How a Capacitor Discharge

How does a capacitor work?

When a capacitor is connected across a power source, it starts charging up as current flows in the circuit.

capacitor

 

Initially when the capacitor has not stored any charge, its potential difference is 0 V. As current flows, charges start storing across its plates, and the potential difference increases. The current also starts to decrease because the capacitor reduces the overall e.m.f of the circuit. When the potential difference of the capacitor equals to the e.m.f. of the power source, current stops flowing.

capacitor2

After the capacitor is fully charged, the switches are set to the figure above. The capacitor starts discharging and a current flows in the left part of the circuit. The potential difference across the capacitor follows an exponential curve.

capacitor3

Wolfram Demonstration

Click this Wolfram Demonstration Project link to access the simulation. This simulation shows how the potential difference across the capacitor decreases during a discharge. The rate of discharge is dependent on the capacitance of the capacitor. A larger capacitor stores more charge. Although the maximum voltage across it is dependent on the power source, a large capacitor produces a larger current.

Procedure:

  1. Adjust all the controls to the left such that d = 0.4, L = 1.5 and time = 0.
    1. Observe that the capacitor discharges almost all its stored charge after about 50 s.
    2. The capacitor has a capacitance of 0.481\mu \text{F} and its maximum stored charge is 21.6 \mu \text{C}
  2. Increase the separation d to 0.7.
    1. Observe that the capacitance is now 0.275\mu \text{F} and maximum charge 12.4 \mu \text{C}. This means that increasing the distance between the plates decreases the capacitance, and correspondingly the charge stored, since Q=CV .
    2. Lesser stored charge means that the current stops flowing after about 30 s.
  3. Increase the length of side to 3.0.
    1. Observe that the capacitance is now 1.1 \mu \text{F} and that the charge stored is now 49.4 \mu \text{C} . Increasing the cross-sectional area increases the capacitance and the stored charge.
    2. Since the stored charge is larger, the capacitor takes a longer time to discharge to 0 V.

Summary

  1. Capacitance depends on the physical property. The capacitance is larger is the distance between the plates is small and the cross-sectional is large.
  2. A large capacitor stores a larger charge, and takes a longer time to discharge. Conversely, it should take a larger time to charge a large capacitor to its maximum voltage.
print