Using Electricity Block 8

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Using Electricity Block 8
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Charging by Contact

• When two good insulators are rubbed together
  electrons can be rubbed from one to the other.

becomes negatively charged
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becomes positively charged
POSITIVE CHARGES NEVER MOVE
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Electrostatic Forces

• Two positive charges repel
• Two negative charges repel
• A positive charge and a negative charge attract

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Charging by Induction
Higher

• An object can be charged by induction i.e.
  without losing charge from the original charged
  object.
• If the cans are now separated the left one will
  be negatively charged and the right positive.

electrons attracted

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Charging by Induction 1
Higher

• Step 1 put a charged strip near to
• the cap of the g.l.e.

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electrons attracted upwards

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Charging by Induction 2
Higher

• Step 2 touch the cap with your finger

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electrons flow onto gle
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Charging by Induction 3
Higher

• Step 3 remove finger and then the strip

• -
• -

electrons spread out over gle
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Uses of Electrostatics Photocopying
The key to photocopying is a drum coated with
selenium. This drum is charged in the dark. An
image of the original document is reflected onto
the drum. The charge stays on the dark part of
the picture and disappears from the light
part. The drum is sprayed with a dark dust
called toner. The toner sticks to the charged
areas the dark parts of the picture.
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Uses of Electrostatics - Photocopying
Then a charged piece of paper is rolled against
the drum. It picks up the picture made by the
toner. Finally the paper is heated to fix the
toner in position.
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Uses of Electrostatics - Precipitator
An electrostatic dust precipitator is used in
coal fired power stations The smoke, from the
burning coal, is passed between a set of grids
and plates. The negatively charged grids charges
the dust in the smoke. This charged dust then
sticks to the earthed plates. These plates are
periodically cleaned by hitting them with a
hammer.
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Other Uses of Electrostatics

• Electrostatic paint spraying produces an
  excellent finish, reduces the amount of paint
  needed and environmental damage.
• Static electricity can be used to reveal finger
  prints on banknotes and similar paper objects.
• Cling film sticks using static electricity.

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Problems Caused by Electrostatics

• When liquids or powders, e.g. aviation fuel or
  flour, flow through modern plastic pipes large
  charges can build up and cause explosions.
• Charged surfaces such as television screens or
  mirrors can attract dust.
• A static charge build up, in an operating theatre
  can have tragic consequences.

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Charge
Highers Learn

• Charge Current x Time
• Q I x t
• Charge is measured in coulombs (C)
• Current is measured in amps (A)
• Time is measured in seconds (s)

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What are Currents?
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Electrical Power
Learn
Power Current x Voltage P I x
V Power is measured in watts (W) Current in
measured in amps (A) Voltage is measured in volts
(V)
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D.C. / A.C.

• Direct Current always flows in one direction

• Alternating Current flows to and fro.

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Those 3 Wires
These three wires have to be insulated from each
other.
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Fuses 1

• If working properly a kettle does not need an
  earth wire.

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Fuses 2

• But if the element becomes faulty the case can
  become live and electrocute you.

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Fuses 3

• If however a fuse and earth wire are connected
  then a large current will flow to earth blowing
  the fuse.

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Paying for Electricity

• We pay for electrical energy in units called
  kilowatt hours (kW hr) or often called units.
• One kilowatt hour is how much energy is needed to
  run a one kilowatt device for one hour.
• For example if we use a 100W light bulb for 10
  hours this equals 1/10 kW for 10 hr
  i.e. 1/10 x 10 1 kW hr.