Name: Student ID 1. (5 pts.) Drift velocity The resistivity of seawater is about

Question

Name: Student ID 1. (5 pts.) Drift velocity The resistivity of seawater is about 25 .crn. The charge carriers are chiefly and ions, and of each there are about 3 10/cm If we fill a plastic tube 2 meters long with seawater and connect a 12-volt battery to the electrodes at each end, what is the resulting average drift velocity of the ions, in cm/s? The fundamental charge is 160 C Hint for unit conversio (5 pts.) Resistance and Resistivity A 3000-km long cable consists of five copper wires, each of diameter 0.7 mm, bundled together and surrounded by an insulating sheath. Calculate the resistance of the cable. Use 3 × 10-6 ·cm for the resistivity of the copper. 2. (5 pts.) The magnitude of the charge on the plates of an isolated parallel plate capacit doubled. Which one of the following statements is true concerning the capacitance this parallel-plate system? 3. (b) The capacitance remains unchanged. (c) The capacitance is increased to twice its original value. (d) The capacitance depends on the electric field between the plates. (e) The capacitance is decreased on the potential difference between the plates.

Explanation / Answer

1.

2. Resistance of a single wire is :

R = resistivity * (L/A)

resistivity = 3 e-6 ohm-cm = 3 x 10^-8 ohm-m

L = 3000Km = 3000,000 m

A = pi*R^2 = pi*.00035^2 sq m

R = 233979 ohm

For a bunch of 7 wires which are parallel, it is R/7 = 33425 ohms

3. Since Q=CV,

the capacitance is doubled when the charge is doubled.

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