Electric Charges

1. Electric Charges Conduction: Transfer of a charge easily. Induction: Influence transfer of a charge. (polarization of a charge) Insulator: Does not transfer a charge easily. Semiconductor: Inhibited transfer of a charge.

2. Electric Charges Sources of electric charges is: The Atom Protons (+) constant Electrons (-) variable Loss electrons result in a net (+) charge Gain electrons result in a net (-) charge The SI unit for a charge is coulomb, C. One coulomb = 6.24 x 1018 electrons or protons

3. Coulomb’s Law Defines the force acting between two charged bodies. (explains gravitational, magnetic and electric forces) Felectric = Kc(q1q2) Kc = Coulombconstant r2 9.0 x 109 Nm2/C2 Electric force that acts between two bodies is equal the produce of the charges on the bodies and inverse the square of the distance between the bodies.

4. Coulomb’s Law In Coulomb’s Law the force is: * a vector quantity * vector act along a line between the bodies . * force applies to a point charge. * distance between the bodies in from center of mass to center of mass.

5. Samples • Consider the force F1 = +F and F2 = -F acting on two charged particles separated by a distance, d. Explain the following: a. the distance between the particles doubles. b. the charge on one particle doubles c. the charges on each particle doubles. d. the charge on each and the distance between doubles.

6. Sample 2. Which is the stronger force: electrical force or gravitational force? Why?

7. Electrical v. Gravitational Electrical Gravitational • attract and repel attractive only • objects are + or - mass always + • stronger weaker • overcome gravity no match for easily electrical force

8. Sampler Coulombs Law 3. The electron and the proton of a hydrogen atom are separated, on average, by a distance of 5.3 x 10-11 m. What is the electrical force acting between the two charged subatomic particles. The charge on each is respectively -/+ 1.60 x 10-19 C.

9. 4. A negative charge of -2.0 x 10-4 C and a positive charge of 8.0 x 10-4 are separated by 0.3 m. What is the force acting between the two charged bodies?

10. 5. A balloon rubbed against wool gains a charge of -8.0 μC. What is the force between the balloon and the wool when the two are separated by a distance 5 cm?

11. 6. A negative charge of -6 x 10-6 C exerts a force of 95 N on a second object when each are 0.05 m apart. What is the magnitude of the second object?

12. 7. Two electrostatic point charges of +60 μC and +50 μC exert a repulsive force of 175 N. What is the distance between the two charges?

13. Vector Sum of Electrical Force Review vector addition from Section 3-2 The resultant force on a single charge equals the vector sum of the individual forces exerted on that charge by all the other individual charges that are present.

14. Vector Sum of Electrical Forces Steps: 1. Find the force exerted between individual bodies using Coulomb’s Law. 2. Use the charge to define the direction. 3. Find the x and y components. 4. Add up the total x and y force component 5. Use the Pythagoream Theorum to find the resultant. 6. Use tan to find direction: Tan q = Fytotal/Fxtotal

15. Sample 3. Four equal charges of 1.5 mC set at each corner of a square 5 cm on each side. Find the net force on a 5th charge placed in the center of the square if that 5th charge is: • -1.5 mC • +3.0 mC

16. Linear Vector Forces • Forces in the same direction, add magnitudes • Forces in the opposite directions, subtract magnitudes Note: Focus on the point charge in question

17. Sample 4. Three point charges of +5 mC at x = 0m, +2 mC at x = 3m, and -4 mC at x = 5m. What is the net force acting on the point charge at x = 3m?

18. Equilibrium Forces acting on a point charge will balance or reach a point where the net charge is equal to zero. Depends on the point charge and the magnitude of that defines the force.

19. Sample 6. Will a point of equilibrium be reached if: • + point charge between + and + • + point charge between + and – • + point charge between – and – • - point charge between – and –

20. Sample 7. Where will equilibrium be reached for: • Two +6 charge at x = 0 and x = 10 when a + point charge is set between. • Two charges of +4 and -2 at x = 0 and x = 10 when a + point charge is set between. • Two charges of -2 and -4 at x = 2 and x = 10 when a + point charge is placed between.

21. Sampler Three charged bodies. q1 = +6µc at 0 m ; q2 = - 2 µc at 4m and q3 = -1 µc at 6m. With respect to q2, what is the force total?

22. Electric Field Area of influence around a charged body. E = Felectric / test charge (qo) NOTE: • Felectric = Kc (q1 q2 / r2) Substitute • Strength of E measured against the effect on qo • Unit is N/C • E is a vector

23. Electric Field 5. q is (+) radiates outward 6. q is (-) radiates inward toward

24. Sample 8. A proton has a charge of 1.6 x 10-19 C. A proton is moved into an electric field of 600 N/C. What force does the proton experience? 9. The point within a electric field is to be +4.5 x 10-6 c. The test charge is measured to be 0.18 N at an angle of 20o. What is the magnitude and direction of the electric field strength?

25. Electric Field Lines Lines of influence radiating from a point charge 1. (+) outward • (-) inward • The number of line define the strength • Like field line will NOT cross.

26. Sample 10. What conclusion can be reached about (a) and (b) on page 648. 11. Two point charges set some distance apart: a. Draw the field lines with the (+) point twice that of the (-) point b. both charge are (-) c. both charges are equal (+) and (-)