# đường sức từ

From Wikipedia, the miễn phí encyclopedia

Bạn đang xem: đường sức từ

This article is about the modern use of "field lines" as a way lớn depict electromagnetic and other vector fields. For the role of these lines in the early history and philosophy of electromagnetism, see Line of force. Field lines depicting the electric field created by a positive charge (left), negative charge (center), and uncharged object (right).

A field line is a graphical visual aid for visualizing vector fields. It consists of an imaginary integral curve which is tangent lớn the field vector at each point along its length. A diagram showing a representative phối of neighboring field lines is a common way of depicting a vector field in scientific and mathematical literature; this is called a field line diagram. They are used lớn show electric fields, magnetic fields, and gravitational fields among many other types. In fluid mechanics field lines showing the velocity field of a fluid flow are called streamlines.

## Definition and description The figure at left shows the electric field lines of two isolated equal positive charges. The figure at right shows the electric field lines of two isolated equal charges of opposite sign.

A vector field defines a direction and magnitude at each point in space. A field line is an integral curve for that vector field and may be constructed by starting at a point and tracing a line through space that follows the direction of the vector field, by making the field line tangent lớn the field vector at each point. A field line is usually shown as a directed line segment, with an arrowhead indicating the direction of the vector field. For two-dimensional fields the field lines are plane curves; since a plane drawing of a 3-dimensional phối of field lines can be visually confusing most field line diagrams are of this type. Since at each point where it is nonzero and finite the vector field has a unique direction, field lines can never intersect, ví there is exactly one field line passing through each point at which the vector field is nonzero and finite. Points where the field is zero or infinite have no field line through them, since direction cannot be defined there, but can be the endpoints of field lines.

Since there are an infinite number of points in any region, an infinite number of field lines can be drawn; but only a limited number can be shown on a field line diagram. Therefore which field lines are shown is a choice made by the person or computer program which draws the diagram, and a single vector field may be depicted by different sets of field lines. A field line diagram is necessarily an incomplete mô tả tìm kiếm of a vector field, since it gives no information about the field between the drawn field lines, and the choice of how many and which lines lớn show determines how much useful information the diagram gives.

An individual field line shows the direction of the vector field but not the magnitude. In order lớn also depict the magnitude of the field, field line diagrams are often drawn ví that each line represents the same quantity of flux. Then the mật độ trùng lặp từ khóa of field lines (number of field lines per unit perpendicular area) at any location is proportional lớn the magnitude of the vector field at that point. Areas in which neighboring field lines are converging (getting closer together) indicates that the field is getting stronger in that direction.

In vector fields which have nonzero divergence, field lines begin on points of positive divergence (sources) and over on points of negative divergence (sinks), or extend lớn infinity. For example, electric field lines begin on positive electric charges and over on negative charges. In fields which are divergenceless (solenoidal), such as magnetic fields, field lines have no endpoints; they are either closed loops or are endless.

In physics, drawings of field lines are mainly useful in cases where the sources and sinks, if any, have a physical meaning, as opposed lớn e.g. the case of a force field of a radial harmonic. For example, Gauss's law states that an electric field has sources at positive charges, sinks at negative charges, and neither elsewhere, ví electric field lines start at positive charges and over at negative charges. A gravitational field has no sources, it has sinks at masses, and it has neither elsewhere, gravitational field lines come from infinity and over at masses. A magnetic field has no sources or sinks (Gauss's law for magnetism), ví its field lines have no start or end: they can only size closed loops, extend lớn infinity in both directions, or continue indefinitely without ever crossing itself. However, as stated above, a special situation may occur around points where the field is zero (that cannot be intersected by field lines, because their direction would not be defined) and the simultaneous begin and over of field lines takes place. This situation happens, for instance, in the middle between two identical positive electric point charges. There, the field vanishes and the lines coming axially from the charges over. At the same time, in the transverse plane passing through the middle point, an infinite number of field lines diverge radially. The concomitant presence of the lines that over and begin preserves the divergence-free character of the field in the point.

Note that for this kind of drawing, where the field-line mật độ trùng lặp từ khóa is intended lớn be proportional lớn the field magnitude, it is important lớn represent all three dimensions. For example, consider the electric field arising from a single, isolated point charge. The electric field lines in this case are straight lines that emanate from the charge uniformly in all directions in three-dimensional space. This means that their mật độ trùng lặp từ khóa is proportional lớn , the correct result consistent with Coulomb's law for this case. However, if the electric field lines for this setup were just drawn on a two-dimensional plane, their two-dimensional mật độ trùng lặp từ khóa would be proportional lớn , an incorrect result for this situation.

## Construction

Given a vector field and a starting point a field line can be constructed iteratively by finding the field vector at that point . The unit tangent vector at that point is: . By moving a short distance along the field direction a new point on the line can be found Xem thêm: mục đích cuối cùng của cạnh tranh là

Then the field at that point is found and moving a further distance in that direction the next point of the field line is found. At each point the next point can be found by By repeating this and connecting the points, the field line can be extended as far as desired. This is only an approximation lớn the actual field line, since each straight segment isn't actually tangent lớn the field along its length, just at its starting point. But by using a small enough value for , taking a greater number of shorter steps, the field line can be approximated as closely as desired. The field line can be extended in the opposite direction from by taking each step in the opposite direction by using a negative step .

## Examples

If the vector field describes a velocity field, then the field lines follow stream lines in the flow. Perhaps the most familiar example of a vector field described by field lines is the magnetic field, which is often depicted using field lines emanating from a magnet.

## Divergence and curl

Field lines can be used lớn trace familiar quantities from vector calculus:

• Divergence may be easily seen through field lines, assuming the lines are drawn such that the mật độ trùng lặp từ khóa of field lines is proportional lớn the magnitude of the field (see above). In this case, the divergence may be seen as the beginning and ending of field lines. If the vector field is the resultant of radial inverse-square law fields with respect lớn one or more sources then this corresponds lớn the fact that the divergence of such a field is zero outside the sources. In a solenoidal vector field (i.e., a vector field where the divergence is zero everywhere), the field lines neither begin nor end; they either size closed loops, or go off lớn infinity in both directions. If a vector field has positive divergence in some area, there will be field lines starting from points in that area. If a vector field has negative divergence in some area, there will be field lines ending at points in that area.
• The Kelvin–Stokes theorem shows that field lines of a vector field with zero curl (i.e., a conservative vector field, e.g. a gravitational field or an electrostatic field) cannot be closed loops. In other words, curl is always present when a field line forms a closed loop. It may be present in other situations too, such as a helical shape of field lines.

## Physical significance

While field lines are a "mere" mathematical construction, in some circumstances they take on physical significance. In fluid mechanics, the velocity field lines (streamlines) in steady flow represent the paths of particles of the fluid. In the context of plasma physics, electrons or ions that happen lớn be on the same field line interact strongly, while particles on different field lines in general bởi not interact. This is the same behavior that the particles of iron filings exhibit in a magnetic field.

The iron filings in the photo appear lớn be aligning themselves with discrete field lines, but the situation is more complex. It is easy lớn visualize as a two-stage-process: first, the filings are spread evenly over the magnetic field but all aligned in the direction of the field. Then, based on the scale and ferromagnetic properties of the filings they damp the field lớn either side, creating the apparent spaces between the lines that we see.[citation needed] Of course the two stages described here happen concurrently until an equilibrium is achieved. Because the intrinsic magnetism of the filings modifies the field, the lines shown by the filings are only an approximation of the field lines of the original magnetic field. Magnetic fields are continuous, and bởi not have discrete lines.