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The LED Torch - Resistors and Ohm's Law
1 Colour Codes
- 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.
- 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:
- Brown - 1 (First number is 1)
- Black - 0 (Second number is 0)
- Yellow -4 (4 zeros)
- 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
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
Vis for Voltage,
Ifor current and
Ris for resistance.
- We can rearange the equation to make
Ithe 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
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)