CK-12-Physics - Intermediate

http://www.ck12.org Chapter 15. Electrostatics

attractive or repulsive. The FBD can be used to determine whether the magnitudes of the forces must be added or
subtracted, as well as the direction of the net force.

F 1 =k(q+^10 r 1 )( 2 q−^5 )=

(

8. 99 × 109 N∗m

2

C^2

)

( 10. 0 × 10 −^6 C)( 5. 0 × 10 −^6 C)

( 0. 25 m)^2 =^7.^19 →^7.^2 N

The distance between the− 5. 0 μCcharge and the− 6. 0 μCcharge is 0. 65 m−. 25 m= 0. 40 m.

F 2 =k(q−^6 r)( 22 q−^5 )=

(

8. 99 × 109 N∗m

2

C^2

)

( 6. 0 × 10 −^6 C)( 5. 0 × 10 −^6 C)

( 0. 40 m)^2 = +^1.^69 →+^1.^7 N

The net force on the− 5. 0 μCcharge is thereforeF 1 +F 2 = + 7. 2 N+ 1. 7 N= 8. 9 N, directed toward the left.

Illustrative Example 3

Three charges form a right triangle as shown inFigure15.15. Determine the net force acting on the 20μCpoint
charge.

Answer:

The forces acting on the 20μCpoint charge are displayed inFigure15.15. Since the forces are not collinear, it is
necessary to use vector resolution.

FIGURE 15.15

The distance between the− 80 μCpoint charge and the 20μCpoint charge can be found using the Pythagorean
Theorem

√

( 0. 30 m)^2 +( 0. 40 m)^2 = 0. 50 m.

The FBD below show the forces on the 20μCpoint charge along with the vector resolution of forceF 1.