Resolved Problems about Electric Current

Problem 73-1 Source: Problem 9 - page 160 - HALIDAY & RESNICK - Book: Fundamentos da Física - 8ª edição - Ed. LTC - 2009.

How long does it take electrons to travel from the battery of a car to the starter? Suppose the current is 300 A and the copper wire connecting the battery to the starter it is 0.85 m long and has a straight section of 0.21 cm² . The number of load carriers per volume unit is 8.49 x 10²⁸ m^-1.

Solution of the Problem 73-1

Rearranging the eq. 73-05 , we can write that the electron drift speed is given by:

V_d = I / (e n_e A)

Replacing with numeric values

V_d = 300 / (1.6 x 10^-19 x 8.49 x 10²⁸ x 2.1 x 10^-5 )

Performing the calculation we find the value of V_d, or

V_d = 1.05 x 10^-3 m/s

As we know the speed and distance (given by the length of the wire, L = 0.85 m ) using the basic principles of mechanics, we have:

t = L / V_d = 0.85 / 1.05 x 10^-3 = 810 s = 13.5 minutes

Back to Top

Previous Problem

Next Problem