Chapter 17 Current and Resistance CHAPTER 17 Conceptual Questions C1 A wire 1 mm in diameter is connected to one end of a wire of the same material 2 mm in diameter of twice the length A voltage source is connected to the wires and a current is passed through the wires If it takes time T for the average conduction electron to traverse the 1 mm wire how long does it take for such an electron to traverse the 2 mm wire a T 4 b T c 4T d 8T C2 A wire has resistance R A second wire has twice the length twice the diameter and twice the resistivity of the first wire What is its resistance a 8 R b R c R 4 d The resistance is not given C3 Resistor A has twice the resistance of resistor B When individually connected across a given potential difference which one dissipates the most power and when connected in series across the same potential difference which one dissipates the most power a A A b A B c B B d B A C4 If a light bulb has half the resistance of a 100 W lightbulb what would be its wattage Assume both bulbs are attached to the same 120 V circuit a 200 W b 50 W c 25 W d More information is needed C5 When the voltage across a nonohmic resistor is doubled the current through it triples What happens to the power delivered to this resistor a This cannot be answered with the information given b The power decreases to 2 3 of the original amount c The power increases to 1 5 times the original amount 35 Chapter 17 Current and Resistance d The power increases to 6 times the original amount 36 Chapter 17 Current and Resistance 17 1 Electric Current 1 The current in an electron beam in a cathode ray tube is measured to be 70 A How many electrons hit the screen in 5 0 s e 1 6 10 19 C a 2 2 1011 electrons b 8 8 1013 electrons c 2 2 1015 electrons d 8 8 1018 electrons 2 A wire carries a steady current of 0 1 A over a period of 20 s What total charge passes through the wire in this time interval a 200 C b 20 C c 2 C d 0 005 C 3 In a certain material there is a current of 16 A flowing through a surface to the right and there is an equal amount of positive and negative charge passing through the surface producing the current How much negative charge passes through the surface a 8 C s toward the right b 8 C s toward the left c 16 C s toward the right d 16 C s toward the left 17 2 A Microscopic View Current and Drift Speed 4 If the current in a wire is tripled what effect does this have on the electron drift velocity in the wire a It stays the same b It triples c It decreases by a factor of three d It increases by a factor of nine 5 Wire A and Wire B are each carrying the same current If the diameter of Wire A is twice that of Wire B how does the drift velocity vdA in Wire A compare to that in in Wire B a vdA vdB b vdA 2 vdB c vdA vdB 2 37 Chapter 17 Current and Resistance d vdA vdB 4 38 Chapter 17 Current and Resistance 6 The number density of conduction electrons in a metal can be found from the density of the metal the mass per mole M of the metal the number of conduction electrons per metal atom and Avogadro s number NA If we assume one conduction electron per atom which of the following gives the number density of conduction electrons for a given metal a NA M b NA M c NA M d NA M 7 If a metallic wire of cross sectional area 3 0 10 6 m2 carries a current of 6 0 A and has a mobile charge density of 4 24 1028 carriers m3 what is the average drift velocity of the mobile charge carriers charge value 1 6 10 19 C a 3 4 103 m s b 1 7 103 m s c 1 5 10 4 m s d 2 9 10 4 m s 8 The size of the electric current in a electrical conductor is a function of which of the following a velocity of charge carriers b conductor cross sectional area c density of charge carriers d All of the above choices are valid 9 When an electric current exists within a conducting wire which of the following statements describes the condition of any accompanying electric field a must be zero b must be parallel to current flow c must be anti parallel opposite direction to current flow d must be perpendicular to current flow 10 When you flip a switch to turn on a light the delay before the light turns on is determined by a the number of electron collisions per second in the wire b the drift speed of the electrons in the wire c the speed of the electric field moving in the wire d none of these since the light comes on instantly 39 Chapter 17 Current and Resistance 11 A high voltage transmission line of diameter 2 cm and length 200 km carries a steady current of 1 000 A If the conductor is copper with a free charge density of 8 1028 electrons m3 how long does it take one electron to travel the full length of the cable e 1 6 10 19 C a 8 102 s b 8 104 s c 8 106 s d 8 108 s 17 4 Resistance Resistivity and Ohm s Law 12 Materials having resistance changes as voltage or current varies are called a ohmic b inohmic c nonohmic d deohmic 13 You measure a 25 0 V potential difference across a 5 00 resistor What is the current flowing through it a 125 A b 5 00 A c 4 00 A d 1 00 A 14 The unit of electric current the ampere is equivalent to which of the following a V b V c m d V s 15 The unit of electric resistance the ohm is equivalent to which of the following a V A b V m c A s d A m 16 A flashlight bulb operating at a voltage of 4 5 V has a resistance of 8 0 How many electrons pass through the bulb filament per second e 1 6 10 19 C 40 Chapter 17 Current and Resistance a 3 7 1016 b 1 8 1021 c 9 4 1017 d 3 5 1018 41 Chapter 17 Current and Resistance 17 If a certain resistor obeys Ohm s law its resistance will change a as the voltage across the resistor changes b as the current through the resistor changes c as the energy given …