Static electricity

Objectives Describe the historical development of the concepts of electrostatics. Identify examples of electric forces in everyday life. Identify the two types of electric charge and how they interact.

Assessment Name one of the founding fathers of the United States who also contributed to our understanding of static electricity. Describe two examples of electric forces in everyday life.

Assessment Which statement about electric forces is true? A negative charge attracts a negative charge. A positive charge repels a negative charge. A negative charge repels a negative charge. A positive charge attracts a positive charge.

Physics terms static electricity electrostatics electric charge electrically neutral electric force electrostatic induction electroscope

Static electricity Benjamin Franklin was America’s first famous scientist. He and many others were studying the properties of static electricity in the 1700’s. Static electricity results from a tiny excess of positive or negative electric charge on objects. The study of electric charges is called electrostatics. Franklin’s portrait appears on the $100 bill.

Brainstorm Where have you seen the effects of static electricity? Think of a couple examples and share them with the student next to you.

An example Rub a balloon against a wool sweater. This transfers some negative charges from the sweater to the balloon. The sweater now has a net positive charge and the balloon has a net negative charge. They will attract and stick together.

What makes the zap? Shuffling your feet on the carpet can also transfer charges. Touching a metal conductor, like a doorknob, causes the excess charge you’ve collected to be attracted to its opposite in the metal. A tiny electric current flows … and that’s what makes the zap!

Other everyday examples Clothes in a dryer without a dryer sheet make sparks. A balloon rubbed on your hair sticks to the wall. Open a package and the packing materials stick to your hands. Pull a wool sweater over your head and your hair stands on end.

Investigation Does matter contain one kind of electric charge, or two different kinds? Turn to Investigation 18C on page 526.

Investigation Part 1: Charging the transparent tape Take 2 pieces of tape about 3-4 inches long. Fold over a small part on each to make a handle. Put both pieces, sticky-side down, on the table (but not touching each other). Grab the handles and lift both tape strips up quickly.

Investigation Questions for Part 1 What happens when you bring the two tape strips close to each other (but not touching)? What can you infer about the similarities or differences in the electric charges on the two strips? What happens when you bring one strip near your hand? Why?

Investigation Part 2: Charging the tape another way Put one piece of tape on the table, sticky-side down. Place the other piece of top of it. Use the lower handle to pull both strips off the table. Rub the tape with your fingers to remove any charge. Pull the tape strips apart by holding the handles.

Investigation Questions for Part 2 What happens when you bring these two strips near to each other? What happens with each strip when you bring it close to your hand? What difference(s) between this and the previous experiment might have caused different behavior?

Electric charge There are two kinds of electric charge: positive (+) and negative (–).

Electric charge There are two kinds of electric charge: positive (+) and negative (–). In electrically neutral objects, positive (+) charges are balanced by an equal number of negative (–) charges.

Electric charge There are two kinds of electric charge: positive (+) and negative (–). In electrically neutral objects, positive (+) charges are balanced by an equal number of negative (–) charges. Most objects are electrically neutral … otherwise there would be lightning everywhere all of the time!

Opposites attract As with magnetic poles, unlike charges attract:

Opposites attract As with magnetic poles, unlike charges attract: Like electric charges repel:

Opposites attract As with magnetic poles, unlike charges attract: Like electric charges repel: Neutral particles feel no electric force at all from positive or negative charges. in neutral objects, induction can cause charge separation that does result in an attractive force.

Electric force Electric forces of attraction and repulsion can be enormously strong. These are the forces hold atoms and molecules together.

Electric force Electric forces of attraction and repulsion can be enormously strong. These are the forces hold atoms and molecules together. Electric forces are so strong that, outside of atoms, positive and negative charges are rarely separated for long!

The Van de Graaff generator A Van de Graaff generator is a device that can separate electric charge. Its sphere can reach thousands or even millions of volts, but it can only create a big spark – not a large electric current – if you touch it.

Charge distribution If an object has a net charge, how are the excess charges distributed?

Charge distribution Like charges repel, so charges will push each other apart as far as possible. If the charges are on a Van de Graaff generator, repulsive forces will push them to the outside edge of the sphere.

Charge distribution Touching the charged Van de Graaff sphere will transfer some of the excess charge to you. If the charges are on your head, the repulsive forces will push them out onto your hair, and push your hair apart!

Electrostatic induction It is also possible to charge an object without touching it.

Electrostatic induction It is also possible to charge an object without touching it. Here, a negatively charged rod repels negative charges and attracts positive charges. The sphere is now “charged”.

Electrostatic induction It is also possible to charge an object without touching it. Here, a negatively charged rod repels negative charges and attracts positive charges. The sphere is now “charged”. This process of separating charge is called electrostatic induction.

Demo: electrostatic induction Charge a plastic rod or ruler by rubbing it with wool. Bring the rod close to an uncharged metal object such as a sphere or can. Is the object attracted to the rod? Try it with other combinations of rods and materials.

Demo: the electroscope An electroscope uses induction to detect electric charge.

Summary: charging an object There are three ways that objects typically become charged. conduction: A charged object contacts an uncharged object and transfers charges to it.

Summary: charging an object There are three ways that objects typically become charged. conduction: A charged object contacts an uncharged object and transfers charges to it. friction: Different materials have different affinities for charge. Rubbing these materials together causes charges to transfer between the materials.

Summary: charging an object There are three ways that objects typically become charged. conduction: A charged object contacts an uncharged object and transfers charges to it. friction: Different materials have different affinities for charge. Rubbing these materials together causes charges to transfer between the materials. electrostatic induction: A charged object is brought close to a neutral object, resulting in charge separation in the neutral object.

Assessment Name one of the founding fathers of the United States who also contributed to our understanding of static electricity.

Assessment Name one of the founding fathers of the United States who also contributed to our understanding of static electricity. Benjamin Franklin studied the properties of static electricity.

Assessment Describe two examples of electric forces in everyday life.

Assessment Describe two examples of electric forces in everyday life. Answers could include lightning striking a tree, clothes in a dryer making sparks, a balloon rubbed on your hair sticking to the wall, packing materials sticking to your hands, etc.

Assessment Which statement about electric forces is true? A negative charge attracts a negative charge. A positive charge repels a negative charge. A negative charge repels a negative charge. A positive charge attracts a positive charge.

Assessment Which statement about electric forces is true? A negative charge attracts a negative charge. A positive charge repels a negative charge. A negative charge repels a negative charge. A positive charge attracts a positive charge.