Drawing resistance
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| You'll need these materials. | |
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| Use a graphite pencil to draw a box. | |
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| Touch one leg of the LED to one end of the box. |
You will need
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A green or yellow ‘light emitting diode’ (LED)
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9V battery
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4 crocodile clips
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2 x 20 cm lengths of insulated wire
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Scissors
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Smooth writing paper
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Graphite pencil (2B or darker)
What to do
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Most electronics stores sell wires with crocodile clips already attached. Get these if you prefer, or use the scissors to carefully strip about 1 cm of insulation from each end of the wire. Attach a crocodile clip to each end. Repeat with the second wire.
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Clip one end of the wire to the ‘+’ terminal of your battery, and the other end to the long leg (the anode) of your LED.
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Take the second wire and attach it to the ‘–’ terminal of your battery. Briefly touch the other end to the short leg (cathode) of the LED to make sure it works. Do not attach the second clip to the LED – it could burn out your LED if linked together for too long.
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On your paper, draw a box 1 cm wide and 5 cm long and colour it in with your graphite pencil, making it as heavy and dark as possible.
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Touch the cathode of your LED to one end of the box. Touch the loose end of the second crocodile clip to the opposite end of the box, and slowly bring them closer together. What happens to the LED’s light?
What’s happening?
Your veins have a rather nifty way of stopping blood from flowing back the wrong way – small flaps called ‘valves’ maintain a direction of blood-flow through your body. Some electronics also need to ensure the current only ever flows in a single direction. To do this, they use a component called a diode that acts a bit like an electrical valve.
The diode in this activity also emits light as electricity flows through it. The faster the electrical current, the brighter the light. However, too much current will cause the conducting material inside the diode to burn up, destroying the LED. What is needed is a simple way of controlling the amount of electricity passing through it. The answer? A resistor, which slows down the current to a level that the circuit can handle.
The box you drew acts as a variable resister; the distance between the circuit’s anode and cathode can vary with the amount of graphite, changing the amount of resistance in the flow of electricity.
The graphite in your pencil is little more than sheets of carbon atoms joined together in such a way that there is a spare electron available, allowing electricity to pass through. The longer the distance the electricity has to travel, the more graphite it needs to pass through, which only slows it down further.
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More distance equals more resistance. |
Applications
Resistance is measured in units called ‘ohms’ (pronounced like ‘homes’ without the ‘h’). The law describing the relationship between ohms, volts (or electrical push) and the amps (current, or speed) is:
Ohms = Volts ÷ Amps
This makes sense if you think about it; if you push with more force without making something move faster, there will be more resistance. Likewise, slowing the current without changing how hard it’s pushed will also increase the resistance.
Different materials will conduct electricity at different speeds. Graphite isn’t as good at conducting electrons as most metals, so is a good resistor. However, resistors can also be made out of long lengths of wire. By making the electricity travel further, it is forced to slow down.
Resistors have a vital role in any circuit by controlling the precise amount of power that passes through its components, stopping them from overheating and breaking.
