ch19_aktv

**Chapter 19: Current and Resistance** [|http://physics.bu.edu/~duffy/PY106/Resistance.html] Example problems and equations adapted from Physics textbook Section 1: Electric Current //Q// is the amount of charge that passes through this area in a time interval, //t//, and the current, //I//, is the ratio of the amount of charge to the time interval. This is represented by the equation math \displaystyle I=\frac{\Delta_{Q}}{\Delta_{t}} math or, math \displaystyle electric\ current = \frac{charge\ passing\ through\ a\ given\ area}{time\ interval} math
 * Current**: The rate at which electric charges move through a given area

The unit for current is called an ampere, and the symbol A is used to represent this.

An example problem for the electric current equation would be as follows: math \displaystyle \Delta_t=\frac{\Delta_{Q}}{I} math math \displaystyle \Delta_t=\frac{1.74\ C}{0.762\ A} math = **2.28 s**
 * The current in a light bulb is 0.762 A. How long does it take for a total charge of 1.74 C to pass a point in the wire?**
 * Given**: //Q// = 1.74C //I// = 0.762 A
 * Find**: //t//


 * Charge Carriers** - positive and negative charges in motion


 * Conventional Current** - the current consisting of positive charge that would have the same effect as the actual motion of the charge carriers, regardless of whether the charge carriers are positive, negative, or a combination.

There are two types of current - direct current (**DC**) and alternating current (**AC**).
 * DC** moves in only one direction, while **AC** continuously moves between forward and reverse directions.

Section 2: Resistance
 * Resistance**: The opposition to the flow of current in a conductor

Resistance (R) is defined as the ratio of potential difference (V) to current (I): math \displaystyle R=\frac{\Delta_{V}}{I} math or, math \displaystyle resistance=\frac{potential\ difference}{current} math

The unit for resistance is called an ohm and is represented by the Greek letter Omega.

math \displaystyle \Delta_{V}=IR math
 * Ohm's Law**:

Ohm's Law only holds true when the resistance is constant, when dealing with what are called ohmic materials.

Length, cross-sectional area, material, and temperature all affect resistance.
 * Shorter length** offers less resistance.
 * Greater cross-sectional area** yields less resistance.
 * Copper** has less resistance than aluminum.
 * Lower temperature** means lower resistance.

An example problem for Ohm's Law would be as follows: math \displaystyle I=\frac{\Delta_{V}}{R}\ =\ \frac{120\ V}{10.0\ ohms}\ =\ 12\ A math
 * The resistance of a resistor is 10.0 ohms. What is the current in the resistor when it is connected across a potential difference of 120 V?**
 * Given**: //R// = 10.0 ohms //V// = 120 V
 * FInd**: //I//


 * Superconductor**: a material whose resistance is zero at or below some critical temperature, which varies with each material.

Section 3 : Electric Power
 * Electric power**: The rate of conversion of electrical energy; the rate at which charge carriers do work; the rate at which charge carriers convert electrical potential energy to nonelectrical forms of energy

The equation for electric power (P) is expressed as current (I) multiplied by potential difference (V). math \displaystyle P=\frac{I}{\Delta_{V}} math The unit of power is the watt, and the symbol W is used.

When electrical power is dissipated, the equation is: math \displaystyle P=\frac{\Delta_{{V}^2}}{R} math

An example equation for electric power would be as follows: Given**: //V// = 120 V //P// = 1320 W math \displaystyle R=\frac{\Delta_{{V}^2}}{P} math math \displaystyle R=\frac{(120\ V)^2}{1320\ W}\ =\ 10.9\ ohms math
 * An electric space heater is connected across a 120 V outlet. The heater dissipates 1320 W of power in the form of electromagnetic radiation and heat. Calculate the resistance of the heater.
 * Find**: //R//

Electrical companies charge for energy used in a unit known as a **kilowatt-hour**. A kilowatt-hour is the energy delivered in 1 hour at the constant rate of 1 kilowatt. In order to find out how much electrical energy would cost, you first convert watts to kilowatts, then multiply the kilowatts by the number of hours to get kilowatt-hours. Then, multiply the kilowatt-hours by the rate the electrical company charges for energy consumption to get the total cost.