Capacitors and their electrical fields

 

The electrical field is a property of each point in space and is defined as proportional to the force experienced by a charge placed at that point.

The greater the potential difference between two points fixed in space, the greater the field at each point between them.

Formally, the electrical field is a vector defined as the negative of the spatial derivative of the potential.

 

The concept of the electrical field is important for understanding membrane function. Biological membranes are typically less than 10 nm thick. Consequently, a transmembrane resting potential of about 100 mV produces a very sizable electrical field in the membrane of about 10up5 V/cm.

This is close to the value at which most insulators break down irreversibly because their atoms become ionized!!!!

 

Of course, typical electrophysiological equipment cannot measure these fields directly. However, changes in these fields are presumably sensed by the gating domains of voltage-sensitive ion channels, which determine the opening and closing of channels, and so the electrical fields underlie the electrical excitability of membranes. Another consequence of the membrane's thinness is that it makes an excellent capacitor.

Capacitance (C; measured in farads, F) is the ability to store charge Q when a voltage Delta V occurs across the two "ends," so that

 

Q = C.Delta V

The formal symbol for a capacitor are two parallel lines.