Electricity and Magnetism Overview

ELECTRICITY AND

MAGNETISM

Electricity

I. What is Electricity?

A. We use electricity every day.  Our homes,

stores, and 

workplaces

 depend on

electricity.

B. Electricity usually means the 

flow

 of

something called electric current in wires,

motors, light bulbs, and other devices.  

C. Electric current flows through solid 

metal

so we can’t usually see it.

I. What is Electricity?

D. Electric current can carry 

energy

 over great

distances.  That’s why you can have an

electric company hundreds of miles away

from your house and still have electricity.

E. Electric current can be very 

powerful

.  An

electric motor the size of a basketball can

do as much work as 15 strong men or

women.  This great power can also be very

dangerous

.

II. History of Electricity

A. The understanding and use of electricity is

relatively recent history. In 1733, French

scientist 

Charles DuFay

 published a book

describing how like charges repel and

opposite charges attract.

B. DuFay theorized that there were two 

fluids

that caused electricity - a positive fluid and

a negative fluid.

II. History of Electricity

C. Later in the 1700’s, 

Benjamin Franklin

invented his own theory that electricity is

the presence of a single fluid in different

amounts.   Franklin thought if there was too

much of this “electric fluid” in an object, it

would have a 

positive

 charge.  Not enough

of this “electric fluid” and an object would

have a negative charge.

II. History of Electricity

D. In the 1800’s, inventors and

scientists began to harness the

power of 

electricity

. 

E. Michael Faraday discovered the

principles of the electric motor in

1830, and 

Thomas Edison

invented the light bulb in 1879. 

Charges

❖

Charges come in 3 main forms;

➢

Objects can be positive, negative, or neutral.

1.

Positive

 - 

A positive object has lost electrons, so it has

more protons 

than electrons.

2.

Negative

 - 

A negative object has gained electrons, so it

has 

more electrons

 than protons.

3.

Neutral - 

A neutral (No charge) object has an equal

amount of protons and electrons.

➢

The unit of charge is the coulomb (C).

III. Charge

A. To understand electricity,

people first studied events like

lightning

 and the sparks that

can occur with static

electricity.

B. The source of the shock and

the sparks from static

electricity is 

electric charge

.

Electric charge is a

fundamental property of

matter.   

III. Charge

C. Charge comes in two forms - 

positive

 and

negative

 charge.  

D. Like charges repel each other, and unlike

charges attract.  That means that a positive

charge and a positive charge will push away

from each other.  The same thing happens

with a negative charge and a negative

charge.  A positive and a negative charge

will 

come together

.

III. Charge

E. The forces between positive and negative

charges are called 

electrostatic

 forces.

Electrostatic forces are extremely strong -

electrostatic forces hold all matter together.

F. Most matter is electrically 

neutral

 - The

amount of positive charge in an object is

exactly equal to the amount of negative

charge in an object.  The total charge is

zero.

III. Charge

G. The unit of charge is the 

coulomb

- it is

abbreviated C.  

H. The protons in an atom carry a positive

charge and the electrons in an atom carry a

negative charge.

●

In an atom, protons are held together in the nucleus

by 

strong nuclear forces

, but electrons are free to

move around.

●

Electricity is caused by 

moving electrons.

●

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Electric charges are from protons which are

positive

      and electrons which are 

negative

   .

Atoms become charged by gaining or losing

electrons.

+

-

Law of Conservation of Charge 

– electric charges

can be transferred from object to object, but it

cannot be created or destroyed.

Positive and negative charges exert forces on each

other.

opposite charges

attract

like charges repel

Static Electricity 

– accumulation of excess electric

charges on an object

*When you rub your socks on the carpet electrons are flowing

from the ground to your socks 

building up static electricity

.* 

Static Discharge

 - is the release or balancing effect of excess

charges that have 

accumulated on an object

. 

*When you touch or “Shock” someone, you are 

discharging

 that

accumulation of charge.*

Lightning is 

static discharge

 on a 100x level.

Electric Current

I. Voltage

A. Review - Remember that a 

circuit

 is something

that provides a path through which energy in the

form of electricity moves.

B. A circuit contains several parts - an 

energy

source

, wires, and sometimes switches or things

that use energy, like light bulbs.

*Pushes Electricity through a circuit.

I. Voltage

C. We can measure the energy level of any place

in a circuit.  To measure electrical energy, we

use a unit called 

volts

. 

D. Voltage is related to 

potential energy

, just like

height is related to pressure in water flow.

I. Voltage

E. Imagine you have two tanks of 

water

, one is higher than

the other.  The higher tank has more potential energy

than the lower tank. The water flows 

downhill

, from

high energy to low energy.  A greater difference in

height means that the water has more 

potential energy

.

F. The two water tank scenario is just like a difference in

voltage.  Electricity flows from an area of higher 

voltage

to an area of lower voltage. 

I. Voltage

G.Voltage is a measure of 

energy

.  The voltage of

a circuit will not change unless there is

something on the circuit that uses energy, like a

light bulb.

F. Anything that uses energy 

lowers

 the voltage of

a circuit since it takes energy away from any

moving charges in the 

circuit

.

II. Current

A. In a circuit, energy flows from places of high

voltage to places of low voltage. 

Electrical

current

 is the term used to describe this flow of

energy.

B. Current is actually the flow of 

negative charges

through a circuit. Current can be thought of as

how much charges flows through a wire per

second

.

II. Current

C. The unit for current is the 

ampere

.  An ampere,

or amp, is a flow of one coulomb per second.

Remember that coulomb is the unit for charge,

so an amp is one charge moving through a wire

per second.

D. For example, a current of 

10 amps

 means that

10 coulombs of charge flow through the wire

every 

second

.

II. Current

E. When you look at a wire, you can’t see 

current

.

The particles that carry charge are electrons.

Electrons

 are the negatively charged part of

atoms, and they are so small that they can flow

in the spaces between 

atoms

. 

II. Current

F. Current occurs in a circuit with a battery

because electrons in the battery 

repel

 electrons

in the wire, which repel other electrons in the

wire, and so on. 

G. Remember: A voltage 

difference

 supplies

energy to make charges flow.  Current carries

energy and does

 work

III. Current in Your Home

A. You use electric current  in your 

house

 every

day.  When you plug in an electrical appliance,

you connect it to a circuit created by wires in the

walls.  The wires eventually connect to power

lines outside your house that brings the current

from a 

power station

.

III. Current in Your Home

B. The electricity in your house uses alternating

current, or 

AC current

.  This means the direction

of current goes back and forth.  In electrical

systems in the US, the current reverses direction

60

 times per second.  In Europe, the AC current

reverses direction only 

50

 times per second, and

the voltage is different, so adapters are needed

for electric devices made in the US.

III. Current in Your Home

C. Each wall socket you see in your house has

three holes - and each hole has a 

wire

 attached to

it inside the wall.  One wire is the 

hot wire

,

which carries 120 Volts AC.  The second wire

stays neutral and carries no volts.  The third wire

is a

 ground

 wire which is connected to the actual

ground near your home - if there is a short

circuit in your appliance, the current will flow

through the ground wire instead of through you!

III. Current in Your Home

D. The current from a battery does not 

alternate

.

A battery only makes current that flows in one

direction.  This is called direct current, or 

DC

current

.

E. For all large amounts of electricity, AC current

is used because it is easier to transmit and

generate

.

IV. Conduction and Resistance

A. Charge flows very easily through some kinds of

materials, like 

copper

.  Material that easily

conducts, or carries, electrical current is called a

conductor.

B. Other materials, like glass or 

plastic

, do not

allow charge to flow.  These materials are called

insulators

 because they insulate or block the

flow of current.

IV. Conduction and Resistance

C. A third category of materials are not as easy-

flowing for current as conductors, but they are

not quite insulators either.  These materials are

named 

semiconductors

.  Semiconductors are

very important for computer chips.

V. Conduction

A. The property of a material to allow charge

to flow is called its electrical 

conductivity

.

All materials have some degree of

conductivity - those with 

high

 conductivity

are conductors.  Materials with low

conductivity are insulators.

V. Conduction

B. Examples of conductors include 

silver

,

copper, gold, aluminum, and tungsten.

C. Examples of insulators include rubber,

paper, 

plastics

, and glass.

VI. Resistance

A. The resistance of an object measures how difficult it is

for 

charges

 to flow through something.  High resistance

means it is difficult for current to flow.  Low resistance

means it is 

easy

 for current to flow.

B. Devices that use electrical energy have 

resistance

.  For

example, light bulbs have resistance.  If you string more

light bulbs together, the resistance adds up and the

current goes 

down

.

VI. Resistance

C. Electrical resistance is measured in units called

ohms

.  The unit is abbreviated with the Greek

letter omega (

Ω).

D. Voltage and resistance 

together

 determine how

much current flows in a circuit.  If voltage goes

up, 

current

 goes up.  If resistance goes up,

current goes 

down

.

Electric Current 

– net movement or

flow of charges in a single direction

through a wire or conductor.

Potential Difference (Voltage)

– force

that causes electric charges to flow;

charges flow from high voltage low

voltage

Resistance

 – tendency for a material to oppose

the flow of electrons, changing 

electrical energy

into 

thermal and light energy

.

Circuit

 – a closed path that electric current follows.

Battery, wires, and

voltage difference

allows  current to

flow.

BATTERIES

Dry Cell

Chemical reactions occur in a

moist paste causing transfer of

electrons

Wet Cell

Contains two connected plates

made of different metals in

conducting solution.

Ex. Car Battery

Making wires thinner, longer or hotter increases resistance

All materials have some electrical resistance measured in 

ohms

.

(Ω)

Ohms Law 

– current in a circuit equals 

Potential 

difference

(Voltage)

 divided by resistance.

VII. Summary

A. Voltage is a measure of energy of a system.

Energy flows anytime there is a voltage

difference

.  Current flows from high voltage to

low voltage.

B. Current is a measure of how many 

charges

 flow

through a circuit per second.  Current is

measured in Amperes, or amps.

VII. Summary

C. Materials that easily carry charge are called

conductors

.

D. Materials that resist carrying charge are called

resistors

.

I. Review

1.

We have covered three important electrical

qualities so far – voltage, current, and

resistance

.

1.

Remember that voltage is a measure of the

energy level of a system.  Any time there is a

voltage difference a 

current flows

.  Voltage is

measured in volts.

II. Review

1.

Current is a measure of how many charges

flow through a circuit per 

second

. The unit for

current is 

amps

.

1.

Resistance is a measure of how difficult it is

for current to flow.  The unit for resistance is

the 

ohm

. 

III. How it all fits together

1.

Voltage, current, and resistance are all

related.  If you increase the 

voltage

 of a

circuit, the current goes up.  If you increase

the

 resistance 

of a circuit, the current goes

down.

III. How it all fits together

B. The relationship between voltage, current, and

resistance can be summarized in a mathematical

relationship called Ohm’s law:              

Where I is current in amps, V is voltage in volts,

and R is 

resistance

 in ohms

I

V

. Ohm’s Law

A.

You can manipulate ohm’s law to find

either current, voltage, or resistance of a

circuit if you know any two of the three

components in a circuit.

B.

To find voltage of a circuit, use

V = I x R

C. To find resistance of a circuit, use

R = V/I

I

V

. Ohm’s Law

D. Lets try some problems: What is the

voltage of a circuit that has a current of 20

amps and resistance of 5 ohms?

A: 

Voltage

G: 

Current, resistance

E: 

V = I x R

S: 

V = 20 x 5

U: 

Volts 

 Answer:________

I

V

. Ohm’s Law

E. What is the current of a circuit that has

voltage of 100 volts and resistance of 5

ohms?

A: 

Current

G: 

Volts, resistance

E: 

I = V/R

S: 

I = 100/5

U: 

Amps

 Answer:________

I

V

. Ohm’s Law

F. What is the resistance of a circuit that has a

voltage of 20 volts and a current of 5 amps?

A: 

Resistance

G: 

Voltage, Current

E: 

R = V/I

S: 

R = 20/5

U: 

Ohms 

                    Answer:________

Electrical Circuits

Electrical energy 

enters your home at the 

circuit breaker

 or

fuse box 

and branches out to appliances, wall socket and

lights.

Circuit breakers 

– small piece of metal that

bends when it gets hot, opening circuit and

stopping current flow.

Electronic fuse 

- small piece of metal that

melts if current becomes to high, opening

circuit

Short Circuit

•

A 

short-circuit

 (also called a “short”) is a

wire that by-passes a device in a circuit.

–

Electricity always chooses the path of

least resistance.

–

Since wires have virtually no

resistance, electricity will go through a

wire instead of a device. 

•

This is known as a 

short-circuit

.

Examples Of A Short Circuit

•

Electricity always chooses the path of least resistance.

IV. Circuits

A. An electric circuit is something that

provides a 

path

 through which electricity

travels.

B. Although we think of circuits in terms of

man-made items, circuits are in nature as

well.  For example, the 

nerves 

in your body

create electric circuits!

IV. Circuits

C. Electric circuits are similar to pipes and

hoses for 

water

.  You can think of wires as

pipes for electricity.  The big difference is

that you can’t get the electricity to leave the

wire.  If you cut a water pipe, the water

comes out.  IF you cut a wire, the electricity

immediately stops 

flowing

.  Electric current

cannot flow except in complete circuits.

IV. Circuits

D. Circuits are made up of wires and electrical

parts, such as

 batteries

, light bulbs, motors,

or switches.  

E. Circuits must be complete for the circuit to

work.  This means that there must be an

unbroken

 wire or combination of wires and

electric devices - basically the electricity

needs a complete path for a circuit to be

complete.

V. Circuit Parts

A. Circuits can have many parts.  To make

circuit drawing easier, there are some

common symbols used in drawings called

circuit 

diagrams

. These electrical symbols

are quicker to draw and can be read by

anyone familiar with electricity.

B. 

Wires

, batteries, light bulbs, and switches

are commonly used in electric diagrams.

You will be learning their symbols today in

lab.

V. Circuit Parts

C. Because a circuit needs a 

complete path

 for

electricity to flow, a switch works by

breaking or completing the circuit path.

When the switch is on, the circuit path is

complete.  When the switch is off, the

circuit path is broken.

D. In many circuit diagrams, any electrical

device is shown as a 

resistor

.  A resistor is

an electrical component that uses electricity,

such as a light bulb.

VI. Circuit Types

A. A circuit with a switch turned to the off

position or a circuit with any break in it is

called an 

open circuit

.  Electricity can’t

travel through an open circuit.

B. A 

closed circuit

 is when a switch is turned

to the on position, there are no breaks in the

wire and the electricity can travel easily

through a closed circuit.

VI. Circuit Types

C. A common problem found in circuits is

that an unintentional 

break

 occurs.  When

building circuits it is a good idea to trace

your finger around the wires to tell if the

circuit is open or closed.  If there are any

breaks, the circuit is 

open

.  If there is a

complete loop, then the circuit is closed.

VI. Circuit Types

D. A short circuit is a circuit path with zero or

very low 

resistance

.  You can create a short

circuit by connecting two ends of a battery.

Short circuits are extremely dangerous

because they can cause huge amounts of

current

.

IV. Complex Circuits

A.

In our homes and workplaces, we often

have many electrical devices on at one

time.  This is only possible by building

complex circuits

.  The circuits we built in

lab 7.1 were simple circuits.

IV. Complex Circuits

•

The two type of complex

circuits are called 

series

circuits and 

parallel

circuits. In series circuits,

the current flowing through

a circuit can only take one

path.  In parallel circuits,

the circuit has branching

points and the current has

multiple paths.

V. Series Circuits

A. In a series circuit, the current can only take

one 

path

.  All the current flows through

every part of the circuit.  What we have

studied so far have been series circuits.  For

example, if you have a battery, a light bulb,

and one switch, everything is connected in

series

 because there is only one path

through the 

circuit

.

V. Series Circuits

B. To find the voltage, 

current

, or resistance in a

series circuit, you would use Ohm’s law 

C.

However, finding 

resistance

 in a series circuit

is a little different because there are multiple

resistors in a series circuit.

V. Series Circuits

D. To find the resistance in a series circuit,

you must add all the resistance together.

R

total

 = R

1 

+ R

2 

...

This means that the total resistance in a series

circuit is the sum of all resistance.

V. Series Circuits

Lets try a problem:  What is the total

resistance of a series circuit with three light

bulbs,   each with a resistance of 1 ohm

A: 

Total resistance

G: 

Three resistors, with 1 ohm resistance each

E: R

total

 = R

1 

+ R

2 

S: 

R = 1 + 1+1

U: 

Ohms 

 Answer:________

V. Series Circuits

What is the current in a series circuit with two

light bulbs at 1.5 ohms resistance each and a

voltage of 30 volts?

A: 

Current

G: 

Resistance, volts

E: I =   V/R  R

total

 = R

1 

+ R

2 

S: 

R = 1.5 + 1.5 = 3 

I = 30/3

U = 

Amps 

    Answer:________

VI. Parallel Circuits

A. A parallel circuit has at least one point

where the circuit 

divides

, creating more

than one path for current to flow.  Each path

in the circuit is called a 

branch

.

VI. Parallel Circuits

B. To find the voltage, current, or resistance

in a 

parallel circuit

, you would use Ohm’s

law  I = 

V

         R 

C. However, finding resistance in a parallel

circuit is a little different because there are

multiple 

resistors

 in a parallel circuit.  

VI. Parallel Circuits

D. To find resistance in a parallel circuit, you

must use the formula

R

total

 = 

1 

  +   

1

            R

1

      R

2

VI. Parallel Circuits

Lets try a problem:  What is the total resistance of a

parallel circuit with three light bulbs,   each with a

resistance of 2 ohms

A: 

Total resistance

G: 

Three resistors, with 2 ohm resistance each

E: R

total

 = 

1

   +   

1

                R

1

      R

2

S: 

R = ½ + ½ + ½  = 1.5

U: 

Ohms 

 Answer:________

VI. Parallel Circuits

What is the current in a parallel circuit with two light bulbs

at 2 ohms resistance each and a voltage of 30 volts?

A: 

Current

G: 

Resistance, volts

E: I = V/R      R

total

 = 

1

   +   

1

                                  R

1

      R

2

S: R = ½ + ½  = 1

    I = 30/1

U = 

Amps

   Answer:________

Magnetism

Magnetism

 – refers to the properties and interactions of

magnets in which there is a  force of attraction or

repulsion  between like or unlike poles.

Magnetic field 

– exerts a force on

other magnets and objects made of

magnetic materials (strongest closed

to magnet)

Strength of force between two magnets depends on the

distance between them.

All magnets have a north pole and a south pole.

LIKE POLES REPEL

UNLIKE POLES ATTRACT

MAGNETIC MATERIALS

Only few metals, such as 

iron

, 

cobalt

, and 

nickel

 are

attracted  to magnets or can be made into

permanent magnets.

Magnetic domains 

– group of atoms with aligned

magnetic poles (too small to be seen with eye)

Permanent magnets 

are made by placing a

magnetic material in  a strong magnetic field,

forcing magnetic domains to line up.

Magnetism

And

Electricity

When electric current flows through a wire, a

magnetic  field forms around the wire.

Strength of magnetic field depends on the amount

of current flowing in the wire.

Electromagnets

 – a temporary

magnet made by wrapping a wire

coil carrying a current around an

iron core.

Electric motor 

– a 

device

 that

changes 

electrical energy

 into

mechanical energy

Increases strength 

of the magnetic field by adding more

turns to wire coil (

solenoid

) or 

increasing the current

passing through the wire

In a simple electric motor, an electromagnet rotates

between the poles of a permanent magnet.