Experiment of the Week
- #139 Static and Humidity
This week's experiment is an important part of my work. When
demonstrating static electricity in my science shows, I always keep the
hair drier nearby and I pay close attention to the weather forecast. To
see how the weather and a hair drier fit in with static electricity,
you will need:
* a hair drier
* several balloons
* small pieces of paper
* your hair (or a piece of cloth if you are "hair
challenged" like me)
* a wet cloth or paper towel
First, blow up a balloon and tie it off. Small, cheap balloon work the
best, but any sort should do the job. Then, tear some tiny bits of
paper and place them on a flat surface. The pieces should be smaller
than your fingernail. Rub the balloon briskly on your hair or a piece
of cloth and then bring it near the pieces of paper. If you generated
enough static electricity, then some of the pieces of paper should jump
up to the balloon.
If the paper did not jump to the balloon, then turn on the hair drier
and use it to dry your hair and the balloon. Be careful not to get the
balloon hot enough for it to pop. Once the balloon is dry, try it
again. This time, the paper should jump very well for you.
Next, take the wet cloth and rub it gently over the surface of the
entire surface of balloon. You want the balloon to be damp. Then rub
the wet cloth lightly over your hair, to make it damp as well. Try
rubbing the balloon on your hair again and bring it near the bits of
paper. This time, you will get very little reaction, if any at all.
Once again, dry the balloon and your hair with the hair drier and the
paper will once again jump up to the balloon.
Why would water cause this? When you rub the balloon against your hair,
you are transferring electrons (tiny, negatively charged pieces of
atoms) from your hair to the balloon. Because electricity does not flow
easily over rubber, the electrons are trapped there, building up a
strong, negative static charge. It is this charge that attracts the
bits of paper.
Rubbing the damp balloon against your wet hair still moved electrons
from your hair to the balloon, but the water formed a conducting
pathway. Instead of remaining trapped on the balloon, the electrons
flowed across its surface to your skin and then to the ground. You
never built up enough of a static charge to attract the paper bits.
When you used the hair drier to dry the balloon and your hair, you
removed this pathway, and once again the static charge could build up.
As the weather gets colder, the air is usually drier. That is why you
get a lot more static shocks in the winter than in the summer. That is
also why many science teachers save their unit on electricity until the
coldest month. It makes the experiments much easier to do successfully.