02_basic_electricity.md

2. Basic electricity

Definitions

• Law of the Charges: Like charges repel and unlike charges attract
• Electricity is electron flow in one direction
• Insulators do not conduct electricity as their electrons are tightly bound to their nucleus
• Conductors are materials in which the electrons are loosely bound
• Primary cell produces an electrical voltage by means of a non-reversible chemical reaction
• Secondary cell uses a reversible chemical reaction
• When two points in an electrical circuit do not have the same voltage they are said to have p.d. (potential difference)
• E.M.F. (Electro-motive force) is electrical pressure from a supply of cell or a generator
• Wattage rating is the value of resistance
• Laws of magnetism states that like poles repel and unlike poles attract
• Maxwell’s right hand corkscrew rule gives the direction of this field: If a right hand corkscrew moves forward in the direction of the current then its rotation will indicate the direction of the magnetic field.

Units of measurement

• Coulomb or C is the quantity of electricity. Symbol of charge is Q
• Ampere or A is the unit of current flow. Symbol of current is I
• Voltage or V is the unit of electrical pressure/potential
• Watt or W is the unit of power dissipated
• Ohms or Ω is the unit of resistance

Resistor values

• 2.4 Ω is 2R4
• 3.3 KΩ is 3K3
• 5.6 MΩ is 5M6

Circuit symbols

• resistor symbol (ANSI)

  

• resistor symbol (IEC)

  

• variable resistor symbol (adjusted during operation)

  

• preset variable resistor symbol (adjusted by a screwdriver when the equipment is being aligned or maintained)

  

Formula

• Q = It, where Q is charge is Coulomb, I is current, t is time in seconds
• V = RI where V is the voltage, R is the resistance, I is the current
• P = IV where P is power, I is current, V is voltage
• P = I²R where P is power, I is current, R is resistance
• P = V²/R where P is power, V is voltage, R is resistance
• Total resistance
  • in series: R = R₁ + R₂ + R₃ + ...
  • in parallel: 1/R = 1/R₁ + 1/R₂ + 1/R₃ + ...
• Total power rating of resistors in parallel: P = P₁ + P₂ + P₃ + ...

Notes

Atoms

• There are about 118 different types of atoms (excluding isotopes)
• The atom, as a whole, is normally electrically neutral
• Each atom comprises a nucleus (in the centre) and one or more orbiting electrons
• The electrons each have one negative charge
• The number of positive charges, protons in the nucleus will equal the number of orbiting electrons
• There may also be some neutrons in the nucleus, but as their name suggests they have no charge and only add weight to the atom

Moving electrons

• escaping electron (-) is normally attracted to the parent atom
• free electron is attracted to another atom that has lost an electron
• if an electron is "lost" then its parent atom will become positively charged
• the electrons will jump around randomly from one atom to another

Electricity

• If a voltage is connected across the material then the movement of electrons changes from random to an organised drift in one direction
• Electrons, being negatively charged, are attracted to the positive terminal of the supply voltage
• Electrons jump from one atom to the next making their way across the material

Conductivity

Material categories:

• Good conductors
• poor conductors
• semiconductors
• poor insulators
• good insulators

It is often convenient to use liquid insulators. For example, mineral oils are often used for insulation in very high power switches. It will prevent arcing by excluding all air. It also conducts away any heat but does not conduct electricity.

Generating electricity

• mechanical energy into electrical energy by dynamo (direct current)
• mechanical energy into electrical energy by alternator (alternating current)
• generator = dynamo or alternator

Resistors

Materials

• Most resistors are made of carbon
• Resistance of the carbon is controlled during manufacturing
• Past: solid rods of carbon
• Present: a film of carbon or metal oxide over the surface of the non conducting rod
• Resistance is dictated by the resistance and thickness of the coating or film

Band colors:

• 1st ring is first digit, 2nd ring is second digit
  • Black = 0
  • Brown = 1
  • Red = 2
  • Orange = 3
  • Yellow = 4
  • Green = 5
  • Blue = 6
  • Violet = 7
  • Grey = 8
  • White = 9
• 3rd ring is the number of zeros
• 4th ring is the tolerance
  • Brown = 1%
  • Red = 2%
  • Gold = 5%
  • Silver = 10%
  • No Colour = 25%

Power ratings

Material	Rating (W)
Carbon and metal oxide	0.125, 0.250, 0.500, 1.00
Wire is wound on an insulating rod and often covered in vitreous enamel	1, 3
Resistance wire is wound on a fibre glass core and protected by a ceramic outer body	4, 7, 11, 17
Several resistors are connected in parallel	even hotter

Dummy load

• When testing a transmitter, the antenna may be often replaced by a dummy load
• It is a resistor/set of resistors designed to match the output impedance of the transmitter and be capable of absorbing its full power output
• Wire-wound resistors have a high power dissipation capacity, but are not used as RF dummy loads as it presents a reactive load to the transmitter.
• In order that the heat is efficiently dissipated, It is usual to immerse the resistors in oil
• oil will conduct the heat but not electricity
• a small breathing hole in the can prevents the build up of pressure as the temperature rises

Potentiometers

• If connections are made to the wiper and both ends of the variable resistor, It is then called a potentiometer (Pot)
• 2 types:
  • operated by the user
  • mounted on PCB and known as preset
• Example uses: volume control
• By convention clockwise == increase the output

Internal resistance

• Voltage at the terminal of the cell would drop a little when the current is flowing
• Cell, although producing the voltage, is not a perfect conductor
• high internal resistance - dry cell
• low internal resistance - lead acid secondary cell, generators

Magnetism

• A freely suspended magnet will align itself with the earth’s magnetic field
• End towards the north pole is known as the north seeking pole (north pole)
• End towards the south pole is known as the south seeking pole (south pole)
• Experiment to show the pattern of magnetic field: A postcard is placed on the top of a small bar magnet and then some iron filings are sprinkled on to the card
• 2 types of materials:
  • Nickel - Iron is difficult to magnetise. But once achieved, will retain it almost permanently.
    • Application: Loud speakers, headphones, cathode ray tube focusing
  • Soft iron is very easy to magnetise but lose their magnetism as soon as the magnetising force is removed
    • Application: Relays, Electromagnets, Transformers

Electricity and Magnetism

• A wire carrying an electric current will have a magnetic field around it
• Strength of a magnetic field ∝ Current flowing through the wire
• Strength of the electromagnetic field will depend on:
  • number of turns in the coil
  • current in the coil
  • core material (coil is wound around a ferrous / iron core)

Uses

Earphone

• Permanent magnet, with its pole pieces, attracts the diaphragm giving a slight concave shape
• When a speech carrying current, flows through the coils the magnetic attraction will vary
• diaphragm move in sympathy with the speech

Loud speaker

• The coil ( and cone ) is mounted so that it will move freely in the strong magnetic field produced by the permanent magnet
• Circular magnet has its north pole in the centre and its south pole round the edge
• Current in the speech coil generates a magnetic field that will interact with the fixed magnetic field
• Coil and the cone will move in sympathy with the speech currents
• Movement of the diaphragm (or the cone) vibrates the adjacent air
• We hear these vibrations and call it sound

Electromagnetic Relay

• An electromagnetic switch
• when a current passes through the coil the magnetic field attracts the armature
• The movement of the armature operates the relay contacts