ELECTROSTATICS(Ch. 20)

What is electrostatics?

The study of STATIC ELECTRICITY.

Static electricity is electrical charges that can be collected and/or held in one place.

This is different from current electricity which is what powers your refrigerator, lights, batteries, etc.

What is charge?

Charge is divided into two categories: positive and negative charges.

Something is said to be charged when it has an unequal amount of positive/negative charges. If there are more positive charges, it is

positively charged. If there are more negative charges, it is

negatively charged. If there are the same amount, it is said to be

neutral.

Opposites attract!

Positive charges like negative charges and will attract each other.

Similar charges (negative-negative and positive-positive) repel each other.

Where does it come from?

Electrons! Electrons! Electrons!

Only electrons move around from one object to another to charge objects. (Why is that?) Just like heat, electrons move from high concentration to low concentration.

Quick!

What happens if a positively charged object touches a negatively charged object?

Electrons flow from negatively charged object positive charged object until they are both neutral.

Quick!

How many electrons does something have if it is neutral?

The same amount as the number of protons!

GROUND.

In electricity the “ground” is very important. This is generally a huge body that will be electrically neutral.

The Earth is a great example of “ground.” The human body can also serve as a ground for many cases.

Example: By touching the charged object, you ground it.

Quick!

What happens if a negatively charged object is grounded?

What happens if a positively charged object is grounded?

How do I get some of that?!

THREE WAYS TO OBTAIN CHARGE: Conduction (two object directly touch) Induction (bringing a charged object near a

neutral object will induce a charge in the neutral object).

Friction (rubbing electrons onto something)

Measure!

The SI unit for charge is the Coulomb. It is abbreviated with a C.

Examples: 1.0 C 3.5 μC 6.3 nC

The charge of 1 electron (called the elementary

charge) is: 1.602 x 10-19 C

Quick!

If an object has 6.022 x 1023 more electrons than protons, what is its net charge?• 6.022x1023 electrons X 1.602x10-19 C/electron• = 96,472 C. [that’s A LOT!!]

How many electrons are there in 1 μC ?

• 1x10-6 C / 1.602x10-19 C/electron• = 6.24x1012 electrons. [that’s A LOT!!]

Conductors vs. Insulators

Conductors allow electrons to move about freely. Metal. Graphite. Water.

Insulators do not allow electrons to move about freely. Plastic. Rubber. Glass. Cloth. Wood.

May the force be with you.

Like charges repel. Opposites attract. The attraction and repulsion is a FORCE.

Electric force is a non-contact force. (like gravitation!)

Electric force in an inverse square law. (like gravitation!)

Coulomb’s Law

Coulomb’s law: The force between two charges is proportional

to the magnitude of the two charges and inversely proportional to the square of the distance between the two.

F = k q1q2 /r2

k is a constant = 9.0x109 N•m2/C2

Your turn to Practice

Please do Ch 20 Rev p 477 #s 1, 4, 7, 9, 14, 20, 21, 24-27.