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The LED Torch - Resistors and Ohm's Law


1 Colour Codes


Learn It

  • Resistance is measured in Ω
  • You know that the narrower the wire through which electricity passes, the more the resistance it has.
  • The most common type of resistors called Carbon Film resistors are made up of a carbon tube into which a grooves have been cut.
  • The grooves mean there is less carbon for the current to pass through, so the resistance can be increased by having a deeper groove.


Learn It

  • Resistors are colour coded so we can tell what there resistance is.
  • Most resistors have 4 bands painted on the outside.


  • Starting at the opposite end to the Gold or Silver band (In this image that would be the right hand side):
    • The first band gives us the first number
    • The second band give us the second number
    • The third band gives us the number of zeros.
    • The fourth bands gives us the tolerance.

  • So for example, the resistor pictured above has the colours:
    1. Brown - 1 (First number is 1)
    2. Black - 0 (Second number is 0)
    3. Yellow -4 (4 zeros)
    4. Gold - +/-5%
  • This means it is a 100000Ω resistor, or what we would normally call a 100kΩ resistor.
  • The 5% tolerance means that the actual resistance can vary by + or = 5%, which in this case would be 0.05 X 100000 = 5000Ω

2 Assessment


Badge It - Silver


Progress ladder Red, Identify several electronic components, with assistance.

  • Look at the resistors in this photograph. 

  • For each resistor, use the colour table to identify it's resistance.

3 Ohm's Law

  • You've seen how increasing the resistance in a wire can effect the current passing through the wire.
  • It is therefore possible to reduce the amount of current flowing in a circuit, by adding in resistors.
  • To do this we use Ohm's Law
  • V is for Voltage, I for current and R is for resistance.
  • We can rearange the equation to make I the subject, which will allow us to calculate the current in a circuit.
  • Look at the animation below to see how.


  • So there are three different ways of writing Ohm's Law


4 Assessment


Badge It - Gold


Progress ladder Orange, Identify several electronic components, without assistance.

  • Copy you own version of this table shown below
  • Look at the simple circuit detailed below, click on the 'get values' button for 2 of the 3 values needed to calculate the third.


  • Hit the button to generate a new question. Fill in the provided values into your copy of the table and then caluclate the third value for yourself.
  • Sometimes the current is described in mA. This is short for milliamps, which are a thousandth of an amp. (Just divide it by 1000 to calculate the current in amps)