Physical Science
Chp 22: 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
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
Ohm's law magic
triangle
Voltage measured in volts, symbolized
by the letters "E" or "V".
Current measured in amps,
symbolized by the letter "I".
Resistance measured in ohms,
symbolized by the letter "R".
a
A
The force
or
pressure
behind
electricity
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?
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.
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.
• 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
• 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.
• 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?
• Most patients, when they become pulseless go
through a brief period in which the heart goes into a
chaotic quivering due to erratic electrical activity. This
is called ventricular fibrillation (VF). VF eventually
deteriorates into a total absence of electrical activity,
often around ten to fifteen minutes after arrest. The
best chance to regain a pulse is when a patient in
recent VF is shocked quickly - ideally in less than 4 - 6
minutes after arrest.
• An AED delivers electrical current through heart
muscle, temporarily ceasing all electrical activity in the
heart, hoping that when that electrical impulse returns,
it will return in an organized pumping action instead of
VF.
• Why do you think
most gas pump
handles are
coated in either
rubber or plastic?
Electric Power
The rate at which electrical energy is
converted into other forms of energy
Electric Power Equation
P=IV
Power (watts)= current (amperes) X voltage (volts)