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3. Regarding the microscopic picture of the local current in a wire as shown in Figure Q6-2, the local electric current j is given by j = (-e)ny (1) where e is the elementary charge, n is the number density of the electrons, and vis an average velocity of the electrons. Electron Atom Figure Q6-2. a) The force acting on the elementary charge is given by F = -eE where E is the electric field. By applying Newton's law, ma = -eE where m and a are mass and acceleration of the electron respectively, calculate the average electron velocity when the mean free time (the time it takes between the collisions with the atoms) is given by t. (Hint: When electrons collide with the atoms, the velocity becomes zero.) (10 Marks) b) Explain Ohm's law from this microscopic picture in terms of causality, and explain how material properties affect the local electric current [10 Marks]