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NUE 052 - Units of Measurement and Meters

The four basic units of electrical measurement are Volts, Amps, Ohms and Watts.

  • Volts is electrical pressure - the potential difference measured between 2 points in a circuit
  • Amps is electrical current - the flow of current in a circuit from positive to negative
  • Ohms is electrical resistance - the opposition to current flow in a circuit
  • Watts is electrical power - the electrical energy in a circuit
  • Click here to see Ohms law using Volts, Amps, Ohms and Watts
  • To measure electrical units we need to use an instrument called a meter.

    Meters come in all shapes and sizes, each for a different purpose. We can have special purpose meters that measure just one type of unit such as watts or an instrument called a multimeter which is capable of measuring volts, amps and ohms.
    Even though the multimeter can measure three electrical units, the meter must be connected differently in the circuit.
  • To measure Volts - connect the meter in parallel with the circuit under measurement while connected to the supply.
  • To measure Amps - connect the meter in series with the circuit under measurement while connected to the supply.
  • To measure Ohms - connect the meter in parallel with the circuit under measurement with the supply disconnected.
  • Click here to learn more about meters Click here to learn more about Oscilloscopes (CRO)
  • Multiples and sub-multiples

    Meters come in all shapes and sizes, each for a different purpose. We can have special purpose meters that measure just one type of unit such as watts or an instrument called a multimeter which is capable of measuring volts, amps and ohms.
  • Electrical measurements can be from thousands of amps to millionths of an amp.
  • Terms used for these measurements 'kilo' and 'milli' are called multiples and sub-multiples.
  • A few examples are listed in the following table:
    Term
    Symbol
    Multiplier
    Example
    mega
    M
    1,000,000
    107MHz = 107,000,000 hertz
    kilo
    k
    1000
    5kV = 5000 volts
    milli
    m
    0.001
    6mA = 0.006 amps
    micro
    u
    0.000001
    2uA = 0.000002 amps

    Conversion Examples:

    Question: A resistance reading is 4.7 Megohms. How many ohms does this represent?
     Answer: Mega is one million so multiply 4.7 by 1,000,000 by moving the decimal point six places to the right 4,700,000 ohms
    Question: There is 2.45 mA flowing in a circuit. How many amps is this?
     Answer: Milli is one thousand so divide 2.45 by 1,000 by moving the decimal point 3 places to the left 0.00245 amps

    Scientific Notation

    Scientific Notation is a way of expressing multipliers (see previous table) in mathematical form. Each multiplier is treated as a power of 10. Following are examples scientific notation using the examples from the previous table:
    Term
    Symbol
    Multiplier
    Example
    Scientific Notation
    mega
    M
    1,000,000
    107MHz = 107,000,000 hertz
    106
    kilo
    k
    1000
    5kV = 5000 volts
    103
    milli
    m
    0.001
    6mA = 0.006 amps
    10-3
    micro
    u
    0.000001
    2uA = 0.000002 amps
    10-6

    Conversion Examples:

    Question: Convert 10kV to volts
    Answer:
    10kV = 10 x 103 V (k = 103)
      = 100 x 102 V (1 decimal place right)
      = 1000 x 101 V (2 decimal places right)
      = 10000 x 100 V (3 decimal places to right)
    10kV = 10,000 V  
    Question: Convert 0.10mA to microamps (uA)
    Answer:
    0.10mA = 0.10 x 10-3 A (m = 10-3)
      = 1.0 x 10-4 A (1 decimal place right)
      = 10 x 10-5 A (2 decimal places right)
      = 100 x 10-6 A (3 decimal places to right)
    0.10mA = 100 uA (u = 10-6)
    Practice Tests are available on the practice test page  
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