You generate static electricity every time you walk accross a carpet, pull tape from a roll, remove your clothing or dry clothes in a drier. Much of the time you don't even realize it unless the air is dry and the static charge suddently crackles, pops and flashes its way to a new home. These static charges are caused by mechanical friction. Back in 600 BC, thales of Greece experimented with the static electricity produced when amber is rubbed with wool.
Once upon a time sap flowing from trees hardened into clear golden nodules which were eventually buried in the earth. Sometimes, before it hardened into amber, the sticky sap entombed bits of plant metter, insects and even droplets of water. A kind of natural casting plastic, amber is easily electrified by friction. It then attratc bits of paper.
Electrified Plastic and Glass
Run a plastic comb through your hair on a dry day and you'll transfer electrons from your hair to the comb. Rub a glass rod with a silk or the synthetic fibers of a paint brush and you'll remove electrons from the class. Both the negatively charged comb and the positively charged glass rod will, like amber, attract bit of paper. You can electrify or charge many materials by rubbing them with fur, wool, etc. Metal? No, the charge leaks away.
Opposite and Like Charges
How do we know the cob and glass rod have opposite charges? A fundamental rule of electricity is like charge repel and unlike charges attract. Here's an experiment that proves the rule and answers the question.
- Unlike charges attract
- Like charges repel
The first instrument design to detect and measure static electricity was the electroscope. You can easily make one.
Be sure the folded foil strip is clean and dry. When you touch a charged object to the wire, the two halves of the foil strip will be given the same charge and will therefore fly appart.
Conductor and Insulator
You can use your electroscope to prove that electrons travel through some materials but not others. Hint: try this on a dry day! Electrons can travel through moist air so the charge on your electroscope will quickly leak away on humid day.
This demonstration shows that electrons can travel through some materials but not others. Materials through which electrons travel are conductors. Material through which electrons travel poorly or not at all are insulator.