# Week 26 (Feb. 19 - 23)

Reading: Current and resistance (Chap. 27)

Key Topics:
electric current, resistance, resistivity, ohm's law, joule heating

Homework Problems: Due Feb. 28 at noon
1. Power lines: High-voltage power lines deliver electrical current to your neighborhood. The aluminum wire used for these lines has a cross sectional area of about 5 square centimeters. What is the resistance of ten kilometers of this wire? (Answer: resistivity of aluminum * length / area = 0.52 ohms ) Hint: you will need to look up the resistivity of aluminum.
2. Car battery: A car battery has a rating of 220 ampere-hours. This rating is an indication of the total charge that the battery can provide to a circuit before failing. What is the total charge (in coulombs) that this battery stores? (Answer: an ampere is a coulomb per second. So 220 ampere-hours * 60 minutes/hour * 60 seconds/minute = 792,000 coulombs). What is the maximum current that the battery can provide for 38 minutes (Answer: 792,000 coulombs / 38 minutes = 347 amperes).
3. Mystery resistor: The current in a series circuit is 15.0 amperes. When an additional 8.00 ohm resistor is inserted in series, the current drops to 12.0 amperes. What is the resistance of the original circuit? (Answer: Since the voltage didn't change, the product of the current and the resistance remains constant. Thus, 15 amps * R = 12 ams * (R + 8). Solving for R gives R = 32 ohms)
4. Resistor combinations: Eight different values of resistance can be obtained by connecting together three resistors (1, 2 and 3 ohms) in all possible ways. What are they? (Answer: 6 (1, 2 and 3 all in series), 6/11 (1, 2, and 3 all in parallel), 11/5 (1 in series with 2 and.3 in parallel), 11/4 (2 in series with 1 and 3 in parallel), 11/3 (3 in series with 1 and 2 in parallel), 5/6 (1 in parallel with 2 and 3 in series), 3/2 (3 in parallel with 1 and 2 in series), 4/3 (2 in parallel with 1 and 3 in series))

General College Physics