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Electricity - PowerPoint PPT Presentation

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Electricity. Atoms Basic laws Ohm’s Law Kirchoff’s Voltage Law AC & DC. 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

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Basic laws

Ohm’s Law

Kirchoff’s Voltage Law


Atoms electrons
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

Electron bands
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


M shell







Voltage current
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


  • 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


  • 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

Speed of electrons
Speed of Electrons

  • Electrons donot 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

Voltage difference
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

Alternative ways of electricity
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

Electrical circuit
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 Ohm’s Law

V = IR


electron flow direction


Ohm s law
Ohm’s Law

  • How the factors of voltage, current and resistance are related is perfectly defined by a very simple mathematical equation called:

  • Ohm's Law - V=IR- the same equation rearranged different ways looks like I=V/R & R=V/I & P=VI & P=I2R & P=V2/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)

Kirchoff s voltage law
Kirchoff’s 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 stated:Vsupply = Vdrop1 + Vdrop2 + Vdrop3 + ...... + Vdropn

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 @ 50Hz

  • This rate of reversing polarity (number of reversals per second / 2 ) is called the 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 220VAC 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

Ac dc components of a signal
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