ch19_arom


 * CHAPTER 19 ~ CURRENT AND RESISTANCE!**

I: Current (A) math \displaystyle \Delta{Q} math : Charge (C) math \displaystyle \Delta{t} math : time (s) R: Resistance (ohms) math \displaystyle \Delta{V} math : Potential Difference (V) P: Power (W) math \displaystyle \Delta{PE} math : change in potential energy (J)
 * KEY VARIABLES:**

math \displaystyle I=\frac{\Delta{Q}}{\Delta{t}} math math \displaystyle R=\frac{\Delta{V}}{I} math math \displaystyle \Delta{V}=IR math math \displaystyle \frac{\Delta{V}}{I}=constant math math \displaystyle P=\frac{W}{\Delta{t}}=\frac{\Delta{PE}}{\Delta{t}}=\frac{q\Delta{V}}{\Delta{t}} math math \displaystyle I=\frac{q}{\Delta{t}} math math \displaystyle P=I\Delta{V} math math \displaystyle P=I^2R math math \displaystyle P=\frac{\Delta{V}^2}{R} math
 * KEY EQUATIONS:**

SECTION 19-1 SECTION 19-2 SECTION 19-3
 * KEY CONCEPTS:**
 * Electric current (I) is the rate at which positive charges move through a conductor past a fixed point.
 * Current is measured in amperes (A) which are a fundamental SI unit.
 * Regard current as being the movement of positive charges, but in common conductors such as metals it is actually the negatively charged electrons that are moving. In such a case the current is considered to be positive charges moving in the opposite direction.
 * Batteries convert chemical energy into electrical energy and generators convert mechanical energy into electrical energy.
 * Batteries always produce a fixed potential difference between their terminals (direct current), generators can either produce the same type of potential difference or one that reverses itself many times a second (alternating current).
 * The current through some circuit depends on the potential difference in the current source, a larger potential difference produces a larger current.
 * The opposition to the flow of current through a conductor is called resistance (R).
 * Resistance is measured in ohms.
 * Ohms Law: the resistance is constant over a wide range of applied potential differences.
 * Materials that follow this relationship are said to be ohmic, others (such as semiconductors) are non-ohmic. Materials may be ohmic at one temperature and non-ohmic at another.
 * R depends on length, cross-section, material, and temperature. Longer conductors, smaller cross-sections, and higher temperatures all increase the resistance of a conductor and vice versa, replacing the material making up the conductor with another material will also change the resistance.
 * The resistance of the human body is about 500 000 ohms if the skin is dry and can drop to 100 ohms if the skin is wet.
 * Some materials have zero resistance below a certain critical temperature, these materials are known as superconductors.
 * Just as power was studied previously as the rate of conversion of mechanical energy it is also the rate at which electrical energy is transferred.
 * When electric utilities sell energy they use the derived unit kilowatt-hour.
 * The power lost to resistance in a wire is proportional to the square of the current.
 * Transformers are used to reduce potential difference to 120V close to the point where it will be put to use.

The organization and overall structure match the 2002 edition of __Holt Physics__ by Serway and Faughn and many of the expressions and the ideas come from there as well. Much of wording and ideas also come from __Honors Physics Review Notes 2008-2009__ by Strong.

Citations:

Faughn, Jerry S. and Raymond A. Serway. __Holt Physics.__ New York, Holt. 2002. Strong, Tom. Course notes. Honors Physics. Dept. of Science. Mt Lebanon High School. May and June 2009.