Electric forces


Triboelectric effect

No! not those! Those are trilobites.

Tribology - Greek tribo='I rub', -ology=study of.

When some materials are rubbed against others, each acquires an electric charge. Static electricity.

In the in-class demonstration of rubbed objects, questions which we'll apply to our study of nuclear physics:

  1. Were the objects statically charged before rubbing with cloths? How would you know?
  2. Do statically charged objects always repel each other? Or always attract each other?
  3. Two objects which have been charged in the same way: Do they attract or repel?
  4. Do two statically charged objects have to touch each other in order to interact with each other?
  5. Does the force between statically charged objects get bigger or smaller as the objects are brought closer to each other?
  6. Does most matter, most of the time appear to be statically charged or not?
  7. (unrelated to nuclear physics): How is static electricity different/the same as gravity?

Rub rabbit fur on pvc and hollow transparent plastic tube to get same charge. Rub glass or plastic rod with silk to get a different charge.

Electric charge

Two objects of the same material, both rubbed with the same kind of cloth are said to have the same kind of charge. Do they repel or attract?

Ejiri in the Suruga Province

What is this a picture of?

Ejira

Ejiri in the Suruga Province, by Hokusai, Katsushika. See also Jeff Wall's "A Sudden Gust of Wind".

Making the invisible visible

boys dayA flying carp can make the direction of the wind visible. (There's more to it than just the direction...also wind speed.)

What can we use to make the direction of the gravitational force visible?

A "plumb bob" (What's the point of doing this in golf?)

...the direction of an electric or magnetic force?

Force fields

There's a way of drawing the forces:

  1. Put an arrow on your drawing indicating the direction of the force on a small "test object".
  2. The length of the arrow is proportional to the strength of the force.
This one shows the force of gravity calculated at each point on a square grid close to the red "mass".

Sometimes we just draw the direction of the force, and connect the arrow, and not how much force there is...

What do you get when you try to sketch this out for the balloon "plumb bob" near the VdG generator?

The planetary model of the atom

Is most matter statically charged or not?

This is the planetary model of the atom. What are the particles in this diagram? Are any of them charged?

How is it possible that matter is is not charged if it's made up of these electrically charged particles??

An ion is any atom/molecule which is not neutral.

In solid materials the nuclei and most of the electrons are immobilized by chemical bonds and cannot move very far.

There are just a few solid materials--mostly metals--in which a few of the outermost electrons are loosely enough bound that they can move far from their nucleus, and even "hop" from one nucleus to another.

These are called conductors. An example is copper.

Balloon attracted to a wall?

A charged balloon may stick to a wall which does not appear to have been rubbed. Why?

PhET balloon / static electricity simulation.

Reading assignment:

Here is Chapter 6 (Electromagnetism) of Richard Muller's "Physics for Presidents" - Part 1, Part 2, Part 3

I only care that you read about the electric force. You may stop reading in Part 2 when he starts talking about magnetism. (But I'll leave the rest of it up in case you are curious!)

Brief reading quiz on Wednesday over this
Some particular questions that you should read for:

  • If you compare the absolute values of the gravitational force attracting two protons to each other and the electrical force repelling two protons from each other, which force has the greater absolute value?
  • What are the units of electrical charge?
  • Can an object have any charge, or is there some fundamental amount of charge?
  • What is the relationship of the charge of the electron and the proton? Have scientists figured out why this is?

Read Chapter 8 of Art Hobson [course textbook] - just sections 8.4-8.8

How is the electric force the same as / different from gravity?

Bring: pvc pipe, length of nylon rope, leather gloves, magnet --- or van de Graaf and balloons.

Reading quiz

  1. If an electron (negative charge) and a proton (positive charge) are placed 1 cm from each other, there is an attractive electric force between them that is...
    1. smaller than the gravitational attraction between them.
    2. approximately the same as the gravitational attraction between them.
    3. larger than the gravitational attraction between them.
    4. There is a force, but it's repulsive, not attractive.

  2. Mary Shelley wrote Frankenstein, a fictional account in which Dr. Frankenstein tries to bring a dead body to life with lightning. Her story was inspired by the real-life research of Luigi Galvani who passed electricity through the leg of what animal? When it jerked, he thought he had brought it to life.

  3. There is a connection between electricity and magnetism. All magnetic field arise due to
    1. circulating electrical currents.
    2. the destruction or creation of electrons in nuclear reactions.
    3. magnetism would not exist without lightning.

  4. Thomas Edison and Nikola Tesla had a feud over...what?

Image credits

Andrew Becraft, vaaccess,