Two 1.5-volt batteries in series were connected to a capacitor consisting of two very large metal disks placed very close to
each other, so that the disks became charged (left disk positive). The batteries were then removed.
Next, thick copper wires and a thick-filament bulb were attached to the charged capacitor, as indicated in Figure 20.71. Two
magnetic compasses lying on top of the copper wires initially pointed north. You find experimentally that when this thickfilament bulb is connected directly to the two batteries in series you observe a compass deflection of 15°.
(a) Briefly describe in detail what you would observe about the bulb and both compasses during the next minute. You
do not need to explain yet why this happens. Just describe in detail what you would see.
(b) At a time that is 0.01 second after assembling this circuit, draw and label the electric field at the 6 locations marked
× on the diagram. Pay attention to the relative magnitudes of your vectors. Make sure you have clearly labeled your
electric field vectors (for example, Ebulb, etc.).
(c) Sketch roughly a possible charge distribution on the diagram. If necessary for clarity, add comments on the diagram
about the charge distribution.
(d) The length of the left copper wire is L, the length of the right copper wire is also L, the length of the bulb filament
is Lbulb, and the gap between the capacitor plates is s. The diameter of the copper wires is d, and the diameter of
the bulb filament is dbulb. The mobility in the copper is u, and the mobility in the hot tungsten is ubulb. The number
of mobile electrons per unit volume is approximately the same in the copper and the tungsten. At a time that is 0.01
second after assembling this circuit, write two valid physics equations involving the magnitudes of the electric fields,
using the labels that you gave for the electric fields in part (b). What are the principles underlying these physics
equations?
(e) In part (a) you described what you would see happening with the bulb and compasses over the course of a minute.
Now explain qualitatively but in detail why this happens. Be brief but be complete in your explanation.