A line of force is an imaginary path straight or curved such that the tangent to it at any point gives the direction of electrostatic field at that point. A field line is an imaginary line along which a unit positive charge would move when set free. The lines of force are drawn such that the number of lines per unit area of cross-section, (area held normally to the field lines) is proportional to magnitude of ®E
I. PROPERTIES OF FIELD LINES
(a) Field lines always come out of positive charge and enter the negative charge.
(b) Field lines never cross each other.
(c) Field lines never form closed loops.
(d) Field lines are always directed from higher potential to lower potential.
(e) Field lines never exist inside a conductor.
(f) If N1 is the number of field lines coming out of a charge q1 and N2 is the number of field lines entering q2 then |q1| / |q2| = N1 / N2
(g) If N1 is the number of field lines coming out of a charge q1 and N2 is the number of field lines coming out of charge q2 , then q1 / q2 = N1 / N2
This relation also exists if field lines are entering both the charges
(h) Tangent to field line at a point gives the direction of field at that point.
(i) Field lines exhibit longitudinal (length wise) contraction, thus indicating that unlike charges attract each other. (See Figure 1)
(j) Field lines exhibit lateral (sideways) expansion, thus indicating that like charges repel each other. (See Figure 2)
(k) Field lines always enter or leave a conducting surface at right angles.
(l) Since electric field inside a conductor in electrostatics is zero, electric field lines do not exist inside the conductor.
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