Physics Chapter 16: Static Electricity
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16.1: Introduction
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Electrostatics is the study of non-moving electrical
charges, also known as static electricity.
Negative charges are known as electrons and positive
charges are known as protons.
IMPORTANT: Protons do not move.
An object is neutral when it has an equal number of
positive charge and negative charge.
a. An object will have a net positive charge when
there are more protons through loss of electrons.
b. An object will have a net negative charge when
there are more electrons than protons through
the gain of electrons.
16.5: Insulators vs Conductors
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16.2: Laws of electrostatics
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Like charges repel: 2 electrons will repel each other if
they come close to each other.
Unlike charges attract: An electron will be attracted to
a proton (Note: Protons do not move)
Note: A neutral charge will not attract or repel
anything.
16.3: Electric charge
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In physics, the space surrounding an electric charge has
a property called an electric field. This electric field
exerts a force on other electrically charged objects.
a. An electric field is a region where an electric
charge experiences an electric force.
Electric fields exert forces on a positive charged
particle, i.e. Proton.
Electric fields do not touch each other due to the
repulsion of positive charges.
The direction of the field is defined as the direction of
the force on a small positive charge.
When a positively charged object and a negatively
charged object is placed near each other, a positive
charge (proton) will travel from a positive charge, as it
repels it towards a negative charge as it attracts it.
Materials that do not allow the electrons to flow freely
inside them are called electrical insulators. It has
electrons that are in fixed positions.
a. Therefore, the addition and removal of electrons
in any part of the insulator does not result in the
electrons in the other part of the insulator to
move.
b. Thus, the charge is localized or confined to the
region. (Charge occurs when you rub it).
A charged insulator can be discharged by passing it
quickly over the flame.
a. In electrical conductor, the valence electrons are
loosely bound and are delocalized.
b. When electrons are gained or lost, other
electrons will flow in automatically so that the
electrons will redistribute equally in the
conductors.
All conductors can be discharged by earthing.
16.6: Methods of charging
1.
Electric charge is a fundamental conserved property of
some subatomic particles, which determines their
electromagnetic interaction.
SI unit of measuring electric charge is the coulomb (C).
As it is difficult to count the number of electrons as the
number is too large, a coulomb is used for a big group
of electrons (6.25 x 1018) electrons.
As each electron has a negative charge of -1.602 x 1019
, when each electron with this amount of charge is
added up, 6.25 x 1018 electrons are needed to form a
coulomb of charge.
The symbol of this is Q, used for both positive and
negative charges.
Formula for charge, Q = It, where I = electric current,
Q = charge and t = time.
16.4: Electric field
The closer the field lines, the stronger the field. (The
closer the proton is to the positive charge, the stronger
the force exerted.)
Method 1: Charging by Friction
a. By rubbing two materials, the materials will get
different charges.
b. The table below shows some combinations the
materials will produce specific charges.
Glass wool rubbed with silk
Ebonite rod rubbed with
fur
Perspex rod rubbed with
wool
Rubber balloon rubbed
with hair
Polythene rod rubbed with
wool
2.
Positive
Glass
Fur
Negative
Silk
Ebonite
Perspex
Wool
Rubber
Hair
Wool
Polythene
Method 2: Charging by induction
Charging 2 conductors by induction
1. Place two conductors side
by side.
2. Bring a negatively charged
rod near Sphere A. This
will cause the electrons in
the metal spheres to be
repelled to the far end of
sphere B, creating an
induced positive charge
on A and a induced
negative charge on B.
3. Leaving the rods intact,
separate the two spheres.
4. Remove the rod. The
charges now spread
throughout the entire
sphere.
Charging 1 conductor by induction
1. Place a positively charged
rod near the sphere. This
causes a negative charge
to be induced on the left
side and a positive charge
to be induced on the
right side of the sphere.
2. Leaving the rod intact,
earth the conductor.
Electrons will flow
away/towards the finger,
making the overall charge
of the rod and the sphere
neutral.
3. Remove the finger first,
before removing the rod.
4. The remaining charges
will redistribute
themselves evenly.
Note: In both cases,
1. Protons do not move at all. Only the electrons can
move.
2. For the first case, there are still some protons left in
sphere B.
3. Always remove the finger before removing the rod for
the second case, if not, once the rod is removed the
sphere will be earthed again.
4. The type of charges present in the rod used will
determine the type of charge in the conductor. A
positively charged rod will create a negatively charged
insulator.
16.7: Neutralizing charged insulators and
conductors
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When an object is charged, it can be discharged, which
is the removal of excess charges.
Discharging a charged insulator: Heating a charged
insulator over a Bunsen flame can neutralize it as the
intense heat causes the air surrounding the glass rod to
be ionized which will neutralize the excess charges.
Discharging a charged conductor: A charged conductor
can be discharged through earthing, which is providing
a path for excess electrons to flow away/into the
conductor, causing the conductor to lose its charge and
become neutral.
If something connected to a charged object is earthed,
it is represented by the symbol:
Thinking Questions:
In case 2, what determines whether the electrons
move in or out of the sphere?
Ans: The charged rod. For instance, if the charged rod
is positively charged, electrons flow in so as to
neutralize the overall charge in both the rod and the
sphere.
2.
Does it matter whether you touch anywhere on the
sphere?
Ans: No. It determines on external influence.
16.8: Electroscope
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An electroscope is used to test for charge and sign of a
charge (positive/negative).
When it is uncharged it can be used to see if the
material is charged or uncharged.
If the charged rod is brought near to a positively
charged electroscope, electrons are induced to the top
of the rod and thus the gold leaves become positively
charged which will diverge.
If the electroscope is charged then a like charge will
cause the gold leaves to diverge.
Note: Repulsion is the only test for the sign of a charge.
16.9: Hazards/Applications of
electrostatics
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Lightning: Lightning is due to the discharge of a large
quantity of electric charge built up in the
thunderstorms. The thunderclouds are charged by
friction between the water molecules. When the
charge becomes large enough, it ionizes the air which
allows huge quantity of electric charge to be
discharged to the nearest object on the ground.
Refueling Planes safely: This can be done by joining
the delivery tanker and the fuel tank electrically with a
metal wire. The wire provides a path for electricity to
flow along, and so prevents the build up of any
potentially dangerous static charge.
Photocopier:
a. The original (the page you want copied) is placed
onto a sheet of glass. An image of this page is
projected onto a positively charged drum.
b. The drum has a coating which conducts electricity
when light falls on it. The parts of the drum which
are lit by the projected image lose their
electrostatic charge when they start to conduct.
c.
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A black powder (called toner) is negatively
charged.
The toner is attracted to the positively charged
parts of the drum. The drum rotates and rolls
against a piece of copier paper. The toner is
transferred from the drum to the paper
making a black and white image of the original.
d. Finally, the paper is heated which makes the
toner stick to it. This is called "fixing" the image.
When you use a photocopier you can feel that the
copier paper is still warm.
Electrostatic precipitator:
a. Pollution from industrial chimneys (for example a
coal burning power station) can be reduced by
using electrostatic charge. As well as the waste
gases from burning coal (CO2, SO2), the chimney
contains many small particles of unburnt fuel (ash
and carbon).
b. The chimney has a high voltage negative grid
across it and this gives the small particles a
negative charge as they go past. Further up the
chimney there are positively charged plates
which attract the negatively charged particles.
The particles of pollution build up on the plates
until they are heavy enough to fall down into
containers. The containers and the plates are
cleaned periodically.
c. In this way, much of the smoky pollution is
removed from the chimney before it can get out
into the atmosphere.
Spray painting
a. Millions of cars are made each year
and the car bodies must all be painted to prevent
them from going rusty. The paint is sprayed onto
the car bodies and the process is made more
efficient by using electrostatic charge.
b. The paint spray goes past a high voltage positive
needle as it leaves the spray gun and the tiny
droplets of paint pick up a positive charge.
Remember, they do this by losing negative
electrons. It is only the electrons which can move.
The car body is then given a high voltage negative
charge which attracts the positively charged paint
droplets.
c. This is good for two reasons. Firstly, the paint
droplets spread out more as they leave the gun.
This happens because they all get the same
positive charge and so they all repel each other.
This is better than coming straight out of the gun
as the paint will cover a wider area more evenly.
d. Secondly, the paint droplets are attracted to the
negative metal car body, and so less paint will be
wasted on the floor or the walls of the paint shop.
Crop sprayer:
a. Crops (plants grown for food) are sometimes
sprayed from an aircraft with an insecticide to
reduce the amount of the crop which gets eaten
by insects.
b. The advantage of spraying crops from an aircraft
is that large areas can be sprayed very quickly.
c. The disadvantage is that it is difficult to control
where the spray will fall on the fields. Some parts
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of the field will receive more insecticide than
others. Some insecticide is blown away on
the wind and does not fall on the crop at all.
If the insecticide is given a static charge as it
leaves the aircraft then much more of
the spray reaches its target and
the spray droplets are spread out more evenly.
This happens because
the insecticide droplets with the static charge
are attracted to the crop even though
the crop is neutral (uncharged).
The insecticide droplets spread out
more evenly because they all have the same
charge.