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.
These 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.
Gravitational effects 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.
But, there is
another force.
Some particles exhibit non-gravitational forces between
them; forces that are much larger than gravitational forces.
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".
The 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.
The force law
for charge is similar to the gravitational force law. For two charges, q1
and q2, the force between them is:
Proportional to the product of the two charges q1
and q2, and
Inversely proportional to the square of the distance, r (in
meters), between them.
So, the force is given by an expression:
F1,2
= q1 q2/(4peo
r2)
Here,
eo
is a fundamental constant of nature, = ~8.885419x10-12 F/m.
Like
every other physical quantity, when you deal with charge you must account
for units.
Charge is measured in
coulombs.
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 expression:
F1,2
= q1 q2/(4pe>o
r2)
Charge is measured in couloumbs (for q1
and q2).
Force is measured in newtons (for F1,2
).
Distance is measured in meters (for r).
Consider this.
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.
Questions
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?
Q4
The concepts
of potential energy would be the same.
Q5
Just like two
masses - like the earth and the moon - can orbit each other, charges can orbit
each other.
Q6
Just like mass,
charge is always positive.
Q7
Just like every
particle has mass, every particle has charge.
Q8
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, not negative.
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.
Where 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
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.
Batteries dis
charge
(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.
There 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!
F1,2
= q1 q2/(4peo
r2)
