Science High School Reviewer Physical Science: Electricity and Magnetism

What is electricity?
All matter is made up of atoms. All atoms, in turn,
are made of different numbers of protons, neutrons,
and electrons. Although electrons are much smaller
than protons, they carry the same amount of electric
charge. When positively charged particles (protons)
and negatively charged particles (electrons) are equal
in number, the charges cancel one another out.
Electrons spin around the outside of an atom’s
nucleus, and they can sometimes separate from it. If
a “stray” electron is lost by one atom and picked up
by another, then the charges of both atoms change.
The atom that lost the electron becomes positively
charged, and the atom that gained the electron
becomes negatively charged. As electrons move from
one atom to another, this charge moves from atom to
atom as well. The transfer of charged particles builds
a series of electrically charged atoms. Electricity refers to
the movement and transfer of the energy of charged
particles. The flow of electricity is energy that is
available for doing work. This energy is used to power
motors, lights, appliances, and many other devices.

Static Electricity
The transfer of electrons from one
place to another causes an imbalance of
positive and negative charges. This results
in static electricity. Static electricity is
the buildup of a positive or negative
electric charge on a material’s surface.
Static electricity can be produced by
many different nonmetallic materials.
Some materials lose electrons easily, and
others tend to attract electrons.

Objects with the same electric charge
repel one another, and objects with
opposite charges attract one another. If
a negatively charged material touched a
positively charged material, the opposite
charges would attract one another, and
the materials would stick together. This is
sometimes referred to as “static cling.”
A difference in electric charge
produces an attractive force which
is similar to gravity, only stronger.
Static charge on a comb can make a
substance such as confetti move against
the pull of gravity and jump to the comb.
Using the comb can affect the overall
charge of your hair as well.

A difference in electric charge
produces an attractive force which
is similar to gravity, only stronger.

How can electricity jump?

A discharge corrects an imbalance, or
difference, in charge through the rapid
movement of electrons. Discharges are
responsible for static shocks.

An induced charge is a static charge
caused by the presence of an object
that itself has a net positive or
negative charge.

Conductors and Insulators
A conductor is a material through
which an electric charge flows easily.
Most conductors are made of atoms
from which some electrons are likely
to become unattached. Metals such as
copper are the best conductors. Your
skin is not a good conductor. If it were,
even a small shock could be dangerous.
An insulator is a material that does not
allow an electric charge to transfer easily.
Conductors and insulators of electric
charge are very similar to conductors and
insulators of heat energy.

How can electricity flow?
The charges that make electrical
energy available to do work can flow
through conductors along different
paths. Each path for electric charge
is an example of a circuit. In circuits,
electric charges move within wires,
bulbs, and other devices.
A simple circuit consists of an energy
source such as a battery, a device such
as a lamp, and connecting wires. The
flow of an electric charge through a
circuit is called current electricity. Light
bulbs light up because an energy source
pushes an electric charge through them.

In a circuit, energy from a
source such as a battery causes an
electric charge to flow through the
wire. Electrons that are not strongly
attached to the atoms inside the wire
move, causing current electricity.

Although the movement of negatively
charged electrons is most often referred
to when studying current electricity in
wires, current is always said to flow
from the positive to the negative terminal
in a circuit. This is called conventional
current. This way of describing the
movement of electric current originated
before scientists fully understood
electricity. However, it is still the way
used to describe how circuits operate.
A switch can control the flow of a
charge in a circuit. When the switch is
opened, the flow is halted. The circuit
is incomplete and is then called an
open circuit. When the switch is closed,
the electric charge resumes its motion.
When current flows once again,
the circuit is called a closed circuit.
!!!!! Electrons flow from the negative terminal to the positive. Conventional current or simply current, behaves as if positive charge carriers cause current flow. Conventional current flows from the positive terminal to the negative.

Direct and Alternating Current
The simple circuit in the diagram on
the opposite page uses direct current.
Direct current, or DC, refers to current
that flows in one direction. Batteries
provide DC, as do solar-powered cells.
The very first commercial electric
power stations also used DC.

Inventor Thomas Edison was a
strong advocate of DC. Rival scientists
such as Nikola Tesla and George
Westinghouse promoted the use of
a different type of current. These
scientists advocated systems that
produced alternating current, or AC. In
AC, the electric charge does not flow
through the circuit in one direction. AC
power is transmitted when the charge
changes direction, moving back and
forth at regular intervals.
It might surprise you to learn
that the power in the outlets in your
home is not the type that Thomas
Edison promoted. Although each type
of current has both advantages and
disadvantages, AC power was the
better choice. The main advantage of
AC is that this type of current can be
transported over long distances with
far greater efficiency.

Resistors
Sometimes, people want to reduce
the amount of electric charge that flows
through a circuit. Current can be reduced
by adding a resistor to the circuit.

Resistors lower the amount of electric
charge that flows through a device.

Lights and other devices connected in a circuit act
as resistors, because they too reduce
current flow. A light bulb converts
electrical energy to both heat and light.

In a series circuit, there is only one
path along which current electricity can
flow.

In a parallel circuit, there are
multiple paths along which current
electricity can flow. For example, in
a string of lights wired in a parallel
circuit, when one bulb burns out, there
are other paths along which electric
charge can flow to all the other bulbs.
Parallel circuits are used everywhere
that we use electricity, including homes,
stores, and offices. If any one device on
the circuit burns out, the other devices
on the circuit will keep working.

Short Circuits
Sometimes, an additional type of
backup is added to protect against high
levels of current electricity. A short
circuit is a path for current electricity
that has little or no resistance. Current
flowing in a short circuit can reach
dangerously high levels and will also
generate heat. A fuse is a device that
prevents dangerous levels of current
from continuing to flow through a
circuit. A fuse contains a piece of metal
that melts if it is heated. This melting
breaks, or opens, the overloaded
circuit.

What are magnets?

Magnets are surrounded by a
magnetic field, an invisible area where
the forces of magnetic attraction or
repulsion can be detected. Scientists
think that magnetism comes from
the spinning motion of electrons in a
magnet’s atoms. Each atom behaves
like a tiny magnet. Because atoms of
iron, nickel, and cobalt tend to point
their magnetic fields in the same
direction, these elements form strong
magnets.

Properties of magnets
All magnets have two poles: a
north-seeking pole (N) and a southseeking
pole (S).

Electromagnets
The flow of electric current
produces a magnetic field. A wire
carrying electric charge has a magnetic
field surrounding it. It can even pick up
certain metal objects, just as a common
magnet can. The magnetism around
a wire carrying an electric current
becomes much stronger when the wire
is wrapped into a tight coil. Each turn
of the coil makes the magnet stronger.
If an iron core is placed within the
wire coils, the magnet becomes even
stronger. A device that is magnetized
by current electricity is called an
electromagnet. Electromagnets are
convenient, because their magnetism
can be turned on and off by stopping
or starting the flow of electric charge.

Applications of Electromagnetism
Electromagnets can be found in
motors and even in sound equipment
such as speakers and electric-guitar
pickups. The strings of an electric
guitar vibrate within a magnetic field.
The pickup “picks up” these vibrations
and converts them into a signal, which
you then hear as an amplified sound.
Electromagnets play an important
role in medicine. Doctors use a technique
called magnetic-resonance imaging,
or MRI, to produce images of human
tissue. In an MRI, the patient lies
inside a long tube and is surrounded
by electromagnets. In the patient’s body,
the nuclei of certain atoms tend to line
up with the magnetic field. When radio
waves are beamed at the patient’s body,
the hydrogen nuclei gain energy and
send out signals that a computer detects.

How do we use
generators?
Electromagnets play an important role
in producing the electricity that we use.
A generator is a device that converts
mechanical energy—supplied by a hand
crank, turbine, magnets, or motors—
into electricity. A generator uses
energy to cause electric charge to flow.
Electric power plants use generators
to produce electric power for homes
and businesses.

Steam turbines are commonly used
to produce current electricity. The
turbines are powered by high-pressure
steam produced inside giant boilers.

Hydroelectric power plants spin their
turbines with water that is stored behind
dams and therefore has a great deal of
potential energy.

From Generators to Homes
Generators at power plants produce
current electricity. How does this
energy make its way into your home?
Current electricity is first conducted
to a transmission substation. The
substation has many towers with power
lines leaving them. Step-up transformers
in the substation increase the voltage of
the current electricity. This allows the
electric charge to be transmitted over
long distances more efficiently.

The high voltages that are used in
transmitting current electricity over
distances are dangerous. Therefore,
before current electricity enters
your home, the voltage is decreased
at several stages. Transformers are
again used to change the voltage of
the current electricity. Step-down
transformers decrease the voltage
of the electric charge. Another local
substation reduces the voltage from
more than 155,000 volts down
to about 10,000 volts. Additional
reductions take place in transformers
on power lines and outside homes.

Saving Energy
Electricity costs money. In addition, producing
electricity uses up fuel and may pollute the environment.
Many generators are run by steam. The steam is
produced by burning fossil fuels such as coal and oil.
You can save fuel and save money by conserving
energy. Wearing warm clothes instead of turning up
the heat saves energy. Opening a window instead
of using a fan saves energy. Turning off lights and
appliances when they are not in use also saves energy.