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.

Problem 73-2  Source:   Problem developed by the author of the site.

Knowing that the resistivity of copper is 1.7241 x 10^-2 ohms.mm² /m and that of aluminum is 2.8264 x 10^-2 ohms.mm² /m , what should be the relation between the areas of copper wire and aluminum so that they have the same strength, considering the wires of the same length?

Problem 73-3  Source:   Problem 21 - page 161 -    HALLIDAY, RESNICK, WALKER, Jearl -   Book: Fundamentos de Física - Vol 3 - Ed. LTC - 8ª edição - 2009.

A wire with a resistance of 6.0 ohms is stretched in such a way that its length makes it three times larger than the original. Determine the wire resistance after operation assuming the resistivity and density of the material remain unchanged.

Problem 73-4  Source:   Problem developed by the author of the site.

On a television screen, whose anode voltage is 25 kV, it attracts an electron beam to the formation of an image with an intensity of 320 uA. What is the amount of electrons that arrive on the screen per second and what is the power consumed?

Problem 73-5  Source:   Problem developed by the author of the site.

In the Figure 73-05.1 we see an electrical cable used in high voltage transmission lines. Notice the core made of a special carbon compound of radius a, being more resistant and light than steel and non-conductive. The conductive material is aluminum with external radius b. This cable has a rated current of I = 1.00 kA.

a) If a = 4.5 mm and b = 14.5 mm, find the current density.

b) Calculate the Ω/Km ratio of the cable

c) Find the Joule loss of the cable per Km for the rated current.

Problem 73-6  Source:   Problem developed by the author of the site.

Considering the results of the previous problem (Problem 73-5), let's assume that this cable is used to transport electrical energy between two locations that are 100 Km apart. If the line cannot have a loss greater than 4% , what the line's rated power and line working voltage should be?

Problem 73-7  Source:   Problem 29-4 - page 249 -    OMAN, Robert & OMAN, Daniel -   Book: How to Solve Physics Problems.

What is the current and the current density in a gas discharge pipe 1.2 cm in diameter, where they circulate in a cross section of the tube, every second, 3.0 x 10¹⁷ electrons and 2.0 x 10¹⁶ positive ions?