Lines of Force
Electrostatics of Class 12
LINES OF FORCE
The electric field lines or lines of force are helpful in visualizing field patterns. They provide the following basic information:
- The direction of the field is along the tangent to a line of force.
- The strength or magnitude of the field is proportional to the number of lines that cross a unit area perpendicular to the line.
The electrostatic field lines have the following properties
- They always start on positive charges and end on negative charges.
- The number of lines that originate from or terminate on a charge is proportional to the magnitude of the charge.
- Lines of force never cross because field cannot be in two directions at the same point.
- Field lines always meet a conducting surface normally.
Note that the lines are crowded together where the field is strong and spread far apart where number of lines passing through a unit area are normal to the field direction.
Suppose N lines emerge from an isolated point charge. At a distance r from the charge the lines are spread over a spherical surface of area 4πr2. Thus, the density of lines is N/4πr2, and decreases as 1/r2.
This is exactly the way the field strength varies according to equation (1.5).
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In Fig. (1.11), the field is strong at A, while at B it is weaker. Since no lines cross the area at C, one might think that the field strength there is zero. However, we have drawn just a few lines for clarity. If the number were increased tenfold, some would surely pass through the area at C. The number of lines that emerge from a unit charge is a matter of choice; what really matters is the relative density of lines at various points. |
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