1. Electricity! Part I: electric charge

2. Atomic structure review
Nucleus
Protons: mass of 1 AU and +1 elementary electric charge
Neutrons: mass of about 1 AU and no charge, electrically neutral
Electron cloud
Electrons: mass nearly 0 Au and -1 elementary charge

3. Electric charge is measured in Coulombs (C)
A positive value for charge indicates a net positive charge
A negative value for charge indicates a net negative charge
6.25 x 1018 protons have a charge of 1 Coulomb (an elementary charge is very small in magnitude)

4. Charges exert forces on each other
Two charges exert electromagnetic forces on each other
Like gravity, the force is exerted without direct contact, acting over a distance
Unlike gravity, the force can either be attractive (between opposite charges) or repulsive (between same charges)

5. Charges exert forces on each other
Like gravitational fields for things with mass, electromagnetic fields exist for things that are not neutral
Electromagnetic fields extend infinitely in space
They are drawn as lines/arrows pointing toward negative charges and away from positive charges
Arrows indicate the direction a positively charged particle/object would move due to the field
Draw the field lines for a positive charge, a negative charge, and a positive and negative charge near each other:

6. Charges exert forces on each other
The electromagnetic force is much stronger than gravity (1039 X)
The gravitational force between two small particles is not noticeable
The electromagnetic force between two small charges is noticeable

7. Coulomb’s Law
𝐹= 𝑘 𝑒 𝑞 1 𝑞 2 𝑟 2
F is electromagnetic force, in Newtons
ke is Coulomb’s constant, 8.99 x 109 N·m2/C2 [no need to memorize]
q1 is the first charge (in Coulombs), q2 is the second charge
r is the distance between the charges (in m)

8. Coulomb’s Law
𝐹= 𝑘 𝑒 𝑞 1 𝑞 2 𝑟 2
if charges have the same sign, F is positive and the force is repulsive
if charges have opposite signs, F is negative and the force is attractive

9. PRACTICE
Two charges of known magnitude are placed a known distance apart. What will happen to the magnitude of the force if…
1. the distance between them is doubled?
2. The distance between them is reduced to 1/3 the original distance?
3. The magnitude of one of the charges is tripled?
4. The magnitude of both of the charges is halved?

10. Law of conservation of charge
Charge can be transferred from object to object, but cannot be created or destroyed

11. Law of conservation of charge
Static Electricity
Accumulation of excess (unbalanced, non- neutral) electric charge on an object
Some substances hold on to electrons weakly and can give them up to substances that have atoms that hold onto electrons more tightly

12. Law of conservation of charge
Charging by Contact
Process of transferring charge between two objects by touching or rubbing
Examples from above: balloon on hair, laundry, shoes on carpet

13. Law of conservation of charge
Charging by Induction
Rearrangement of charge on a neutral object
Caused when an unbalanced charge (say, a negatively charged balloon) induces the negative charges (electrons) on another the object (like a wall) to move away because of repulsion, leaving the area near the negatively charged object with a positive charge.
The object still has a net neutral charge—the charges just became separated

14. Charging by Induction

15. Conductors A material through which electrons move easily
Examples: metals, salt (electrolyte) solutions

16. Insulators A material in which electrons are not able to move easily
Electrons are held tightly, so that they cannot move freely
Examples: plastics, glass, plastic coating of wires, dry wood

17. Grounding Providing a path for a charge to reach the earth
The earth can conduct and is large enough to absorb charge differences

18. Grounding Birds can land on a powerline because they are not grounded
Lightning rods are grounded and guide charge to the ground away from other parts of a building.