Electricity Force, Field Then Voltage, Potential, Resistance, Current, & Power Chapter 16, then 17 & 18

Atoms & Charge Parts of atom Charge on the parts Unit of charge Transfer of charge?

Human Hands (if very dry) Leather Rabbit Fur Glass Human Hair Nylon Wool Fur Lead Silk Aluminum Paper Cotton Steel (neutral) Wood Amber Rubber Nickel, Copper Brass, Silver Gold, Platinum Polyester Styrene (Styrofoam) Saran Wrap Polyurethane Polyethylene (scotch tape) Polypropylene Vinyl (PVC) Silicon Teflon MORE POSITIVE = less electrons + When two items on this list are rubbed together, the higher one will become positive.

Electric Charge quantity “e” represents the charge of a single electron or proton Robert Millikan 1909

Charging objects To charge an object negatively, you must ? To charge an object positively, you must ? Conductors & Insulators, ground, semiconductors Ways to charge an object 1. Conduction 2. Induction Y:\Science Dept\Physics\Ohls videos\PP movies\charge flow.mpg Y:\Science Dept\Physics\Ohls videos\PP movies\physics videos\Gas_Pump_Fire.mp4

Charging by friction and contact: “Conduction” Contact required

Charging by Induction: on conductor Y:\Science Dept\Physics\Ohls videos\PP movies\q by ind.mpg Y:\Science Dept\Physics\Ohls videos\PP movies\q polarize.mpg Normal Bring a charge near causes separation

Grounding After separation of charge is induced Grounding far side of conductor gives one sign of charge a place to go. Then object has net charge

Static “Stopped” Electricity

What do you think will happen when the balloon is moved closer to the wall? Neutral wall Negatively charged balloon

What do you think will happen when the balloon is moved closer to the wall? A.Some positive charges in the wall will move towards the balloon B.Some negative charges in the wall will move towards the balloon C.Some positive charges in the wall will go onto the balloon D.Some negative charges on the balloon will go to the wall E.None of these?

Question If a suspended object A is attracted to object B, which is charged, we can conclude that A)Object A is uncharged B)Object A is charged C)Object B is positive D)Object B is negative E)Object A may or may not be charged

Why is it safe in a car during a lightning storm? Y:\Science Dept\Physics\Ohls videos\PP movies\Car_Lightning_-_Top_Gear_-_BBC_autos.mp4 Tesla cageTesla cage musicmusic Y:\Science Dept\Physics\Ohls videos\PP movies\physics videos\Tesla_coil_-_Faraday_cage_.flv

lightning

Hit by Lightning Every plane gets hit by lightning on average once a year.plane

Example How many excess electrons are placed on a lead block that is charged to -9  C? (  = x10 -6 ) What if the block is charged +9  C? [1 electron has 1.602× C of charge] 5.62 ×10 13 electrons force

Electric Force Depends on? How much charge How close together Which type of charge ( gives direction) Multiple charges means use  F

All of the pucks feel a force to the right. True or False ?

The puck in C feels a greater force to the right than the puck in D. True or False

The net force on the puck in A is zero. True or False

For which of these choices is the green puck most likely not to move? (All -) A B C equation

Coulomb’s Law don’t put charge sign in equation Y:\Science Dept\Physics\Ohls videos\PP movies\coulomb.mpg Y:\Science Dept\Physics\Ohls videos\PP movies\coulomb strength.MOV Electric Force

Example What is the force between q 1 = 10 nC and q 2 = -18 nC if they are 2.5 m apart? (n = )

History: Millikan Oil Drop F = Eq where q is the charge on the oil drop and E is the field strength. F = Vq/d where V is the voltage on the plates and d is their separation. The drop is being pulled down by its weight, mg. When the drop is suspended and stationary, the net force is zero. So: Vq/d – mg = 0 Vq/d = mg and Determined electron charge! 1 electron has 1.602× C of charge

Example An attractive force of 2 N exists between two charges. If we double the one charge, quadruple the second charge, and triple the distance between the charges, what happens to the force between the charges?

Example What is the force on charge 3 due to the other two charges shown below? q 1 = 4  C q 2 = -3  Cq 3 = 8  C L = 2.25 m L = 1.5 m

Another way to … Write charge on an object, usually small particles. Written in terms of “e” the fundamental quantity of charge. Examples: +3e = +3(1.602x C) = +4.81x C -5e = -5(1.602x C) = -8.01x C

example If object 1 with a charge of -4e is placed 0.72 m to the left of object 2 with a charge -7e, how much electric force is there, and what direction is object 2 pushed?

Example Where does a -15 nC charge need to be placed between a 9 nC and a 5 nC charge that are 2.5 m apart in order for it not to move?

Electric Field What does the Force around a charge look like? Force field! “Electric force field” is a system developed to try and model what the force around a charged object looks like (shows strength and direction of force)

Electric Field --space surrounding an electric charge -- shows direction of force on a positive test charge --density of lines shows strength of force = electric field for positive vs negative charge = calculation of electric field only needs one charge = means Efield is independent of other “test” charges Y:\Science Dept\Physics\Ohls videos\PP movies\e field.mpg

Question Rank (from smallest to largest) the magnitudes of the electric field at points A, B, and C in the following diagram. A B C

Lightning Rod

Typical Electric Fields Electric Field is zero inside any conductor Electric field is perpendicular to the surface of a conductor

Example What is the magnitude of the electric field detected 1.8 m from a 0.9 nC charge?

Electrostatic Photocopier

Example What is the electric field detected at point P due to the two charges shown below? + - q 1 = 2 nC q 2 = -7 nC L = 2.25 mL = 1.5 m P