Physics Chapter 19: Current and Resistance

 Electric Current (I)  The Flow of Electrical Charge (Charged Particles) Within a System  Charge Flows from Higher Electric Potential to Lower Electric Potential

Current and Resistance  Electric Current (I)  A Ratio of the Change in Charge Per Change in Time

Current and Resistance  Electric Current   q, Coulombs   t, Seconds  1 Coulomb / Second = 1 Ampere (A)  “Amp”

Current and Resistance  Electric Current  Problem: “Zapped”  A Careless Electrician Accidentally Touches the Positive and Negative Wires of a 440kV Circuit for 0.2 seconds. The Current on the Circuit is 0.02mA. What is the Electrical Energy Felt by the Electrician?

Current and Resistance  Electric Current  Solution: “Zapped”  V = 4.4x10 5   t = 0.2 s  I = 2.0x10 -5 A  q = ?

Current and Resistance  Electric Current  Problem: “Really Zapped!”  A Really Stupid Electrician Touches the Positive and Negative Wires of a 440kV Circuit for 0.2 seconds. The Current on the Circuit is 50A. What is the Electrical Energy Felt by the Electrician?

Current and Resistance  Electric Current  Solution: “Really Zapped!”  V = 4.4x10 5   t = 0.2 s  I = 50 A  q = ? Yes, That’s 4.4 Million Joules!!!

Current and Resistance  Alternating Current (AC)  Generated Electricity  “Normal” Household Electricity  Voltage “Alternates” between + and –

Current and Resistance  Alternating Current (AC)  Generator Causes Sinusoidal Fluctuations in Voltage +V 0 -V 0 0 t + to - - to +

Current and Resistance  Direct Current (DC)  Electricity Released from Chemical Reactions  Lead-Acid Batteries  “Dry Cells”

Current and Resistance  Direct Current (DC)  Batteries have No Fluctuations in Voltage +V 0 -V 0 0 t

Current and Resistance  Current Running Through a Circuit is like Water Being Pumped Through a Hose  The Higher the Water Pump Pressure, the Greater the Flow of Water Through the Hose  The Higher the Voltage, the Greater the Flow of Current (amps) Through the Circuit  Double the Voltage, and the Current Doubles

Current and Resistance  Current Running Through a Circuit is like Water Being Pumped Through a Hose  So what if we take our garden hose (with the same pressure) and replace it with large diameter fire hose?  Less Resistance = Increased Flow (Current)

Current and Resistance  Current Running Through a Circuit is like Water Being Pumped Through a Hose  So what if we take our 25’ garden hose (with the same pressure) and replaced it with a 250’ garden hose?  More Resistance = Decreased Flow (Current)

Current and Resistance  Ohm’s Law  R = Electrical Resistance  V = Circuit Voltage  I = Circuit Current (Amperage)

Current and Resistance  Ohm’s Law  Resistance is Volts/Amp  Units are Ohms (  )  1 Volt/Amp = 1 Ohm

Current and Resistance  Ohm’s Law  Some Materials are More Resistant to Current Flow than Others  Conductors  Al, Cu, etc…  Semi-Conductors  C, Si  Insulators  Plastics, Rubber, Wood, etc…

Current and Resistance  Electric Power  Electric Energy Delivered to a Circuit per Second  V = Volts (Joules/Coulomb)  I = Amperes (Coulombs/Second)  P = Power (Joules/Second)

Current and Resistance  Electric Power  Units are Watts  1 Joule/Second = 1 Watt

Current and Resistance  Homework  Pages  Problems:  19 (1.0s)  29 (3.4A)  42 ($.08)