Charge (q) __ results from an excess of protons (+) or electrons (-). Units are coulombs which measures how much excess + or - charge is present. A coulomb is thus a measure of a quantity of charge in a system. The charge on one electron is equal to 1.602 x 10-19 coulombs.
Current (I, A) __ the movement of charge through a medium in response to a potential gradient. In solids (e.g., a copper wire), current is usually the result of moving of electrons, but in solutions, it's generally cations and/or anions that are moving. Thus, physiologists will speak of a "sodium current", a "potassium current", a "cholide current", and so forth. Units are amperes (1 amp = coulomb sec-1). In biological systems, currents are usually measured in units of picoamperes (1 pA = 10-12 amp)
Resistance (R) __ a measure of how difficult it is for charges (electrons or ions) to move through a medium; units are Ohms. As defined by Ohm's Law, a resistance of one Ohm will produce a potential difference of one Volt between two points if a current of one ampere is passing through the resistive medium connecting the two points. Resistance per se is rarely used by physiologists, who generally prefer to use conductance.
Conductance (g) __ the reciprocal, or inverse, of Resistance (i.e., g = 1/R = R-1), conductance represents a measure of how easy it is for charges (electrons or ions) to move through a medium. Units are mhos ( = the inverse of ohms. Get it? Ha, ha, ha! Except it's not a joke…….) or, more commonly in biology, Siemens. In biological systems, currents are usually measured in units of picoSiemens (1 pS = 10-12 Siemen)
Potential (V, y, E, e) __ the form of energy that is capable of causing charges to move. In biological systems, results from the separation of + and – charges, usually on opposite sides of a barrier such as a plasma membrane. Defined from Ohm's Law:
E = IR Units are Volts (1 volt = the energy required to cause a current of 1 ampere to move through a medium whose resistance is equal to 1 ohm)
Capacitance ( C ) __ the ability to store charge, in particular + and – charges stored separately from each other. Units are farads (1 farad = 1 coulomb volt-1). In biological systems, capacitance is usually measured in units of microfarads (1 mf = 10-6 farad), and the capacitance of plasma membranes is usually in the range of 0.8 to 3 mf. Something with the ability to store charge is called a capacitor. When a capacitor is fully charged, the following relationship holds:
V = q/C where V is the potential difference between the two plates of the capacitor, q is the total charge stored by the capacitor, and C is its capacitance. Note that because of the equals sign, changing parameters on either side will change the value of parameters on the other side. Thus, changing the value of q will change the value of V, and changing the value of V will change the value of q (C is usually assumed to be constant). That last sentence is a very important concept to keep in mind when trying to understand membrane potentials.
Power (W) __ the rate at which work is done or energy is being 'used'. The standard unit is the watt (W), which is defined as 1 J s-1. In electricity, we can express 1 watt as the energy (heat) released when a current of 1 ampere flows 'through' a potential differeence of 1 volt, allowing us to calculate the power (in watts) from the relationship:
watts = volts * current
or
W = V*I = V2/RR-C Circuit __ an electrical circuit consisting of a Capacitor connected to a battery or other electromotive force through a Resistance. The electrical and temporal 'behavior' of an R-C circuit is determined by the capacitance (uf) and resistance (ohms) of its components. The electrical properties of R-C circuits are important to understand, because the plasma membranes of cells behave electrically as though they were composed of a number of parallel R-C circuits, one circuit for each kind of ion that is diffusing through the membrane. This R-C circuit behavior of plasma membranes is explicitly represented by the Equivalent Circuit Model of the plasma membrane.