Introduction to Electricity

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Introduction to Electricity

• John R. Ebden

Seneca College
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What is electricity?

• Static electricity
• Current
• Voltage
• Resistance
• AC
• DC

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Atoms and electrons

• Electronics is based on controlling the movement
  of electrons

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Static electricity

• Rubbing objects can cause electrons to be added
  or removed from the object.
• The object then becomes charged because it no
  longer has the same number of electrons as
  protons.

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Unlike charges attract
Like charges repel
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Static electricity

• When a plexiglass rod is rubbed with a silk
  cloth, electrons move from the rod to the cloth,
  leaving the rod positively charged, and the cloth
  negatively charged.

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Static electricity

• If the now positively charged rod is placed near
  your hair, it will cause the electrons in your
  hair to move towards the end of the hair.
• Your hair will become attracted to the positive
  charged rod making it stand up.

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Static electricity
He is charged up !!
He just lost his charge !!

• Static electricity occurs when electric charges
  build up on an object, but the electric charges
  cannot move around.

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Static electricity

• Static electricity is a big problem for companies
  that deal with non-conducting materials like
  plastics.
• The high voltages can destroy sensitive
  electronic components
• At companies like Celestica personnel must wear
  ESD coats, work on ESD surfaced tables, and wear
  grounded wrist straps

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Current electricity

• There is another type of electricity, current
  electricity, which consists of a continuous flow
  of electrons.
• It requires a source of electrons and a pathway
  to carry or conduct them.
• The source of electrons might be a battery or a
  generator.

G
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Current electricity

• Current electricity must flow along a pathway,
  usually metal, called a conductor.
• Copper , aluminium , and gold are common
  conductors.
• Some materials prevent the flow of electrons and
  are called insulators.

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Current electricity

• A bulb or an electric appliance may be part of
  that pathway. Electrons are returned to the
  battery or generator along the pathway completing
  the electric circuit.

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Voltage

• Current electricity flows because there is a
  difference in electrical pressure or potential
  between two points along a conductor.
• This potential difference is called voltage.

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Direct current (DC) and alternating current (AC)

• Electric circuits can be connected to either a
  direct current (DC) or an alternating current
  (AC) electricity supply.

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Direct current (DC)

• The stream of electrons flow in only one
  direction around the circuit, from the negative
  terminal to the positive terminal

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Direct current (DC)

• In the early days, the concept of electron flow
  was not fully understood so scientists randomly
  decided that current in a conductor flowed from
  the positive terminal and into the negative
  terminal.
• It is still convention today to show current
  flowing in this direction (ie. opposite to
  electron flow).

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Alternating current (AC)

• The electrons move backwards and forwards around
  an electric circuit in this case

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How electricity is made

• AC (most common) is made using a generator.
• DC is made by
• batteries (chemical reaction)
• power supplies (rectification of AC)

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DC Sources - Batteries
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DC Sources Power Supplies
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Making AC

• AC Electricity can be made or generated by
  moving a wire (conductor) through a magnetic
  field.

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Magnetism

• A bar magnet has a north and south pole. It is
  placed under a sheet of paper and iron filings
  are sprinkled over the top of the paper.
• These lines of filings show the magnetic field
  around the magnet.

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Making AC

• If a coil of wire is moved within a magnetic
  field electricity is produced in the coil.
• As loop is cutting more lines of magnetic field
  when it is horizontal, the current is a maximum.

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Making AC

• When loop is vertical no magnetic field lines
  are being cut, and current is zero.

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Making AC

• When loop has rotated 180o it is flat again.
• Current is now maximum but in the opposite
  direction.

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Making AC

• The current produced changes direction every half
  turn (180 degrees ). This is called alternating
  current or AC.
• The generators at large power stations produce
  nearly all the electricity we use in this way.

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Making AC

• The voltage in the loop varies with angle as
  shown.

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Making AC

• 1 rev 1 cycle 360 degrees
• domestic supply 60 cycles/sec 60 Hz
• ? loop rotates at 60 rev/sec 3600 rpm

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Making AC

• In many parts of Europe the frequency employed is
  50 Hertz (cycles/sec)

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Making AC

• 3 phase current is used to supply commercial
  buildings as it is more efficient.
• Produced by 3 loops rather 1

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Current

• Can compare water flowing through a pipe with
  current in a conductor flow of electrons

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Current

• A flow meter measures the water flow in
  litres/sec
• An ammeter measures the current in A
• (1 A 6.24 ? 1018 electrons/sec)

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Current

• 1 Coulomb (C) charge carried by 6.24 ? 1018
  electrons
• ?1 Ampere 1Coulomb/second (A C.sec-1)

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Current

• The movement of charge is called electric
  current.
• The more electrons per second that pass through a
  circuit, the greater the current.
• Current is the rate of flow of charge.
• I Q/t

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Voltage

• A pump can be used to cause the water to flow in
  the pipe.
• The force that makes the water flow is called
  pressure (measured in Pa or psi)

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Voltage

• A generator is an electrical pump
• The electrical pressure is called electrical
  potential or voltage (measured in V)

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Voltage

• Force between charges is
• F kQ1Q2/r2
• To separate unlike charges (or bring like charges
  together) work is done
• W Force x distance
• W ?Fdx

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Voltage

• When two objects have a difference in charges, we
  say they have a potential difference or voltage
  between them.
• The unit of voltage is the volt.
• Thunderclouds have hundreds of millions of volts
  between them.

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Voltage
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Voltage

• A difference in potential energy is defined as
  voltage.
• The voltage between two points is one volt if it
  requires one joule of energy to move one coulomb
  of charge from one point to another.
• V Work/Charge or V W/Q
• Volt Joule/Coulomb
• Voltage is defined between points.

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Ammeters and voltmeters

• Ammeters are connected into the circuit. The
  circuit must be broken to put one in.
• Voltmeters are connected across components to
  measure voltage. No need to break the circuit

Note voltage does not flow, current does
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Measuring voltage

• Voltmeter is placed in parallel
• No need to change circuit

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Measuring current

• Ammeter is placed in series
• Will need to break the circuit

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Resistance

• When water flows in a pipe, its flow is impeded
  by factors such as the size of the pipe,
  roughness of the pipe, etc.
• Anything that impedes or resists current flow in
  a circuit is called resistance.

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Resistance

• Electrical resistance is measured with an
  ohmmeter. Units are ohms (?)
• The power must be off before making any
  measurements.

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Ohms Law

• Relates current, voltage and resistance
• I V/R or, V IR or, R V/I

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Example - What is the current?

• Lamp has resistance of 96 ? and battery is 12 V
• I 12V/96? 0.125A 125x10-3A 125mA

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Meters - Analogue (VOM)
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Meters - Digital (DMM)
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Meters - Oscilloscope

• Used for analysing waveforms