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Chapter 32: Electrostatics

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Mr. Entwistle

on 10 February 2016

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Transcript of Chapter 32: Electrostatics

Electrostatics involves electric charges, the forces between them, and their behavior in materials.

Electrostatics refers specifically to electricity at rest.
Electrons cannot be created nor destroyed, only transferred from one material to another.
In a neutral atom, there is no net charge; (+) = (-). If electrons are added or removed the atom becomes charged, and we call that an
If you drag your feet on a carpet and pick up electrons, are you positively or negatively charged?
What are electrostatics?
Conservation of Charge
Conservation of Charge
Most electrons in an atom are bound tightly, but the outermost electrons are bound loosely and can be easily removed.

How much energy required to tear off an electron varies for different materials.
Conservation of charge is a cornerstone of physics, just like conservation of momentum and energy!

Electrons cannot be divided, so they are transferred in whole numbers.
Thursday, February 4, 2016
Vol 32
The Atom
Electrical Forces and Charges
Yes, there will be some math.
Coulomb's Law
Electrical Forces
come from particles in atoms. These are billions of times stronger than the force of gravity.

To the right is a simple model of the atom. It shows a positively charged nucleus holding the electrons in orbit.

This electrical property of of mutual attraction or repulsion is called
. Electrons are negatively charged (-), while protons are positively charged (+).

It is key to remember that opposite charges attract and like charges repel.

Why is that?...
In 1784 French physicist, Charles Augustin de Coulomb discovered the strength with which charged objects either attract or repel each other.

When you do that, you get fancy laws and equations named after you, this is Coulomb's:

Coulombs & the Electrical Proportionality Constant
hmmm... doesn't Coulomb's Law look familiar?...
There are four fundamental forces in nature; Gravity, Electromagnetic, Strong, and Weak. You've already learned about gravity, and the electromagnetic force is determined by k, the Electrical Proportionality constant.

But let's call it an even 9,000,000,000.

The SI unit of charge is the
, represented by C. One might think 1C is the charge of 1 electron, but it isn't. It's actually the charge of 6,240,000,000,000,000,000 electrons.

While that seems like a massive number, it's actually no more charge than what passes through a 100W bulb every second.
How does the electrical force between the proton in and electron in a hydrogen atom compare to the gravitational force between those two particles?
Let's start with what we know.
Next, let's plug in our constants and solve for the electrical force.
Lastly, we will solve for the gravitational force and compare.
Conductors and Insulators
Electrons are more easily moved in some materials than in others. For example, in metal the outer electrons are not bound to any particular atom, but are free to roam the surface of the material. In other materials, like rubber or glass, even the outer electrons are tightly bound and must stay with individual atoms.

Materials in which charge flows through are called

Materials in which charge does not flow are called

Mr. Kaiser has some awesome animations that show how electrons move inside conductors and insulators.
Some materials are capable of conducting charge under certain conditions, but not in others. These are called

Because one can control the conductivity of the material, they are often used to control electric current.
Magic or Science?
Charging by Induction
If a charged object is brought near a conducting surface, the electrons will move in that conducting surface.


Image A shows two neutral spheres. When a negatively charged rod is brought near the charge spreads out across both spheres, and then the two spheres are separated. This is called an

They have been charged through
, which means without direct contact.
Rearrangement and Organization
Charge Polarization
Why do you get zapped when you touch a doorknob?
Charging by Friction and Contact
We have seen many instances of charge by friction. Electrons can be transferred when two objects rub against each other.
Electrons can also be transferred by touching.

Conductors = spread out
Insulators = close by
Polarization is basically induction, but in insulators instead.

The rearrangement of the positions of charges make one side of the atom to be slightly more positively (or negatively) charged than the other is called
electrical polarization
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