Electricity

Static electricity The buildup of electric

charges on an object

Negative charge Rub a rubber rod

with wool Rod collects

electrons Positive charge Rub a glass rod with

silk Rod loses electrons

Bubble demo Why did this happen? Induced charge: When one object makes a

nearby neutral object act like it has a charge.

electric dipole Any object with different

charges on each end. Boab24's

Build an electroscope. 1. Bring your balloon near

the aluminum foil, what happens to your leaves? Why?

2. Touch your balloon to your aluminum foil ball, what happens to the leaves, why

This is called conduction

3. While the leaves are apart touch your finger to the aluminum foil ball, what happens to the leaves?

This is called grounding

4. Now bring the balloon close to the electroscope so the leaves spread but not too close where conduction happens. While the leaves are spread keep the balloon where it is and touch the foil. Now take the balloon and your finger away. Why does the electroscope stay spread?

5. Before grounding your electroscope, bring your balloon close to it again, what happens to the leaves? Why?

2 laws Law of electric charges Like repel, unlike attract Coulombs law: The force between two charged objects is directly

proportional to the product of their charges and inversely proportional to the square of the distance between them.

F=k(q1q2)/r2

Rough translation: the closer two charged objects are, the stronger the force between them.

Magnets Anything with a north and south pole 2 types of magnets Permanent Temporary Magnetic field Area around a magnet where the

magnetic force can be felt

Who discovered magnets? Greeks They called it lodestone because it

always pointed to the lodestar or North Star

How Magnets work All atoms have electrons Electrons spin making a magnetic field Clockwise = North pole on top Counterclockwise = North pole on

bottom Atoms with pairs of electrons cancel

each other out

Good magnets are made mostly out of odd numbered elements

Atoms with magnetic poles gather into domains

If we can get domains to line up we have a magnet

2 ways Run electricity around ferromagnetic

material Rub with a magnet

The Earth as a magnet Compasses are little

magnets When the north pole

points north it is actually pointing to the south (magnetic) pole of the earth

Earth’s poles flip every 50,000 years

Cirque de circuits Electric circuits: Path that electrons can take

Dude tests electric shock collar on himself

Dude 2 charges himself with cathode ray tube and touches stuff

Volts (E) Potential difference How bad the –

wants to give electrons and the + wants them

Amps (I) How many electrons pass a point in 1

second 1 amp = 6.25 x 1018 electrons

Ohms (R) Resistance to electron flow Often used to make light, heat or work

Ohm’s law

Watts = volts x amps

Practice problems A 1.5 volt AA battery is running a light,

the light uses 1.1 amps, what is the resistance of the light?

My headlights have 1.8 ohms of resistance and use 6.66 amps. What kind of battery should I put in my truck?

How many amps does a 100 watt light bulb run?

What is the resistance of this light bulb? What is the resistance of a 300 watt

hair dryer?

Patterns of circuits Series: Circuit where there is only one path for

electrons to take Light bulbs, what happens to resistance?

What happens to the voltage? A 9 volt battery has 3 lights on it, what is the

voltage drop for each light? Batteries, what happens to voltage? Rtotal = R1+R2+R3…

Parallel Branching the path that electrons can

take Light bulbs, what happens to resistance Batteries, what happens to voltage 1/Rtotal = 1/R1+1/R2+1/R3…

I have three 9 volt batteries wired in series, what is my total voltage? If I have two light bulbs wired in series in this circuit what is the voltage drop at each light bulb?

I have three 9 volt batteries wired in parallel, what is my total voltage?

I have three light bulbs wired in series that have 10 ohms of resistance each, what is my total resistance?

Marathon problem The lights in my garage are wired in

parallel. I have three 60 watt incandescent light bulbs and one 20 watt fluorescent light bulb. What is the total resistance of my circuit?

Transforming electricity Transformers: change the amount

of volts and amps in electricity.

The power company puts out power at 200,000 volts and little amps

Your house transforms it into 120 volts and 15-20 amps

High voltage transformer short

Appliances also change the voltage higher or lower

How transformers work If primary coil has less coils then

voltage is increased If the primary coil has more coils

the voltage will drop. Equation: Ep/Es = Tp/Ts

E= voltage T= turns (coils) P= primary S= secondary

Questions My alarm clock has 400 coils on the primary

side and 100 coils on the secondary side, how many volts does it run off of?

My printer is plugged into a wall socket, it runs on 32 volts, the primary coil has 45 coils on the primary, how many coils are on the secondary?

Effects of transforming electricity on current In a circuit the watts must stay the

same.

Amps and Volts Equation: Ep/Es = Is/Ip

E= voltage I= amps P= primary S= secondary

Amps of a transformer

A plasma TV steps up the 120 volts to 1000 volts. The current in the primary (before the transformer) is 2 amps. What is the current at the secondary?

Marathon problem A transformer for a

vibrating chair has a primary coil of 150 turns and a secondary coil of 350 turns. It draws 4 amps incoming (primary). What is the resistance of the chair?