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Natural Science - Year II

Unit 50: Static Electricity, Conductivity, and Electric Fields

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Science Weblecture for Unit 50

This Unit's Homework Page History Lecture Science Lecture Lab Parents' Notes

Science Lecture for Unit 50: Static Electricity

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Electrical Charge and Static Electricity

In this section we are making a leap of faith. Even though we haven't gotten to the point in history when people realized that the atom could be broken into parts, we are going to have to talk about electrons. For now, you need to know that Dalton's unbreakable atoms are breakable, and that one of the parts of the atom, the negatively-charged electron, is actually rather easy to pull off and move around. The mass of data about electrical phenomena helped physicists realize that atoms might not be such compact, solid objects after all.

This week, I get to send you to an excellent site originally developed for ThinkQuest by students at the Collegio Franklin Delano Roosevelt as part of a challenge in the 1990s. The site is now maintained as part of the Easy Physics website, and is an excellent introduction to all facets of physics.

Before we start, let's get some basic concepts in place.

Static electricity

  1. Causes: Friction
  2. Two kinds of charge
    1. Like charges repel
    2. Different charges attract
  3. Force
    1. Proportional to the amount of charge
    2. Weakens with distance
    3. Coulomb's law

Static charges are easily created by friction, by simply rubbing two appropriate objects together -- fur and amber, a balloon and your hair. Friction strips electrons from one of the objects, and deposits them on the other, creating a net positive charge on the object losing electrons and leaving a net negative charge. Electrical charges create an electrical field, just as mass creates a gravitational field, but there is a difference. Gravitational forces are always attractive: every mass attracts every other mass. Electrical forces between charged objects are attractive when the objects have opposite charges, that is, when one is positive and the other is negative. If the objects both have a positive charge, or both have a negative charge, the electrical force is repulsive. We say "opposites attract" for electrical charges.

The strength of electrical force depends on two factors: how much charge is present on each object, and how far apart they are. Like gravity, electrical force is an "inverse square law" force: the force goes down as distance increases, and is proportional to the square of the distance. The law that governs this force is named after Coulomb, and, as we saw in the history weblecture, is F electricity   =   k   q 1 q 2 r 2

Read the Introduction to Electricity, chapter 11 of the Learn Physics Today! Easy Physics site. [1 web page]

Be sure that you understand the answers to the following questions.

  • What are the two kinds of charges?
  • When is an object "charged"?
  • What is the difference between conductors and insulators?
  • How does something become charged by induction? by conduction?
  • What is the Coulomb?
  • According to Coulomb's law, what happens to the electric force on one charge if you increase the distance between charges?
  • What happens if you increase one of the charges?

Electricity and Magnetism

Magnetic fields are a product of moving electrical charge. While we can speculate on and sometimes produce an electrical field without a surrounding magnetic field if we charge and object and keep it motionless, we can never have a magnetic field without some moving electrical charge. All electric current gives rise to a magnetic field.

Read the Introduction to Electricity, chapter 12 of the Learn Physics Today! site. [1 web page]

Can you answer these?

  • How are electricity and magnetism alike?
  • Is the electric field dependent on two charges or only one?
  • How is the electric field dependent on distance from the charge?
  • What is an electric field line?
  • What is voltage? How is it related to electric force?

How Batteries Work

Because any moving charge produces electric current, whether the charge is on an electron or an atom or molecule (ion), we can harness chemical interactions between ions to create electron flow in wires, and then use their kinetic energy to power electrical devices like lights, buzzers, motors, and computers.

Now read about how a battery works.

[Four links: Anatomy, Chemistry, Performance, and Inside a Battery (the last requires Shockwave)]

  • What parts of the battery are like the zinc and copper plates of Volta's pile?
  • Trace the path of an electron from the zinc anode through the lightbulb, different solutions in the battery and back home again. Notice that part of the time the electron is flowing through wires, and part of the time it is participating in an acid-base chemical reaction.
  • How do you get more power out of a set of batteries?
  • How can you increase the life of a set of batteries?

Study/Discussion Questions

Further Study On your Own (Optional)