Electricity and Magnetism II Griffiths Chapter 7 Maxwell’s Equations Clicker Questions 7.1

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  • Slide 1
  • Electricity and Magnetism II Griffiths Chapter 7 Maxwells Equations Clicker Questions 7.1
  • Slide 2
  • In the interior of a metal in static equilibrium the charge density is: A)zero always B)never zero. C)sometimes zero, sometime non-zero, depending on the conditions. 7.2
  • Slide 3
  • Charge Conservation Which of the following is a correct statement of charge conservation? A) B) C) D) E) None of these, or more than one. 7.3
  • Slide 4
  • For everyday currents in home electronics and wires, which answer is the order of magnitude of the instantaneous speed of the electrons in the wire? A.more than km/s B.m/s C.mm/s D.m/s E.nm/s 7.4
  • Slide 5
  • An electric current I flows along a copper wire (low resistivity) into a resistor made of carbon (high resistivity) then back into another copper wire. In which material is the electric field largest? A.In the copper wire B.In the carbon resistor C.Its the same in both copper and carbon D.It depends on the sizes of the copper and carbon I 7.5
  • Slide 6
  • Rank order (from greatest to smallest, e.g. A=C>B) Magnitude of E field Conductivity Current Current Density A copper cylinder is machined to have the following shape. The ends are connected to a battery so that a current flows through the copper. University of Colorado, 2011 A C B 7.6
  • Slide 7
  • Inside this resistor setup, what can you conclude about the current density J near the side walls? A)Must be exactly parallel to the wall B)Must be exactly perpendicular to the wall C)Could have a mix of parallel and perp components D)No obvious way to decide!? I I J?? V0V0 V=0 7.7
  • Slide 8
  • Inside this resistor setup, (real world, finite sizes!) what does the E field look like inside ? A)Must be uniform and horizontal B)Must have some nonuniformity, due to fringing effects! I I E?? V0V0 V=0 7.8
  • Slide 9
  • In the necking down region (somewhere in a small-ish region around the head of the arrow), do you think A) B) Recall the machined copper from last class, with steady current flowing left to right through it University of Colorado, 2011 II 7.9
  • Slide 10
  • In steady state, do you expect there will be any surface charge accumulated anywhere on the walls of the conductor? A) Yes B) No Recall the machined copper from last class, with steady current flowing left to right through it University of Colorado, 2011 II 7.10
  • Slide 11
  • The resistivity, is the inverse of the conductivity, . The units of are: A)Amps/volt B)V/m C)m/ohm D)ohm*m E)Joules/m 7.11
  • Slide 12
  • A steady electric current I flows around a circuit containing a battery of voltage V and a resistor R. Which of the following statements about is true? A.It is zero around the circuit because its an electrostatic field B.It is non-zero around the circuit because its not an electrostatic field C.It is zero around the circuit because there is no electric field in the battery, only in the rest of the circuit D.It is non-zero around the circuit because there is no electric field in the battery, only in the rest of the circuit! E.None of the above 7.12
  • Slide 13
  • EMF is the line integral of the total force per unit charge around a closed loop. The units of EMF are: A)Farads B)Joules. C)Amps, (thats why current flows.) D)Newtons, (thats why its called emf) E)Volts 7.13
  • Slide 14
  • Imagine a charge q able to move around a tube which makes a closed loop. If we want to drive the charge around the loop, we cannot do this with E-field from a single stationary charge. q + + Can we drive the charge around the loop with some combination of stationary + and charges? A)Yes B)No 7.14
  • Slide 15
  • A circuit with a battery with voltage difference V is attached to a resistor. The force per charge due to the charges is E. The force per charge inside the battery is f = f bat + E How many of the following statements are true? A)0 B)1 C)2 D)3 E)4 A B 7.15
  • Slide 16
  • ++++++++++++++++ Is there a nonzero EMF around the (dashed) closed loop, which is partway inserted between two charged isolated capacitor plates. A)EMF=0 here B)EMF0 here C) ? I would need to do a nontrivial calculation to decide - - - - - - - - 7.16
  • Slide 17
  • One end of rectangular metal loop enters a region of constant uniform magnetic field B, with initial constant speed v, as shown. What direction is the magnetic force on the loop? A.Up the screen B.Down the screen C.To the right D.To the left E.The net force is zero B B v L 7.17
  • Slide 18
  • One end of rectangular metal loop enters a region of constant uniform magnetic field B, with initial constant speed v, as shown. What direction is the magnetic force on the loop? A.Up the screen B.Down the screen C.To the right D.To the left E.The net force is zero B B v L 7.18
  • Slide 19
  • One end of rectangular metal loop enters a region of constant uniform magnetic field B, out of page, with constant speed v, as shown. As the loop enters the field is there a non-zero emf around the loop? A.Yes, current will flow CW B.Yes, current will flow CCW C.No B B v L 7.19
  • Slide 20
  • A rectangular metal loop moves through a region of constant uniform magnetic field B, with speed v at t = 0, as shown. What is the magnetic force on the loop at the instant shown? Assume the loop has resistance R. A.2L 2 vB 2 /R (right) B) 2L 2 vB 2 /R (left) C) 0 B.D. Something else/not sure... B B v L w 7.20
  • Slide 21
  • Consider two situations: 1) Loop moves to right with speed |v| 2) Magnet moves to left with (same) speed |v| What will the ammeter read in each case? (Assume that CCW current => positive ammeter reading) B A A) I 1 >0, I 2 =0 B) I 1 = I 2 > 0C) I 1 = -I 2 > 0 D) I 1 = I 2 = 0 E) Something different/not sure 7.21
  • Slide 22
  • Faraday found that EMF is proportional to the negative time rate of change of B. EMF is also the line integral of a force/charge. The force is A)The magnetic Lorentz force. B)an electric force. C) the strong nuclear force. D)the gravitational force. E)an entirely new force. 7.22
  • Slide 23
  • A time changing B creates an electric field via Faradays Law: A)Now I have no idea how to find E. B)This law suggests a familiar way to find E in all situations. C)This law suggests a familiar way to find E in sufficiently symmetrical situations. D)I see a path to finding E, but it bears no relation to anything we have previously seen. 7.23
  • Slide 24
  • A stationary rectangular metal loop is in a region of uniform magnetic field B, which has magnitude B decreasing with time as B=B 0 -kt. What is the direction of the field B ind created by the induced current in the loop, in the plane region inside the loop? A) Into the screen B) Out of the screen C) To the left D) To the right E) other/?? B B w 7.24
  • Slide 25
  • A stationary rectangular metal loop is in a region of uniform magnetic field B, which has magnitude B decreasing with time as B=B 0 -kt. What is the direction of the field B ind created by the induced current in the loop, in the plane region inside the loop? A) Into the screen B) Out of the screen C) To the left D) To the right E) other/?? B B w 7.25
  • Slide 26
  • A rectangular metal loop is moving thru a region of constant uniform magnetic field B, out of page, with constant speed v, as shown. Is there a non-zero emf around the loop? B B v A.Yes, current will flow CW B.Yes, current will flow CCW C.No 7.26
  • Slide 27
  • On a piece of paper, please write down your name, Stokes theorem, and the Divergence Theorem 7.27
  • Slide 28
  • A loop of wire is near a long straight wire which is carrying a large current I, which is decreasing. The loop and the straight wire are in the same plane and are positioned as shown. The current induced in the loop is A) counter-clockwiseB) clockwiseC) zero. I to the right, but decreasing. loop 7.28
  • Slide 29
  • The current in an infinite solenoid with uniform magnetic field B inside is increasing so that the magnitude B in increasing with time as B=B 0 +kt. A small circular loop of radius r is placed NON- coaxially inside the solenoid as shown. What is the emf around the small loop? (Assume CW is the direction of dl in the EMF loop integration) B r A.kr 2 B.-kr 2 C.Zero D.Nonzero, but need more information for value E.Not enough information to tell if zero or non-zero 7.29
  • Slide 30
  • The current in an infinite solenoid with uniform magnetic field B inside is increasing so that the magnitude B in increasing with time as B=B 0 +kt. A small circular loop of radius r is placed outside the solenoid as shown. What is the emf around the small loop? (Assume CW is the positive direction of current flow). A.kr 2 B.-kr 2 C.Zero D.Nonzero, but need more information for value E.Not enough information to tell if zero or non-zero B r 7.30
  • Slide 31
  • The current in an infinite solenoid of radius R with uniform magnetic field B inside is increasing so that the magnitude B in increasing with time as B=B 0 +kt. If I calculate V along path 1 and path 2 between points A and B, do I get the same answer? A.Yes B.No C.Need more information B R A B Path 1 Path 2 7.31
  • Slide 32