F RONTLINE
Where teachers share ideas and teaching solutions with the wider physics teaching community: contact [email protected]
TEACHING PHYSICS
WITH
FOOD
AND
DRINK
Making sweet, static electricity: (above) charging with sugar and discharging; the equipment (bottom).
You can make
sweet electricity
in your kitchen
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 plastic 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 funnel and the lower part as a container for sugar.
Put about 150 g of sugar into the bottom part of
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P H YS I C S E D U C AT I O N
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 outside 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-
January 2004
F RONTLINE
Where teachers share ideas and teaching solutions with the wider physics teaching community: contact [email protected]
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 positive charge
(~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 experiment 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 relationship between charge (Q), voltage (V) and capacitance (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 r is 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 magnitude of this approximation is correct.
This activity can be used to introduce electrostatics or as an accompanying experiment when
dealing with capacitance in problem solving.
Gorazd Planinšič
VINTAGE PHYSICS
Dieting isn’t the only way to lose weight
Spring
Wooden tray
(high friction)
Wooden
blocks
Look, no friction: this demonstration, which uses
inexpensive equipment, has stood the test of time.
Here is a lovely, simple demonstration of weightlessness. 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-
January 2004
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
P H YS I C S E D U C AT I O N
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