FR O N T L I N E
36 PH YS I C S ED U C AT I O N
Where teachers share ideas and teaching solutions with the wider physics teaching community: contact ped@iop.org
January 2004
Static electricity is everywhere, so it must be in my kitchen. It took me some time, but finally I found it – and plenty of it, as you’ll see.
For this demonstration you will need:
●a plastic plate;
●a metal can;
●aluminium foil;
●a 1 litre plastic bottle;
●some sugar;
●a Pyrex beaker;
●an enamel-coated metal pan with a plastic handle.
Take a clean, dry metal can and put it on a plas- tic plate, which has been placed upside-down on a table. Cut some strips of aluminium foil and hang them round the rim of the can. Cut a 1 litre plastic bottle in half – you will use the upper part as a fun- nel and the lower part as a container for sugar.
Put about 150 g of sugar into the bottom part of
the bottle and pour it through the funnel into the can. The aluminium strips should rise outwards, indicating that the can has become charged (as with a school electroscope). Touch the can and the strips will go back down again. You may hear a spark and feel a small shock, depending on the weather out- side and the humidity in your kitchen.
Empty the can and this time pour the sugar from an enamel-coated pan with a plastic handle directly into the can. The foil strips will rise again, possibly even higher this time.
What happened? The can is insulated from the table by the plate, so charge enters the can with the sugar. When the sugar moves against the funnel walls, it takes charge from the walls or charge is removed from the sugar by the wall. Precisely what happens depends on the combination of materials involved (sugar and plastic or enamel, in this case).
This phenomenon is known as triboelectric charg- Making sweet, static electricity: (above) charging with sugar and discharging; the equipment (bottom).
You can make sweet electricity in your kitchen
TE A C H I N G PH Y S I C S W I T H FO O D A N D DR I N K
FR O N T L I N E
PH YS I C S ED U C AT I O N 37 Where teachers share ideas and teaching solutions with the wider physics teaching community: contact ped@iop.org
January 2004
Here is a lovely, simple demonstration of weight- lessness. Take two wooden blocks attached by a spring and place them on a wooden tray, with the spring stretched but the wooden blocks unable to move because of friction due to the normal contact force with the tray.
Now drop the tray. Suddenly there is no contact force with the blocks, so there’s no friction. No fric-
tion means that the spring pulls the blocks together.
You can then make one of the blocks a light piece of wood, such as a domino. Trap the domino by placing a heavy block on top of it to increase the friction. This time when you drop the tray, the domino will fly out dramatically.
I set this up at school using two 1kg masses joined by an expendable spring. I placed them on rubber mats on a piece of MDF and dropped the lot onto sandbags. Most of the students worked out what was going to happen after a bit of thought. Then I asked what would happen if I threw the whole lot up into the air. Inevitably, most of them thought that the blocks would fly together when they reached the peak of their motion.
This is simple and cheap but is the source of lots of good physics.
●This demonstration was adapted from Hamilton (1936).
Further reading
Hamilton G H 1936 Science Masters’ Book Series II Part 1(John Murray) p21
Ken Zetie
Wooden blocks
Wooden tray (high friction) Spring
Look, no friction: this demonstration, which uses inexpensive equipment, has stood the test of time.
VI N T A G E PH Y S I C S
X
ing and it has been known about for more than 200 years, but it is still not completely understood.
I determined the sign and size of the charge on the can using a coulombmeter. The measurements showed that when the sugar was poured through the plastic funnel, the can obtained positivecharge (~50 nC). However, when the sugar was poured from an enamelled pan, the can obtained negative charge (~–65 nC). I also measured negative charge of about the same amount when I poured the sugar from a Pyrex beaker, but not when I used a kitchen glass. The can was 13 cm high and 8.5 cm wide.
Here’s a homework exercise: think of an experi- ment that will demonstrate that the charges in the two cases above really are of opposite sign.
Students are more familiar with volts than coulombs. There is too little charge on the can to measure voltage directly, but you can estimate it.
This is a good opportunity to remember the rela- tionship between charge (Q), voltage (V) and capac- itance (C) (Q=CV) and to estimate the voltage between the charged can and the ground.
First you need to estimate the capacitance of the can. You can approximate the capacitance of the cylindrical can with the capacitance of a sphere of equal volume (Csphere=4πε0r), where ris the radius of the sphere). In my case Csphere= 7pF, which gives the voltage between the can and the ground as about 9 kV. I felt the spark when I touched the charged can, so I believe that the order of magni- tude of this approximation is correct.
This activity can be used to introduce electro- statics or as an accompanying experiment when dealing with capacitance in problem solving.
Gorazd Planinsˇicˇ
