GUIA DE EJERCICIOS Resistencias

1. Determinar la longitud de un alambre de cobre que tiene una resistencia de 0,2 Ω y diámetro de 0,254 cm.

2. Convirtiendo los valores a unidades métricas encuentre: a) Cuál es la resistencia de una placa de cobre con las dimensiones mostradas (T =

20°C) en la figura? b) Resuelva (a) para aluminio y compare los resultados. c) Sin calcular la solución numérica, determine si la resistencia de la placa

(aluminio o cobre) aumentará o disminuirá con un incremento de longitud. Explique su respuesta.

d) Resuelva (c) para un incremento del área transversal.

3. Si la resistencia laminar de una muestra de óxido de estaño es de 100 Ω. ¿cuál es

el espesor de la capa de óxido? 4. Determine el incremento en resistencia de un conductor de cobre si el área se

reduce por un factor de 4 y la longitud se duplica. La resistencia original era de 0,2 Ω. La temperatura permanece constante.

92 ⏐⏐⏐ RESISTANCE

12. Determine the increase in resistance of a copper conduc-tor if the area is reduced by a factor of 4 and the lengthis doubled. The original resistance was 0.2 !. The tem-perature remains fixed.

R

G

FIG. 3.46Problem 11.

4 ft

1 2 in./

3 in.

10. A wire 1000 ft long has a resistance of 0.5 k! and an areaof 94 CM. Of what material is the wire made (T " 20°C)?

*11. a. What is the resistance of a copper bus-bar with thedimensions shown (T " 20°C) in Fig. 3.46?

b. Repeat (a) for aluminum and compare the results.c. Without working out the numerical solution, deter-

mine whether the resistance of the bar (aluminum orcopper) will increase or decrease with an increase inlength. Explain your answer.

d. Repeat (c) for an increase in cross-sectional area.

equation for the resistance R is defined in terms of the variables, and theresult is obtained. The true value of developing in the above sequenceis the fact that you can place the program in memory and, when theneed arises, call it up and change a variable or two—the result willappear immediately. There is no need to reenter all the definitions—justchange the numerical value.

In the chapters to follow, Mathcad will appear at every opportunityto demonstrate its ability to perform calculations in a quick, effectivemanner. You will probably want to learn more about this time-savingand accuracy-checking option.

PROBLEMSSECTION 3.2 Resistance: Circular Wires1. Convert the following to mils:a. 0.5 in. b. 0.01 in.c. 0.004 in. d. 1 in.e. 0.02 ft f. 0.01 cm

2. Calculate the area in circular mils (CM) of wires havingthe following diameters:a. 0.050 in. b. 0.016 in.c. 0.30 in. d. 0.1 cme. 0.003 ft f. 0.0042 m

3. The area in circular mils isa. 1600 CM b. 900 CMc. 40,000 CM d. 625 CMe. 7.75 CM f. 81 CMWhat is the diameter of each wire in inches?

4. What is the resistance of a copper wire 200 ft long and0.01 in. in diameter (T " 20°C)?

5. Find the resistance of a silver wire 50 yd long and 0.0045in. in diameter (T " 20°C).

6. a. What is the area in circular mils of an aluminum con-ductor that is 80 ft long with a resistance of 2.5 !?

b. What is its diameter in inches?7. A 2.2-! resistor is to be made of nichrome wire. If the

available wire is 1/32 in. in diameter, how much wire isrequired?

8. a. What is the area in circular mils of a copper wire thathas a resistance of 2.5 ! and is 300 ft long (T "20°C)?

b. Without working out the numerical solution, deter-mine whether the area of an aluminum wire will besmaller or larger than that of the copper wire. Explain.

c. Repeat (b) for a silver wire.9. In Fig. 3.45, three conductors of different materials are

presented.a. Without working out the numerical solution, deter-

mine which section would appear to have the mostresistance. Explain.

b. Find the resistance of each section and compare withthe result of (a) (T " 20°C).

Aluminum:l = 50 ft,d = 50 mils

Copper: l = 10 ft, d = 10 mils

Silver: l = 1 ft, d = 1 mil

FIG. 3.45Problem 9.