1. 20-2 Electrical Force

2. Electrical Forces vs Gravity
Strong – Some can produce accelerations more than gravity
Repulsive or attractive where as gravity is always attractive
Henry Cavendish showed that electrical forces obey an inverse square law
Did not publish work and was found a century later

3. Forces on Charged Body
Charged objects do not necessarily need to be touching (long range force)
The electrical forces is summarized…
1) Two kinds of charges: Positive and Negative
2) Charges exert force over a distance
3) Force is stronger when charges are close together
4) Like charges repel, Opposite Charges attract

4. Electroscope Metal knob with metal stem and two thin metal leaves
Enclosed to be away from outside influence

5. Charging by conduction
For conduction, objects have to be _____________
When touching the ball on the electroscope, the charges are spread all through the metal causing the leaves to _____________

6. Charging by conduction
How can you determine whether electroscope is positively charged or negatively charged?
Bring a known charge to the end
Leaves spread further apart if same charge is applied
Leaves fall slightly if opposite charge is applied

7. Charging by Induction Charging an object without touching it
When charged object is brought close, the neutral charged item brings the opposite charge close and alike charges move away

8. Conduction and Induction

9. Coulomb’s Law Charges will push or pull on another
Force depends on size of charge and distance of separation
Experiment was designed by French physicist Charles Coulomb

10. Coulomb’s Law Symbol for charge is q
Force is varied inversely with the square distance
F = 1 / d2
Force is varied directly with the charge of the bodies
F = qAqB

11. The Coulomb SI unit of charge is the coulomb (C) Coulombs Law
Symbol to represent charge is (q)
Coulombs Law
F = K (qAqB / d2)
K = 9.0 x 109 Nm2/C2
One Coulomb is the charge of 6.24 x 1018 electrons – about the charge going thru a 100 watt light bulb in about 1 second.

12. Newton’s Third Law
The equation gives the magnitude of the force that charges exert on each other.
Forces are equal in magnitude but opposite in direction

13. Problem Solving Strategy for Electrical Force Problems
1) Sketch the System showing all distances and angles to scale (diagram the vectors of the system)
2) Use Coulomb’s Law to find the magnitude of the force
3) Use your diagram along with trig relations to find the direction of the force
4) Perform all algebraic operations on units as well as numbers

14. Problem
Two Charges are separated by 3.0 cm. Object A has a charge of +6.0 µC, while object B has a charge of +3.0 µC. What is the force on object A?
qA = +6.0 µC
qB = +3.0 µC
dAB = m

15. F B on A = K (qAqB / dAB2)
= (9.0 e9 Nm2/C2) x (6.0 µC)(3.0 µC) / (0.30 m)2
F B on A = 1.8 x 102 N in +x direction

16. Problem
Two charges are separated by an unknown distance. Object A has a +4.5 µC and Object B has a -7.5 µC. If the attractive force of A on B is 250 N, what is the distance that the two Objects are spaced?