University:
Kentucky State University
Course:
PHY 212 | General Physics II
Academic year:

2024
Views:

69

Pages:

4
Author:

omiliar74be

Resistance Vs Resistivity Resistance of a resistor (wire) measures how hard charge flows through it: R = V/I L R A • the longer a wire, the harder it is to push electrons through it • the greater the cross-sectional area, the “easier” it is to push electrons through it • the greater the resistivity, the “harder” it is for the electrons to move in the material Resistivity = material (or substance) property Resistance = object property Ring & Necklace (objects) of gold (substance): same ρ different R Example 4 : Suppose you want to connect your stereo to remote speaker. (a) If the wire must be 20 m long, what diameter copper wire should you use to make the resistance 0.10  ? R = L / A A = L / R A =  (d/2)2 Geometry; Ms. Nabilah, 4th grade!  (d/2)2 = L / R (d/2)2 = L / R d/2= ( L / R )½ d = 2 ( L / R )½ d = 2 [ (1.68x10-8) (20) /  (0.1) ]½ m d = 0.0021 m = 2.1 mm (b) If the current to the speaker is 4.0 A, what is the voltage drop across the wire? V=IR V = (4.0) (0.10) V V = 0.4 V Resistors in circuits • symbol we use for a “resistor:” • • • • in principle, every circuit component has some resistance all wires have resistance for efficiency, we want wires to have low resistance in idealized problems, consider wire resistance to be zero • lamps, batteries, and other devices in circuits also have resistance This is the theme of the next chapter, so stay tuned!