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PEC120 General Physics
Introducing Electrostatics
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The atom
The atom one of the fundamental building blocks of matter
They are the smallest particle that we can ‘image’…
(sorry I forgot where I downloaded this from…probably IBM)
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Inside the atom
The atom, once thought to be the smallest particle is made up of three smaller particles
� proton
� neutron
� electron
Protons (+ve) and Neutrons (no charge) are found in the nucleus.
Electrons (-ve) orbit around the nucleus
The atom
When the number of electrons is equal to the number of protons, the atom is
neutrally charged….that is there is no
NET charge.
If the atom gains an electron, it is said to be negatively charged (negative ion) if it
looses one it is positively charged
(positive ion)
Opposites attract
The idea that opposites attract comes from physics…
� Negative charges attract positive charges
� Negative charges repel negative charges
� Positive charges repel positive charges)
The same principle applies to magnetism
(north attracts south pole, north and
north repel)
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2 socks are observed to attract each other.
Which, if any, of the first 3 statements MUST be
true? (emphasis on MUST)
1 2 3 4
0% 0%
57%
43%
1. The socks both have a non-zero net charge of
the same sign.
2. The socks both have a
non-zero net charge of opposite sign.
3. Only one sock is charged; the other is
neutral.
4. None of the preceding
statements MUST be true.
Symbols and Units of Charge
It seems natural that the basic unit of charge would be the charge on the electron (-e) or
proton (+e), but it isn’t all that practical in the
macro world
e is the elementary charge
We usually q as the symbol and coulombs (C)
as our unit of charge. 1e = 1.602x10-19 C
Why Coulombs?
If we ‘charge’ a balloon by rubbing across your hair, billions of electrons are transferred.
Ie ~109 e = 1.602x 10-10 C = 16nC
However, in most examples, the amount of charge is usually larger than ~µC
When you consider this, it’s easy to see why we use the unit C, rather than e.
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Millikan Oil Drop
Cahill, J. (2005) Heinemann Physics for WA 2nd Edition
Charging by Contact
The Millikan Oil Drop experiment is an example where a material was charged
by contact (or friction)
In it’s simplest form, charging by contact
happens when one object touches or rubs against another.
� The balloon on the board is also an
example!
Charging by contact
�Rubber clogs and
supermarket vinyl
�Van de Graaff
generator
�Shoes on the mats
under the pedals in
cars
�Aircraft
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Charging by Induction
We can also charge something just by bringing a charged object close to it
This is known as charging by induction
Not surprisingly, induction only works with
conductors….we’ll discuss what these
are soon.
Electrostatic Induction
If we look at the example of the balloon, but replace it with a
ruler (it’s easier to draw) and paper bits again
We would see that the ruler has
an excess of electrons.
When it is brought near to the
paper, the electrons in the paper are repelled to the furthest side away from the ruler
- - - - - - - - - - - - - -- - - - - - - - - - - - - -
- - - - - - - - - - - - - -- - - - - - - - - - - - - -
- - -- - -- - -
- - -- - -- - -
++++++++
++++++++++++
++++
When the charge in 1 object causes a charge in
another this is called induction
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Electrostatic Induction
(1) A positively charged rod is placed near a conducting
object attached to an insulating glass pedestal
(but not touching.)
(2) The opposite side of the conductor is grounded for a
short time interval.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Electrostatic Induction
(1) Then the ground wire is removed…
(2) And finally the rod is removed.
+
+
+
+
+
+
+
+
What is the charge remaining on the
conductor in the end?
1 2 3 4
17%
0%
17%
67%1. (+) Positive
2. (-) Negative
3. 0 Neutral
4. Not sure / Can’t
decide
+
+
+
+
+
+
+
+
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Polarisation
If you don’t have free electrons, or electrons which can move throughout
the material, induction still occurs…
But,there is a rearrangement of the electrons
around the nucleus.
Polarisation
If we could see the electrons and the
nucleus what we
would see is a
change in the shape
of the electron cloud
and a change in the
relative position of the nucleus
Dipoles
You can also get a
situation where there is
an uneven distribution
between the –ve and
+ve charges in a
molecule.
This is important in
chemistry!
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What happens if we try the ruler
trick with a metal object???
1 2 3 4
20% 20%20%
40%1. The bits of paper would be attracted
2. The bits of paper
would be repelled
3. Nothing at all
4. We would look
silly
Electrostatics and Materials
What happens if we try the ruler trick with a metal object???
In the case of a metal object, the excess
electrons don’t build up, they travel
through the metal and are dispersed amongst all of the other ‘free’ electrons.
Conductors
Metals (and some non
metals) have a fairly
rigid lattice of metallic
ions which are
surrounded by a ‘sea of
electrons’
Any excess electrons
get absorbed into this
sea of electrons
+ + +
+ + +
+ + +
+ + +
+ + +
+ + + +
+
+
Delocalised ‘sea’ of
electrons
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Glad Wrap and Bowls
Plastic food wrap (ie GladWrap) becomes electrically charged when pulled from its container. As a result, it is attracted to objects such as food containers.
Does the wrap stick better to plastic
or metal containers?
1 2 3 4
67%
0%0%
33%
1. Yes!
2. No!
3. I haven’t really
noticed
4. I don’t use wrap
Glad Wrap and Bowls
Plastic food wrap (ie GladWrap) becomes electrically charged when pulled from its container. As a result, it is attracted to objects such as food containers.
The charged wrap nicely polarizes non conducting plastic rather than metal, resulting in better sticking.
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Conductors
It’s important to note that the electrons in conductors are not completely free of interactions when they move in the ‘sea’.
They still experience some resistance� electrical friction if you let me stretch the
mechanics analogy!
Resistance isn’t good, because it results in energy losses (just like Friction!)
Insulators
In non metals, the electrons are an important aspect of the bonding and are
not ‘free’ to move around.
This means that they do not allow a flow of
electrons and they do not conduct charge.
Examples
Conductors
Good
Insulators
Metals Plastics
Ionic Solutions Glass
Dry fabrics
Water Ice
Body Ok Wood
Semiconductors Paper
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Semiconductors
Some non metals, usually silicon and germanium are part of a group called the
semiconductors.
These are materials which aren’t as
conductive as the metals, but can be made more conductive by adding small
amounts of impurities
Silicon – solar cells
Superconductors
Some materials, particularly at very low temperatures (~0K) are perfect
conductors, they have NO resistance.
This means that once electric current
enters the superconductor the electrons will continue to flow forever…
http://www.superconductors.org/
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Why are Superconductors
important???
Superconductors can handle large currents with no resistance and extremely low
energy losses
� This is a huge benefit over regular
conductors
But,only if we can get them at temperatures
very close to 0K!
Applications of High Temp SC
Application Benefits
Electrical Cabling HTS cables can carry three to five times more
power than conventional utility cables, which
means they can more easily meet increasing
power demands in urban areas.
Motors Motors made with superconducting wire will be
smaller and more efficient.
Generators HTS generators will use superconducting wire in
place of iron magnets, making them smaller and
lighter. They may also get more power from less
fuel.
I’m really sorry about
today’s cartoon….
it just has to be done!
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