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  • Ohm's Law, understanding voltage and current

       Wiring Home > Electrical > Ohm's Law, understanding voltage and current
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    It is extremely important to understand the methods used to control the amount of current in a circuit, and how voltage and resistance effect a circuit. Ohm's Law , is used to show the relationship of current, voltage and resistance in a circuit.


    When electrical pressure ( voltage ) from a power source is applied, it is possible to force electrons out of their circular paths and cause them to pass from atom to atom along the length of the wire. The greater the number of electrons passing a given point in a circuit the greater the amount of current in a circuit.

    The intensity of electrical current is measured in amperes, or amps for short. An (A) is used to represent amperes, and an (I) is used to represent the amount of current in a circuit.


    Voltage is the amount of electrical pressure from a source that is used to generate the current.

    The letter (E) is used to represent the voltage. The letter (V) for volt is the unit used to express the quantity of electrical pressure.


    The property of a material, which causes it to oppose the movement of electrical energy, is called resistance. All materials have some resistance. Materials, which offer little resistance to electrical movement, are called conductors. Those, which offer high resistance, are called nonconductors or insulators.

    Resistance is measured in ohms. The symbol for ohms is the Greek letter omega. The letter (R) is used for representing resistance in formulas.


    The Ohm's law formula is used to show the relationship of current, voltage and resistance. Ohm's law states that in any electrical circuit the current is directly proportional to the voltage applied to the circuit and is inversely proportional to the resistance in a circuit.

    According to Ohm's law, when the resistance of a circuit is constant, changing the voltage can change the current: current will increase when the voltage is increased, and current will decrease when the voltage is decreased. Similarly, when the voltage is constant, current will increase when the resistance is decreased, and current will decrease when resistance is increased.

    I = intensity of current in amperes
    E= Quantity of electrical pressure in volts
    R= The amount of resistance in ohms
    P= The amount of Power or Watts

    Example of using the Ohm's law formula.

    In this example we are going to find the voltage. Your current is 10 amps and your resistance is 5 ohms.

    You would use the following formula:

    E = I x R
    10 amps x 5 ohms = 50volts

    Great Resource - Ohm's Law, Electrical Math and Voltage Drop Calculations

    Ohm's Law Formulas

    ohm's law formulas

    Ohm's Law Calculator
    Enter two known values and press Solve to calculate unknowns.





    Related Resources:
  • Ohm's Law, Electrical Math and Voltage Drop Calculations
  • National Electrical Code Handbook
  • National Electrical Codebook
  • Stallcup's Illustrated Code Changes
  • The Complete Guide to Home Wiring (Black and Decker Complete Guides Series)
  • Wiring Simplified
  • Guide to Using the National Electrical Code
  • Residential Remodeling and Repair Professional Reference

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