What it shows
A foil plate with an insulating handle can be charged over and over from one rubbed plastic sheet without losing the sheet’s charge. It shows charging by induction and earthing, and the plate can make a small spark in a dark room.
The science
Rubbing the plastic with wool gives it a negative charge. When the foil plate is placed on it, the plate is neutral, but electrons in the plate are pushed to the top by the negative charge below, which leaves the lower side slightly positive. Touching the top with a finger lets the electrons flow to the earth, so the plate is left with a net positive charge. When it is lifted by the insulating handle it keeps this positive charge, and the plastic sheet keeps its own charge, since only a small part of it touched the plate. This is induction and earthing.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A polythene or thermocol sheet, about 30 cm by 30 cm
- A piece of wool or dry hair-free woollen cloth
- A foil pie plate or a round plate of card covered with aluminium foil
- A plastic cup or bottle glued upside down at the middle of the plate as an insulating handle
- A small paper strip or a foil ball on a thread to detect charge
- A dry day
Safety
For this project: The sparks are tiny and harmless but can surprise you. Use only on a dry day, away from electronics and away from people with pacemakers. An adult must supervise.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fix the plastic cup upside down at the centre of the foil plate with tape, to serve as an insulating handle.
Place the plastic sheet on a dry table and rub it hard with the wool for a minute in one direction.
Hold the plate only by the handle and put it flat on the charged sheet.
Touch the plate with a finger for a moment, then take the finger away. Do this in this order.
Lift the plate by the handle straight up. Bring the foil ball or paper strip near it and see that it is attracted or jumps.
Touch the plate with a finger and see that it discharges. Place the plate back on the sheet and repeat several times without rubbing again.
Dim the lights and try to see a tiny spark when your knuckle approaches the charged plate.
Draw diagrams of the charge movement in four stages: placed on, finger touches, finger removed, plate lifted.
What you should see
The plate can be charged more than ten times from one rubbing, and each time it attracts the paper strip and may give a small crackle when touched.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why do we touch the plate with a finger?
To let electrons flow to the earth and leave the plate with a net charge.
2What is the charge on the plate when lifted, if the sheet is negative?
Positive.
3Why does the sheet not lose its charge?
The plate touches only a few points, so little charge is exchanged.
4Why is an insulating handle needed?
Otherwise the charge would flow to your body and be lost.
Take it further
Charge the plate and use it to charge a jar electroscope or a hanging foil ball by induction, and compare the deflection.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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