An introduction to electrical circuits, and their use in physiology
An introduction to electrical circuits, and their use in physiology
Introduction
This material represents an introduction to electrical circuits, and their use as models in neurobiology and cardiovascular physiology. It is designed to be helpful to those who have not studied physics recently. It is not necessary to go through the maths if you don't want to, but there is nothing more than simple algebra here, so it should not be hard to follow the equations!
Some of this material will be expanded upon in first-year physiology lectures, but the basic theory behind electrical circuits is something that you would be expected to understand. The pages on capacitance represent more advanced material which is taught within the physiology courses themselves: supervisors will be able to help you with this material when it comes up in your course.
Some definitions
Charge (Q) arises from a net imbalance between the number of positively- and negatively-charged particles in a given place. Net charge may therefore be positive or negative. Charge is measured in coulombs; Faraday's constant, 96,485 coulombs, is the amount of charge carried by one mole of electrons (this value can be useful in calculation questions). Charged particles important in biology include electrons and ions (N.B. a hydrogen ion, H+, is actually a proton!).
Current (I) is a flow of electrically charged particles. It is measured in coulombs per second, otherwise know as amperes ("amps"). In an electrical circuit, the charged particles are electrons, which flow in the wires. In electrically-conducting solutions, such as saline, Ringer solution or the fluids of the body, the charged particles which flow are ions, e.g. Na+, K+ and Cl-.
Voltage (V)
is a measure of the potential difference between
two points, one of which is at a positive potential relative
to the other. If a ball is lifted above the ground, it
possesses potential energy due to the effect of gravity;
when the ball drops, it moves down its potential energy
gradient. In a similar way, an electrical voltage represents
an electrical potential energy gradient, down
which charged particles would like to move, if permitted.
A potential difference can therefore be thought of as
a driving force which tends to push charged particles
around. For example, because they are negatively charged,
electrons would tend to move from a region of negative
potential to a region of more positive potential, whereas
sodium ions (Na+) would move from positive
to negative. The word "potential" reminds us that the
charged particles in question may not actually be able to
move, even though they experience the driving force.
As an example, a household battery generates a potential difference between its positive pole and its negative pole: most batteries have the value of this potential difference in volts printed on the side (e.g. 3V). Of course, if you are simply holding the battery in your hand, the potential difference, although it exists, is not doing anything useful, but it can be used to drive an electrical current if the battery is connected into a circuit (see diagram).
The convention in electrical circuits is to portray current as if it were the flow of positive charge: in the diagram, the current arrows therefore pass from the positive pole to the negative pole of the battery. In a real wire, we know that it is actually electrons which flow, and they are negatively charged – they would, in fact, travel in the opposite direction! The end result, of course, is the same.
If there are other physics-related areas that you would like to see introduced in a similar way on-line, please contact Dr Matt Mason