Electrically charged objects attract or repel each other
Everybody has experienced the shocks of static electricity. This common and startling experience provides a shared experiential base for thinking about and exploring this topic. Most texts attempt to build on and expand these experiences with activities involving rubbing balloons and noting the results. These activities can be an excellent way to explore the topic. However, be aware that static electric effects are most pronounced under conditions of low humidity. It may be difficult to reproduce these effects under conditions of high humidity. The water molecules in the air absorb the electric charges, and prevent the charges from accumulating on the balloons or other rubbed objects.
In the CONCEPT OVERVIEW, we emphasized that electricity and magnetism are fundamental aspects of our universe of matter and energy. We experience specialized, local effects of these general properties of the universe. Static electricity is a case in point. We feel shocking sparks that jump between us and other objects. These sparks can be connected with a deep understanding of matter in our world. All the matter in our world has electrical properties. We usual don’t experience the electrical nature of matter, but it is there all the time.
All atoms have parts that are positively charged electrically and parts that are negatively charged electrically. The negatively charged parts are the electrons that exist around the central positively charged nucleus of the atom. It is these negatively charged electrons that move from one object to another when we rub them. The object that loses electrons because of the rubbing becomes positively charged whereas the other object that gains the electrons becomes negatively charged.
Grade 4 students are not expected to know the terms atoms and electrons. However, they can begin to describe static electricity in terms of negative electrical charges moving from one object to another. They can also make the analogy that both magnetic and electric effects involve like charges/poles repelling each other and unlike charges/poles attracting each other.
Some texts go beyond the basics of static electricity by describing induced charges. This topic is not required for this standard, and it is well beyond grade 4 appropriateness. The basics of static electricity explain that two rubbed objects repel or attract each other because they each either have a net positive or negative electric charge.
Induced electric charge deals with the common phenomenon that a charged object will often attract other objects that have not been rubbed (especially on a dry day). For example, a rubbed balloon will stick to a wall or will pick little up pieces of paper. Neither the wall nor the little pieces of paper have been rubbed so they should be electrically neutral, and should not be attracted to the balloon.
Assume that the balloon has gained electrons because of rubbing and therefore exhibits a net negative charge. As it approaches the wall (or pieces of paper), it repels the electrons on the surface of the wall (or paper) and pushes them deeper into the interior. As a result, the surface of the wall has an induced, temporary net positive charge. Since this is opposite to the net negative charge on the balloon, they are attracted to each other and the balloon sticks to the wall (or picks up the pieces of paper).