Unit D: Electrical Principles & Technology

Review of the Atom

  • protons have a positive charge
  • electrons have a negative charge
  • neutrons have no charge (neutral)
  • atoms may gain or lose electrons and become ionized
  • charged particles can either repel or attract each other

Electrical Charge

  • most objects have an equal amount of positive and negative charge so they are neutral overall

Charges:

  • when there are more protons than electrons, the object has a positive static charge
  • when there are more electrons than protons, the object has a negative static charge

Bringing a charged object near a neutral object may cause charge separation in the neutral object

Law of attraction

Opposite charges attract each other, like charges repel each other (No this does not apply to love)


Electrical Discharge

Build up of charge on one object may cause a transfer of charge to another object the resulting spark is the electrical discharge

  • (ex) “shock” from touching someone, lightning, Van de Graaf generator, Tesla coil
  • a Van de Graaf generator uses friction from a belt being rubbed to build up a static charge

Current Electricity

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Electrical Current

  • static electricity is not useful since it doesn’t flow continuously
  • electric current is the continuous flow of electrons

Circuit:

a circuit is the continuous path that an electrical current flows through

 It is made up of:

  1. conductor – wires
  2. load – device to convert energy eg) light bulb
  3. energy source – battery

conductors:

Allow electrical charge to move eg) copper wiring

  • small devices (cell phones, MP3 players etc) use very little current
  • large devices (cars, generating stations) use a lot of current

 


    Voltage vs. Amperes

    Voltage or potential difference:

    a measure of how much electrical energy each charged particle carries

    • the unit of voltage is the volt (V)

    easiest way to measure volts is with a voltmeter

    Amperes (A):

    The rate at which electrical current flows is measured in a unit called


    Electrical Safety

     REASON WHY YOU SHOULD'NT FLY A KITE NEAR ELECTRICAL WIRES

    REASON WHY YOU SHOULD'NT FLY A KITE NEAR ELECTRICAL WIRES

    Short Circuits

    • a short circuit is an unintended path for electricity
    • electric current will always take the shortest path available
    • if a power line is down, electricity will not flow because the circuit is open
    • if you go too close you will close the circuit and the electricity will flow through you!

    Dangers of Electrical Shock

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    Both voltage and current can be dangerous!!

    • Skin is a bad conductor of electricity therefore a power supply voltage must be higher than 40 volts to “push” a fast flow of charges through a human body and cause electrocution

     

     

     

    • However, high voltage is only dangerous if there is a flow of electrons (current) you can get a zap of 10,000 volts from scuffing your feet on a carpet and then touching a door knob but thankfully death doesn’t happen because there is no current!

     Common Voltages !

    Voltage (volts)

    • human cell 0.08 V
    • microphone 0.1 V
    • photocell 0.8 V
    • electrochemical cell 1.1 V to 2.9 V
    • electric eel 650 V
    • wall outlets in house 120 V, 240 V
    • generators in power stations 550 V

    The current rating must be higher than ten milliamps  (10 mA or 0.01 A)

    At a current lower than 10 mA, even a high voltage power supply cannot electrocute you


     Electric Current Ratings

    Electrical Device

    • electronic wrist watch 0.00013 A (0.13 mA or milliamps)
    • electronic calculator 0.002 A (2 mA)
    • electric clock 0.16  A (160 mA)
    • light bulb (100W) 0.833 A
    • television (color) 4.1 A
    • vacuum cleaner 6.5 A
    • oven element 11.4 A
    • toaster 13.6 A
    • car starter motor (V-8) 500 A

    too much electricity flowing through a person’s body can cause the following:

    • pain
    • loss of muscle control
    • burns
    • damage to the heart
    • death

    Insulators

    • Electricity is more dangerous when current can flow easily, like a copper wire.
    • Current does not flow as easily through insulators such as wood, plastic, rubber and air.
    • To prevent injury and short circuits many conductors are wrapped in an insulator (Hence why all power cords are coated in plastic)
    • Moist materials tend to conduct better than dry materials
    • If an insulator is damaged, shock from a short circuit is possible

    Protection from Electricity

    Our homes and the devices we use are equipped with safety features to protect us from electric shocks many appliances and devices have three-pronged plugs the third prong connects the device to the ground wire of the building to provide another pathway for electricity just in case of a short circuit

    Fuses and circuit breakers interrupt a circuit when too much current flows through it fuses have a wire in them that melts if the current gets too high breakers have a wire that trips a switch if it heats up too much both a blown fuse and a tripped breaker will open the circuit, not allowing electricity to flow

    The current in a lightning strike can be 30 000 A (current as low as 0.1 A can be fatal) lightning rods are placed on the top of tall buildings etc and they are connected to the ground by a wire the lightning strike is then carried to the ground to be discharged


    Cells & Batteries

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    Dry Cells:

    Electricity producing cells we use as batteries are called “dry” because the chemicals inside are in a paste

    3 Parts of a dry cell:

    1. The paste is an electrolyte, which is a chemical that contains ions that can conduct electricity
    2. Two electrodes, which are two different metals 
    3. A conductor which connects the negative electrode to the positive electrode, electrons move through this conductor

    Wet Cells:

    Electricity producing cells, “wet” cells because they use a liquid electrolyte (ex H2SO4(aq)). Use in batteries for cars. 

    Two electrodes are dipped into the liquids

    Cheaper and easier to make than dry cells but you have to be careful not to spill the electrolyte which is corrosive


    Rechargeable Cells

    • The chemical reactions inside a rechargeable cell are reversible in secondary cells
    • Can be used to store energy from an outside source

    Batteries:

    Connecting two or more cells together makes a battery


    Electrochemistry

    Electrochemistry:

    The study of chemical reactions that involve electricity

    electrolysis:

    An industrial process that is used to separate useful elements

    • (ex)  hydrogen and oxygen gases for fuel for the space shuttle

    electroplating:

    Uses current to deposit atoms of a metal onto an electrode

    • (ex) silver plating

    electrorefining:

    Process that removes impurities from a metal


    Controlling the Flow of Electrical Current

    Insulators:

    Electrons in insulators are tightly bound to the positive nucleus of their atoms and they cannot flow

    • Glass
    • Rubber

    Conductors:

    Electrons are not tightly bound and are free to move (with some resistance)

    • Metals

    Superconductors:

    Superconductors are perfect conductors

    • The electrons have no resistance to flow
    • The temperature must be very low (close to absolute zero) for superconductivity
     VARIOUS RESISTORS, THE DIFFERENT COLOUR BANDS CORRELATES TO VARIOUS RESISTANCE AMOUNTS

    VARIOUS RESISTORS, THE DIFFERENT COLOUR BANDS CORRELATES TO VARIOUS RESISTANCE AMOUNTS

    Resistors:

    Resistors allow only a certain amount of electric current to pass

    Resistance:

    Is a measure of how difficult it is for electrons to flow through a substance

    • Resistance is measured in Ohms (Ω)
    • The resistor gains energy from each electron that passes through it
    • This energy can be released as heat or light

    (ex) in a space heater or light bulb, liquids can also be good resistors

    • Adding resistors to a series line consecutively increases the total resistance of that line...

    (ex) Total (Ω) = Resistor1 (Ω) + Resistor 2 (Ω) + Resistor 3 (Ω)....

    (ex) Total 150 Ω = R1 (50 Ω) + R2 (75 Ω) + R3 (25 Ω) 

    Switches:

    Used to control the flow of electricity through a circuit

    • When the switch is on, two conductors are pressed together, closing the circuit and making electricity flow
    • switches are enclosed in an insulating case for protection

    Variable Resistors:

    Allow the gradual adjustment of electric current

     THE 'OLDSCHOOL' METHOD OF DRAWING A VARIABLE RESISTOR

    THE 'OLDSCHOOL' METHOD OF DRAWING A VARIABLE RESISTOR

    • also called rheostats
    • made of one single curved resistor
    • a dial changes the amount of the resistor that is used

    Transistors:

    A switch that is controlled by an electric input, however it does not have any moving parts


    Modelling & Measuring Electricity

    Modelling Electricity

    Because electricity “flows”, we can use water to be a model for how electricity behaves

    A waterfall is a good model for voltage a

    • A change in elevation allows the water to flow because of gravitational potential energy in a circuit, a change in potential difference from a battery allows the electrons to flow
    • The higher the potential difference (waterfall) the higher the voltage (energy)

    A pipe is a good model for resistance and current

    •  (Ex) If you use a pipe to drain a pool, a longer, thinner pipe will have the most resistance and will drain it much slower (lower current)
    • If you use a short, wide pipe, it will have less resistance and will drain faster (higher current)

    Ohm’s Law

    Georg Ohm found a relationship between voltage (V), current (I) and resistance (R):

    1. Resistance in a conductor is constant
    2. Current is directly proportional to voltage

    Thus...Increasing the voltage in a conductor will increase the current since resistance is constant

    where:   V = voltage in Volts (V)

                   I = current in Amperes (A)

                  R = resistance in Ohm’s (Ω)

    V = IR

    Example 1

    Calculate the voltage if the current is 0.5 A and the resistance is 2.0 Ω.

    I = 0.5 A  , R = 2.0 Ω, V = ?            

    V = IR

    V = (0.5 A)(2.0 Ω)

    V = 1.0 V

    Example 2

    Calculate the current if the voltage is 1.2 V and the resistance is 0.6 Ω.

    V = 1.2 V, R = 0.6 Ω, I = ?          

    V = IR or... I = V/R

    I = 1.2V/0.6Ω

    I = 2.0 A

    Example 3

    Calculate the resistance if the current is 1.1 A and the voltage is 2.2 V.

    V = 2.2 V, I = 1.1 A, R = ? 

    V = IR or... R = V/I

    R = 2.2V/1.1 A

    R = 2.0 Ω

    REVIEW QUESTIONS


    Test Meters

     A TYPICAL MULTIMETER

    A TYPICAL MULTIMETER

    All test meters use a small amount of current

    Test Meters:

    Measures the following

    • voltmeter - potential difference between two points in a circuit (voltage)
    • ammeter - electrical current which is the rate of flow of electricity
    • galvanometers - measures small amounts of currents
    • ohmmeters - measures resistance
    • multimeters - all in one measuring tool, measures voltage, current or resistance in a circuit

    Analyzing & Building Electrical Circuits

    Circuit Drawings

    Engineers and scientists use special symbols to draw, plan and analyze electrical circuits

    Circuits have 4 basic parts:

    1. Source – provides energy and a supply of electrons
    2. Conductor – path for current
    3. Switch – controls current flow
    4. Load – converts electrical energy into another form of energy
    • you can draw simple circuits using the common circuit symbols (see pg 312 of text)
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    Examples

    Draw each of the following circuits:

    1.    3 cell battery, 1 motor, 1 switch

    2.    2 cell battery, 1 resistor, 2 lamps, 1 switch

    3.    3 cell battery, 3 lamps, 1 ammeter, 1 switch

    4.    4 cell battery, 1 fuse, 3 lamps, 1 motor, 1 rheostat

    5.    3 cell battery, 2 lamps, 2 switches that must control the lamps from either end of a room (both on/off function)


    Series Circuits

     A SERIES CIRCUIT - CLOCKWISE WE HAVE THE BATTERY, A LAMP, ANOTHER LAMP, A AMMETER, AND A SWITCH.

    A SERIES CIRCUIT - CLOCKWISE WE HAVE THE BATTERY, A LAMP, ANOTHER LAMP, A AMMETER, AND A SWITCH.

    Series circuit:

    A circuit which has all loads in one single loop

    • electrons have only one path the follow
    • an interruption results in no flow of current
    • adding loads increases the resistance in the circuit

    Parallel Circuits

     A PARALLEL CIRCUIT - TOP TO BOTTOM WE HAVE A BATTERY, A LAMP, AND ANOTHER LAMP.

    A PARALLEL CIRCUIT - TOP TO BOTTOM WE HAVE A BATTERY, A LAMP, AND ANOTHER LAMP.

    parallel circuit:

    A circuit which has more than one pathway for the electricity to flow through

    • loads in separate loops will not interfere with each other
    • loads in separate loops reduces the resistance in the circuit

    Applications of Circuits

     CHRISTMAS LIGHTS THAT USE SERIES LIGHTING - YOU GET WHAT YOU PAY FOR

    CHRISTMAS LIGHTS THAT USE SERIES LIGHTING - YOU GET WHAT YOU PAY FOR

    • House wiring uses parallel circuits…you don’t want one light bulb burning out to shut down all power!
    • Houses also have a circuit in series so that you can turn off all power at once if needed
    • Christmas lights use parallel wiring so the bulbs will stay lit even if one burns out
    • Microcircuits (microelectronic circuits) are extremely small circuits that are made up of transistors and resistors