OBJECTIVE:  I will be able to describe how a point charge exerts a force on another point charge after today's class

WORDS/FORMULAE FOR TODAY

Terms:

• conductors - materials where electrons can roam
• insulators - materials that keep their electrons close to home
• coulomb - a unit of electrical charge (see below)

Constants:

• e^-_mass = 9.1 x 10^-31 kg
• e^-_charge = 1.60 x 10^-19 coulombs (C)
• k_e = 8.987 x 10⁹ Nm²/C²

Formulae:

• Electrical Force: F_e = (k_eq₁q₂)/r²

WORK O' THE DAY: 

Let us BRIEFLY review our last test

FRanswer

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Most of us are familiar with the term 'electromagnetic radiation' or EM radiation for short.

As the term might suggest, electromagnetic radiation, EM waves and other references to "EM" refer to phenomena of nature associated with electricity and magnetism.

In such cases, we are usually talking about radiation along the EM spectrum from radio waves (very low energy) to gamma rays (very high energy) which are characterized by waves with both electric and magnetic components.

HOWEVER.... before we get to that, it's best to examine the nature of electric fields first (and separate and distance from EM radiation)

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Electric Charge

Recall from your understanding of the basic guts of an atom that electrons inhabit a region of space surrounding the nucleus of an atom.

If you've taken chemistry, you may also recall that electrons DO NOT really circle their host nuclei in perfect circular orbits like mini planets but instead exist in regions of space in areas of probability dictated by quantum mechanical equations.

The important takeaway here is to consider electrons existing away from the nuclei of their respective atoms, and (contrary to your first impressions) those electrons are NOT ALWAYS tightly bound to their nuclei.... in fact (and in varying degrees) electrons are not at all tightly bound by most metals, but are in fact 'shared' by other nearby atoms and are free to roam from atom to atom to atom...

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Basic Guts:

When, and if, electrons shift away from some part of a material on the MACRO scale (we're not talking about ions here folks) that material becomes negatively charged.

When, and if, electrons shift towards some part of a material on the MACRO scale (aging, we're not talking about individual atoms gaining or losing electrons) that material becomes negatively charged.

Materials that have GAINED electrons are said to have a NEGATIVE charge.

Materials that have LOST electrons are said to have a POSITIVE charge.

When two such materials are in close enough proximity, the positively charged material attracts the negatively charged material.

From our conversations of Newton's Laws, such an interaction clearly involves a force... Since this interaction occurs as a result of electrons traveling to or away from part of a material (on a macro scale) we call this the ELECTRIC FORCE.

Not surprisingly, the ELECTRIC FORCE is a conservative force, in other words, charge is neither lost nor gained in the interaction, but rather transferred.

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Talk in your group and suggest a method to explain why a certain type of sock always sticks to a certain type of shirt in the laundry (be sure to talk like Newton)

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A material can become charged by direct contact with another material (conduction) --- such as sliding your stocking feet across a carpet. Obviously some materials work better at this than others.

Also, some materials where electrons are less tightly held can have an induced charge simply by being close enough to another charged materials causing electrons to roam.

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So.... now that we have an idea of how the electric force works to attract electrons on a macro scale, let's take a look on the micro scale.

The electric force is, like gravity, an inverse square force. In other words, the strength of the force decreases with the square of the distance between two charged particles:

F_e = (k_eq₁q₂)/r²

• k_e relates to a constant called "The permeability of free space" (eg a vacuum) => k_e is actually an easier to use version of that constant. So... if you prefer:

  k_e = 1/(4πε_o)

  ε_o represents the permeability of free space and is pronounced 'epsilon not' (personally I prefer to just just use k_e

• q₁ = the magnitude of charge of the first object (in coulombs)
• q₂ = the magnitude of charge+e of the second object (in coulombs)
• r² = the square of the distance between q₁ and q₂meters

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1. The book indicates that the gravitational force is Negligible compared to the electric force... why is that a GROSS understatement?
2. Does the electric force work in reverse? In other words, can we measure the REPULSIVE force between two electrons using that equation?

HOMEWORK:

Part I)

Water droplets inside a thundercloud experience violent wind gusts that move those droplets throughout the cloud. Those violent interactions of water molecules can result in a build up of charge resulting in a discharge of lightening.

My question for you to research-- how does either (pick 1)

1) Water droplets freezing into ice crystals at the top of the cloud

or

2) Water droplets colliding violently and/or ice crystals colliding violently within the cloud

lead to a build up of positive or negative charge resulting in lightening.

Also, I'm very interested in precise, scientific descriptions so please omit explanations such as "the water droplets rub violently against each other resulting in positive charge". Please be more precise on the actual mechanics of conduction.

 Problems: (Chapter 23 probs begining on page 474)

PART II a:

Since we didn't get to it in class today, review 23.1 -- it's pretty straightforward but please, still follow out review steps

1) Write a sketch

2) Provide a qualitative analysis of the problem

3) Write down initial conditions

4) Close your book and do the work

5) Check your work

Part b)

Please review example 23.2 IN DETAIL. Do it slowly, there's a LOT of good stuff in there. That means:

1) Write a sketch

2) Provide a qualitative analysis of the problem

3) Write down initial conditions

4) Close your book and do the work

5) Check your work

Be prepared to answer questions about that tomorrow.

PART III:

Develop a brief (~ 1 minute) presentation/demonstration/lesson plan (whatever) to explain to a bright, articulate GHHS 10th grade chemistry student why it is so grossly inadequate to explain the difference between the gravitational force and the electrical force as "negligible" (best works will include math but won't depend on the math)