Unit 4
1. In terms of electric pressure, describe a charged capacitor.
A charged capacitor has higher-than-normal electric pressure on one plate
because of excess charge, and lower-than-normal pressure on the other plate
because of depleted charge. The amount of pressure difference depends on
the pressure difference of the terminals of the battery used to charge the
capacitor.
2. In terms of electric pressure, explain why a capacitor stops charging.
A capacitor stops charging when the compression of charge in its (+) plate
raises its pressure to that of the high-pressure source (a battery, another
capacitor) connected to it, and when the depletion of charge in the (-) plate lowers
its pressure to that of the low-pressure source connected to it.
3. For this question, assume that you start with a circuit containing a battery, two long
bulbs and a charged capacitor (Figure 3a).
L L
L L
Figure 3a Figure 3b
CHARGED CAPACITOR SECOND BATTERY ADDED
Predict what you would observe when an additional identical battery is added to the circuit
in Figure 3a (as shown in Figure 3b):
The bulbs will light as brightly and for the same length of time as when the
capacitor was first charged.
Explain your reasoning:
The difference between the electric pressures on the outer battery terminals is
now twice what it was before. Additional charge must travel to and be
compressed in the (+) capacitor plate, and leave the (-) capacitor plate before
the pressure difference between the plates is again equal to the pressure
difference between the battery terminals.