Electricity

1
Electricity

• Atoms
• Basic laws
• Ohms Law
• Kirchoffs Voltage Law
• AC DC

2
Atoms Electrons

• All matter is composed of atoms
• All atoms have a nucleus composed of one or more
  positive protons and may have one or more
  neutrons
• Orbiting the nucleus may be one or more negative
  electrons
• Electrons are said to be separate from the
  nucleus but hang around it in spinning orbits
• because the positive charge of the nuclear
  protons and the negative charge of the electrons
  attract each other

3
Electron Bands

• Electrons circle nucleus in defined shells
• K 2 electrons
• L 8 electrons
• M 18 electrons
• N 32 electrons
• Within each shell, electrons are further grouped
  into subshells
• s 2 electrons
• p 6 electrons
• d 10 electrons
• f 14 electrons
• Electrons are assigned to shells and subshells
  from inside out
• Si has 14 electrons 2 K, 8 L, 4 M

L
K
M shell
10
d p s
6
2
4
Voltage Current

• When an electromotive force, called a potential
  difference or voltage (measured in Volts) is
  applied to the material, some of the electrons
  get pushed or yanked away from their nuclei and
  drift through the material in the same general
  direction
• The direction they go depends on the direction or
  polarity of the applied voltage
• An applied voltage is commonly referred to as
  having a negative and a positive terminal
• The quantity of electrons that move by any point
  in a material over a given amount of time is
  called the current
• It is measured in Amperes

5
Resistance

• The ability a material has to allow electron
  migration through it is a measure of its
  conductivity
• Alternatively speaking, the material's inability
  to allow electrons to flow is called its
  resistance which is measured in Ohms
• The usual symbol for ohms is the Greek symbol
  Omega ?
• When a material has a very low resistance to
  electron flow, it is called a conductor
• When a material has a very high resistance, it is
  called an insulator

6
Resistors

• Wires are made of metal (usually copper) which
  are conductors because of their low resistance
• The plastic sheathing covering the wire is an
  insulator because of its very high resistance
• We produce circuits by assembling conducting
  materials and insulating materials in a way as to
  produce a specific path for the electrons to flow
• Some materials (such as those used in electronic
  components) can carry electrons at some medium
  level
• These materials are called resistors
• They are infinitely varied and are used in some
  fashion in virtually all electrical circuits

7
Speed of Electrons

• Electrons do not move through a conductor at the
  speed of light
• They bump along rather slowly, but they're all
  pushing each other along
• The effect of their pushing each other however,
  does travel at the speed of light which is why
  you can talk on the phone with somebody on the
  other side of the planet without any noticeable
  delays

8
Voltage Difference

• Electrons will only flow when there is a
  difference in charge placed across them
• Therefore, when you think of voltage, think of it
  as the difference across something
• Voltage is always measured with respect to
  something
• If you measure, say 5V, it is with respect to
  the negative terminal which is your zero Volt
  point
• The zero Volt point in a circuit is commonly
  referred to as the ground point
• If there were a 12V point, you think of it as a
  12V potential relative to 0V or ground
• You could, however, say there was a 7V potential
  between the 5V and the 12V points
• Whether that potential is said to be positive ()
  or negative (-) depends on which one you are
  considering to be your reference point

9
Alternative Ways of Electricity

• There is another form of electricity besides that
  consisting of moving electrons
• The movement of positive ions is also considered
  electricity
• You might understand this form better if you
  think of it as a chemical reaction where the
  energy or work used to create the reaction comes
  from an applied potential difference (voltage)
• Also, the opposite is true
• A chemical reaction can also generate a potential
  difference
• The reaction produces a migration of positive
  ions in one direction and electrons in the other
  direction
• This type of electricity occurs in electrolytes
  such as in batteries, electrolysis and the neural
  synapse of the brain

10
Electrical Circuit

• Electricity is the flow of electrons
• Good conductors (copper) have easily released
  electrons that drift within the metal
• Under influence of electric field, electrons flow
  in a current
• magnitude of current depends on magnitude of
  voltage applied to circuit, and the resistance in
  the path of the circuit
• Current flow governed by Ohms Law

V IR

electron flow direction
-
11
Ohms Law

• How the factors of voltage, current and
  resistance are related is perfectly defined by a
  very simple mathematical equation called
• Ohm's Law - VIR- the same equation rearranged
  different ways looks like IV/R RV/I PVI
  PI2R PV2/Rwhere
• R resistance in ohms
• V potential difference in Volts (V)
• Note that many books use the symbol "E" to
  represent voltage while others prefer "V
• The "E" stands for electromotive force
• I current in Amperes (A)
• Most people say "Amp" as a short form
• P power in Watts (W)

12
Kirchoffs Voltage Law

• Simply stated, it says that the sum of the
  voltage drops in a circuit is always equal to the
  voltage applied
• For example, if you apply 5 volts (5V) to a
  circuit, then all voltages which the components
  in the circuit 'use up' or 'drop' will always add
  up to be equal to 5V
• This applies to each and every circuit
• Mathematically statedVsupply Vdrop1 Vdrop2
  Vdrop3 ...... Vdropn

13
AC DC

• AC means 'alternating current'
• DC means 'direct current
• When a steady amount of voltage is applied to a
  circuit, it is called DC power
• The level of voltage can be varying, but as long
  as its polarity does not change, it is referred
  to as DC
• The output of a battery is always DC because it
  has a terminal which always remains positive with
  respect to the other
• When an applied voltage continues to change
  polarity in a repetitive fashion, it is called AC
  power
• The output of the 220V wall outlets in your home
  are AC
• because the live terminal in the outlet changes
  its polarity with respect to the neutral terminal
  100 times every second
• 2 transitions per cycle _at_ 50Hz
• This rate of reversing polarity (number of
  reversals per second / 2 ) is called the frequency

14
Frequency

• Each pair of reversals is called a cycle and
  there are 50 of them every second
• The frequency is the number of cycles per second
• The unit of measure for frequency is called Hertz
  after the dude who defined this
• The unit symbol is 'Hz
• The frequency of the 220V AC power in Hungary is
  50Hz
• AC power can be any voltage with reversing
  polarity and it can oscillate (reverse regularly)
  at any frequency
• It's all still AC
• Radio transmitters use AC

15
AC DC Components of a Signal

• AC waves have often been said to be riding along
  on top of a DC level
• It is possible to think of an AC signal as being
  superimposed on the DC signal or as riding on it
• because if you add the voltage value of an AC
  wave to the value of a DC level, the sum that
  results can be an entirely DC level which varies
  just as the AC wave did, but always maintains the
  same polarity by never crossing the zero volt
  line (ground)
• This happens if the DC level is greater than the
  whole (peak-to-peak) AC waveform
• AC is referred as the AC component of the varying
  DC level
• A device called a capacitor separates any DC
  component from an AC waveform
• Capacitors can only transfer the change in
  voltage and cannot pass or conduct the steady or
  non-changing DC voltage