1. Unit 9 Introduction to Electricity (Filled In)
Name: _____________________ Date: ________
Period: ___________
Unit 9 Introduction to Electricity
(Filled In)
“Physics is learning by iteration – by going over and over the same material - each time making more sense.”
- Paul Hewitt
Essential Questions:
How do we know there are only 2 types of charges in the world?
How can we “charge” an object?
Why do clothes have “static cling” after the dryer? Why do we shock ourselves on a door knob after walking on a carpet?

2. Vocabulary
Electricity – Electrical phenomena (similar to gravity). Examples: lightning, spark from grabbing a door knob, force that holds atoms together in molecules
Charge – Property of matter, like mass. There are 2 kinds of charge, “positive” and “negative” (defined by Benjamin Franklin).
Coulomb (“C”) – SI unit to measure the amount of electric charge 1 C = 6.25 x 1018 electrons
Conductor – Material that allows electrons to flow through easily
Insulator – Material that does not allow electrons to flow easily
Electric Current – Motion of electrons. We build to technology to control current for our use, such as computers, toasters, and smart phones.

3. First things first… What is an atom?
We can’t see inside the atom directly
We can only build models of an atom, and test these models with experiments
Electron
Positive “+” charge
Nucleus is made up of particles called protons and neutrons.
Proton
Negative “-” charge
Bohr model: Electrons circle the nucleus, much like planets circle the sun. Electrons are at different distances from the nucleus.
Quantum model: Electrons do not have a specific location around the nucleus.
Densest dots on the picture show how likely it is for an electron to be at that location.

4. Cool Claims About Atoms All claims must be tested by experiments!
Electrons are all the same – same mass and charge
Protons have1800 times more mass than electrons
The positive charge of each proton is equal to the negative charge of each electron
On Earth, most atoms are neutral – equal number of protons and electrons. In space, stars are plasma – electrons are free from atomic nuclei.
Neutrons have mass, but no charge
Helium atom model:
2 electrons
2 protons
2 neutrons

5. What is an electric conductor?
Conductor: Any metal or material that allows electric current – moving electrons (Metals are also good heat conductor!)
Why are metals good conductors (electric & heat)? Electrons in the metal atoms are “loose” and free to move around the metal
Examples:
Silver, gold, platinum – expensive, don’t corrode, used in high-value electronics
Aluminum, copper – cheap, copper is commonly used in electrical wiring

6. What is an electric insulator?
Insulator: Material that is a poor conductor of electric current (moving electrons)
Why are insulators bad conductors (electric and heat)? Electrons are tightly bound to atoms and stay in place
Examples:
Glass
Most plastics – used to shield wires (rubber)
Air

7. How to “charge” a neutral object by friction: Rubbing insulators separates charges
Rubbing 2 different insulator materials together (friction) transfers electrons from one material to the other
Material that gives up electrons easily becomes positive
Material that gains electrons becomes negative
Electrons move from Wool to Ruler
Electrons move from Silk to Balloon

8. How do we know that there are only 2 kinds of charges?
Repel each other:
Balloon and ruler
Silk and wool
Attract each other:
Balloon and silk
Ruler and wool
Test many different materials.
Note that 1/2 of materials repel each other, and half attract each other.
There must be only 2 kinds of charges!
Balloon and Ruler have extra electrons – “negative” charge.
Silk and Wool have lose electrons – “positive” charge.

9. How to charge an object by induction
How to charge an object by induction? Bringing a charged object nearby (without touching), polarizes the other object
Don’t touch!
Don’t touch!
a) 2 neutral metal spheres, touching
b) Charge a glass rod by friction.
Bring rod near one sphere – don’t touch!
Electrons are repelled by the rod and move to sphere B, leaving A positive.
c) While holding the rod near…
Separate the spheres.
Extra electrons on sphere B are trapped.
d) Remove the rod.
Charges on a metal move around until they are evenly spread out on the metal surface.

10. Can you answer these questions?
After you rub a balloon on your hair, why does it stick to the wall?
After you comb your hair, why does the comb pick up small bit of paper?
Charge on the balloon attracts opposite charge on the wall, and this force causes the balloon to stick to the wall.

11. Practice Problems 1

12. Practice Problems 2

13. Practice Problems 3

14. Parts of the Van de Graaf Generator: Rubber belt peels away charges from plastic roller, and charges conduct to the metal cover
Details about how the Van de Graaf generator works:
Separated charges want to come back together again – see the lightning?