Question

2 Drift Velocity The average velocity of electrons in a material with a scattering time τ is vd = eE me τ. The scattering time also determines the resistivity of the material: ρ = me ne2τ . Here, n is the free electron density of the material — the number of free electrons per cubic meter. (a) Estimate the free electron density in copper. There is one free electron per copper atom. The mass density of copper is 8.92 g/cm3 , and 1 mol of copper has a mass of about 63.5 g. (b) The resistivity of copper is 1.7 · 10−8 Ω· m. Estimate the drift velocity of electrons in a copper wire in an electric field of 9 V/m. How long does it take an electron to travel from one end of a 1 m copper wire to the other? (c) Critical Thinking: How does your result from part (b) compare with the time it takes the lights to come on when you flip a switch? How can you explain the difference?

Answer 1

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