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Lecture 1

Physiology 3140A Lecture Notes - Lecture 1: Electric Field, Lipid Bilayer, Potassium Channel


Department
Physiology
Course Code
PHYSIO 3140A
Professor
Donglin Bai
Lecture
1

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INTRODUCTION ELECTRIC CURRENTS FOR COMMUNICATION
Electric Potential - - the electric potential (electrostatic potential) at a point in space is the electrical
potential energy divided by charge at is associated with the static electric field
- Measured in volts
- electric field exerts a force on charged objects, accelerating them in the direction of the force
- In the same or opposite direction of the electric field
- if the object has a positive charge the force and acceleration will be in the direction of the
field
Electric Current - - the flow of electric charges or the rate of flow of electric charges; in a wire
(conductor) its carried by electrons, in an electrolyte its carried by ions and in plasma its carried by
both ions and electrons
- measured in amperes (charge flowing at one coulomb per second)
Electrical Resistance - - a measure of an objects opposition to a steady electric current; an object of
uniform cross section will have a resistance proportional to its length and inversely proportional to
its cross sectional area (and of course proportional to the resistivity of the material)
- measured in ohms
- resistance is measured as the proportion of voltage to current (R = V/I)
Capacitance - - the ability of a body to hold an electric charge; its also a measure of the amount of
electrical energy stored (or separated) for a given electrical potential
- measured in farads (one coulomb/volt)
Potential (volts) mV (10-3 V), uV (10-6 V)
Current (amperes) nA (10-9 A), pA (10-12 A)
Resistance (ohms) - m (10^6), G (10^9)
Conductance (Siemens) nS (10-9 S), pS (10-12 S)
DIRECTION OF ELECTRIC CURRENT
- the movement of positive charges
- these move from a point of (high) electric potential to low potential
- larger potential (voltage) difference, the larger the current
- current is also dependent on the medium that is carrying the current (ie smaller dismeter
cable offers a higher resistance than a larger diameter cable)
Ball Rolling Down Hill Analogy this is an analogy for current flowing from high to low potential;
the height is the potential, the speed of the ball reproduces the current and the surface the ball rolls
on is the resistance here voltage is seen as analogous to pressure and current is analogous to water
flow
OHM’S LAW (applies to electric circuits) - - the current (I) passing through a conductor is directly
proportional to the voltage difference (V) across the conductor and inversely proportional to the
resistance (R) of the conductor
Therefore I = V/R (or V = IR) and in some cases we use G (conductance) to replace resistance: G =
1/R
Therefore ohm’s law can be written I = GV
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