Introduction to Electrostatics!

Demo Videos

1. Two Plastic Rods2. Two Glass Rods3. Plastic and Glass Rods

Summary of Big Ideas From Glass/Plastic Rods Video Demos

1. Charge by friction (rubbing).2. Opposite Charges Attract.3. Like Charges Repel.

Whiteboard: What is happening during the charging process for the glass and plastic rod? Draw a picture of what you think is happening on atomic level? Be descriptive!You may assume the glass rod becomes positively charged and the plastic rod becomes negatively charged.

Charging…

• Positively Charged = More protons than electrons

• Negatively Charged = more electrons than protons

• Neutral = same number of protons and electrons

• Electrons are either being removed or added.

Demonstration: Pith Ball and Plastic/Metal Rods

• Rub plastic rod with fur. What do you observe?

• Rub metal rod with fur. What do you observe?

Conductor vs. Insulator

Conductors allow electrons to flow through object and are easily removed/gained.

Insulators do NOT allow electrons to move easily through object and are NOT easily removed/gained.

Demo Videos

#3: Charged rod and Aluminum Soda Can

#4: Charged Rod and Plastic Water Bottle

Whiteboard

Draw a picture of what is happening inside the can and bottle that cause them to be attracted. Remember there is no CONTACT!

Show how you think it is different for the two objects.

What is happening?

• Polarization• Because aluminum is a good conductor, when the

negatively charge rod is brought near, the electrons move to the opposite end of the can.

• Because plastic is a poor conductor/good insulator, electrons within are still repelled but they move just within the atom and are not free to move within the object.

• Neutral Object will always attract charged objects through polarization.

Video Clip #5: Conductors

This experiment has two parts. In the first part the charged electrode of a Wimshurst generator is brought near a soda can on the left side. A light styrofoam disc covered with aluminum foil hangs on the other side (on the right side) of the can, touching it. Use the model of the internal structure of metals to predict what will happen to the light disc covered with aluminium as the ball of the generator gets charged and then brought close to the left side of the can. Make sure that you predict what happens to the disc before the ball touches the can and then right after it touches.

In the second part, experiment is repeated with a plastic bottle.

Predict the outcome of each experiment before watching the video. To help, draw a picture representing the behavior of charges inside the metal can and the plastic bottle.

Note that the two metal balls of the Wimshurst generator used in this experiment have opposite charges.

Charging

• Friction• Conduction (Direct contact, like between pith

ball and rod)• Induction…??? Indirect method of Charging.

Induction Example: Electrophorous

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--

- --- - -

Rag

Blegh! I gotta get

outta here!

Oh baby! Me

likee!

Styrofoam

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--

-

---

-

+

+

+ +

+

--- - -

+ --

-+

-+

NOTE: Protons do not move! The net positive charge at the bottom is created by an absence of electrons.

Does anyone know what this separation of charge is called?

Polarization!

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--

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--- -

+

+

++

+

--- - -

+

---

+

-

+

$*@%!

--

--

-

+

+

++

+

--- - -

+

+

+

Okay, show’s over.

Quit crowding

you numskulls!

NOTE: Protons do not move! The charge is evened out by the remaining electrons spacing themselves out.

+

+

++

+

+

+

+

Electroscope

• Indicates if an object is charged• Why do the leaves separate? • What is different about putting a negatively or

positively charge object? • What is different about putting an insulator or

conductor ?

Summary of Electroscope

• Positively Charged object will cause positive leaves

• Negatively Charged object will cause negative leaves

• Uses polarization for an insulator• Uses conduction for a conductor… leaves the

charge on the electroscope

Coulomb’s LawF = kq1q2/r2

• k = 8.99 109 N m2 / C2

• q1, q2 are charges (C)• r2 is distance between the charges (m)• F is force (N)

Does this look familiar??

Homework

1) Coulomb’s Law Practice Q’s2) Read Coulomb’s Law Lab – Be prepared for

tomorrow.3) Bring Utexas #12 tomorrow