If you have a refrigerator, you probably have had experience with
magnets. Magnets are not new technology, but have been around for
over 2000 years. Magnets are an important part of current
electricity, and they are used in a great many devices, from
generators to motors to compasses to cassette tapes (Phil doesn't
have a CD player) to video tapes.
If you've ever seen a compass, you will remember that it has the
interesting property of always pointing to the north. This is
because there is a magnet on the end of the compass needle. If
you bring a bar magnet near to a compass, you will notice that
the needle repels from the bar magnet's north pole. This means
that the compass needle is also a north pole. Like charges repel.
So if the magnet is north, it should point AWAY from the north
pole of the earth, since like charges repel. The answer to this
little riddle is simple- the north pole of the earth is actually
a magnetic south pole. Ah ha! Another mystery explained by
The properties of magnets should not need much explanation. There
is a north and a south end. The north attracts south, and vice
versa. They are usually made out of metal, mostly Iron, Neodymium
or ALNICO (ALNICO is an alloy of ALuminum, NIckle and CObalt).
Much like an electric field around an electric charge, there is a
magnetic field around a magnet. Anything that enters this force
(and is magnetically inclined) will experience a force.
A man named Oerstead once discovered that wires that conduct
electric charge also creat a magnetic field. (This is called
electromagnetism.) If you want to find the
direction of this
field, it is much easier than the electric field method. All you
need to do is grasp the wire in your left hand, with your thumb
in the direction of the flow of charge (negative). Your curved
will be pointing in the direction of the field. This is called
Left Hand Rule. Magnetic Fields have a circular direction around
a wire. This only deals with straight wires. However, sometimes
we make what is called a coil. If you want to make a coil, wrap
some wire around and around a pencil, but don't let the wire
cross itself. Then remove the pencil. Ta-da, you have a simple
There are actually three left hand rules. The
second left hand rule states that if your wrap your left hand
around a coil of wire, with your fingers in the
direction of the flow of charge, your thumb points to the end of
the coil that is the north pole. That's right- coils of current
conducting wire are also magnets!
Since a wire creates a magnetic field of it's own, if we place a
wire into a magnetic field, the two fields will
interact. There will be a force on the wire. According to the
famous scientist Michael Faraday: "The force
on a wire is at
right angles to the direction of the magnetic field." This
statement leads us to the third and final left hand rule. Flatten
your hand. Point your thumb in the direction of the flow of
negative charge. Point your fingers in the direction of the
magnetic field. You palm will be pointing in the direction of the
force on the wire! It's a miracle! Well, maybe not a miracle.
Still, it is an amazingly easy way to find the direction of a
Now that we can find the direction of the force, we need to
calculate it's magnitude. No, don't groan. It's just another
formula, and all you need to do is plug numbers into it. After
you understand the formula, it's a no-brainer. Here it
That's right, the fore equals a guy named Bill. No wait, that's
not it. B is actually the strength of the magnetic field. I us
the current, and L is the length of wire that is inside of the
Click here to observe the motion of a charged particle in a
What do we mean, "strength" of the field? Well, you can probably
use algebra to turn that equation you just learned into an
equation for field strength.
B= F / IL
The field strength is actually called "Magnetic Induction." B is
the magnetic induction, sometimes called the "B field." The unit
of magnetic induction is the telsa (T). One telsa equals one
Newton per Ampere times Meter. It's easier to just remember
Current carrying wires experience force in a magnetic field. We
said that. We also said that current carrying wires have charged
particles flowing through them. So if a single charged particle
was in a field... all by itself... yes, it would get lonely, but
it would also experience force. The equation to find the force on
a single particle is:
F is force (as usual), B is the magnetic induction, q is charge,
and v is velocity. Do NOT confuse the velocity v with a Voltage
V. That could be bad come test time.
Since a wire in a magnetic field experiences a force, we should
be able to use that force. Physics is useful. We have said that
a hundred times. Long ago, people figured out how to put a bend
of wire into a magnetic field. Then they gave it a nudge. The
force took over, and since it was at a right angle to the
magnetic field- it turned the loop. A little bit. However, soon
the wire was turned so that the force was in the other direction.
So what did we brilliant scientists do? Reverse the current. The
motor was born. Every half turn, the current in a motor is
reversed. This makes the loop inside of the field spin. This
makes anything attached to the loop spin. An electric motor-
there are literally millions of uses for an electric motor, as we
are sure you already know.
Electromagnetic Induction isn't a complex concept. Faraday, one
of the principal founders of electricity and physics, discovered
that if he moved a wire through an electric field- it produced a
current. This is electromagnetic induction. The current direction
can be found using the third left hand rule, which you already
When a wire moves in a magnetic field, a force acts on the
charges in the wire. Work is done on the charges. Their potential
energy is increased, thus the potential difference is increased.
This is called inducing an EMF. EMF stands for electromotive
force, but EMF isn't really force at all. EMF is measured in
Volts, just like potential difference. The term EMF is
misleading, but there is nothing you can do about it- it has been
used since before electricity was fully understood.
To calculate EMF, you use a simple formula:
EMF = BLv
This should be easy for an experienced physicist like you to
understand. EMF in volts equals the magnetic induction times the
length of wire times the velocity of the wire.
We hope you still remember what vectors are. When a wire moves in
a magnetic field, only the resolved velocity vector which is
perpendicular to the field produces a current. You need to use
only the perpendicular velocity, so be careful, sometimes
questions can be confusing.
Faraday also invented the generator. Remember the electric motor?
Well, the generator is the same. Exactly. Only instead of
electricity turning the loop of wire, we ue a force to turn the
loop, which produces electricity. Technically, if you spin a
motor you are generating electricity.
If you are particularly intelligent, you may have noticed a bit
of a paradox in what we have taught. Current induces a magnetic
field. Motion in a field induces current. So if a wire moves
through a field, it will have current. This current will create a
field. The fields will interact. The force on the moving wire
will be in the opposite direction of the motion of the wire. The
more you move the wire, the greater the force in the opposite
direction. This is what a scientist named H. F. E. Lenz figured
out. "The direction of the induced current is
such that the
magnetic field resulting from the induced current opposes the
change in the field that caused the induced current." It is
called Lenz's Law. Lenz's Law applies to motors. Once a motor
gets going, it will start to produce it's own magnetic field.
This will create a current which opposes the current to the
motor. There is a current conflict. Luckily, the opposing
current is never strong enough to counteract the original
current. The occurrence of a counter-current in a motor is called
You have reached the final lesson in electricity. If you have
made it this far, you should go back and read it all again. Then
you'll be a REAL expert. We've covered an entire year of physics
(in some schools) in only a few paragraphs. But this is intense
stuff, it is the stuff that makes you into a physics superman.
A while ago you read that the power company transmits electricity
at an extremely high voltage, and a very low current. This is to
minimize heat and energy loss. It makes sense. But say you have
200 Volts, 10 Amps. How do you make the current (I) smaller and
the Voltage (V) greater?
First of all, you know that power is V times I. This means that
2 Amps times 4 Volts has the same power as 4 Amps and 2 Volts.
Right? Right. So:
Power In = Power Out
This is the concept of a Transformer. Transformers are not those
robotic toys. But they really ARE "More than meets the eye". A
transforms electricity, increasing voltage and decreasing
current. If you know anyone who has a large electrical box on
their lawn or street corner, there is probably a transformer
A transformer is a looped conductor, with 2 coils, one on each
side. Just look at the diagram and it should be pretty easy to
understand. Electricity comes in one side, and loops around the
conductor. This coil is the primary coil. The primary coil
induces a current in the conductor, which travels over to the
other coil, the secondary coil. This induces a current in the
coil, which travels out the other side. If the primary coil has
more loops, voltage is reduced. If the secondary coil has more
loops, voltage is increased.
Thr formula for figuring out the voltage output of a transformer
V out = Loops out * V in / Loops in
Now you know all about electricity. Please don't ever stick a
screwdriver into an electrical circuit. It hurts. Lots. Trust us.
Did we mention that it hurts? It does. Lots.