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An unknown wire in a circuit has a circular cross-section with diameter 3.9 mm. It carries a current of 1.3 mA.

Required:
a. What is the current density, in milliamperes per square centimeter, in this wire?
b. If you assume the drift velocity in this wire is 1 Ã 10^-2 cm/s, what is the density of charge carries per cubic centimeter?
c. If the conductivity in this wire is 5.5 à 10^6 Ω^-1 m-1, what is the average time between collisions, in nanoseconds?

Sagot :

Answer:

Explanation:

Given:

diameter of the wire, [tex]d=3.9~mm[/tex]

current in the wire, [tex]i=1.3~mA[/tex]

a)

Current density:

[tex]\o=\frac{i}{\pi.d^2/4}[/tex]

[tex]\o=\frac{1.3}{\pi\times 3.9^2/4}[/tex]

[tex]\o=0.109~mA/mm^2[/tex]

b)

Given drift velocity, [tex]v_d=1\times 10^{-2} ~cm/s[/tex]

From the formula:

[tex]v_d=\frac{i}{n.e.A}[/tex]

where:

n = charge density (here the charge carriers are electron)

q = quantity of charge on a carrier

A = cross-sectional area of the conductor

[tex]n=\frac{i}{v_d.q.\pi d^2/4}[/tex]

[tex]n=\frac{1.3\times 10^{-3}}{1\times10^{-2}\times(1.6\times 10^{-19})\times\pi\times0.39^2/4 }[/tex]

[tex]n=6.80\times 10^{18}~cm^{-3}[/tex]

c)

Given conductivity of wire, [tex]G=5.5\times 10^{6}~\Omega^{-1}.m^{-1}[/tex]

Using formula of average time between collision:

[tex]\tau=\frac{m.G}{q^2n}[/tex]

here:

m = mass of the a carrier

[tex]\tau=\frac{9.11\times 10^{-31}\times 5.5\times 10^6}{(1.6\times 10^{-19})^2\times 6.80\times 10^{12}}[/tex]