Current and Resistance

Homework Assignment

Questions 2, 3, 4, 5, 7, 9, 17, 20

Problems 1, 2, 7, 8, 15, 16, 22, 27, 33, 36, 43, 45, 46, 48, 49, 52

Q2 What factors affect the resistance of a conductor?

Q3 What is the difference between resistance and resistivity?

Q4 Two wires, A and B, of circular cross-section are made of the same metal and have equal lengths, but the reistance of wire A is three times greater that that of wire B. What is the ratio of their cross-sectional areas? How do their radii compaire?

Q5 What is required in order to maintain a steady current in a conductor?

Q7 When the voltage acrosss a certain conductor is doubled, the current is observed to increase by a factor of three. What can you conclude about the conductor?

Q9 Why migh a "good" electrical conductor also be a "good" thermal conductor?

Q17 Two conductors of the same length and radius are connected across the same potential difference. One conductor has twice the resistance of the other. Which conductor will dissipate more power?

Q20 Two lightbulbs both operate from 110 V, but one has a power rating of 25 W and the other, of 100 W. Which bulb carries the greater current?

27.1 In the Bohr model of the hydrogen atom, an electron in the lowest energy state follows a circular path, 5.29 x 10 - 11 m from the proton.

(a) Show that the speed of the electron is 2.19 x 106 m/s.

(b) What is the effective current associated with this orbiting electron?

27.2 In a particular cathode ray tube, the measured beam current is 30 microA. How many electrons strike the tube screen every 40 s?

27.7 A Van de Graaff generator produces a beam of 2.0-MeV deuterons, which are heavy hydrogen nuclei containing a proton and a neutron.

(a) If the beam current is 10.0 micro A, how far apart are the deuterons in the beam?

(b) Is their electrostatic repulision a factor in beam stability?

27.8 Calculate the average drift speed of electrons traveling through a copper wire with a cross-sectional area of 1.00 mm2 when carrying a current of 1.0 A (values similar to those four the electric wire to your desk lamp). It is known that about one electron per atom of copper contributes to the current. The atomic weight of copper is 63.54 and its density is 8.92 g/cm3.

27.15 Calculate the resistance at 20oC of a 40-m, length of silver wire having a cross-sectional area of 0.40 mm2.

27.16 Eighteen-gauge wire has a diameter of 1.024 mm. Calculate the resistance of 15.0 m of 18-gauge copper wire at 20.0oC.

27.27 A resistor is constructed of a carbon rod that has a uniform cross-sectional area of 5.0 mm2. When a potential difference of 15 V is applied across the ends of the rod, there is a current of 4.0 x 10 - 4 A in the rod.

Find (a) the resistance of the rod and (b) the length of the rod.

27.33 If a copper wire has a resistance of 18 ohms at 20oC, what resistance will it have at 60oC?

27.36 A segment of Nichrome wire is initially at 20oC. Using the data from Table 27.1, calculate the temperature to which the wire must be heated to double its resistance.

27.43 A 10-V battery is connected to a 120-ohm resistor. Neglecting the internal resistance of the battery, calculate the power dissipated in the resistor

27.45 Suppose that a voltage surge produces 140 V for a moment. By what percentage will the output of a 120-V, 100-W lightbulb increase, assuming its resistance does not change?

27.46 A particular type of automobile storage battery is characterized as "360-ampere-hours, 12 V". What total energy can the battery deliver?

27.48 In a hydroelectric installation, a turbine delivers 1500 hp to a generator whichin turn converts 80% of thae mechanical energy into electrical energy. Under these conditions, what current will the generator deliver at a terminal potential difference of 2000 V?

27.52 The heating element of a coffee maker operates at 120 V and carries a current of 2.0 A. Assuming that all of the heat generated is absorbed by the water, how long wdoes it take to heat 0.50 kg of water from room temperature of 23oC to the boiling point.

Be sure you do these homework problems before we discuss them in class. Please do not just wait and watch my talk about their solution. That's dangerous! Don't just come watch me work the problems! Do them yourself! That's very important!




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(c) Doug Davis, 2002; all rights reserved