NCEA Physics
Electricity and Magnetism
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Amber is the fossilized resin from some trees. Used in jewellery it was
often vigorously polished with a soft cloth. Ancient Greeks discovered
a strange property; when it is rubbed with a cloth, it can attract nearby
bits of straw or grain. From the Latin word for amber, electrum, we
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get the word electricity.
This mysterious attractive or repulsive property could also be passed to
other materials. In the classic experiment two tiny balls of pith were
hung next to each other without touching. A glass rod was rubbed with
silk and touched to each pith ball. The pith balls would suddenly move
away from each other.
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The traditional unit of charge is the Coulomb, named after French
physicist, Charles Coulomb (1738-1806) He was a French physicist
and military engineer. Because of his expertise with simple
machines, he was able to build an apparatus to measure the
electrical force between two charged objects.
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It is possible to see a similar effect of an electric field. A charged object
placed in fine oil with tiny bits of thin thread will cause the thread to
line up in the field. It might look like the picture below.
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When two opposite charges are placed near each other in fine oil with
fine bits of thread, the situation looks like the picture below.
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The drawing below represents what is happening between the
plates. The top plate has an excess of positive charge and the
bottom plate has an excess of negative charge. The electric field
(E) is directed downward (from positive to negative). The drawing
shows the direction of force (F) the field exerts on a positive
charge and on a negative charge. If the charges were free to move,
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they would accelerate in the direction of the force.
Electric fields can, therefore, cause charges to move. The
movement of charges is an important concept in electricity.
Physicists call the net movement of charges an electric current. In
the late 1700's scientists chose the direction of electric current to be
the direction in which positive charges move in an electric field.
This was before scientists knew that electrons and protons were the
negative and positive charge particles, and that the electron moved
more easily than the proton.
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Van de Graaff
•A Van de Graaff
generator can produce a
very high static charge.
•Unfortunately the air
will conduct some of this
charge.
•A Van de graaff can
only work well at low
humidity.
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How it works
The dome is left
with a large
positive charge.
Electrons then
jump from the
metal dome to the
positive belt.
Electrons are
drawn off from the
belt towards the
ground.
The generator
produces a large
positive voltage.
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Van de graff
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Static hair
When you are
charged by a
van de Graaff
machine each
part of your
body repels
every other part.
The strands of
your hair repel
each other and
spread out from
each other.
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Van de graff hair shot
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Uses and dangers
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Lightning conductors
•Lightning conductors can
protect tall buildings during
electrical storms.
•A long copper strip connects a
spiked object on the roof to a
large metal plate in the ground.
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Lightning conductors
•As the storm increases the
bottom of the cloud becomes
negatively charged.
•These negative charges repel the
electrons in the lightning
conductor, leaving it with a
positive charge.
•The electrons in the cloud will
be attracted to the copper strip
rather than the building.
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Lightning conductors
•During the lightning
strike the charge will pass
through the copper strip
to the ground.
•The building will not be
damaged as long as the
copper strip is in good
condition.
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Refuelling
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Warning sign
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Vapours are the danger
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After a fire
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How big is the problem ?
In 2000 there were 81 refuelling fires in the
USA, in 2001 there were more than 130.
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Year 2000 reports
Remember we said that static
electricity likes low humidity.
When it is cold the humidity
drops and there is a greater
chance of fire.
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Rules for safe refuelling
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Filling a gas can
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Special Pipes
•In NZ many petrol refuelling hoses are made from a
special kind of plastic that can conduct static, this stops
the build up of charge that could cause a fire.
•Unfortunately there are still many fires caused by
smoking during refuelling.
•Some cars have strips fitted to the rear of the car to
discharge the build up of static charge.
•Making a safe tyre that will not charge a car is
important.
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Planes and helicopters
•When a passenger jet refuels a large amount of
petrol travels from one tank to another. This
movement can cause a huge build up of charge.
Before the fuelling starts cables can be attached to
the plane to stop this build up.
•Helicopter blades cause a lot of static to build up.
Sea rescue workers often have metal cables trailing
below them to discharge any static before they have
to touch the sea or boats below them.
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Refuelling planes
During refuelling the fuel gains electrons from the pipe,
making the pipe positive and the fuel negative. The
resulting voltage may cause a spark – bad news!
Solution: Either earth the fuel tank with a copper rod or
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connect the tanker to the plane by a copper conductor.
Photocopiers
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Static
•The key to a photocopier is that the
powder used in the machines is attracted
to static charge.
•How we use this piece of physics to
make a copy of a piece of black and
white paper is a little more complicated.
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Charging the drum
•A photocopier has a drum
that is charged with static
in the dark.
•Light is reflected off the
original paper onto the
drum.
•When light hits the drum
that part of the drum loses
its charge.
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Spraying the toner
•The drum is sprayed with
a dark dust called toner.
•The toner sticks to the
charged areas – the dark
parts of the picture where
the text was.
•Toner is very fine and
extremely dangerous if
breathed in.
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Drum to paper
•We now have a copy of
the original on the drum.
•Next a charged piece of
plain paper is rolled
against the drum.
•The plain paper picks up
the toner from the drum.
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Melting the toner
•The paper is now
covered in toner powder.
•The toner is then heated
to fix it into position.
•This is why the paper is
hot when it comes out of
the machine.
•This is how a photocopy
is made.
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The complete picture
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Photocopying review
Photocopiers
Photocopiers use
use static
static
electricity.
electricity. They
They work
work by:
by: 1)
1)
Copying
thepage
page
Copying an
an ______
image ofofthe
onto
onto aa ______
chargedplate,
plate,2)2)Light
Light
then
then causes
causes the
the charge
charge to
to ____
leak
away,
away, leaving
leaving an
an “electrostatic
“electrostatic
impression”
impression” of
of the
the page,
page, 3)
3)The
The
charges
charges left
left on
on the
the plate
plate ______
attract
small
small drops
drops of
of black
black powder,
powder, 4)
4)
The
The powder
powder isis transferred
transferred from
from
the
5) The
The
the plate
plate onto
onto the
the _____,
paper, 5)
paper
paper isis _____
heatedtoto“fix”
“fix”the
the
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powder.
powder.