URGENT! PHYSICS HELP PLEASE! I need help before my exam...

Question

1. A copper pipe has an inside diameter of 3.00 cm and an outside diameter of 5.00 cm. What is the resistance of 25.0 m of this pipe?

 

2. The wiring in a house must be thick enough so it does not become so hot as to start a fire. What diameter must a copper wire be if it is to carry a maximum current of 39 A and produce no more than 1.5 W of heat per meter of length?

 

3. A rectangular solid made of carbon has sides of lengths 1.0 cm, 2.0 cm, and 4.0 cm, lying along the x, y, and z axes, respectively. Assume the resistivity is ρ=2.7×10−5 Ω⋅m. Determine the resistance for current that passes through the solid in:

(a) the x direction

(b) the y direction

(c) the z direction

 

4. A certain copper wire has a resistance of 19.0 Ω . At some point along its length the wire was cut so that the resistance of one piece is 7.0 times the resistance of the other.

(a) Determine the length of the short piece.

(b) Determine the resistance of the short piece.

(c) Determine the resistance of the long piece.

Answer 1

Hi Leigh.

 

1.0 Imagine a cylindrical piece of wire of length L and area A. The material in which the wire is made determines its resistivity ρ. The resistance R od this piece of wire is given by

 

R = ρL/A

 

The pipe is made out of copper. So, ρ = 1.69X10^-8 Ωm assuming a temperature of 20C, according to my Halliday and Resnick. The length L is 25 m. The cross sectional area is A = 0.25π(0.05 m)² - 0.25π(0.03 m)2 . = 1.26X10^-3 m². So, we are really to plug everything in: R = (1.69X10^-8 Ωm)(25.0 m)/(1.26X10^-3 m²) = 3.35 X 10^-4 Ω.