Electricity

Chapter 20

Types of electric charge Protons ‘+’ charge

Electrons ‘-’ charge

SI unit of electric charge is the coulomb (C)

Interactions between charges Like charges repel

Opposite charges attract

Static Charge Latin word “Stasis” which means “Stays”

Objects are typically “Neutral” w/ the same # of protons and

electrons

They can become “charged” by gaining or losing electrons –

NOT PROTONS! – They stay in the nucleus!

The buildup of these charges is “Static Electricity”

In Static Electricity the charges build up and STAY;

they don’t flow as they do in electric currents

Transferring Static Charge Friction – transferred from rubbing i.e. get shocked after

walking on the carpet

Conduction – transferred by direct contact w/ another object –

hair standing on end w/ Van de Graff machine

Induction – the force field of a highly negatively charged object

pushes the electrons away from nearby objects causing them to

become + charged, they then are attracted to each other. i.e. statically charged balloon attracts small pieces of torn up paper

Static Discharge Objects don’t hold a static charge forever – objects tend

toward equilibrium – they “want” to be neutral

When electrons move toward this equilibrium – static

discharge occurs

Humidity – water vapor in the air pulls electrons off negatively

charged objects, preventing static charges to build up

(remember that water is a polar molecule)

Sparks & Lightning - objects reaching static equilibrium

Wimshurst Generator

Lightning Rods

Electrical Current Water flowing thru a pipe depends on more than the

angle of the pipe. It also depends on the length of the

pipe, diameter of the pipe and if the pipe is clogged or

open.

Electrical Current is how much electric charge flows!

Electrical Current is measured in Amperes

Represented by an I (for intensity)

Amount of Electrical Current (amps) depends on more

than just Voltage, it depends on the Resistance found in the circuit.

Conductors

Allow the easy flow of electricity

loosely bound electrons that are free to move from atom to atom

metals like aluminum, gold, copper and

silver

Insulators Insulators – resist the flow of electrons

hold more tightly to their valence

electrons:

plastic, rubber, glass

Electroscope Experiment

Electrical Resistance the opposition to the

flow of electricity –

measured in Ohms –symbol is the Greek letter Omega -

Electricity will take the path of least resistance Remember the water

pipe analogy?

The greater the resistance, the less current there is for a given voltage.

a. Longer wires have greater resistance than short wires

b. Thin wires have more resistance than thick wire

c. High conductors have less resistance than insulators

Ohm’s Law

V = IR

Let’s look at worksheet 20.2 –

Electric Current and Ohm’s Law

Voltage = Power! Voltage – causes current to flow through an electrical circuit

Volt – unit of measure to measure this electrical potential energy

A Voltage Source (battery or generator) is required to maintain the

electrical potential in a circuit.

Power

P=IV Power = Current x Voltage

Power is measured in watts

Please list which letters and SI units represent

the following:

Electric Charges

C

Coulomb

Electric Current

I

amps

Resistance

R

ohms

Voltage

V

volts

Circuit

Circuit- a complete path through which charge

can flow

There are 2 types:

Series

Parallel

Circuit (Schematic) Diagrams All circuits need at least the following

Power supply, wire, resistors, other items include switches, connectors,

meters, etc.

There is a set of standard symbols used to represent these items

in a diagram of the circuit

Series Circuits Series Circuits – provides only one path for the electrons to

follow

1. A break in the circuit stops the flow of electricity to all other parts of the

circuit

2. With multiple light bulbs (more resistance) the current reduces & the

dimmer the lights become

Parallel Circuits Parallel circuits – the different parts of the circuit are on separate

branches

A break (burned out light bulb) in the circuit doesn’t stop the flow to

the remaining devices

Multiple light bulbs will remain the same brightness since the resistance is not

decreasing as it does in a series circuit.

Each pathway can be separately switched off w/out affecting the others

Household circuits – Wired in parallel, with a standard of 120 volts

Voltmeters are wired in parallel

Parallel vs Series Circuits Demo

Parallel Circuits

The more paths the LESS the resistance

Water example again: added pipes coming from a

large tank will allow more water to flow out than a

single pipe.

Therefore as resistance decreases, current

increases; they are inversely proportional

Electric Energy

E=Pt Energy = Power x time

Energy is measured in kilowatt hours

1kwh= 3,600,000j

Electric safety

In an overloaded circuit, too much current can

cause circuits to explode and catch fire

Fuse: blows when current gets too strong

Circuit breaker: switch that opens when current

gets too strong

Ground: current goes through the ground

instead of you during a short circuit

Have you ever thought about who

changes the light bulbs in radio

towers? http://www.flixxy.com/tower-climbing.htm

http://www.youtube.com/watch?v=rdNbidOh

gDA

1. [OH-] =7.5 x 10-18 M

Series or

Parallel Circuits?

Hello

1.

2. 3. 4.

5.

Amps/Volts can kill

If your body resistance is 100,000 ohms, then the current which would flow would be the 2nd of each Voltage listed, but if you have just played a couple of sets of tennis, are sweaty and barefoot, then your resistance to ground would be lower, therefore the 1st voltage of each example would be used:

.001 A (1 - 100 V) tingle

.005 A (5 - 500 V) small shock (maximum harmless current)

.030 A (10 - 1000 V) painful, lose muscle control

.100 A (100 - 10,000 V) extreme pain, stop breathing

1.00 A (6000 – 600,000 V) nerve damage, heart stops, death possible

10.0 A severe burns, death is certain

If you need more help on Circuits,

please see this!!

Series and Parallel Circuits

Electricity Lab

Circuits!

Chapter 21

MAGNETISM & ELECTROMAGNETISM

Magnetic Poles

Magnetic Poles – the ends of the magnet, area where the magnetic

effect is the strongest.

If a bar magnet is suspended by a thread or string, it will align itself so

that one strong end points north and the other points south, hence

the names for the “North” and “South” poles of the magnet.

Like poles of separate magnets repel – push away from – each other

Unlike poles attract each other

magnet video

Magnets

If you snap a magnet in half, the inside pieces become the opposite

poles:

Magnetic Fields

that region around a magnet that is affected by the magnet. Strongest at the poles, the Force forms lines that go out of the North Pole and wrap back around to enter in at the South Pole.

Attract & Repel Magnets attract because force comes out of North

Pole and goes into the South Pole

Magnets repel because the forces are pushing away from each

other

Attraction Repulsion

Making Magnets

Since Magnetism and electricity are so closely related, it is relatively easy to make magnets

Temporary magnets – materials that become magnetized while in contact w/ strong magnets – ie a paperclip is able to pick up more paper clips when stuck to a strong magnet

Permanent magnets – materials that maintain their magnetism when the magnet is removed from it.

Magnetic Earth

Earth’s core is Iron –

Earth is a giant magnet

Earth’s magnetic north

pole is not the same as

Earth’s axis north pole.

It is about 1250 km (776

miles) away from the

true north pole

Electric Current & Magnetic Fields

An electric current produces a magnetic field

An electric current through a coil of wire around a nail produces a magnet

Solenoid- coil of current-carrying wire (more coils = more magnetic force) (less coils = less magnetic force)

Electromagnet- placing a ferromagnetic material in

the coil of a solenoid

How does a stereo speaker

work?

http://www.youtube.com/watch?v=_otCquvos8o

Generating Electric Current

Induction of electric current - making a current flow in a wire

1. Moving a coil of wire up and down in a magnetic field or

2. Moving a magnetic field up and down through a coil of wire

Alternating Current The flow of an induced current may be constant or may change

direction

Alternating current –AC – as a coil is moved up & down on a magnet

repeatedly the current would reverse direction each time

A current that changes direction

The electricity in our homes is AC

AC generators- simply a backwards motor

a. requires a mechanical source to spin the axle

b. which in turn spins the loop/armature which will induce a current.

c. Attached to each end of the coil loop are Slip Rings – which spin &

d. transfers the electricity to the brushes & the rest of the circuit

Direct Current Direct current – DC – the current resulting in electrons flowing from high

potential to lower potential

a. Therefore it moves in one direction only

b. The electricity stored in batteries is DC

DC Generator

Similar to an AC generator but has a single Commutator

instead of two slip rings

Batteries -Electrochemical cells 1. Converts chemical energy into electrical energy

2. Consists of two different metals – the electrodes

3. Electrodes immersed in a chemical “bath” that conducts electricity called

the electrolyte

4. The part of the electrodes above the electrolyte is the terminal and used to connect the battery to the circuit.

There is a chemical reaction between the electrodes and the electrolyte resulting in

a buildup of electrons on one of the terminals (it becomes the “-“ terminal)

The other terminal gives up its electrons and becomes the “+” terminal.

This difference sets up the electrical potential of the system = Volts

When cells are connected in series the voltages of the cells are added together

Transformers 1. Remember resistance occurs anytime current is sent thru a

wire.

2. Power companies have found that very high voltages can

travel more efficiently thru the wires

3. Once electricity is generated, it is transformed (in a step up

transformer) to a very high voltage ( up to 750,000 volts) then sent along the transmission lines

4. Voltage is then reduced at a substation at a step-down

transformer to a lower voltage ( between 2,000 & 5,000 volts)

5. Electricity is then sent throughout the neighborhood and as

it comes into the home it is step-downed one more time to the

110 volts required for our household appliances and tools

Changing Voltage with a Transformer

1. Transformer – piece of iron with two wires coiled around it. The coils do not come into contact w/ each other

2. Transformers work only with AC currents, not DC currents and is accomplished by Induction

3. The loops are labeled primary coil/winding, electricity coming into the transformer; the secondary coil is the loop/winding coming out.

Changing Voltage with a Transformer Step-up or step-down voltage is directly proportional to the

number of coils present

Electricity and Transformers

#coils secondary/ #coils primary = v out/v in

Ex. A transformer has 50 coils in its primary coil and 200 in its

secondary coil. The input voltage is 25 volts, what is the output

voltage? What type of transformer was this?

Using Electric Power

Electric power – Remember that Power is the rate at

which work is done and the unit of power is the Watt.

Formula is: Power = Voltage x Current

Formula is: Watts = Volts x Amps

Or Amps = Watts / Volts

Or Volts = Watts / Amps

P = Volts x Amps P = 6 x .5 P = 3 Watts

P = Volts x Amps 500 = 120 x A 500/120 = A 4.17 Amps

Calculating Electrical Energy Cost Paying for energy – we are charged by the electric company

for the power we use. It is calculated and billed to us by the

kilowatt hour.

1. The formula used is Energy = Power x Time =

(VoltsxCurrent)xTime

2. The formula used is Kilowatt hours = Kilowatts x Hours