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– 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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