C & S Chapter 7 Notes

 

7.1   Hook up the long bulb and time how long it glows, then do the same for the short bulb.

Notice:  i starts high and then goes down sharply.

 

7.2  Observe the direction of i in charging vs  discharging.  Wrap the wire around the compass several time to magnify the outcome.

 

RBD.  io = current without the capacitor in the circuit.

 

7.3a  Same!

 

7.4 a&b  Get each group to do this and put results in a well organized table.  It's easy.  Use a switch for precise timing.  Turn out the lights.  Just charge and discharge the capacitor with both bulbs 3 or  4 times to get a good average of how long it takes lite to go out when charging  and discharging .  You need to let it continue to charge for at least a minute after it goes out before discharging.  (Make a table of values.)  Then charge using one bulb and discharge using the other bulb several times and compare discharge times with when charged by same bulb.

 

7.5.  Be able to explain this in terms of fringe fields.

 

7.6.1   From page 142 Efringe = Q s/(2e0AR), so E is proportional to Q , s and 1/R.  So the electric field is much smaller per unit charge on a large capacitor than it is on a small one.

7.6.2   E is also proportional to s.  So E is much smaller per unit charge when s is small.  (smaller s the slower the build up of fringe field.)

 

7.6.3  The effects of the plastic, (called a "dielectric")  is caused by polarization of the plastic.  The polarization causes another fringe field opposite the direction of the fringe field due to charge on the plates, which slows down the build up fringe field to oppose the battery.

 

7.6.3b  The greater the polarization the greater the effect.

 

7.6.3c  In this case, polarization is total, or a maximum.  So this would slow down the build up of fringe field the most.

7.6.4  Both the battery and capacitor would be pushing electrons in the same direction, hence we'd expect discharge to be twice as fast. 

7.6.5  Do it, it looks neat!

 

7.6.6  RBD  Caps in series reduce total capacity to hold charge!

 

 7.7b  Not a good answer.  - The energy emitted by the bulb is not necessarily the same amount stored. 

Hwk 7-3  Have them do this  in class!