Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 22

Any charge has a region of influence around it, in which various electrostatic effects are noticed – this region is the electric (electrostatic) field due to the charge. Charge is the fundamental source of static electricity, and the electrostatic field shows the effect of the charge in the world around the charge source. In this field, we find that

 Like polarity charges are repelled.

 Opposite polarity charges are attracted.

 Conductors (e.g. metals) redistribute their charges and experience a change of potential (voltage) in response to the field.

 Particles of many materials may be attracted or repelled within the field.

Static electricity phenomena are due to these basic effects.

Dust particles, and small objects, are attracted or repelled by a field, especially if they are themselves charged (e.g. ionized particles in the air). The force F experienced by a charge q in an electrostatic field E is (Cross 1987)

If equal positive and negative charge are sufficiently close together, from a distance their electric field effects cancel, and no external field is noticed. The charges are said to be neutralized.

Electrostatic fields and potentials around an object are not easy to visualize. One way of doing so is by use of field and equipotential lines. A field line represents the path a small charge would take, if it were free to move under the influence of the force due to the electrostatic field alone. Field lines always leave a conductor at a right angle (90°) to the surface.

In Figure 1.1 a charged spherical conducting object has a potential V. Each point in the surrounding space can also be assigned a potential, according to the work required to move a unit charge to that position. If all the positions of equal potential are marked, an equipotential line (or in three dimensions a surface) is marked out. A system of equipotential surfaces could be marked, forming contours of potential showing the presence of the peak in potential rather like the contours on a map showing the presence of a hill. Equipotential lines are always at a right angle (90°) to the field lines.

Equipotential lines are like contour lines on a map of an area of the earth's surface. Height is a form of potential energy. If a ball is released on a smooth hillside, it will roll down the hill perpendicular to the contour lines. Similarly, if a same polarity charge (e.g. a positive charge, next to a high positive potential) is present in the electrostatic field, it will move away from the peak in potential, in a path perpendicular to the equipotential lines. These paths form lines of electrostatic field. The intensity of the field is given by how close together the field and equipotential lines are.

Figure 1.1 Field lines and equipotential around a charged sphere.

The electric field E (vector, as it has magnitude and direction) is the gradient of voltage V over a distance s. Electric field, therefore, has the units volts per meter (V/m).

In Figure 1.1 if the charged sphere is very small, it is effectively a point of charge. The electrostatic field around the charge Q at a distance r from this point is proportional to the charge present, according to Coulomb's law (Cross 1987)

From this equation, in this case the field strength decreases rapidly with the distance from the charge, with 1/r². This is also indicated by the spreading of the field lines with distance from the charge. Field lines can be considered to begin and end on electrostatic charges, and so a high density of field lines at a surface implies a high charge density as well as high electrostatic field.

For other shapes of charge patterns, the equipotentials will not in general be spherical, and field lines will in general be curved rather than straight lines. Field lines are always perpendicular to the equipotentials and are always perpendicular to conducting surfaces as these are also equipotentials.