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Transcript
Charge
Field Forces
Electricity, like gravity and magnetism, is a field force.
Field forces can act at a distance and through a vacuum.
Masses attract each other by gravitational force
Charges attract and repel by electric force.
And as we shall see later, moving charges become
magnetic and attract and repel one another with the
magnetic force.
We will see many similarities between electricity and gravity.
As a result, electricity will be compared to gravity.
© RHJansen
Particles of Charge
Proton: positively charged particle.
Electron: negatively charged particle.
Neutron: a neutral particle that has no electrical properties.
Variables for Charge
e
used for the smallest charged particle, the electron
1 e = 1.6 × 10−19 Coulombs (the units of charge)
All charge in the universe is a multiple of this charge.
This is also the charge of a proton.
q a small amount of charge, often a point charge or a
spherical object containing charge.
Q a large amount of charge, often the charge of parallel
plates.
© RHJansen
Charge
Unit of charge: Coulombs (C)
Charge on an electron: 1 e = 1.6 × 10−19 C
Charge on a proton: 1 e = 1.6 × 10−19 C
Charge on a neutron: zero
Number of charges in 1 Coulomb: 6.25 × 1018 charged
particles
Not given, but easy to find: It is the reciprocal of 1.6 × 10−19
Attraction and Repulsion
Like charges repel
Opposite charges attract
Neutrons have no effect
© RHJansen
Conserved
Charge cannot be created or destroyed
The number of protons, electrons, and neutrons will not change
(until we get to Modern Physics)
The questions in this chapter will be where are the charges and
what are they doing.
Quantized
Charge comes in exact quantities (packets).
Every charge in the universe is made of protons and electrons.
Since there are no ½ electrons or ½ protons all charge in the
universe will be a multiple of 1.6 × 10−19 .
© RHJansen
Neutral Charge ???
If an object has neutral charge, does it mean that it is missing
electrons and protons.
No
Neutral charge means it has the same number of protons and
electrons, and the positively charged protons are canceling the
negatively charged electrons.
This means we really don’t know how much charge is actually
on a neutral object.
© RHJansen
1 C of charge ???
In fact even when an object has its charge given, we will not
know the actual amount of charges on the object.
When a problem gives you the charge of an object, it is only
telling you how much extra charge is on the object.
An object that has a + 1 C charge could consist of:
256 C of protons and 255 C of electrons, or
78 C of protons and 77 C of electrons.
We can’t really be sure.
However, it does not matter, because only the excess charge is
important.
The excess charge gives an object its electrical properties.
Polarized Objects
A polarized object has a region of positive charge at one end and
negative charge at the other (opposite poles)
These objects may have an overall neutral as shown in the example
below.
One of the most famous objects is the
water molecules, where the oxygen atom
with its higher electronegativity pulls the
electrons closer to it making the oxygen
end of the molecule more negative, while
the hydrogen ends become more positive.
© RHJansen
Conductors
A conductor is a material that allows electrons to move through it
with ease.
Metals are excellent conductors
Metals act like one gigantic molecule that shares the outer (valence)
electrons with all the atoms within the metal.
Excess charges pile up on the outside surface of conductors.
Insulator
An insulator is a material that holds its electrons tightly.
Insulators do not allow charges to move through them.
Plastic is used to coat metal wires so that the charges cannot leave
the wire.
Excess charges can be placed on an insulator, but where they are
placed initially is where they remain until removed.
© RHJansen
Electroscope
An electroscope is a device that shows the presence of charge.
The simplest version consists of frame with a metal plate on top and two pieces
of foil hang next to each other inside the frame.
Once again I am showing all the charges, even though they cancel and the
electroscope is neutral. I just want to show how the charges are moving.
A charge rod is brought near the metal plate of the electroscope.
Metal Plate
Foil leaves
Electroscope
The diagram at the right shows the result.
The negative rod repels the electrons from the metal plate to the foil leaves.
The leaves pick up a net negative charge and repel each other.
The force of electricity is many time stronger than gravity, and the foil leaves
rise indicating the presence of charge in the rod.
Charging by Friction
Rubbing two substances together can transfer outer (valence) electrons
from one to the other. Example walking on a carpet on a dry day
builds excess charge on you which is released in a static shock when
you touch a metal conductor.
Charging by Conduction
This process involves touching a charged object to an uncharged
object. Like charges repel each other and the excess charges on the
charged object want to get away from one another. The uncharged
object represents a lot of empty space where these charges can move
into and decrease the force of repulsion on each other.
Before
After
Charging by Induction
This process involves charging an object without transferring charge
by touching it to another charged object.
Neutral objects do contain charges. The protons and electrons are
equal in number and cancel. Normally we only look at excess charge,
but this time look at all the charges.
Start with 2 neutral spheres that are touching each other. A negatively
charged rod is brought near (but not touching) the two neutral
spheres. What will happen ?
Charging by Induction
This process involves charging an object without transferring charge
by touching it to another charged object.
The negative charges in the spheres are repelled by the negative
charges in the rod. (Note: only electrons can move, as they are on the
outside of the atom. Protons are trapped deep in the nucleus.)
The left sphere gains a net positive charge, while the right sphere gains
a net negative charge. The spheres are technically still one object. They
are now Polarized (one positive pole and one negative pole).
Charging by Induction
This process involves charging an object without transferring charge
by touching it to another charged object.
Now the spheres are separated.
The result is two separate spheres that are charged. This process
occurred without either sphere being touched by the rod.