9-Electrostatics

Slideshows: Electrostatics Slideshow

Textbook: Chapters 14 in Mastering Physics (get online code for registration on about page of google classroom)

Objectives:

• Nature of charge

  • Explain experimental evidence that there are 2 types of charge.

  • Challenge the claim that an electric charge smaller than the elementary charge has been isolated

• Conservation of charge

  • Make claims explaining object interactions where charge is conserved

  • Make predictions about the sign and relative quantity of objects after being charged. 

  • Define and discuss open and closed systems to explain when charge is or is not conserved in a system. 

• Coulomb's Law

  • make qualitative and quantitative predictions about the interaction between 2 electric point charges. 

  • Connect the concepts of gravitational force and electric force to compare similarities and differences

Physics Facts: Things to memorize

• Charge (q) is measured in units of coulombs.

• Charge is conserved – it can neither be created nor destroyed, though positive charge can neutralize negative charge.

• The smallest possible unit of charge is e = 1.6 x 10-19 C.  Experimental results showing a charge that’s not a multiple of e should be rejected.

• Most charged objects in a laboratory have microcoulombs (µC) or nanocoulombs (nC) worth of charge.

• The electric force between two isolated charged objects is given by Coulomb's law (only equation below)

  • q is the amount of charge on each object

  • r is the distance (separation) between the objects' centers

  • k is the coulomb's law constant, 9 x 10⁹ Nm²/C²

The concept of conservation of charge means you sometimes need to write your own equations to determine the charge on objects after they interact. 

Common Misconceptions: (these statements all have errors)

• 1. Misconception: Coulomb's law applies to charge systems consisting of something other than point charges.

     • Correct Principle: Coulomb's law relies on the geometry of 2 point charges. Other shapes/larger objects can be treated as the sum of a lot of coulomb's law interactions or would need a different model to explain them. 

  2. Misconception: The electric force is the same as the gravitational force.

     • Correct Principles: Electric forces can be attractive or repulsive while gravitational forces can only be attractive. Electric forces vary with charge, while gravitational forces vary with mass. Electric forces require very little charge excess charge to cause noticeable interactions while gravitational forces require a significant mass (like the earth's) to cause noticeable effects. 

  3. Misconception: Forces at a point exist without a charge there.

     • Correct Principle: Force is an interaction between 2 objects, so 2 objects must be involved to have a force. Electric fields can exist based on one object, but we save those for Physics 2. 

  4. Misconception: A charged body has only one type of charge.

     • Correct Principle:  All objects are made of atoms, and atoms have balanced numbers of protons (+ charges) and electrons (- charges). Neutral objects maintain this balance while negatively charged objects has gained extra electrons (beyond those needed to balance their protons) and positively charged objects have lost some electrons (leaving some protons without a paired electron). Overall, the excess electrons or lost electrons are very small in comparison to the total number of electrons and protons in any object.