A nichrome wire has a resistance of 15 and a diameter of 3.0 mm. The number density of thefree electrons in nichrome is 9.0 1028 m–3.A potential difference (p.d.) of 6.0 V is applied between the ends of the wire.What is the average drift speed of the free electrons in the wire?A 9.8 10 –7 m s –1B 3.9 10 –6 m s –1C 6.1 10 –6 m s –1D 2.5 10 –5 m s –1
Question
A nichrome wire has a resistance of 15 and a diameter of 3.0 mm. The number density of thefree electrons in nichrome is 9.0 1028 m–3.A potential difference (p.d.) of 6.0 V is applied between the ends of the wire.What is the average drift speed of the free electrons in the wire?A 9.8 10 –7 m s –1B 3.9 10 –6 m s –1C 6.1 10 –6 m s –1D 2.5 10 –5 m s –1
Solution
To solve this problem, we need to use the formula for current, I = nAvq, where n is the number density of free electrons, A is the cross-sectional area of the wire, v is the drift speed, and q is the charge of an electron.
First, we need to find the current in the wire. We can use Ohm's law, V = IR, where V is the potential difference, I is the current, and R is the resistance. Rearranging for I gives us I = V/R = 6.0V / 15Ω = 0.4A.
Next, we need to find the cross-sectional area of the wire. The area of a circle is given by A = πr², where r is the radius. The diameter of the wire is 3.0mm, so the radius is 1.5mm or 1.5 x 10^-3 m. Substituting this into the formula gives us A = π(1.5 x 10^-3 m)² = 7.07 x 10^-6 m².
Finally, we can substitute these values into the formula for current. Rearranging for v gives us v = I / (nAq). The charge of an electron is 1.6 x 10^-19 C. Substituting all the values gives us v = 0.4A / (9.0 x 10^28 m^-3 * 7.07 x 10^-6 m² * 1.6 x 10^-19 C) = 3.9 x 10^-6 m/s.
So, the average drift speed of the free electrons in the wire is 3.9 x 10^-6 m/s, which corresponds to option B.
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