger. This exercise generates the picture in Figure 1.1b.

ELECTROMAGNETIC FIELD AND HEALTH EFFECTS BASIC INFORMATION AND TUTORIALS

What Are The Health Effects Of Electromagnetic Field?


A current flowing through a wire, alternating at 60 cycles per second (60 Hz), produces around it a magnetic field that changes direction at the same frequency. Thus, whenever in the vicinity of electric equipment carrying any currents, we are exposed to magnetic fields.

Such fields are sometimes referred to as EMF, for electromagnetic fields, or more precisely as ELF, for extremely low-frequency fields, since 60 Hz is extremely low compared to other electromagnetic radiation such as radio waves (which is in the megahertz, or million hertz range).

There is some concern in the scientific community that even fields produced by household appliances or electric transmission and distribution lines may present human health hazards. While such fields may be small in magnitude compared to the Earth’s magnetic field, the fact that they are oscillating at a particular frequency may have important biological implications that are as yet poorly understood.

Research on the health effects of EMFs or ELFs continues. Some results to date seem to indicate a small but statistically significant correlation of exposure to ELFs from electric power with certain forms of cancer, particularly childhood leukemia, while other studies have found no effects.

In any case, the health effects of ELFs on adults appear to be either sufficiently mild or sufficiently rare that no obvious disease clusters have been noted among workers who are routinely exposed—and
have been over decades—to vastly stronger fields than are commonly experienced by the general population.

From a purely physical standpoint, the following observations are relevant: First, the intensity of the magnetic field associated with a current in a wire is directly proportional to the current; second, the intensity of this field decreases at a rate proportional to the inverse square of the distance from the wire, so that doubling the distance reduces the field by a factor of about.

The effect of distance thus tends to outweigh that of current magnitude, especially at close range where a doubling may equate to mere inches. It stands to reason, therefore, that sleeping with an electric blanket or even an electric alarm clock on the bedside table would typically lead to much higher exposure than living near high-voltage transmission lines. Measured ELF data are published by many sources.