Announcements Electrostatics – Potential worksheet is due tomorrow [All materials are posted online under the electromagnetism post] Electrostatics Quiz on Monday [Today = last two topics, Tomorrow = Practice, Friday = More practice for P1-P3] No lab today or next week

Electricity Comes Alive 4.2.1 Electrical Current

Definitions current: RATE OF CHARGE FLOW unit: C/s or AMPERE requires: POTENTIAL DIFFERENCE PATH FOR FLOW

Example #1 100 coulombs of charge pass through point A in 4.0 seconds. What is the rate of current flow through point A? I = Δq / t I = 100 C / (4.00 s) I = 25 A A

Example #2 During a thunderstorm a lightning strike transfers 15.0 coulombs of charge in 10.0 milliseconds. What was the electrical current produced in the strike? I = Δq / t I = 15.0 C / (10 x 10-3 s) I = 1.5 x 103 A

Example #3 A wire carries a current of 50 amperes. I = Δq / t How much charge flows through the wire in 10 seconds? How many electrons pass through the wire in 10 seconds? I = Δq / t 50 A = q / (10 s) q = 500 C   3.125 x 1021 e

Electrical Stopping Power 4.2.2 Electrical Resistance

Definitions resistance: OPPOSITION TO CURRENT unit: Ω factors that change resistance: resistivity: MATERIAL length cross-sectional area temperature

Resistance Factors R ρ R A NEXT R L R Temp.

Resistance Factors R R ρ A R R L Temp. To build an “ideal” conductor with the smallest possible resistance you would select one that is: LOW RESISTIVITY, SHORT, WIDE, COLD R L R Temp.

Example #1 Determine the resistance of a 1.0 meter long copper wire with a cross-sectional area of 0.01 meter2. R = ρL / A R = (1.72 x 10-8 Ω·m)(1.0 m) / (0.01 m2) R = 1.72 x 10-6 Ω

Example #2 A piece of wire that has a length of 5.0 x 107 meters and a cross-sectional area of 0.025 meter2 has a resistance of 31.8 ohms. What is the composition of this wire? R = ρL / A ρ = RA / L ρ = (31.8 Ω)(0.025 m2) / (5.0 x 107 m) ρ = 1.59 x 10-8 Ω·m