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by Procheta Mallik


Now that you've played around with some conductors and insulators, this guide explore the idea of electrical resistance and changing the electrical resistance of a material. (Water in this case.)

    • Scissors have functional sharp edges. Contact may result in injury. Always keep blades away from fingers and body. Handle with care.

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  1. Note the terminals(positive and negative) of the two AA Cells.
    • Note the terminals(positive and negative) of the two AA Cells.

    • Now stretch the cycle-tube band over a cell from one terminal to another. Do the same for the other cell.

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    • Now push the head of a safety pin into the space between the positive battery terminal and the band.

    • Repeat this for the other cell.

    • Push the positive terminal "pin" of one cell into the negative of the other cell. This will result in a 'U' shape with one pin sticking out.

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    • At this point your probe should look like this, as shown in the picture.

    • Cut the ice-cream stick into 2 rectangular pieces of about 4 cm each.

    • Push one of the pieces into the space between the band and their cells, on one side.

    What are these "different materials"? Give a few suggestions/examples....

    Procheta Mallik - Reply

    • Do the same on the other side of the batteries as well.

    • Bend the negative pin of the LED at a right angle about 1 cm from its base.

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    • Now insert the negative pin in-between the open negative terminal of the battery and twist it up around the band.

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    • Take the wires and remove a bit of insulation from both the ends.

    • Then take one end of wire and pass it through the loop on the positive pin. Twist it to make a secure electrical connection.

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    • Tie the other wire to the positive terminal of the LED. Then bend it around the ice-cream stick piece.

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    • The wires are now your test leads.

    • If the LED does not glow, it is an example of a negative result, in this case the material is an insulator.

    • If the LED glows, it is an example of a positive result, in this case the material is a conductor.

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    • The connections between the battery and safety pin might be loose,in that case hold the two terminals with your thumb and index finger for a better connection.

    • Make sure all the connections are clean and stable, otherwise the current will not flow.

    • To ensure proper connection, wind the wire several times over the material.

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    • Make a table and note down if the bulb glows for a particular material or not.

    • Classify the materials depending on whether the bulb glows or not, into conductors and insulators.

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    • Did you notice any common characteristic(s) for the materials that conducted electricity? What about those that didn't?

    • Could you find any difference in the brightness of the LED depending on the conducting material you tested?

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    • Setup up the conductivity tester as shown in the picture, with about 4cm of of space between the leads.

    • Now fill the bottle cap half way with tap water.

      • You can try distiled water as well, if available.

    • Now carefully place the cap in-between the test leads and bend them till both are touching the water's surface but not touching each other.

    • The led will either not light at all or be very dimly inuminated depending on various factors such as water purity, battery voltage and led efficiency.

      • In the case of distilled water the led will not light at all because of the high resistance of pure water.

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    • Now add about a teaspoon of salt (5grams) and observe the led.

    • The led will slowly rise in brightness as the salt dissolves in the water and decrease its resistance. The led will not get as bright as with a good conductor like a metal because the water still maintains a resistance to current flow.

    • You can make the salt dissovle faster by stirring the water with a match stick or straw for about 30 seconds.

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Resistance to the flow of electricity can be a very useful property. The method of dimming a load (a light or fan) by placing a resistance in series with it, is how all early dimmers for electrical appliances worked. Some of the excess energy is lost as heat across the resistor. Even in our case, a tiny amount of heat is being lost through the water, changing its temperature. This change is so small in the case of an LED that it's quite tricky to measure, but old-style dimmers for mains use used to get quite hot indeed. It would be interesting to find out why this happens.

Kailash NR

Member since: 05/02/2017

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