Homework Reading: Chap. 30 and Chap. 31 Suggested exercises: 30.1, 30.3, 30.5, 30.6, 30.8, 30.10, 30.12, 30.14, 30.16, 30.18, 30.20, 30.24, 30.26, 30.32, 30.37 Problems: 30.40, 30.43, 30.45, 30.47, 30.49, 30.51, 30.55, 30.58, 30.66, 30.68 (due: Fri., Nov. 6) Chapter 30. Current and Resistance Lights, sound systems, microwave ovens, and computers are all connected by wires to a battery or an electrical outlet. How and why does electric current flow through a wire? Chapter Goal: To learn how and why charge moves through a conductor as what we call a current. 1
Chapter 30. Current and Resistance Topics: The Electron Current Creating a Current Current and Current Density Conductivity and Resistivity Resistance and Ohm s Law Chapter 30. Basic Content and Examples 2
Inside a Computer Chip -Q Conductor +Q 3
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Example (a)the current density in a cylindrical wire of radius R = 2.0 mm is uniform across a cross section of the wire and J = 2.0 10 5 A/m 2. What is the current through the outer portion of the wire between radial distance R/2 and R? (b) Suppose, instead, that the current density through a cross section varies with radial r as J = ar 2, in which a = 3.0 10 11 A/m 4, and r is in meter. What now is the current through the same outer portion of the wire? 11
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These four wires are made of the same metal. Rank in order, from largest to smallest, the electron currents i a to i d. A. i d > i a > i b > i c B. i b = i d > i a = i c C. i c > i b > i a > i d D. i c > i a = i b > i d E. i b = i c > i a = i d These four wires are made of the same metal. Rank in order, from largest to smallest, the electron currents i a to i d. A. i d > i a > i b > i c B. i b = i d > i a = i c C. i c > i b > i a > i d D. i c > i a = i b > i d E. i b = i c > i a = i d 18
Why does the light in a room come on instantly when you flip a switch several meters away? A. Electrons travel at the speed of light through the wire. B. Because the wire between the switch and the bulb is already full of electrons, a flow of electrons from the switch into the wire immediately causes electrons to flow from the other end of the wire into the lightbulb. C. The switch sends a radio signal which is received by a receiver in the light which tells it to turn on. D. Optical fibers connect the switch with the light, so the signal travels from switch to the light at the speed of light in an optical fiber. Why does the light in a room come on instantly when you flip a switch several meters away? A. Electrons travel at the speed of light through the wire. B. Because the wire between the switch and the bulb is already full of electrons, a flow of electrons from the switch into the wire immediately causes electrons to flow from the other end of the wire into the lightbulb. C. The switch sends a radio signal which is received by a receiver in the light which tells it to turn on. D. Optical fibers connect the switch with the light, so the signal travels from switch to the light at the speed of light in an optical fiber. 19
The two charged rings are a model of the surface charge distribution along a wire. Rank in order, from largest to smallest, the electron currents E a to E e at the midpoint between the rings. A. E c > E e > E a > E b = E d B. E d > E b > E e > E a = E c C. E c = E d > E e > E a = E b D. E b = E d > E a = E c = E e E. E a = E b > E e > E c = E d The two charged rings are a model of the surface charge distribution along a wire. Rank in order, from largest to smallest, the electron currents E a to E e at the midpoint between the rings. A. E c > E e > E a > E b = E d B. E d > E b > E e > E a = E c C. E c = E d > E e > E a = E b D. E b = E d > E a = E c = E e E. E a = E b > E e > E c = E d 20
What are the magnitude and the direction of the current in the fifth wire? A. 15 A into the junction B. 15 A out of the junction C. 1 A into the junction D. 1 A out of the junction E. Not enough data to determine What are the magnitude and the direction of the current in the fifth wire? A. 15 A into the junction B. 15 A out of the junction C. 1 A into the junction D. 1 A out of the junction E. Not enough data to determine 21
Rank in order, from largest to smallest, the current densities J a to J d in these four wires. A. J c > J b > J a > J d B. J b > J a = J d > J c C. J b > J a > J c > J d D. J c > J b > J a = J d E. J b = J d > J a > J c Rank in order, from largest to smallest, the current densities J a to J d in these four wires. A. J c > J b > J a > J d B. J b > J a = J d > J c C. J b > J a > J c > J d D. J c > J b > J a = J d E. J b = J d > J a > J c 22
A wire connects the positive and negative terminals of a battery. Two identical wires connect the positive and negative terminals of an identical battery. Rank in order, from largest to smallest, the currents I a to I d at points a to d. A. I c = I d > I a > I b B. I a = I b > I c = I d C. I c = I d > I a = I b D. I a = I b = I c = I d E. I a > I b > I c = I d A wire connects the positive and negative terminals of a battery. Two identical wires connect the positive and negative terminals of an identical battery. Rank in order, from largest to smallest, the currents I a to I d at points a to d. A. I c = I d > I a > I b B. I a = I b > I c = I d C. I c = I d > I a = I b D. I a = I b = I c = I d E. I a > I b > I c = I d 23
Chapter 30. Reading Quizzes What quantity is represented by the symbol J? A. Resistivity B. Conductivity C. Current density D. Complex impedance E. Johnston s constant 24
What quantity is represented by the symbol J? A. Resistivity B. Conductivity C. Current density D. Complex impedance E. Johnston s constant The electron drift speed in a typical current-carrying wire is A. extremely slow ( 10 4 m/s). B. moderate ( 1 m/s). C. very fast ( 10 4 m/s). D. Could be any of A, B, or C. E. No numerical values were provided. 25
The electron drift speed in a typical current-carrying wire is A. extremely slow ( 10 4 m/s). B. moderate ( 1 m/s). C. very fast ( 10 4 m/s). D. Could be any of A, B, or C. E. No numerical values were provided. All other things being equal, current will be larger in a wire that has a larger value of A. conductivity. B. resistivity. C. the coefficient of current. D. net charge. E. potential. 26
All other things being equal, current will be larger in a wire that has a larger value of A. conductivity. B. resistivity. C. the coefficient of current. D. net charge. E. potential. The equation I = V/R is called A. Ampère s law. B. Ohm s law. C. Faraday s law. D. Weber s law. 27
The equation I = V/R is called A. Ampère s law. B. Ohm s law. C. Faraday s law. D. Weber s law. 28