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Make an electroscope

By David, 28 October 2021 Activity

Foil strips dangling inside a cup.

Without an electrical charge the foil strips hang freely.

By Patrick Mahony

Have you ever been surprised by a static shock? This clever device can measure electrical charge!

You will need

  • Clear plastic cup or wineglass
  • Paperclip
  • Aluminium foil
  • Paper
  • Scissors
  • Balloon
  • Something woolly to rub the balloon on. Animal fibres work best – wool, angora, cashmere, or you can use the hair on your head!

What to do

  1. Cutting a piece of paper with scissors.Cut a piece of paper big enough to sit over the mouth of the cup.
  2. Paper clip which has been partially straightened.Straighten the paperclip. Bend one end into a right angle, and the other end into a hook.
  3. Cutting a strip of foil with scissors.Cut two strips of foil about 1 cm wide and 3 cm long.
  4. Hook the paperclip through the ends of the foil strips so they hang freely.Lay one foil strip on top of the other and hook them both onto the end of the paperclip.
  5. Hook the other end of the paper clip through the paper.Pierce the paper with the other end of the paperclip and place the paper over the mouth of the cup.
  6. Paper square over cup opening with foil strips hanging in the centre of the cup.The two strips of foil should hang down inside the cup from the other end of the paperclip. If they touch the bottom of the cup, bend the paperclip so they are hanging freely.
  7. Blowing up a balloon.Blow up the balloon and tie the end.
  8. Rubbing the balloon on a woollen blanket.Charge the balloon by rubbing it quickly against the woolly thing you have ready.
  9. Balloon next to cup with foil moving towards the balloon.Bring the charged balloon near the side of the cup. What happens?

 

What’s happening?

When you rub the balloon against something woolly, some of the electrons from the atoms in the fabric are transferred onto the balloon. These extra electrons give the balloon an overall negative charge. The rubber of the balloon is an insulator. This means the electrons can’t move very easily through the rubber, so the charge stays put.

The negative charge means there is an electric field around the charged surface of the balloon. This field can affect other charged particles.

When the charged balloon is moved close to the jar, the electric field passes through the glass. The aluminium foil strips are conductors. Unlike insulators, conductors are materials that electrons can move through readily. The electrons in the metal foil are repelled by the electric field on the balloon and move away. This creates an imbalance of charge within the metal strips.

If the two strips end up with different charges – one positive and one negative – they will attract each other. If they have the same charge – both positive or both negative – they will push each other away. The movement of the strips shows that a net electric charge (more negative, or more positive) is present. An instrument used to detect and measure electric charge is called an electroscope.

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