Transcript Electricity & Magnetism

Electricity
Static, Currents, Circuits
Electricity?



Electricity is all about electrons, which are the
fundamental cause of electricity
Static Electricity - involves electrons that are
moved from one place to another, usually by
rubbing or brushing
Current Electricity - involves the flow of electrons
in a conductor
Electricity
Everything in the world is made up of atoms. Each
atom has smaller parts in it. One of those parts is
called electrons. Electrons can move from atom to
atom. When an electron moves to a different
atom, it causes another electron to have to move.
When electrons move quickly from one atom to
another is it called Electricity!
Let Us Review

ATOMS………………………………….
Atoms…
 Have
neutrons, protons, and
electrons.
 Protons are positively charged
 Electrons are negatively charged
Electrons…
 Are
located on the outer edges of
atoms…they can be moved.
 A concentration of electrons in an
atom creates a net negative
charge.
 If electrons are stripped away, the
atom becomes positively charged.
The world is filled with electrical
charges:
+
-
+ +
+
+ +
+
+
+
+
-
What is this electrical potential
called?
 Static
-
Electricity
-
- -
+
++
++
Static Electricity
 The
build up of an electric charge
on the surface of an object.
 The charge builds up but does not
flow.
 Static electricity is potential energy.
It does not move. It is stored.
 When static electricity is discharged
a “spark” or “shock” occurs.
Static electricity
Static electricity is caused when certain
materials are rubbed against each other.
 Electrons can be rubbed off one material
and on to another.
 The material that has got extra electrons is
now negatively charged
 The material which has lost electrons is
positively charged.

Where do charges come
from?
Rubbing materials does NOT create
electric charges. It just transfers
electrons from one material to the
other.
Where do charges come
from?
When a balloon rubs a piece of wool...
+
–
+
–
–
–
–
–
+
+
wool
+
electrons are pulled from the
wool to the balloon.
The balloon has more electrons than
usual.
The balloon: – charged,
The wool: +charged
Static Discharge…
 Occurs
when there is a loss of
static electricity due to three
possible things:
Friction
- rubbing
Conduction – direct contact
Induction – through an electrical
field (not direct contact)
Static Discharge
Human body can not feel less than
2,000 volts of static discharge
Static charge built up by scuffing
shoes on a carpet can exceed
20,000 volts?
The Electrostatic Force

The electrostatic force is simply the field
force that exerted by electrical charges

Because of the intensity of the attraction
between charged atoms, it is stronger than
the force of gravity
Static Electricity
Static electricity is the
charge that stays on an
object.
Law of Charges
Unlike charges attract
each other, and like
charges repel each
other.
Electrical Charge

Some materials have a weak affinity
(attraction) for electrons so they easily
loose their electrons due to friction. When
you walk across the floor you pick up
these electrons and become “charged”.
When you touch someone you release
that charge (discharge) and shock them.
Electrical Charge

Most objects are neutral because their
charges are balanced, but when
conditions change so do the electrical
charges

Have you noticed how on drier days you
seem to pick up charges more easily and can
“shock” your friends easily?
Electrical Charge

The reason some materials seem to have
more ability to cause “shock” is because
some materials have a weaker affinity for
electrons
They give up their electrons more easily
 When you walk across carpet your
shoes/socks are able to pick up the electrons
because the carpet easily gives them up

 The
you can SHOCK your friends
Electrical Charge
When an object loses electrons it
becomes positive
 A gain of electrons results in a negative
charge
 Whether a charge is negative or positive,
the net charge is measured in Coulombs
(C)


One Coulomb is the charge carried by 6.24 x
1018 protons or electrons
Electrical Charge

Remember insulators?


The reason they insulate so well is because
they hold so tightly to their electrons (not
allowing them to flow)
Conductors?

Conductors conduct so well because they
hold loosely to their electrons allowing
electrons to flow
Electric Fields

Electrostatic force is a field forceMeaning that it is exerted over an area
 Charged objects can exert a force on distant
charges which they are not in direct contact


Physicists have noticed that electrical charges
flow in a particular pattern
 Lines
of force radiate outward from a charged
object through a path of least resistance
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.
Electrical Induction

Is the creation of charge region when a
neutral object when exposed to a nearby
electrical charge
They do NOT touch
 The charge is temporary
 It is a field force

Electrical Induction
•Dipole- contains a positive and a negative pole (end)
•it is a neutral molecule whose electrons have
shifted from positive to negative
Detecting Charges

Electroscope- an instrument used to
detect the presence of a charge

They do NOT detect type of charge nor
amount
 It
cannot quantify
Conductors vs. Insulators

Conductors – material through which electric
current flows easily.

Insulators – materials through which electric
current cannot move.
Transferring Charges

Electrical conductors- allow electricity to
flow through, because they hold their
electrons loosely

Most often these are metals
 Remember

that sea of electrons
Electrical insulators- do not allow
electricity to flow, because they hold onto
their electrons very tightly

Glass, wood, and rubber
Transferring Charges

Remember when you get rid of your stored
electrons you are discharging the surface

Electrical discharge causes the shock you
experience after a build up of static electricity
Transferring Charges

Semiconductors- allows limited electron
flow or conduct under certain conditions

This is because some materials can change
their electron affinity in response to
environmental conditions
Examples
 Conductors:
 Metal
 Water
(only
because of the
minerals and
metals in the
water)
 Insulators:
 Styrofoam
 Rubber
 Plastic
 Paper
 Wood
Grounding
What is grounding?
An object is grounded when it is
connected to the earth through a
connecting wire.
If a charged conductor is grounded, it
will become neutral.
Grounding

Additional wire in circuit to protect a person
from shock.

Plugs have a 3rd prong. This connects the
metal shell of the appliance to the ground
wire of the building.
Lighting Rods

Lightning rods were used to protect
buildings from lightning strikes

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.
Storing Charges

How can we make an object keep its
charge?

Remember insulators keep their electrons
while conductors allow their electrons to flow
freely……
Storing Charges

So…
 To
store a charge we must use both conductors
and insulators…….
Storing Charges

Leyden jar- one of the first devices used to
store charges

It is made of a jar (glass or plastic) lined and
coated with lead and that used electrical
induction and grounding to greatly increase
storage capacity
 Other
metals will work, with less power
Storing Charges

Capacitors are devices used to store
electrical energy today.

They are constructed very similarly to a
Leyden Jar
How do we apply our knowledge of
static electricity?

Pollution is a huge problem today,
especially in big cities and in homes


Just like every other material even dust and
other pollutants have atoms; and thus protons
and electrons
Ionic purifiers clean the air by using electricity to attract and
trap dust by generating negative ions. The negative ions are
created by running high voltage electricity through thin metal
plates that create a negative charge on surrounding gas
molecules and adhere to the particulates in the air. Now that
the particulates are linked with a negative charge, they are
attracted to their opposite polarity, a positively charged metal
plate that captures the impurities from the passing air flow.
Electrical Current and Ohm’s Law
Electricity that moves…
 Current
(electricity): The flow of
electrons from one place to
another.
 Measured in amperes (amps)
 Kinetic energy
Electrical Current
Electrical potential energy is the ability to
allow electricity to flow
 Conventional current flow- the flow of
positive charges through a conductor, this
is opposite the flow of electrons

Electrical Current

Conventional flow- flow of positive charges
through a conductor

Let us look at this more..
 Now
we know that protons do not move. This idea
does not change that rule
 What occurs when an electron leaves an atom, it
leaves a positive hole. This causes an electron to
move into it and another electron moves out in a
different direction

So we can say that conventional current is movement of
positive “holes” through a conductor
Current

The directed movement of electrons or
the flow. (pushed by voltage)
Unit of measure is the amp or ampere
Measured with an ammeter or amp meter
Conventional theory
Says that current flows from + to Scientists first guessed that it was the proton
that was in motion in the atom
Electron theory
Says that current flows from – to +
When scientists discovered that it was the electron
that was in motion, electron theory was born
Does it matter?
When talking about electronics, it does
matter which way current flows, but for
basic electricity, it doesn’t
 Most automotive texts, and classes still
teach conventional theory, so that is
what we will stick with.

Direct Current
(DC) Flows in one direction, directly/straight
from the source
 Items such as:

 Cell
phone
 Digital camera
 Laptop computer

Are powered by a direct current: because they
have batteries
Direct Current:

Electrical circuit= Path through which
electricity flows

Electrical load- takes the electrical energy and
converts it to another form of energy
 Light
bulb- converts electrical energy to light
energy
D.C. Current

Current always flowing the same way
Sources of a Direct Current

Electrochemical Cells and Batteries

An electrochemical cell uses electrons
released by a chemical reaction
 This
chemical reaction occurs between a metal
and electrolyte paste

A battery consists of one or more
electrochemical cells
Batteries
Batteries contain one or more electric cells
 Electric cell- a wire connects 2 metals that
are in contact with an electrolyte, it uses
chemical energy to produce an electrical
current


Electrolyte- a liquid or paste substance that
conducts electricity
Batteries



Wet Cell- uses a liquid electrolyte to conduct
electricity
Dry Cell- uses a paste electrolyte to conduct
electricity
When a battery is “dead” it is because the metal
in the battery no longer reacts with the
electrolyte.

The electrolyte may be depleted
or

The metal is used up
What is a generator?
– a machine that
changes mechanical energy to
electrical energy
 Usually use moving magnets to
create currents in coils of wire.
 Generator
Sources of Direct Current

Photovoltaic Cells- solar cells that use
energy absorbed via light rays to power
electrical devices

The light ray is absorbed by a semiconductor,
its electrons then gain kinetic energy and
jump about creating energy!
Alternating Current

The current electricity flows from a source
through many conductors to several
outlets

It periodically changes direction at a specific
frequency
A.C. Current

Current flows first one way, then the
other
Direct current versus alternating current –
AC vs DC : What’s the difference?
Direct current is electrical current which comes from a
battery which supplies a constant flow of electricity in
one direction.
Alternating current is electrical current which comes
from a generator. As the electromagnet is rotated in the
permanent magnet the direction of the current alternates
once for every revolution.
.
How do we Measure Electricity?
 Volt–
the measurement of the amount
of electrical push or force in a circuit
 Watt– the measurement of power, or
how fast work is done
 Ampere– a unit used to measure how
much current flows through a given
part of a circuit in one second
What is Voltage?
 The
measure of energy given to
the charge flowing in a circuit.
 The greater the voltage, the
greater the force or “pressure”
that drives the charge through the
circuit.
Voltage

A VOLT is the SI unit for potential
difference

The difference in electrical potential between
two points
 The
amount of work needed to move a charge
between them
 1V = 1J
1C
Voltage

Electrical pressure (pushes current)
Atoms that are short electrons and atoms
with extra electrons
Unit of measure is the volt
Measured with a voltmeter
Note! A good voltmeter won’t
have any flow thru it
Ways to make voltage
Magnets
 Chemical
 Pressure
 Heat
 Light
 Friction

Most common
Calculating Volts
If an electric motor operates at 2880 watts
and 12 amps, what would be the voltage
requirement for that motor?
Volts = 2880 Watts / 12 Amps
2880 / 12 = 240 Volts
How much is one amp?

One amp is 6.25 X 10 to the 18th power
of electrons past a given point per
second. (one Coulomb)
6,250,000,000,000,000,000
The Amp

SI unit of electrical current is the ampere
(A) or amp
 1A=
1C
1s
 An
ammeter is used to measure amps
Calculating Amperage
If we have a 100 watt lamp plugged into a 120
volt receptacle, we can determine the rate of
flow or the amperes for that circuit.
Amps = 100 Watts / 120 Volts
100 / 120 =.833 Amps
Difference b/t Volts and Amps
 Example
 Amps
– you could say that…
measure how much water
comes out of a hose.
 Volts measure how hard the water
comes out of a hose.
Ohms Law

Current flow is strictly a result of how
much voltage and resistance there is

To get more current
 Increase
voltage
 Decrease resistance
 Or both

To get less current to flow
 Decrease
voltage
 Increase resistance
 Or both
OHM’S LAW

When the voltage and resistance are
equal in a circuit, ONE amp will flow

One volt will push one amp through one
ohm of resistance
A picture is worth a
thousand words
Electrical Calculations – What is Ohm’s Law?
I=
3V
2Ω
I = 1.5 amps
Ohm’s law formulas
Voltage is represented by the letter “V”
 Amperage is represented by “I”
 Resistance is represented by “R”

V=IxR
Amps Volts Watts
The following relationship exists
between Amps, Volts and Watts.
Amperes are a measure of the rate of flow of
electricity in a conductor.
Volts are a measure of electrical pressure.
Watts are a measure of the amount of energy
or work that can be done by amperes and volts.
Formulas
Watts = Volts x Amps
Volts = Watts / Amps
Amps = Watts / Volts
What is Resistance?
 The
opposition to the flow of an
electric current, producing heat.
 The greater the resistance, the
less current gets through.
 Good conductors have low
resistance.
 Measured in ohms.
Resistance

Opposition to current flow (anything that
slows down current)
Unit of measure is the ohm
Measured with an ohmmeter
Factors that affect the
resistance of a circuit

Type of material used

Conductor / Insulator / Semi-conductor
Length of the circuit
 Diameter of the circuit
 Temperature
 Connections

Calculating Watts
If a water heater operates at 20 amps on a 240
volt circuit, what is the wattage of the appliance?
Watts = 240 Volts x 20 Amps
4800 Watts =240V x 20A
Watts=4800
How is Electrical Power calculated?
Electrical Power is the product of the current (I) and the
voltage (v)
The unit for electrical power is the same as that for
mechanical power in the previous module – the watt (W)
Example Problem: How much power is used in a circuit
which is 110 volts and has a current of 1.36 amps?
P=IV
Power = (1.36 amps) (110 V) = 150 W
Here is an easier way to remember
The three formulas
When you know the amperage and
resistance of a circuit you can figure
the voltage
V
Voltage = amperage times resistance
When you know the voltage and
amperage of a circuit you can figure
the resistance
V
Resistance = voltage divided by amperage
When you know the voltage and
resistance of a circuit you can figure
the amperage
V
V
Amperage = voltage divided by resistance
Ohm’s Law

A cars headlight draws a current of 4.2 A
at the car battery’s voltage of 12 V. What
is the resistance of the headlight?
 I=
4.2 A
 V= 12 V
 R=?
 R= V
I
 R=
12V
4.2 A
 R=
2.85 V/A~ 2.9Ω
Direct Current and Power

Power measures how fast work is done
Work = joules
 Time= seconds
 Power= J/s
 Power= Watts (W)

Direct Current and Power

Power is directly related to energy

In fact when the electric company charges a
home for “power” they are actually charging
for energy used, not how fast you use the
energy
Electrical Power and Energy

Electrical energy is measured in kilowatthours (kWh)


Energy= Power x Δt
Electrical Power is measured in Watts

Power = voltage x current
 P=
VxI
DC Electrical Power

A flashlight bulb draws 4.17 A when lighted. If it
has a resistance of 1.44 Ω, what power does it
draw from the 6.00 V battery?



V= 6.00 V
I= 4.17 A
R= 1.44 Ω

P= IV
P= (6 V)(4.17 A)= 25.02 W
DC Electrical Power

The following formulas will also work:
 P= I2R

Because P= VI and V= IR




 P=

So if I replace IR for V
P= I x R x I
P= I2R
P= (4.17 A)2(1.44 Ω)= 25.04 W
V2/R
Because P= V x I and I= V/R




So if I replace V/R for I
P= V x V/R
P= V2/R
P= (6.0 V)2/(1.44 Ω)= 25.00 W
How can we control currents?
 With
circuits.
 Circuit: is a path for the flow of
electrons. We use wires.
Direct Current

So how do we control a current
With a SWITCH- a conductor that can open
and close a gap in a circuit
 When the switch is open the circuit is open
and electricity does not flow
 When the switch is closed the circuit is closed
and the electricity flows

SWITCH

A switch is a conductor that bridges the gap in
a circuit
 It
is like a drawbridgeWhen the bridge is down the people can pass- you
have a closed circuit
 When the bridge is up no one can pass- you have
an open circuit

What is the difference between an open circuit and a
closed circuit?
A closed circuit is one in which the pathway of the electrical
current is complete and unbroken.
An open circuit is one in which the pathway of the electrical
current is broken. A switch is a device in the circuit in which the
circuit can be closed (turned on) or open (turned off).
Direct Current

Two types of Circuits:
Series Circuit
 Parallel Circuit


Circuits can be controlled by a switch
A
switch is a metal conductor that is able to open
and close a circuit
What are electric circuits?
Circuits typically contain a voltage source, a wire
conductor, and one or more devices which use the
electrical energy.
What is a series circuit?
A series circuit is one which provides a single pathway
for the current to flow. If the circuit breaks, all devices
using the circuit will fail.
There are 2 types of circuits:
 Series
Circuit: the components
are lined up along one path. If
the circuit is broken, all
components turn off.
Series Circuit
Series Circuit Rules
Only one path
 Amperage stays the
same
 Each resistance adds
up to the total

 R1

+R2 =Rt
Voltage divided
between the loads (all
used up or dropped)
There are 2 types of circuits:
Circuit – there are several
branching paths to the
components. If the circuit is
broken at any one branch, only
the components on that branch
will turn off.
 Parallel
Parallel Circuit
What is a parallel circuit?
A parallel circuit has multiple pathways for the current to
flow. If the circuit is broken the current may pass through
other pathways and other devices will continue to work.
Electrical Safety
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.
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.

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.

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.
That’s It !!!!