Matter is made of charged particles called protons and electrons. Protons are positively charged, and electrons are negatively charged. The interaction between charged particles causes objects to become charged as well. Two of the same charges repel; opposite charges attract. Charged objects behave in the same way. Electric charges may move, or they may stay put. Electric charges at rest have static electricity. Friction and induction cause the charges to move between objects. Static electricity builds up until the objects lose their charges. A shock, flash of light or crackling sound signals when a sudden electric discharge occurs.
An electroscope detects static electricity charges. Students can construct a simple electroscope and use it to test whether objects are charged. Students will need a glass soda bottle, modeling clay, stiff wire, aluminum foil, a foil wrapper, a piece of silk or wool and objects to test, such as a plastic pen, comb, balloon or other small, nonmetal objects around the classroom. To build the electroscope, make a hook at one end of the wire and tape a foil gum wrapper to it. Lower the hooked end of the wire into the bottle and secure at the opening with clay. Make a ball of aluminum foil and stick to the top of the wire. Charge an object by rubbing it with silk or wool then hold it near the ball of foil on the wire. Students can experiment with charged and uncharged objects and perform the tests on dry and humid days.
Objects can become charged by friction. When objects rub together, the electrons from one object move to the other object, and the objects acquire opposite charges. Objects with opposite charges attract each other. Students can investigate this further by rubbing two balloons with wool cloth and observing what happens when they bring the balloons close together. Students can experiment with a charged balloon and other objects, such as their hair, bits of paper, pieces of string, salt or an empty soda can. The experiment will help students understand that an object's electric field exerts force on the electric field of other objects. Alternately, students can use a comb rubbed with wool instead of a balloon.
Electrons can move between objects even when the objects do not touch. Students can investigate how an uncharged object becomes charged when it approaches a charged object with a Styrofoam plate, disposable metal pie pan, Styrofoam cup and a piece of wool cloth. Place the cup upside down on the inside of the pie pan and secure with tape or a hot glue gun to make an insulating handle. Rub the wool on the bottom of the Styrofoam plate to induce a negative charge then turn it upside down. When a student holds the metal pan over the Styrofoam plate, the plate's negative charge repels the electrons on the bottom of the pan. The electrons move away from the plate to the rim of the pan, and the bottom surface of the pan becomes positively charged. When a student touches the rim, electrons move from the rim to the student's finger and cause a shock. Pull the aluminum plate away from the foam plate and the overall charge becomes neutral again.