1. 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

3. Electricity Comes Alive
Electrical Current

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

5. 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

6. 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

7. 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

8. Electrical Stopping Power
Electrical Resistance

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

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

11. 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.

12. 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

13. Example #2
A piece of wire that has a length of 5.0 x 107 meters and a cross-sectional area of 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