ch19_adag

**Chapter 19: Current and Resistance**

__**19-1: Electric Current**__  - **Current**  is the rate at which charges move through the cross section of the wire and is represented as I and found with this equation: where Q is the charge passing through a given area. -Current is measured in //amperes//, A. 1 A=1C/s -Conventional current is defined in terms of positive charge movement: -Positive and negative charges in motion are sometimes called //charge carriers//. //-Conventional current// is defined as 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 of the two. -//Electrolytes// are solutes that consists of charged carriers.
 * //I=//Δ//Q///Δ//t//

- **Drift velocity**  (Vdrift) is the net velocity of a charge carrier moving in an electric field. -When a conductor is in electrostatic equilibrium, electrons move randomly. -The average energy gained by the electrons as they are accelerated by the electric field is greater than the average loss in energy due to the collisions. Individual electrons move slowly along the conductor in a direction opposite the electric filed, E, with a velocity known as the //drift velocity// (Vdrift) -The magnitudes of drift velocities are very small, drift speed is much less than the average speed between collisions. -Batteries and generators supply energy to charge carriers. -A difference in potential maintains current in a circuit. -Current can be direct (dc) or alternating (ac).

__**19-2: Resistance**__  - **Resistance**  is the opposition to the motion of charge through a conductor and can be found using this equation: -Resistance is measured in **Ohms**  ( <span style="color: rgb(0,0,128); font-family: Georgia,serif;">**Ω** <span style="color: rgb(161,0,255); font-family: Georgia,serif;">) and is constant over a wide range of materials. -Ohm's Law is: <span style="color: rgb(161,0,255); font-family: Georgia,serif;">-But, Ohm's Law does not hold for all materials, only materials that are //ohmic//, or have a constant resistance over a wide range of potential differences. -//Non-ohmic// materials are the opposite. -The amount of resistance depends on length, cross-sectional area, material, and temperature:
 * <span style="color: rgb(161,0,255); font-family: Georgia,serif;">//R=//ΔV/I
 * <span style="color: rgb(161,0,255); font-family: Georgia,serif;">ΔV/I=constant

-If ΔV remains constant, current decreases when resistance increases. -A <span style="color: rgb(0,0,128); font-family: Georgia,serif;">**resistor** <span style="color: rgb(161,0,255); font-family: Georgia,serif;"> is a simple electrical element that provides a specified resistance. -Salt water and sweat lower the body's resistance. Salt water (sweat) especially lowers resistance because ions in the salt readily conduct charge. - <span style="color: rgb(0,0,128); font-family: Georgia,serif;">**Superconductors** <span style="color: rgb(161,0,255); font-family: Georgia,serif;"> have no resistance below a critical temperature (aluminum, tin, lead, and zinc). -Once a current is established in superconductors, the current continues even if the potential difference is removed. -The //Meissner effect// is when a magnet floats in the air over a superconductor because of the interaction between the current of the superconductor and the magnet.

__**<span style="font-size: 120%; color: rgb(0,255,0);">19-3: Electric Power **__ -**<span style="color: rgb(0,0,128);">Electric power ** (P) is the rate of conversion of electrical energy. -To find electric power, use this equation: <span style="color: rgb(161,0,255); font-family: Georgia,serif;">//-//The conversion of electrical energy to internal energy in a resistant material is called //joule heating// and is referred to as I^2R loss. -Electrical energy is transferred at high potential differences to minimize energy loss.
 * <span style="color: rgb(161,0,255); font-family: Georgia,serif;">//P=I//Δ//V//

<span style="font-size: 120%; color: rgb(161,0,255); font-family: Georgia,serif;">Questions: <span style="color: rgb(161,0,255); font-family: Georgia,serif;"> 1. The current in a light bulb is 0.835 A. How long does it take for a total charge of 1.67 C to pass a point in the wire?

2. An electric space heater is connected across a 120 V outlet. The heater dessipates 1320 W of power in the form of electromagnetic radiation and heat. Calculate the resistance of the heater.

<span style="font-size: 120%; color: rgb(161,0,255); font-family: Georgia,serif;">Answers: <span style="color: rgb(161,0,255); font-family: Georgia,serif;"> 1. <span style="color: rgb(161,0,255);">Δt=2.00 s

2. R=10.9 ohms

Bibliography: "Holt Online Learning." __Holt Online Learning__. 5 June 2009 <http://my.hrw.com/tabnav/controller.jsp?isbn=0030724864>. All pictures taken from "Holt Online Learning." __Holt Online Learning__. 5 June 2009 <http://my.hrw.com/tabnav/controller.jsp?isbn=0030724864>.