1. Coulomb’s Law Physics 102 Professor Lee Carkner Lecture 9

2. PAL #8  Force on 2 from 1:  F 12 = kq 1 q 2 /r 2  F 12 = (8.99X10 9 )(5)(1.6X10 -19 )(2)(1.6X10 -19 )/(3 2 ) = 2.56X10 -28 N  Force is to left, make negative  Force on 2 from 3:  F 23 = (8.99X10 9 )(7)(1.6X10 -19 )(2)(1.6X10 -19 )/(3 2 ) = 7.31X10 -28 N  Force is to right, make positive  F net = F 12 + F 23 = - 2.56X10 -28 N + 7.31X10 -28 N = 4.75X10 -28 N  Direction is to the right +5e+7e-2e 3 m2.1 m F 12 F 23

3. Inverse Square Law  Electric force:  Gravitational force:  Both are inverse square laws   Distance is more important than charge or mass  Decreasing the distance will greatly strengthen a force, increasing the distance will greatly weaken the force

4. Computing Electrical Force  Consider a pair of charges   Draw an arrow from the experiencing charge either towards or away from the proving charge  Note that the forces are equal in magnitude and opposite in direction (F 21 =-F 12 ) +q 1 -q 2 F 21 F 12

5. Resultant Force  To find the net force from several forces add the force from each vectorially:  Put an x-y axis on the system   draw as arrow  Find the magnitude each force  F 1, F 2, F 3 ….   F 1x, F 1y …  Sum the x and y components  F x = F 1x + F 2x …   F 2 = F x 2 + F y 2

6. X and Y  Remember vector addition rules:  Components along axis computed from:    Total F:   Angle to x axis:  tan  = (F y /F x ) q F F x = F cos   F y = F sin 

7. Spherical Charge Distribution   An electron is a good example   but,  A uniform spherical distribution can be treated as a point source located at its center

8. The Electric Field  Electrical and gravitational forces act at a distance   The area near the charge is said to be occupied by an electric field   The test charge is small enough so that its field does not affect the main one

9. Defining the Electric Field  The electric field is defined as the force on the test charge per unit charge or: E = F/q 0  E = k q/r 2  i.e., the electric field at any point a radial distance r away from a charge q   We define the direction of the electric field based on the direction a positive test charge would move  Positive charges move with the field, negative against

10. Calculating the Electric Field   Need to find the magnitude and direction of the field from each charge and add vectorially  Note that once you find the value of the electric field, the force on a charge at that point is just F=Eq 0 

11. Next Time  Read 16.8-16.10  Homework Ch. 16: P 14, 17, 26, 30

12. Plates A, B and D are charged plastic plates. C is an originally uncharged metal plate. The electrostatic forces between some of the plates are shown. Will the remaining two pairs attract or repel each other? A)CD repel, BD attract B)CD attract, BD repell C)Both repel D)Both attract E)You cannot tell from the information given

13. If you rub a balloon on your head and the balloon becomes negatively charged, your head becomes, A)Negatively charged B)Positively charged C)Neutral D)Its charge does not change E)It depends on the type of balloon

14. How could you make a ordinary object positively charged? A)By adding protons B)By subtracting electrons C)By transforming neutrons D)By adding nuclei E)Objects cannot become positively charged

15. A negatively charged rod is brought near one end of an uncharged metal bar. What will happen to the end of the metal bar furthest from the rod? A)It will become positive B)It will become negative C)It will become neutral D)It depends on the type of metal E)Nothing