Transcript Physical Science - Pleasant Hill High School

Physical Science
Ch. 7: Electricity
• Yes, we all know what electricity is, but
exactly what is it?
-where does it come from
-can you see it
-how is it created
Electricity
• Electricity is a force created by a difference in
charges (+ & -) due to gained or lost
electrons. (an electron is a negatively
charged particle.)
• When electricity is flowing between two
points, this is actually electrons moving from
point A to point B. This is called an electrical
current.
• In order for these electrons to flow,
however, there must be a difference in
charges (# of electrons built up) between
the 2 points. Just like heat flow needs a
difference in temperatures.
• Electricity always flows from a location
with a negative charge to a location with a
positive charge. (like charges repel, opposites attract)
-
+
• Think of a battery, the
top has a + charge and
the bottom has a
negative charge. So
when they are
connected, electrons
flow from the bottom to
the top.
Static Electricity
• You may have noticed that if you
walk across the carpet in socks,
and then touch your cat’s nose, it
will shock him/her (it).
• This is because, you picked up
some free electrons walking over
the carpet (kind of like dust), and
therefore gained a negative
charge.
• Touching kitty’s nose allows you to dissipate that
charge (send the extra electrons to an area with
fewer electrons, negative to positive)
• The shock will continue until both surfaces are at
the same charge (neutralized)
• Static electricity is a temporary 1 time
charge produced by an excess of
electrons.
• Electrons in the air will attach to moisture
in the air. That’s why very dry air (like in
the winter, or in the dryer) may hold a
large number of free electrons, because
there’s not much moisture for them to
attach to.
i.e. There’s more static in the air in the winter
because there are more free electrons just
floating around
Van der Graff generators basically induce a strong
electrical field (can be either positive or negative depending on
the setup) into a hollow metal sphere. This field can
the release electrons into the surrounding air when
the voltage becomes great enough.
Voltage
• The difference in charges between 2
points is called the voltage.
• 2 points can both have a tremendous
charge, but if the charge is the same then
there will be no flow of electrons between
them.
- - - - - - - - - - - - -
• The number of electrons actually flowing
through a conductor is called the current,
and is measured in amperes (amps).
1 amp = 6.25 x 1018 electrons per sec.
(6,250,000,000,000,000,000!)
• So let’s say that using a
voltmeter you determine
that there are no electrons
flowing from point A to point
B through an extension
cord connecting the two.
Does that mean it is safe
for you to touch either of
the 2 points? Hmmm……
• No! There may be no electrons flowing
because the charges are the same.
However, your charge may be much
different, which would allow the electricity
to flow into you.
Conductors and Insulators
• Although an electric current can
occasionally jump from 1 point to another,
it normally travels through a conductor. A
conductor is any material which allows
electrons (electricity) to flow easily through
it.
• Metals in general are good conductors of
electricity.
• Insulators are materials which prevent the flow of
electrons (electricity).
Some good insulators include rubber, glass,
wood, and plastic.
Any type of electric cord usually consists of a
metal conducting wire surrounded by a layer of
rubber or plastic insulation.
Resistors

Materials called resistors can slow the flow of
electricity, without stopping it all together.

Almost all materials, even good conductors,
provide at least a small amount of resistance to
an electric current.
Resistance is measured in ohms.
W
Direct Current (DC)
• In most electrical
circuits, the current will
flow in only 1 direction.
This is called direct
current or DC.
• Examples of direct
current include
batteries, lightning, and
static electricity.
Lightning
• Lightning is a form of direct current (DC)
produced by static electricity in clouds.
• The static is formed when air molecules
move past each other (just like clothes in a
dryer).
• The negative charges group at the
bottom of the cloud and transfer
electrons to the ground, which has taken
on a positive charge.
• Why do you think the ground has taken on a
positive charge?
• Does lightning always travel from cloud to
ground? Hmmm……
Alternating Current (AC)
• An alternating current will send a
flow of electrons in 1 direction
through a circuit, and then it will
reverse the flow in the other
direction.
• Household outlets are an example
of AC current. They reverse the
direction of the current about 120
times per second.
• What would
happen if you stuck
one finger in one
side of an outlet?
• One finger in each
side?
Transformers
 The AC which comes from your household
outlets usually has a voltage of about 120V.
The voltage of the power lines outside,
however, is much higher. Before entering
your house, the electricity from the power
lines must pass through a transformer. This
is a device which can increase or decrease
the voltage which exists between 2 points.
• Transformers outside your house make the voltage
in your outlets safe for household appliances.
• Occasionally a lightning strike make take out a
transformer, and send a power surge into your
house.
Step-up & step-down transformers
• Transformers can also increase the
voltage between 2 points.
An amplifier and a stun gun are examples
of step-up transformers.
Electric Generators
• The electricity supplied to
your home is produced by
an electric generator. This
is a device which changes
mechanical (kinetic) energy
into electricity.
• Most electric power which is
generated in Missouri is
either from hydroelectric or
fossil fuel power plants.
Electric Motors
Electric motors are devices which convert
electricity to mechanical energy.
 Most household appliance are examples of
electric motors.

Ex.: washing machine, fan, refrigerator,
VCR, dishwasher, hair dryer, etc.
• Electric cars have engines which run on
electrical energy rather than chemical potential
energy (gasoline).
• These cars are more efficient, cleaner to
operate, and decrease our dependency on fossil
fuels. However, the engines are not as powerful
and the cars have a limited range.
Batteries


Electric motors are usually specified to run on
either alternating current or direct current
(produced by batteries).
A battery is a device which produces a
continual direct current, through a chemical
reaction.

Batteries can be composed of a liquid electrolyte
(a liquid which will conduct an electric current)
(wet cell) or a paste electrolyte (dry cell).
Whether liquid or paste, the electrolytes are
usually acidic.
 Car and boat batteries are examples of wet cells,
flashlight batteries are examples of dry cells.
• A chemical reaction inside
the battery pulls electrons
away from 1 of the
terminals and gives them
to the other. This causes
1 terminal to be negative
(-) and the other to be
positive (+).
• When these terminals are
connected through a device,
they produce a flow of
electrons from negative to
positive, which power the
device.
Electric Circuits
• A circuit is a pathway along which an
electric current can travel, en route to a
device.
• Locating a device along that pathway will
allow it to be powered by the electric
current.
• There are 2 different types of circuits:
series and parallel.
Series Circuit
• A series circuit is one
which has only 1 pathway
for electricity to flow. All
devices are located along
this single pathway, and
any break in the circuit will
stop the flow of electricity.
• Ex.: old type Christmas
tree lights
Parallel Circuits
• A parallel circuit has
multiple paths which the
electricity can travel down,
with different devices
along the different paths.
• If one pathway is open or
broken, the others can still
carry electricity.
Ex.: newer Christmas tree
lights, household circuits
Schematic Diagrams
• A schematic diagram
uses symbols to
show electrical
circuits and wiring.
• A short circuit
occurs when the
current stops short
of the intended
device, usually
because of the
circuit being
completed prior to
reaching the device.
• A circuit breaker is a device which prevents too
much electricity from flowing through a circuit.
Breakers can be reset usually with the flip of a
switch.
• Fuses do the same thing as a breaker, but they
use a small wire connector which will burn out if
the flow of electricity is too high. Fuses usually
have to be replaced after they burn out. Fuses
are often found in electric appliances also.
Electromagnets
• An electromagnet is a temporary
magnet made by passing an
electric current through a wire
coiled around a metal core.
Electromagnets have 2 advantages
over normal magnets:
1. They can be turned on
and off.
2. Their strength can vary
based on the amount of
current flowing.
• Examples of electromagnets include
cranes in scrap yards, telegraphs, and
certain types of doorbells.
History of Electricity

600 B.C.-Thales discovered
static electricity.

1600 - William Gilbert names
the force electricity

Mid 1700's - Ben Franklin shows
that lightning is made of
electricity, and does numerous
experiments
1800 - Volta makes
first battery
1878 - T. Edison creates
electric light bulb

1943 - First computer
(ENIAC) created
1942 - Fermi produces the
first fission reaction
• The Van De Graff generator in class produced
around 15,000 v of electricity, while a typical
household outlet produces only 120 v.
Therefore, you’re safer sticking your finger in
an outlet.
Right…………?????
Explain.
• An automated external defibrillator (AED),
which is similar to the manual defibrillator
often seen used by EMT’s, is a portable
device used to diagnose and treat victims of
cardiac arrest.
How do you think this device works?
• Luigi Galvani was
cooking frog legs in
an iron skillet. He
noticed that when he
picked one up with
copper tongs to flip it
over, sometimes it
would “kick” as if it
were still alive. Why
do you think this
occurred?
• Draw a schematic diagram of a circuit with
a power source, 1 switch, 2 individual
devices along a series circuit, and 3
devices on separate branches of a parallel
circuit.
• The step-down transformer shown below has 8
coils of wire on the primary side and 4 coils
going out on the secondary side. If this
transformer is hooked up to a typical household
outlet, how many volts will be produced?
• Why do you think
most gas pump
handles are
coated in either
rubber or plastic?
• What purpose do you think the yellow connectors
on the electric fence shown below serve?
• The device shown below is an electronic
body fat tester. The subject holds the tester
by the handles, and a weak electric current is
passed through their body. How do you think
this is able to determine their percentage of
body fat?
Although both
dangerous, a lightning
strike near the beach in
Miami may be more
potentially hazardous
than the same strike at
Truman Lake. Why do
you think this is so?
• A car battery (wet cell) works similar to the
voltaic piles which we built in class.
Do you think it would be possible to build a
voltaic pile out of our
lab materials big
enough to have
the same power
as a car battery?
Why or why not.
• What is the object shown, and how do
you think it works?
Shown below is a ground fault circuit interrupter
(GFCI) outlet. What’s special about it, how
does it work, and where might one be found?
• In the picture below, what is the charge of
the ground? How do you know?
• Is the man in the car safe or in danger? Explain.