In Chapter 22 we learned about static electricity and the forces between electric charges, but we did not address what happened when electric charges were allowed to move in response to those forces. Guess what? Now we learn about what happens when charges move—a phenomenon known as electric current.
pp. 436–437. Flow of Charge. Charge flows only as long as there is an electric potential difference—and flowing charge acts to deplete that potential difference. Therefore, to maintain a flow of charge, power must be supplied to maintain the potential difference.
p. 437. Electric Current. This section is short, but you should be able to find some key information in it.
Recommmended workbook exercise: p. 89.
pp. 437–438. Voltage Sources. This is a quick read. Make sure after reading this that you can answer:
p. 439. Electrical Resistance. Read this quickly, because it is very, well, conceptual. It tells you a little about what electrical resistance means without actually defining it.
pp. 439–440. Ohm’s Law. This is the key formula for understanding the properties of circuits you will analyze in this course. Read this section conceptually, and then make sure that you understand every relation described in the text. Also carefully read footnote 5 at the bottom of page 439.
Recommmended workbook exercise: p. 90.
pp. 440–442. “Ohm’s Law and Electric Shock.” This briefly describes the health effects of electric shock, and explains how shocks occur and how they can be prevented. This is beyond the scope of this course, but it would be a good idea to read it anyway!
pp. 442–443. Direct Current and Alternating Current. Most of what we will do in this course analyzes direct rather than alternating current. The one nod we will give to alternating current regards electrical transformers, which we will address in due course.
As far as this class is concerned, the important part of this section is the stuff on page 442. At this stage, be able to define and describe direct current and alternating current.
Ignore the “Converting ac to dc”stuff at the bottom of page 443.
pp. 444–445. Speed and Source of Electrons in a Circuit. Skip this; we will not address it in class.
pp. 446–447. Electric Power. Here we get another formula! This one is for the power in an electric circuit. Read this short section carefully, and make sure you understand all of it.
Don’t skip footnote 7 at the bottom of page 446. In fact, read it carefully! It shows how the units of current and potential multiply to give power units.
Recommmended workbook exercise: p. 91.
p. 447. “Fuel Cells”box. Read this if you are interested. We won’t address it in class.
Compare the formula for Ohm’s law to the formula for Newton’s second law. They are mathematically of the same form, but their quantities are different.
The formula for electric power in terms of potential and current also follows this mathematical form. However, the three quantities cannot be identified as cause, effect, and moderator. What is different?
Copyright © 2008, Richard Barrans
Revised: 11 January 2010. Maintained by Richard Barrans.
URL: http://www.barransclass.com/astr1070/rguides/P1050F10rg_04-06.html