lesson you're going to learn about charge. Before we start, here's
a quick question for you.
Have you ever used charge? Click on your answer.
You are using charge now, believe
it or not.
These lessons have been
designed to be read electronically, not on paper.
To read them electronically
means using a monitor.
A monitor uses charge
to make the screen emit light of various colors, and to control that light
so that it forms letters and windows, etc.
produce the light that the monitor emits, and to control the appearance
of that light, voltage is used to control the path of charge that is emitted
at the back of the monitor and which strikes the monitor screen.
A caveat! If you
are reading this lesson using a flat panel display, then these statements
are not true. In that case imagine that you are using a CRT monitor.
Here is a little pictorial
model of what happens. Click the appropriate phrase to show how the
electrons (which carry charge!) move up, move down, or travel a flat path.
Note the following:
When there is positive
charge on the top plate there is also negative charge on the bottom plate.
The electron is simultaneously attracted to the positive charge and repelled
by the negative charge.
The same attraction/repulsion
is obtained when the charges are reversed.
Here is what happens.
When you want a dot in
the top portion of the screen you put positive charge on the top
plate and negative charge on the bottom plate, and the positive charge
attracts the electrons and the negative charge repels the electrons as
they fly by so they hit the screen above the center.
When you want a dot in
the bottom portion of the screen you put a positive charge on the top plate
and a negative charge on the top plate to repel the electrons, moving them
With no charge on the
plates, the electron travels a flat path.
are a few questions for you to answer.
If there is more positive charge on the top plate, which way will the charge
If the moving charge is positive, which way will it move compared to the
direction the electron moves?
get the idea that monitors are the only devices that use charge.
Let's review a few other places where you may have used charge.
You may have spoken of
charging your car battery - or charging any other re- chargeable battery.
Charging a battery is doing exactly what the phrase says. When you
charge a battery you are putting charge into your battery, and the battery
stores the charge for later use.
When you discharge that
battery you are also using charge. Charge flows through the devices
you attach to your battery - the lights in your car, the electronics that
control your car, the CD player you put the charged batteries into, etc.
use charge all the time. You may not think about it much, but you
do. Here are some examples of times when you use charge.
Every time you run electronic
gear - a TV, a stereo, a computer - from an AC wall plug outlet, you use
a device called a power supply that stores charge in a capacitor.
That stored charge allows the electronic circuits you use to run during
the very short times when the AC voltage goes through zero - and it does
that 120 times a second on a 60hz power line.
You not only use charge,
but charge should be feared - at times. When clouds get charged they
can discharge by producing large lightning bolts that are very destructive.
When there's a solar flare,
the sun emits a stream of charged particles that pour down on the earth
at a million miles per hour. A flare can disrupt satellite communications
affecting the whole world.
Comets have a tail of
charged particles and provide one of the most awesome sights in the heavens.
What Do You Need To Learn
There are many things that you might need to know about charge. Here
is a partial list.
Two charges can interact
and produce forces on each other. That effect is used when we deflect
a stream of electrons to produce spots at different points on a monitor
screen. The same process occurs in ink-jet printers. You need
to learn about forces between charges.
You need to learn about
how charge flows. Remember, it flowed into the battery and out of
the battery. Charge flow is important.
You will need to learn
about units for charge and charge flow, and you will need to learn about
charge related energy concepts like voltage.
For This Lesson
This lesson introduces you to some simple concepts about charge.
At the end of the lesson, you want to be able to do the following.
a question involving charge
able to compute amounts of charge.
able to predict how charge moves - when charges attract and when they repel.
Between Charges & Facts About Charge
In this section you will begin by learning about charge - a basic electrical
quantity. We start with a short discussion of the force between charges.
Classical Greeks were
the first to note that small pieces of material were attracted to rubbed
amber. That's the first recorded instance of an observation of force
due to charge.
You have seen electrical
effects if you have noticed the attraction of small bits of paper to a
recently used comb.
Those effects are evidence
of a force that exists - a force that is not a gravitational force.
That force is one of the fundamental forces of nature, and, along with
gravity, it is one of the two forces that we humans can experience directly.
tiny effects have gradually been studied and put to use, especially in
the last century and a half. Starting from observing these tiny effects,
scientists and engineers have learned basic principles and discovered other
electrical effects that have led to the industries we rely on today including
the power industry, the electronic communication industry and the whole
world of computers.
The effects these forces have in the world are no longer tiny. The
major moving forces in society - the ability to communicate instantaneously
and the ability to compute solutions to large problems - are directly attributable
to what we know about electricity. And, what we know about electricity
starts with charge - the invisible quantity that produces electrical forces.
There are two large forces
that we can experience - gravitational forces and electromagnetic forces.
Both of these forces act
through space, sometimes over large distances.
cause the moon and planets to take elliptical orbits around a larger body.
Mass causes this gravitational attraction. However, no one can give you
a really good explanation of exactly what mass is except to say that it
is a property of matter that causes this gravitational attraction.
there is another force.
Some particles exhibit
non-gravitational forces between them; forces that are much larger than
Not all particles experience
this force, but those that do are said to possess a property called charge.
Force due to charge obeys
an inverse square law, of exactly the same form as the gravitational force.
Again, charge, like mass, is perhaps ultimately unexplainable, but some
bodies possess it and are said to be "charged".
force law for charges is somewhat different because charge comes in two
different types, positive and negative charge.
Two like charges (two
positive charges or two negative charges) will repel each other,
whereas two masses always attract each other. This interactive demo
gives an idea of how two unlike charges move.
force law for charge is similar to the gravitational force law. For
two charges, q1 and q2, the force between
Proportional to the product
of the two charges q1 and q2, and
to the square of the distance, r (in meters), between them.
So, the force is given
by an expression:
= q1 q2/(4peo
is a fundamental constant of nature, = ~8.885419x10-12 F/m.
Like every other physical quantity, when you deal with charge you must
Charge is measured in
Coulombs are named after
Charles Augustin Coulomb who was the first person to determine that the
force law for charges was an inverse square law.
Charge not only comes
in two varieties, it also comes in discrete sizes. Electrons and protons
each have the same size charge (but of opposite polarity) of magnitude
1.6x 10-19 coulombs (+ or - as appropriate), where a
coulomb is the MKS unit of charge.
Note, the electron's charge
is usually counted as negative, and the proton's charge as positive, although
that is only a convention and there is no "lack" or "surplus" associated
with negative and positive charges.
When you use the force law
= q1 q2/(4peo
Charge is measured
in couloumbs (for q1 and q2).
Force is measured
in newtons (for F1,2 ).
Distance is measured
in meters (for r).
many electrons does it take to produce -1 coulomb of charge?
Remember that the charge
on one electron is -1.6x 10-19 couloumbs, so you just
need to pile up enough electrons to get one couloumb.
If charge obeys an inverse square law it obeys a force law just like the
gravitational force law. The gravitational force law depends inversely
upon the square of the distance between two masses, so mass plays a role
somewhat similar to the role charge plays in the force law.
Because of the similarity between the laws there are going to be some concepts
that work the same in both cases. There will also be some differences.
Two positive charges repel each other whereas two masses attract each other.
Charge comes in two varieties that we call positive and negative.
We don't know that that happens for masses. Anti-matter probably
does not have negative mass, although it interacts with matter explosively.
It doesn't look like two masses could repel each other. The possibility
of attraction and repulsion makes charge unique.
If the force law between charged particles is the same as the force law
between two masses, then what phenomena of gravitiation fields would you
expect to be the same for charged particles?
The concepts of potential energy would be the same.
Just like two masses - like the earth and the moon - can orbit each other,
charges can orbit each other.
Just like mass, charge is always positive.
Just like every particle has mass, every particle has charge.
Just like mass, two charged particles always attract each other.
There's one last set of facts about charge that you should know.
The charge on an electron
is always the same. It's has exactly the same value for every electron
in the universe.
The proton has the same
absolute value of charge as the electron, but it has a positive charge,
If you have just one electron
and one proton (a hydrogen atom perhaps) then you have no net charge.
The two charges cancel! And, they cancel exactly!
Other fundamental particles
also have exactly the same charge as an electron, although it can be either
positive or negative. The charge on an electron is a fundamental
quantity - a constant of nature.
Do You Use Charge?
You may be tempted to think that charge is somewhat obscure and that you
don't ever use charge. You're wrong. You use charge constantly,
and you buy lots of things that store charge.
When you plug an electrical
device into a wall plug you use charget. One example is a light bulb.
Charge flows from the wall plug, through the connecting wire and through
the bulb. In the process, the flowing charge heats up the filament
in the bulb generating light - unless it is a fluorescent lamp, and then
a different process creates the light. Actually, when charge flows
it is called current. Click here to go
to the lesson on current.
If you own a car, you
own a storage battery. The battery stores enough energy to allow
you to start your car. The battery stores energy by storing charge on the
battery plates. When you use the battery, charge flows out of the
battery. That's current flowing from the battery.
Actually, when you buy
a battery for a toy, a radio, a CD player, etc., you are buying stored
energy, and the energy is stored as charge with potential energy.
(lose their charge) and some can be recharged
(You can put charge back into them.).
Every time you run electronic
gear - a TV, a stereo, a computer - from a wall plug outlet, you use a
device called a power supply that stores charge in a capacitor. That
stored charge allows the electronic circuits you use to run during the
very short times when the AC voltage goes through zero - and it does that
120 times a second on a 60hz power line.
is some late breaking news
Recently physicists have
discovered more basic entities, quarks, that may have one third of the
charge of an electron. Again, it is exactly one third, and charge
always comes in multiples of that quantity. Quarks come in groups
that have no net charge, and form some of the fundamental particles like
protons and neutrons. For those atomic particles charge always comes
in integral multiples of the charge on an electron, or they have no net
charge at all!
this lesson you have started to learn about charge, a basic electrical
quantity. However, charge in motion - charge flowing through a wire
for example - is current, and that is something else you need to learn
about. Click here to go to the lesson
on current. If you want to go directly to the lesson on voltage, click