http://www.ck12.org Chapter 15. Electrostatics
the negative charge.
The electric field lines for an isolated positive charge and an isolated negative charge are shown inFigure15.17.
Notice that the field lines for both charges radiate out in straight lines. The positive charge has field lines that point
away from the positive charge. The negative charge has field lines that point toward the negative charge.
FIGURE 15.17
(left) The field lines for a positive charge.
(right) The field lines for a negative
charge.Rules for Drawing Field Lines when More Than One Charge is Present
- Field lines always start on a positive charge and end on a negative charge.
- Field lines never cross.
- The more field lines passing through a perpendicular area, the greater the magnitude of the electric field.
- Field lines are always drawn in proportion to the magnitude of the charge. For example, if there is three times
as much positive charge as negative charge, then three times as many field lines are drawn from the positive
charge. - Any line drawn tangent to a field line indicates the direction of the field at that point.
Field lines are also referred to as lines of force, because they indicate the direction of the force on a positive charge
q. The electric field~E=Fqcan be rewritten in the form~F=q~E. The chargeqin this case is not restricted to a
positive point charge.
Check Your Understanding
- What is the magnitude and direction of the electric field at a point 0.75m from a positive charge of+ 25 μC?
Answer:The magnitude of the electric field isE=krq 2 = ( 8. 99 × 109 )^25 ×^10
− (^6) C
( 0. 75 m)^2 =^399 ,^556 →^4.^0 ×^10
5 N
C
The electric field is directed radially outward.
The field pattern for a pair of opposite charges of equal magnitude (known as “electric dipole”) is shown inFigure
15.18.
- What is the direction of the force on a positive test charge in the electric field ofFigure15.18?
Answer: A positive test charge would be repelled by the positive charge inFigure15.18. The electric field lines
point in the same direction as the electric force.
- What is the direction of the force on a positive test charge in the electric field ofFigure15.18?
Answer: A positive test charge would be attracted by the negative charge inFigure15.18. The electric field lines
point in the same direction as the electric force.