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Electricity
Static Electricity
Review of the Atomic Structure of Matter:
(1) protons = + charge, nucleus
(2) neutrons = no charge, nucleus
(3) electrons = - charge, probability cloud
Electrons are the only particles that are free
to move from atom to atom. This movement
of electrons gives rise to electricity.
Any object that has an unequal number of
protons and electrons is said to have a static
charge.
Examples of static electricity:
Van de Graaff Generators
If you walk across a nylon carpet with a
rubber-soled shoe you can generate a static
charge when you touch a doorknob (the
electrons are transferred from the tips of your
fingers to the doorknob).
Other examples of static electricity include:
(1) the static cling of laundry
(2) the static cling of plastic wraps
(3) lightning – a sudden static discharge
produced when the charge on an object
is too great and electrons rapidly move
from the object to the air molecules that
touch the object
Cloud to ground lightning occurs when a
negatively charged cloud induces a positive
charge at the earth’s surface.
When the cloud becomes sufficiently
charged, electrons are explosively
transferred to the earth’s surface.
Thunder is produced when the gases in the
atmosphere are heated by the lightning and
expand rapidly.
Laws of Static Electricity
(1) Opposite static charges attract. Like
static charges repel.
(2) Charged objects can induce a charge in
uncharged objects by attracting or
repelling electrons.
(3) Electric charges concentrate and leak
from points.
(4) Positive charges that result from a loss
of electrons are only temporary.
Gradually, electrons flow from other
sources to balance the excess positive
charges.
Electroscopes are used to detect static
charges.
The pictures above are of a gold-leaf
electroscopes.
The amount of charge that an object has is
measured in coulombs, c. One coulomb is
defined as an electric charged of 6.25 x 1018
electrons or protons.
If an object gains 6.25 x 1018 electrons or loses
6.25 x 1018 electrons (and has an excess of 6.25
x 1018 protons) it has a charge of one coulomb.
Current Electricity
Most nonmetals are insulators (poor
conductors). They can hold an electric charge
for some time but the charge does not flow
away because the electrons in most nonmetals
do not readily move from atom to atom.
Metals are good conductors because they have
one, two, or three electrons in their outermost
energy levels. These electrons are not as
tightly held and are free to move from one
atom to another.
Metals such as Au, Pt, and Cu are some of the
best conductors of electricity.
Materials like plastic, rubber, glass, and
porcelain are some of the best insulators or
nonconductors.
Semiconductors are materials that have a
conductivity between conductors and
insulators.
Semiconductors can be pure elements, such as
silicon or germanium, or compounds such as
gallium arsenide or cadmium selenide.
In a process called doping, small amounts of
impurities are added to pure semiconductors
causing large changes in the conductivity of
the material.
Although many electronic devices could be
made using vacuum tube technology, the
developments in semiconductor technology
during the past 50 years have made electronic
devices smaller, faster, and more reliable.
Semiconductors are used in:
radio
watch
computer
electronic
balances
television
CD player
lights
calculator
telephone
microwave oven
diagnostic
equipment
car
clock
security devices
video games
VCR
stereo
air conditioner
musical greeting
cards
refrigerator
stove
The flow of electrons through a conductor is
referred to as current electricity or electric
current.
The path along which the movement takes
place is called an electric circuit.
Open vs. Closed Circuits
A circuit is said to be open when a break
exists in a complete conducting pathway.
Example: turn a light switch off
A closed circuit (light switch on) allows for the
free flow of electron along a conduction
pathway.
Iboth switches a
The flow of electrons can be compared to the
flow of water.
Electric current is a measure of the number of
electric charges, or coulombs (6.25 x 1018 e-),
that pass a given point in a wire per second.
One coulomb of electrons flowing through a
wire per second is an ampere, A, of current
(amp).
The ampere is measured with an instrument
called an ammeter.
Current is usually represented by the letter I
in equations and graphs.
What keeps electrons moving in a circuit?
In an electric circuit, a battery or generator
acts as a “pump”, taking electrons from
atoms.
The electrons create a difference in the level
of charge between the two binding posts of a
cell or battery creating a potential difference
between the two posts.
Electrons flow through a wire because the
excess electrons at the negative terminal have
a higher potential energy than electrons at the
positive terminal.
The difference in potential energy between an
electron at the negative terminal and one at
the positive terminal is called potential
difference.
Potential difference between terminals is
measured in volts, V.
Potential difference is often referred to as
voltage.
Examples:
flashlight battery = 1.5 volts
car battery = 12 volts
electrical outlets = 110 volts
A voltmeter measures potential difference.
Modern batteries use a variety of chemicals
to power their reactions. Typical battery
chemistries include:


Zinc-carbon battery - Also known as a
standard carbon battery, zinc-carbon
chemistry is used in all inexpensive AA,
C and D dry-cell batteries. The electrodes
are zinc and carbon, with an acidic paste
between them that serves as the
electrolyte.
Alkaline battery - Used in common
Duracell and Energizer batteries, the
electrodes are zinc and manganeseoxide, with an alkaline electrolyte.






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Lithium photo battery - Lithium, lithiumiodide and lead-iodide are used in
cameras because of their ability to supply
power surges.
Lead-acid battery - Used in automobiles,
the electrodes are made of lead and leadoxide with a strong acidic electrolyte
(rechargeable).
Nickel-cadmium battery - The electrodes
are nickel-hydroxide and cadmium, with
potassium-hydroxide as the electrolyte
(rechargeable).
Nickel-metal hydride battery - This battery
is rapidly replacing nickel-cadmium
because it does not suffer from the
memory effect that nickel-cadmiums do
(rechargeable).
Lithium-ion battery - With a very good
power-to-weight ratio, this is often found
in high-end laptop computers and cell
phones (rechargeable).
Zinc-air battery - This battery is
lightweight and rechargeable.
Zinc-mercury oxide battery - This is often
used in hearing-aids.
Silver-zinc battery - This is used in
aeronautical applications because the
power-to-weight ratio is good.

Metal-chloride battery - This is used in
electric vehicles.
When electrons have a steady flow in one
direction, the flow is called direct current
(dc).
An example is a dry cell battery, where
current travels in the same direction for the
life of the cell.
When the direction of the current keeps
jumping back and forth in a circuit, it is
called an alternating current (ac).
Electric generators (dynamos) and power
plants are sources of alternating current.
Capacitors are metal plates separated by an
insulator that store an electrical charge.
When you connect a capacitor to a battery,
here’s what happens:


The plate on the capacitor that attaches to
the negative terminal of the battery
accepts electrons that the battery is
producing.
The plate on the capacitor that attaches to
the positive terminal of the battery loses
electrons to the battery.
Resistance is a measure of how much a
conductor resists or holds back the flow of
electrons.
Current is greater if the resistance of the
circuit is reduced.
Good conductors have low resistance, while
poor conductors have higher resistance.
The electrical resistance of a conductor
depends on:
(1) thickness – Thick wires have less
resistance. Resistance varies inversely as
the square of its diameter.
(2) length – Longer wires have greater
resistance. Resistance varies directly
with length.
(3) temperature – Resistance increases as
the conductor becomes hotter and
decreases as it becomes cooler.
In superconductors molecular motion has
been slowed to the point that it does not
hinder electron flow.
Resistance is measure in ohms, .
 = omega
The relationship between electric current,
voltage, and resistance is described in Ohm’s
Law.
Ohm’s Law states that the amount of electric
current flowing in a circuit increases as
voltage increases and decreases as resistance
decreases.
current = voltage / resistance
I=V/R
Units: I(amps, A), V(volts, V), R(ohms, )
V=AxR
Examples:
A circuit has a resistance of 100 ohms and a
potential difference of 110 volts. What is the
resulting current in the circuit in amperes?
(a) How large a current will 120 volts send
through a resistance of 20 ohms?
(b) What is the resistance of an electric
toaster if 120 volts sends a current of 5
amperes through it?
Electric power is a measure of the rate at
which the energy of flowing electrons is
used.
A current of one ampere flowing through a
circuit with a potential difference of one volt
produces one watt of power.
Power = current x voltage
P = IV
Units: P(W), I(A), V(V)
Example:
At 110 volts, 0.25 amperes of current flow
through a small black and white TV. How
much power is used by the TV?
You pay for the energy you use!
The electric energy you use in your home is
measure in units called kilowatt-hours,
kWh.
Electric devices are rated in watts according
to the amount of energy per second they
need to operate.
The charges are based on:
(1) power – how fast electrical devices
consumer energy
(2) time – how long the devices are used
energy = power x time
E = Pt
E = VIt
P = VI
Units: E(Wh or kWh), P(W or kW), t(h)
They use a kilowatt-hour meter. The electric
bill is calculated on a price per kilowatthour plus assorted fees.
Example:
How much energy is used by a 750-watt
room air conditioner in 8 hours?
If we used it for 8 hours a day for 15 days,
what is the operating cost?
Paths For Electrons
(1) A series circuit is one in which all
components are connected in tandem.
The current at every point of a series
circuit stays the same. In series circuits
the current remains the same but the
voltage drops may vary. The total
resistance is the sum of the parts.
There is only one path for current. If one bulb
in a series burned out all of the bulbs would
go out.
(2) Parallel circuits are those in which the
components are so arranged that the current
divides between them. In parallel circuits the
voltage remains the same but the current may
vary. The circuits in your home are wired in
parallel.
Current that passes through one electrical
device does not have to pass through another.
Overloaded wiring may overheat and cause a
fire.
Short Circuits create a pathway of very low
resistance.
In most cases, short circuits are caused by
electron flow between two wires that are
touching.
Example:
Overloads can occur when the amount of
current used by all of the devices exceeds the
safe limits of the circuit.
Fuses and circuit breakers offer protection.
A fuse contains a short, Pb-alloy wire that
melts if the circuit carries a load that is too
large. The circuit breaks and the current
stops. They are consumable.
good fuse
blown fuse
A circuit breaker acts like a switch to open a
circuit if the current is too great. It can be
reset and reused.
Generating Electricity
(1) cells and batteries
(2) Electromagnetic induction – process of
producing a current by moving a
magnet near a conductor or conductor
near a magnet.
A generator produces electricity by turning
coils of a wire in a magnetic field.
(1) Steam (burning coal) or falling water
(2) blades of the turbine are turned
(3) coils of wire are turned in a magnetic
field
(4) electricity is produced
(5) transmitted at high voltages (step-up
transformer) by power lines
(6) voltage decreased (step-down
transformer) prior to use
A transformer is a device that changes the
voltage of alternating current.
Heat and Light From Electricity
Resistance causes heat. The greater the
resistance the greater the heat produced.
Electricity produces light. The following are
types of electric light:
(1) electric arc – A small gap is present in a
circuit. With sufficient voltage the
current jumps the gap and produces a
bright light.
(2) incandescent bulb – heat a wire or
filament to the point it glows
Example: tungsten filament
(3) gas – At low pressures gases are
conductors. A high voltage strips
electrons from the gas.
Examples: Ne – red, Na –yellow, Hg –
blue/green
(4) fluorescent – bulb is coated with
phosphors (powered minerals that
convert UV light to visible light –
fluorescence)