Download Notes: Electricity and Magnetism

ISCI 2002
Notes – Electricity and Magnetism
Electricity
What exactly is electricity, and how is it produced?
To understand electricity it is important to remember what you learned about
atoms. An atom can be in its stable state or electrons equaling protons. This
balances the atom or makes it electrically neutral. Also remember that metal
atoms tend to lose electrons and become positive ions called cations. Nonmetal
atoms tend to gain electrons and become positive ions or anions. Some
materials, like metals, lose these electrons very easily (conductors). Plastics or
wood tend to hold electrons very tightly (insulators). Believe it or not electrons
are dislodged easily from your hair. Electrons are transferred to plastics like a
comb. Your hair stands on end, and in the meantime the plastic comb picks up
the electrons. The protons in the comb are attracting the electrons dislodged
from your hair.
So electricity can be described in one way as a transfer of electrons from one
substance to another. This refers to the conservation of charge (see book).
Electrical forces can be compared to gravitational forces also. Remember how
the inverse square law applied to gravity? Remember Newton’s Law of
Universal Gravitation? Many of the same rules apply to electrical force. The
force of electricity is directly related to (q) charge of the particle and inversely
related to the distance between them. This is called Coulomb’s law. See the
formula on page 121. The force of the electrical current acts along a straight
line from one charge to another. The unit of charge is the coulomb or (C). One
coulomb equals 6.25 billion billion electrons. This is the amount of charge that
flows through a 100 watt light bulb in a little more than a second. The charge
on a single electron equals (after dividing the value of 1 C by the number of
electrons) is 1.60 x 10-19.
So the Universal law of Gravitation and Coulomb’s law are very similar except
that gravitational forces are only attractive where as electrical forces can be
both attractive and repulsive.
If you take a balloon and rub it against your hair and then place it on the wall,
it will usually stick to the wall. When the charged balloon touches the wall it
effectively alters the charge distribution on the wall by altering their centers of
charge. Positive (protons) are attracted to the balloon where as the electrons or
negative charges move away or are repelled. The wall and its electrons have
been polarized (charges move to opposite ends).
What is an electrical field? What is Electrical Potential?
Remember that a gravitational field is produced when two objects with
specified masses are a specific distance apart. There is a space which
surrounds each mass. This is called a gravitational field. The spaces around
the masses are altered. So each mass interacts with the field, not directly with
the other mass.
How is this similar to an electrical force? Any space around an electric charge
is filled with an electric field (extends through space similar to the gravitational
force). This is a vector field (has magnitude and direction). The magnitude is
the force per unit charge. If a charge (q) experiences a force (F) then the electric
field becomes E = F/q. Direction is away from the positive charge and towards
the negative charge.
When a charged particle is placed in an electric field will experience a force.
The direction of the force on a positive charge is the same direction as the field.
This can be seen as field lines (figure 7.8). Field lines also show the intensity
of the field. If the lines are tightly bunched together, this shows a very strong
force. The field and field lines about an electron point towards the electron
and about a proton points in the opposite direction or away from the proton.
Any charged object has potential energy depending on its location in an electric
field (similar to an objects potential in a gravitational field or GPE = mgh. Work
is required to push a particle against the electrical field of a charged body. This
changes the electric potential of the charged particle. If a positive particle is
pushed closer to a positively charged sphere (such as a Van de Graff generator
we will work with in class) increases the potential energy of the charged
particle. The particle now has a certain amount of electric potential energy. If
released, it travels away from the positive sphere with a specific amount of KE.
If you push a particle with twice the charge you do twice the work. Twice the
charge also possesses twice the potential energy. When working with electricity
you consider the electric potential energy per charge. You do this by dividing
the amount of energy by the amount of charge. This refers to electric potential:
Electric Potential = electric potential / charge. The unit for Electric Potential is
the volt and referred to as voltage. One volt (V) equals one joule (J) of energy
per (C) of charge or:
1 Volt = 1 Joule / (C) - 1.5 volt battery give 1.5 joules of energy to every one
coulomb of charge flowing through the battery.
What is the significance of Electric Potential or Voltage? Definite values for it
can be assigned to a location. For example in an electrical circuit, 12V higher
charge is maintained at the positive terminal as compared to the negative
terminal. This is important because when these terminals are used (connected)
the charges will move between each location due to this difference (current).
*Be sure to read about conductors and insulators on page 124!
Voltage Sources and Electrical Current
For charges to ‘flow’ between terminals, there must be a potential difference
between them (as described above). Flow always moves from an area of higher
potential to lower potential. To sustain a current flow requires a pumping
device to maintain a proper difference in electric potential. Examples are
batteries that do this by pulling negative charges apart from positive charges.
Chemical batteries do this by disintegrating lead or zinc in acid. Generators
separate charges by a process called electromagnetic induction.
When this occurs the charged particles then flow in a specific direction. This is
called an electrical current. In metals electrons move from the metal atoms
move freely around the other metal atoms. In solids, the protons are fixed, but
in fluids they flow as well (automobile batteries). So when you apply voltage to
a metal wire you are basically setting them in motion in a certain direction and
this is an electrical current.
The rate of electrical flow is measured in amperes (rate of flow of 1 (C) of
charge per second or 6.25 billion-billion electrons per second). The speed of the
flow of electrons is slow. They do not move unobstructed. They bump into
other atoms constantly. The drift speed is less than one centimeter per second.
What is the difference between Direct and Alternating Currents?
Electrical devices use both AC and DC devices. If you plug it into the wall you
are using AC current. But if are using batteries you are using DC current.
DC refers to direct current. Charges only flow in one direction. In batteries
flow is from or away from the negative terminal to the positive terminal. It is
also one way through a circuit (explain later).
AC or alternating current works by charges moving in one direction and then
moving in the opposite direction. Generators or alternators switch signs at
terminals to achieve this movement. Most all AC currents in this country
alternate at 60 cycles per second or 60-Hertz (Hz) current. Other countries may
have 50, 25 cycles per second.
We should mention circuits before we move on. Most circuits are either
series or parallel. As you see in figure 7.21 a simple series circuit is a one-way
operation with a switch at some point between the lights and battery. Current
flows through each light or the same current exist in all three lights at the same
time. Charges move from the negative terminal through the lights and
eventually back to the positive battery terminals. The path again is one way. If
the circuit is broken at any point the whole circuit is shut down! If one light
burns out and breaks the circuit, then all of the lights in that circuit go out.
Some older Christmas tree lights are series circuits.
Lights in our house are not wired this way. Most lighting in homes are done via
a parallel circuit. See figure 7.22. You see three lamps connected to the same
two points A and B. When the electrons leave the negative terminal of the
battery they only have to travel through one lamp before returning to the
positive terminal. You have three separate pathways. If one pathway is broken
it does not affect the other circuits.
Electrical current does meet resistance in wires and other substances used to
promote electrical current. Thin wires produced more resistance than thicker
wires. Copper has a low level of resistance as compared to other metals.
Rubber is used as an insulator because it has a very high level of resistance to
electrical flow.
Ever heard of a superconductor? This is any substance that when placed in low
temperatures its resistance will equal zero!
An Ohm is the unit for electrical resistance (Ω). Ohm’s Law explains how
resistance is measured. It explains the relationship between current flow,
voltage and resistance. Current is directly proportional to voltage and inversely
proportional to resistance. Makes sense right!
Current = (V)/ (Ω); or Amperes = volts / ohms; or I = V/R
In lamp cords there is a resistance of about 1 ohm and the light bulb provides
more than 100 ohms of resistance. Inside each of these are resistors which
regulate the flow of current.
Please read about how one receives an electrical shock and what produces
the shock on pages 127-128.
You should be able to work the following math applications:
 Ohm’s law problems on page 127, 129
 Electrical Power on page 130
Magnetism
What is a magnetic force?
Very similar to electrical forces except the magnetic poles produce magnetic
forces instead of electrical forces. Magnets have two distinct poles, north and
south. Like poles attract each other and opposite poles attract.
There is a big difference in electrical charges and magnetic poles. Electrical
charges can be separated and are separate entities (electrons and protons)
where as magnetic poles are like the heads and tails of a coin. If you break a
magnet in the middle there will still be a north and south pole. See figure 7.25.
The broken pieces will still act as a whole magnet.
What are magnetic fields?
Magnetic fields are produced by moving electric charges. The electrons that
make up the magnet are in constant motion. The spin of the electrons and the
revolution produce this movement. When an electron spins it produces a
magnetic effect. If they spin in the same direction it makes a strong magnet. It
is important to note that in most substances electrons spin in opposite
direction cancelling the effect out. Only in iron, nickel and cobalt do we see
electrons that spin in the same direction. So iron atoms are in effect a tiny
magnet. Magnets that you buy are mostly made of iron, nickel, cobalt and
aluminum (alloy).
Spaces around a magnet produce a magnetic field. Place a piece of paper on
top of a magnet and the spread iron filing on top of the paper. You see the
lines of the magnetic field on the paper (figure 7.27). Distinct patterns are
seen. Direction of the field is from the north to the south pole.
What is a Magnetic Domain?
When the atoms in iron spin in the same direction they produce a strong
magnetic field. The atoms line up in the same direction producing magnetic
domains. See figure 7.30. Again the direction is from north to the south pole.
How are electric currents and magnetic fields related?
When electric charges move or a current of charges is produced this produces a
magnetic field. This field that surrounds a wire makes a pattern of concentric
circles. The needle of a compass will be pointed in one direction. When the
current is reversed, the needle will reverse its direction. If you place iron in a
current-carrying coil of wire the alignment of the domains in the iron produces
an electromagnet which is a very strong magnet. Ever seen a car or large piece
of metal lifted up? This is the work of an electromagnet.
Magnets and Electric Motors
A simple motor works like this. Current in a motor is made to change direction
of a coil which makes a half rotation. It produces a continuous rotation. See
page 136. A magnet produces a magnetic field in an area of a loop of wire
which is mounted to turn about an axis. Current passes through the loop it
flows in the opposite direction in the upper and lower sides of the loop. One
part is forced to the left and the other to the right. Current is then reversed or
alternated during each half revolution. Rotation is continuous (so long as
current is supplied). This would be example of a DC motor. In larger motors a
simple magnet is replaced with an electromagnet.
Important Terms
1. Diamagnetic materials – metals which have weak and negative
susceptibility to magnetic fields. Repelled by magnetic fields. Solids
with paired electrons that have opposite spins (most metals)
2. Paramagnetic – metals such as Mg, Li. Somewhat susceptible to a
magnetic field. Slightly attracted to the field, but does not retain
magnetic properties when the field is removed. In these type substances
there are some unpaired electrons present in the metals.
3. Ferrromagnetic- metals which have a great amount of unpaired electrons
with opposite spins. Strong attraction to the magnetic field and retains
magnetic properties when the field is removed. Iron, cobalt and nickel
are examples. When the field is applied domains are produced (atoms line
up).