EE115, Capacitive reactance

1. Discuss why it is that for a discharging capacitor, the discharge current is in the direction opposite from the charging current.

2.Explain why capacitive reactance is measured in ohms.  Also, state two differences between capacitance and capacitive reactance.


1. A capacitor is a system made of two plates separated by a dielectric. This physical arrangement has the property to store a certain charge Q under a certain voltage U. By DEFINITION the capacitance C = Q/U. Now imagine you want to charge up this system. You need, let say to bring a positive charge to the upper plate while depleting the lower plate of positive charges. This will charge the capacitor with positive charge on the upper plate and negative charge on the lower plate and means in turn a positive current INTO the upper plate and also a positive current OUT of the lower plate (remember by physical convention a positive current is made of positive charges). You just charged you capacitor with a positive voltage on the upper plate and a negative voltage on the lower plate.

Suppose now you discharge the capacitor. After discharging the voltage on the capacitor will be zero. When discharging the capacitor will behave exactly like a Voltage source (+ to upper plate, – to lower plate). Therefore the positive current will flow from OUT of the upper plate and INTO the lower plate. This way the discharge current is opposite to the charging current.

2. By DEFINITION resistances, reactances and impedances are all measured in ohms. I repeat this is by their physical DEFINITION. It means that regardless you have a resistance, a reactance or a capacitance all 3 will have the same effect on the flow of an electric current (i.e. they oppose the flow) and this means all 3 have the same measuring unit.

First: Capacitance means the property of a system to store charge. The smaller the voltage under which the charge is stored the higher the capacitance. Capacitive reactance is the property of a capacitor to oppose the flow of an alternating current through it.

Second: Capacitance is measured in FARADS while capacitive reactance is measured in OHMS.

Third. While the CAPACITANCE is denoted by the symbol C (and therefore it is equal to C) the CAPACITIVE REACTANCE is equal to 1/(omega*C) (omega is the current angular speed) and therefore it is inversely proportional to C (the Capacitance)