Electrostatics Chapter 9

9.2 The Electrical Nature of Matter O Rubbing items together does not create electrical charges. The charges were already there! O STATIC ELECTRICITY refers to the buildup of electric charge on the surface of objects. The static charges remain on an object until they either bleed off to ground or are quickly neutralized by a discharge. O The study of static electric charge is called ELECTROSTATICS.

The Law of Electric Charges: O Like charges repel one another, and unlike charges attract one another.” O Charged objects will also attract neutral objects. (comb and paper)

Water and Balloon Video

9.3 Charging By Friction O Rubbing two different materials together, a process known as charging by friction, is the simplest way to give something a charge. O This is what you do every time you drag your feet along a carpet so you can reach out and zap someone's ear. O Your feet in socks and the carpet are doing charging by friction.

O Since the two objects are made of different materials, their atoms will hold onto their electrons with different strengths. O As they pass over each other the electrons with weaker bonds are “ripped” off one material and collect on the other material O See page 275 in book.

Example 1 Rub a piece of ebonite (very hard, black rubber) across a piece of animal fur. Explain what happens… The fur does not hold on to its electrons as strongly as the ebonite. At least some of the electrons will be ripped off of the fur and stay on the ebonite. Now the fur has a slightly positive charge (it lost some electrons) and the ebonite is slightly negative(it gained some electrons).The net charge is still zero between the two... remember the conservation of charge. No charges have been created or destroyed, just moved around!!!!!

Example 2: Rub a glass rod with a piece of silk. Explain what happens… Example 3:

9.5 Charging by Conduction (Contact) O Conduction just means that the two objects will come into actual physical contact with each other (this is why it is sometimes called “charging by contact”). O Charging by Conduction Video Charging by Conduction Video O Swinging Cereal Experiment

PROJECT 1 - Swinging cereal What you need: a hard rubber or plastic comb, or a balloon thread small pieces of dry cereal (O-shapes, or puffed rice of wheat)

What to do: 1.Tie a piece of the cereal to one end of a 12 inch piece of thread. Find a place to attach the other end so that the cereal does not hang close to anything else. 2.Wash the comb to remove any oils and dry it well. 3.Charge the comb by running it through long, dry hair several times, or vigorously rub the comb on a wool sweater. 4.Slowly bring the comb near the cereal. It will swing to touch the comb. Hold it still until the cereal jumps away by itself. 5.Now try to touch the comb to the cereal again. It will move away as the comb approaches.

What Happened?? Combing your hair moved electrons from your hair to the comb. The comb had a negative charge. The neutral cereal was attracted to it. When they touched, electrons slowly moved from the comb to the cereal. Now both objects had the same negative charge, and the cereal was repelled.

9.6 – Insulators O Insulators: Substances which do not allow electrons to move very freely from atom to atom. (They do not carry electricity very well. Wool, glass, rubber, wood, etc.) O Dust furniture or your TV, the electric charges from the cloth remain on the surfaces and attract uncharged dust particles. O These materials keep us safe from electric shocks (plastic/rubber insulating substance on electronics’ wiring.

Conductors Conductors: Substances which allow electrons to move very freely from one atom to another. (They carry electricity very well.) Winter air (dry and acts as an insulator) More Shocks! Summer air (moist and redistributes electric charges on objects) Less shocks! See Table 1 on pg. 280

9.7 - Grounding O When a charged object is connected, or grounded, to Earth, (by a conductor) it shares its charge with the entire Earth. The soil is a good conductor, and Earth is so large that is effectively removes all the excess charge from the object.

Discharging at a Point O A sharp pointed conductor that causes electrons to get pushed off the object at that point because of the repulsion between the electrons. (The tail end of an airplane.)

9.8 – Charging by Induction O Let’s look at a dust particle. When it is near a charged TV screen, the charges on the screen, cause the electrons on the dust particle to change position slightly. This slight shift of the electrons makes the side of the dust particle facing the screen have the opposite charge to that on the screen, and the dust is attracted toward the charged TV screen. O This is called induced charge separation

Video: Charging by Induction Charging Conductors by Induction:

Remote Control Roller What we need: O empty soda can O balloon O your hair