1. Electric Charge

2. Electric charge Fundamental property of matter Positive or negative
Determines electric force

3. The Electric Force

4. Coulomb’s Law kq1q2 F = d2 Nm2 k = 8.992  109 C2
C = coulomb (unit of electric charge)

5. Coulomb’s Law kq1q2 F = d2 Force is attractive for opposite charges
Force is repulsive for like charges

6. Discussion Question
The electric force is like the gravitational force because
a. Opposites attract.
b. Both decrease as 1/d2.
c. Both involve charge.
d. Both were identified by Isaac Newton.
e. All of these.

7. Discussion Question
The electric force is different from the gravitational force because
a. Gravity is only attractive.
b. The electric force is much stronger.
c. Electric charges can cancel.
d. All of the above.

8. Example Problem
The electron has a charge of –1.6  C and the proton has a charge of 1.6  C. Their average separation in a hydrogen atom is 5.3  10–11 m. What is the force between them at this distance?

9. Quick Question
What is the force between the electron and proton of a hydrogen atom when the electron is twice as far away?
a. Twice as much.
b. The same.
c. Half as much.
d. One fourth as much.

10. tells us something about matter
Charge Polarization
tells us something about matter

11. Scenario
A bag contains equal numbers of positive and negative charges. The charges can move around inside the bag, but they cannot leave the bag. The bag is placed near a very large, immobile + charge. + – +

12. Quick Question
What sort of force exists between the + charges in the bag and the large + charge?
The + charges are attracted to the charge.
The + charges are repelled by the charge.
The + charges are neither attracted nor repelled. + – +

13. Quick Question
What sort of force exists between the – charges in the bag and the large + charge?
The – charges are attracted to the charge.
The – charges are repelled by the charge.
The – charges are neither attracted nor repelled. + – +

14. Quick Question
In which direction do the + charges in the bag accelerate due to the large + charge?
Toward the charge.
Away from the charge.
The + charges will not accelerate. + – +

15. Quick Question
In which direction do the – charges in the bag accelerate due to the large + charge?
Toward the charge.
Away from the charge.
The – charges will not accelerate. + – +

16. Poll Question
After the charges re-distribute, which force to the external + charge will be stronger?
The attraction to the – charges.
The repulsion to the + charges.
The attraction and repulsion will exactly cancel. + – +

17. Poll Question
What sort of force exists between the bag overall and the large + charge?
The bag is attracted to the charge.
The bag is repelled by the charge.
The bag is neither attracted nor repelled.
Demo: balloon and uncharged objects: wall, aluminum can, soap bubbles, 2”x4” + – +

18. Group Poll Question
What sort of force on the bag will exist if the external charge is negative?
The bag is attracted to the charge.
The bag is repelled by the charge.
The bag is neither attracted nor repelled. + – –

19. Coulomb’s Torsion Balance

20. Principle of Superposition
Force from several charges = sum of forces from individual charges c b Fb Fc a
F = Fb + Fc + Fd Fd Fb Fc Fd F d

21. Vector Fields

22. Electric field
Direction is the direction of the force exerted on a positive charge
Vector arrows point away from positive charges and toward negative charges
Magnitude is the force in N on a +1 C charge

23. Electric Field
Unit = N/C

24. Visualizing a Field
Magnitude of the force on a charge is greater where field lines are close together
Direction of the force is parallel to field lines
Force on a positive charge is along field lines
Force on a negative charge is opposite field lines

25. Field Lines

26. Question A B C D
Particles A–D have the same charge. Which experiences the greatest force from the field? (Green arrows are field lines.)

27. Example
Draw field lines for the electric field about these two objects with opposite charges. + -

28. Group Work
Draw field lines for the electric field about these two objects with equal charges. + +