Course:SCIE001/Physics/Reading Guides/Chapter 26

This is a combination of a quick read of the textbook and my (Haley Patrick) old AP notes. Be sure to read the text in depth to fully understand everything.

Learning Objectives for Electrostatics

• Contrast the different behaviours of charged conductors and insulators
• Explain why a neutral object may be attracted to a charged object
• Apply the principle of superposition to determine the electric field due to multiple charges
• Predict the motion of a charge in an electric field
• Use symmetry and Gauss’s Law to predict the electric field due to some continuous charge distributions
• Describe the relationship between electric potential energy and electric potential and to calculate their values for a system of point charge
• Explain what an electron Volt represents
• Describe the relationship between equi-potential surfaces and electric field

Basic Concepts (26.1)

This section is mostly examples of static electricity experiments you may have done in high school. It boils down to a few key concepts:

1. There are two types of charge: positive and negative.
2. Opposite charges attract, like charges repel.
3. Charged objects attract neutral objects.
4. The repelling, attracting abilities of charged objects acts at a distance. The force is stronger the closer the two objects are.
5. The only way to transfer charge is by touching. Conductors easily allow charge to flow while insulators do not.

In fact, sections .2 and .3 are discussions of these core ideas.

Charge (26.2)

From here, the textbook goes into a more in-depth discussion of what charge is. We all know our atomic particles from chemistry, like you can remove electrons from an atom to ionize it and etcetera . Protons and electrons have what is called the fundamental unit of charge, which is the smallest charge that can possibly exist. Since you can’t have a fraction of a proton or electron, you can only have certain amounts of charge on an object. In other words, charge is quantized. An object with more protons is positive, and one with more electrons is negative.

Also, look at the instructions in this section on how to draw a charge diagram.

Insulators and conductors (26.3)

The textbook discusses the difference between insulators and conductors in this section, but it doesn’t do a great job explaining why electrons in conductors (AKA metals and ionic solutions) are loosely bound to their nucleuses. Check out 7.7 of our chem textbook, “the band theory of solids”, where they do a much better job of explaining this. If this is too much chem for a physics reading, just know that the electrons in conductors are highly mobile.

While you can charge an insulator by rubbing, you can’t do that for metals. You can however touch a charged object to a metal. When this happens, the electrons move from the less favourable insulator to the more favourable conductor. When they do this, they displace the electrons of the conductor in order to reach static equilibrium. The extra electrons end up at the surface. Discharging happens when a grounded (touching the ground) conductor like a wire or yourself touches a charged object, causing the charge to spread between the two materials before trying to distribute evenly with the earth. But since earth is so huge, practically all the charge ends up on it.

There is more in this section on polarization, dipoles, and charging by induction. This is when a charged object is held close to an uncharged one. The electrons in the uncharged object distribute themselves so that opposite charges are closer and like are farther apart. Check out the pictures on page 798 and the induction diagrams on 800. Know the difference between induction and conduction.

Coulomb’s law (26.4)

If you are comfortable with the equation for gravity, you’ll like Coulomb’s law.

${\vec {F}}={\frac {q_{1}q_{2}}{r^{2}}}{\hat {r}}$

Where F is the electrostatic force, k the electrostatic constant, the Qs the charges on two objects and R the distance between the two charges.

• Charge is measured in Coulombs (C)
• If you’re working with 3 or more point charges, find the force between each pair of particles and add them together. Remember, Force is a vector!

Check out the problem examples in this section and be comfortable with the formula.

Electric Field (26.5)

An electric field is the area around a charge where the effects of the charge can be felt. An easy example is you can pick up tissue paper with a comb without touching the two together.

${\vec {E}}={\frac {\vec {F}}{Q}}$

Or, it can also be expressed as

${\vec {E}}=k{\frac {q}{r^{2}}}{\hat {r}}$

This is like the $GM/R^{2}$ part of the gravitational force equation, which is g, the gravitational field strength (:

• SIGN CONVENTION FOR ELECTRIC FIELD: The direction of the electric field is always in respect to which way the electrostatic force will act on a positive charge. So a field arrow pointing right would make a proton move right, but an electron move left.

Be sure to read over the parts on vector notation and the field of a point charge. You should also be able to interpret an electric field diagram.

Relevant Links

That’s it for this chapter. Do the stop to thinks and be sure to understand the concepts described above. Merry December!! (: