ELECTRIC CHARGES & CURRENT

By Mr. Frank Todd Notes provided by Mr. Joe Martonik

A. Electric Charges & Static Electricity

•

1. Types of Electric Charge & Interaction

• Protons = + charge • Electrons = - charge • Like charges repel (+ +, - -) • Unlike charges attract ( + - )

•

2. Electric Fields

• The area around electric charges that has the

force

of the charge exerted on it.

• When a charge is placed in an electric field, it is

pulled or pushed

.

• The field is the

strongest

near the charged particle.

•

3. Static Charge:

Results from a

build up

or redistribution of charge on an object.

•

Charged Objects

: – Some materials hold their electrons well (

insulators

). Some do not and the electrons move freely from atom to atom (

conductors

).

– A neutral object can become charged by gaining or losing electrons.

– A build up of charges is called

static electricity .

– (The charges build up but do

NOT FLOW )

Give up

electrons

easily Triboelelectric series Gain

electrons

easily •Human hands (usually too moist, though)

Very positive

•

Rabbit Fur

•

Glass

•

Human hair

•

Nylon

•

Wool

•

Fur

•

Lead

•

Silk

•

Aluminum

•

Paper

•

Cotton

•

Steel Neutral

•

Wood

•

Amber

•

Hard rubber

•

Nickel, Copper

•

Brass, Silver

•

Gold, Platinum

•

Polyester

•

Styrene (Styrofoam)

•

Saran Wrap

•

Polyurethane

•

Polyethylene (like Scotch Tape)

•

Polypropylene

•

Vinyl (PVC)

•

Silicon

•

Teflon Very negative

•

Transferring Charge:

Charges can be transferred by the following ways.

–

Friction

—by rubbing materials against one another

•

Transferring Charge:

Charges can be transferred by the following ways.

–

Conduction —

by direct contact between two objects

•

Transferring Charge:

Charges can be transferred by the following ways.

–

Induction

—movement of electrons to one side of an object. (Caused by an electric field of another object)

•

Static Cling:

Clothes stick together because each piece of clothing contains opposite charges. What caused the clothes to be charged?

•

Rubbing between clothes as they tumble about in the dryer.

•

4. Static Discharge:

– When there is a loss of static electricity as electric charges move off an object, it is called

static discharge.

–

Humidity:

Water molecules in the air can pick up electrons which may be in the process of being transferred. Charge then

cannot

build up on an object.

•

4. Static Discharge:

–

Sparks & Lightning:

– A spark results when

electrons jump

from one object to another.

– Lightning is a giant spark created when water droplets become charged by swirling wind during a storm. Electrons collect in the lower part of the clouds. To restore a neutral condition, electrons jump from the

clouds

to the

ground.

•

5. Detecting Charge:

– An

electroscope

is a device used to detect electric charge.

– The device cannot tell if it is storing + or – charges. – Can only detect the presence of a

charge

.

B. Circuit Measurements

•

1. Electrical Potential:

• It is the

potential

energy per unit of electric charge.

• Electrons flow from places of

higher

potential to places of

lower

potential.

– This called

Potential Difference.

•

2. Voltage & Sources:

• Voltage is measured by a

voltmeter

.

• Potential difference provides the

force

that pushes charge through a

circuit.

• This force is called the

Volt.

• Electrons will flow in a circuit as long as there is a potential difference or voltage between

2 parts

of a circuit.

• • As voltage increases the flow of electrons increases.

Batteries and Generators

serve as voltage sources.

•

3. Resistance:

The

opposition

to flow of electric charges in a circuit.

• The

greater

the resistance in a circuit, the less

current

or flow of electrons for a given voltage.

• • Influences on Resistance: – A wires length and thickness – Thick + Short wires =

low resistance

– Temperature—

Low

temperature= lower resistance

Ohms

= unit for measuring

Resistance

•

4. Current:

Is the number of electrons passing a point in a

given amount of time

.

• Current always flows through a path of

least resistance.

• Current will increase/decrease based on the battery construction or source of electrons.

•

Amps

=unit of measuring current • Measured by an

Ammeter.

Common tools and their Amperage bug killer 1-2 fan = 1-3 electric drill 3-6 lawn mower 6-12 saber saw 4-8 grinder 7-10 hedge trimmer 2 3 weed trimmer 2-4 sander 4-8 chain saw 7-12 band saw 5-12 drill press 7-14 belt sander 7-15 router 8-13 shop vac 8-14 lawn edger 9-10 air compressor 9-15 table saw 12-15 snow blower 12 15 circular saw 12 15 1/4 HP motor = 6 amps 1/2 HP motor = 10 amps 3/4 HP motor = 14 amps 1 HP motor = 16 amps

• • • •

5. Ohm’s Law Resistance Ohms R

= V/I • V= RxI • I = V/R = Voltage/Current = Volts/Amps

:

V

:

R x I

• • • •

Sample Problem:

• A 0.40 amp current is produced when a 12 Volt battery is connected to a headlight. How much resistance is produced by the headlight?

R=V/I R= 12 V/0.40Amps

R= 30 Ohms

C. Series and Parallel Circuits

•

1. Series Circuits

• A circuit where all the parts are connected “one after the other” creating only

one path

for the flow of electrons.

• If one bulb in a series circuit burns out or is removed the

circuit is broken

and all the bulbs go out.

• As more bulbs are added, they get dimmer b/c the resistance

increases

.

• Ammeters should be wired in series.

C. Series and Parallel Circuits

•

2. Parallel Circuits

• Different parts of the circuit are on

different

branches. This type of circuit provides different paths for the electric

current

. (Each bulb has it’s own path.) • If one bulb is removed or burns out there are other paths for the

current to take

so the other bulbs stay lit.

• Parallel circuits

glow brighter

flows to the bulbs.

because more current • As more branches are added to a parallel circuit the resistance

decreases.

• Voltmeters should be wired in parallel.

Label each type of circuit below.

Bulb

Series Parallel

•

3. Household Circuits:

• Must be wired in

Parallel

• • Electricity is fed into a home by Thick and Heavy wires called

lines.

These have

low

resistance.

• Parallel branches extend from the lines to the

wall sockets

and

Appliances

.

Switches

are placed to control branches of circuits one at a time.

• Voltage in house circuits is

120

Volts.

D. Electrical Safety

•

1. Becoming Part of a Circuit

• Short

Circuit

—A connection that allows a current to take an

unintended

path.

• If you touch an exposed wire in the house, 120 Volts of current will pass into your body. This is called a

SHOCK

.

•

2. Grounding:

• Additional wire in circuit to

protect

a person from shock.

• Plugs have a 3 rd

prong

. This connects the metal shell of the appliance to the

ground wire

of the building.

• Lightning rod metal rod mounted to the roof of a building. If lightning strikes the rod the energy flows down the rod to a

ground wire

and then into the

Earth.

• • •

3. Fuses and Circuit Breakers:

• When a wire carries more current than it is designed to carry it will get

HOT

. The

insulation

will then burn.

Fuse-

A device with a thin strip of wire (metal) that will

melt

if too much current flows. This is part of the circuit. (When the fuse melts or “blows” the circuit is

broken

.)

Circuit Breaker

- device which uses an

electromagnet

to turn off a circuit when it is overloaded.

•

4. Electric Shocks:

• The human body depends on tiny electrical pulses to control many processes (ex. Heart beat) An electrical shock may

disrupt

these processes.

• The severity of the shock depends on the current. A current of

0.2amps

will burn and travel across the body and could stop the heart.

• Current of an electric shock is related to

voltage

and

resistance.

• Your body has a low

resistance

(ions in fluids) • When

wet

your bodies resistance is hundreds of times

lower.