Electric Charge Chapter 17-1 page 628

17-1 Objectives Understand basic properties of electric charge Differentiate between conductors and insulators

Properties of electric charge What happens when you run a comb through your hair on a dry day? What about when you rub your hair with a balloon? What if we were to take that charged balloon and hold it against the wall?

When materials behave this way … We call it electric charge Why does it work best on dry days? Because excessive moisture can provide a pathway for charge to leak off a charged object

There are two kinds of electric charge Like charges repel Unlike charges attract Opposites attract Benjamin Franklin (1706-1790) named the two different kinds of charge positive and negative

So, When you run a balloon against your hair, the charge on your hair is positive and the charge on the balloon is negative And, opposites attract When there are equal amounts of positive and negative charge, then there is no net charge

What about electrostatic painting? Electrostatic painting uses the principles of attraction Paint is given a negative charge The object to be painted is positive So, opposites attract and more paint hits the object

How about cosmetics? Some cosmetic products contain chitin Organic; found in crab, lobster, and butterflies Chitin is positively charged Human skin is naturally slightly negatively charged So, it helps makeup stick

Electric charge can be transferred How does all this happen? Think back to biology and chemistry …

Subatomic Particles Protons Neutrons Electrons Positively charged; located in the nucleus Neutrons Neutrally charged; located in the nucleus Electrons Negatively charged; located outside nucleus & move around it Easily transferred When # protons = # electrons, then there is no net charge When electrons are transferred, the atom gaining the electron becomes negatively charged and the atom loosing the electron becomes positively charged Charged atoms are called ions

Transferring charge Charge has a natural tendency to be transferred between unlike materials Like a balloon and hair Human hair and wool (like when you where a hat)

Electric charge is conserved Charge is not created nor destroyed – the charge is just transferred from one object to another

Electric charge is quantized 1909 – Robert Millikan Experiment at University of Chicago Oil droplets between metal plates Discovered that the charge of a charged object is always some multiple of a unit of charge This unit: e Electrons have a – e; protons have a + e e = 1.602 x 10-19 C C = coulomb (unit for electric charge)

Transfer of electric charge We can classify substances in terms of their ability to transfer electric charge Conductors: Materials in which electric charge moves freely Copper, aluminum Insulators: Materials in which electric charge does not move freely Glass, rubber, silk, and plastic

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