What it shows
Rubbing a balloon with different materials gives different amounts of static charge, and a charged balloon picks up small paper bits. Counting the bits lifted ranks the materials from weak to strong charging.
The science
All materials have positive and negative charge. When two different materials are rubbed together, electrons move from one to the other, so one becomes negatively and the other positively charged. Which way and how many depends on the materials, so they can be listed in a series. A charged object attracts neutral paper bits by polarising them: the nearer side of each bit gets the opposite charge, which is pulled more strongly than the far side is pushed. On damp days water films let charge leak away.
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.
- 3 balloons of the same size
- Pieces of wool, cotton cloth, silk or nylon, polythene bag and paper
- About 50 small paper bits, each 3 mm square
- A ruler
- A tray
- A notebook
Safety
For this project: Balloon pieces are a choking risk for young children, so keep burst balloons away from them. Do not do this near a computer or phone, and avoid very humid days.
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.
Put 50 paper bits on the tray in a flat heap.
Rub a balloon on your hair or wool 20 times in the same direction and with the same speed.
Hold the balloon 2 cm above the bits and count how many are lifted, then discharge it by touching it with a wet finger.
Repeat for the cotton, silk or nylon, polythene and paper in turn, always rubbing 20 times.
Write the number of bits lifted by each material in a table and repeat each three times for the average.
Draw a bar chart ordering the materials from strongest to weakest charging.
Hold the balloon near a thin stream of tap water or near your arm hair and see the bending and the lifting.
Write a sentence saying which materials gave the strongest charge, and what the weather was like that day.
What you should see
Wool and synthetic cloth usually pick up the most paper bits (20 or more), cotton and paper the least, and the ranking is the same on repeated runs.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What moves when objects are charged by rubbing?
Electrons, which move from one material to the other.
2Why does a charged balloon pick up neutral paper?
It polarises the paper, and the nearer opposite charge is pulled more strongly.
3Why does it work poorly on humid days?
Moisture on surfaces lets the charge leak away.
4What are the two kinds of charge?
Positive and negative; like charges repel and unlike attract.
Take it further
Test the same balloon rubbed on a plastic ruler and on steel and compare their sign by bringing them near a hanging charged foil ball.
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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