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.
Colour 1st Band 2nd Band 3rd Band (Multiplier) Tolerance
Black 0 0 0  
Brown 1 1 1  
Red 2 2 2  
Orange 3 3 3  
Yellow 4 4 4  
Green 5 5 5  
Blue 6 6 6  
Violet 7 7 7  
Gray 8 8 8  
White 9 9 9  
Gold       5%
Silver       10%
  • 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. You can magnify them by hovering with your mouse cursor.
  • 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 = I R
  • 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 this table shown below
Voltage(V) / V Current(I) / A Resistance(R) / Ω
  • Look at the simple circuit detailed below.
  • 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)