The Solution to Solutions

Entry Task

• Your teacher gives you a mystery substance to test. You observe that the substance is made of hard brittle crystals, has a high melting point, and is soluble in water. What do you expect to find when you test the substance’s conductivity in solution? Explain your reasons.

Previously we…• Grouped things into catagories based on macroscopic

observations• Hard, crystalline solids with high melting points do not conduct

electricity as solids, but do conduct electricity when dissolved in water

• Soft, oily materials with low melting points do not conduct electricity and are not soluble in water.

• What’s happening on a microscopic level to explain what we see at the macroscopic level?

The Solution to Solutions• Create a new title/thread “The Solution to Solutions” 11/7/13• Learning Target: I can use my prior knowledge of electrical

charge to make predictions about how charged objects will interact with other objects.

• Update your Table of Contents!

The Solution to Solutions

• Read the intro on p. 111-113• Take notes on anything important!

VOCABULARY• Electroscope:• An electroscope is instrument that is used to

detect electric charge on a body. It was the first electrical measuring instrument. The first electroscope was invented by British physician William Gilbert around 1600.

Concept

• Charging by friction: separating positive and negative charge by rubbing two different materials together

• Since the two objects are made of different materials, their atoms will hold onto their electrons with different strengths.

• As they pass over each other the electrons with weaker bonds are “ripped” off of that material and collect on the other material.

Triboelectric Series(Grouping material according to their Tendency to Give up Their Electrons

Human Hands Most Positive Asbestos Rabbit Fur Glass Mica Human Hair Nylon Wool FurLead Silk Aluminum Paper Cotton Zero Steel Wood

Amber Sealing Wax Hard Rubber Nickel, Copper Brass, Silver Gold, Platinum Sulfur Acetate, Rayon Polyester Styrene (Styrofoam) Saran WrapPolypropylene (Thermal clothing) Vinyl (PVC) Silicon Teflon Most Negative

Demonstration & Investigation• Styrofoam Cup electroscope• Sketch and label the electroscope. • Explain each observation…

What I see What it meansWhen the charged plastic knife is brought close to the pin, the pin _______________When the charged plastic knife touches a pin, then is brought close to the same pin, the pin __________________When the charged test tube is brought close to the same pin that the knife touched, the pin ___________________

Busted

Electroscope Demo

•Make a sketch of what is happening at the microscopic level to cause what you observed.• S&T #2-5 p. 115

Busted

Tasks• Complete Part II: Paper Charges p. 115-116.• Follow directions in your book!• S&T #1-2 p. 116• All observations/answers should have a have a heading a page

number!• Complete Part III: Bending the Rules p. 117-118• Follow directions in your book!• S&T #1-3 p. 118-119

The demo didn’t work, now what?• The main idea of the demo was to

demonstrate different charges.• There are two types of charges: positive

(+) and negative (-).• Opposite charges attract:• (+) and (-)• (-) and (+)

+ -

+-

I love you! I love you too!

The demo didn’t work, now what?• Same charges repel (want to move away

from each other)• (+) and (+)• (-) and (-)

++

--

Get away from me! Gladly!

The demo didn’t work, now what?• A charged object can attract a neutral

object because the charged object can induce a temporary charge on the neutral object.• How is this similar towhat you saw withthe water and paper?

Metal Sphere

++++++++++++++

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The demo didn’t work, now what?• A charged object must be close to another

object to exert a force on it.• Did you see that with the water and

paper?• Electroscope demo

Part II: Paper Charges

• S&T #1 p. 116: How did the pieces of paper interact with differently charged objects (the plastic and glass)?• The charged plastic knife (-) attracted paper

bits when it was brought close to the pile.• The charged glass test tube (+) attracted paper

bits when it was brought close to the pile.

Part II: Paper Charges

• S&T #2 p. 116: How would your observations change if you could cut up the pieces of paper to the size of Democritus’s particles?• The observations would not change since the

atomic-sized particles possess the same charges as the visible pieces of paper.

Part III: Bending the Rules• Show how water molecules attract both

positively and negatively charged objects.

Water molecules have both a positive and negative end. Since they are free to rotate, the end with the opposite charge can rotate to be near the charged object (opposites attract).

S&T Part III #1 p. 118• 1. How is a water

molecule like a compass needle?

Features of Water Molecule

Feature of Compass Needle

Why are they similar

Molecule has (+) and (-) end

Needle has (N) and (S) end Ends have opposite characteristics

Molecule rotates when influenced by external electric field

Needle rotates when influenced by external magnetic field

Both move in response to external fields

There is a balance of charges (overall neutral)

There is a balance of poles Poles and charges come in pairs

S&T III #2

• 2. What does it mean to call water a polar molecule?• Polar means that water has ends that

exhibit pole-like behavior. One side exhibits a slight positive charge, and the other side exhibits a slight negative charge. It behaves like a compass needle and can rotate when exposed to different external charges.

Ben Franklin’s kite

• Mythbusters