CN2011 Handout 1 Electrical circuits

CN2011 Handout 1 Electrical circuits - BIO5571 Electrical...

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BIO5571 Electrical circuits, Page 1 2011 I. ELECTRICAL CIRCUITS A circuit consists of a number of parts or elements hooked together. The connections between the elements (wires) are assumed to be perfect conductors so that charges can move through the wires completely freely. To make charges move, energy must be supplied in the circuit by providing a source of electrical potential energy between two points so that a voltage appears. A cell does this across its membrane by establishing an electro-chemical gradient. Another way to provide energy is to inject charges at some point in the circuit (that is, “pass a current into the cell”). The energy is provided by a battery or power supply that the experimenter hooks up. The cell resembles an electrical circuit in many ways, and the relationships between the charge on the cell membrane, the current through the membrane and the voltage across the membrane can be analyzed in terms of an “equivalent circuit.” One aspect of neuronal function which is very interesting is that the cell changes the equivalent circuit in response to stimuli. Another way of saying this is that the cell changes the ion channels which are open in its membrane in response to stimuli. Ions move across the membrane down their electrochemical gradients, change the charge on the cell membrane capacitance, and so change the membrane potential. Determining how the cell controls ionic currents across its membrane, and how the resulting voltage changes influence other processes in the cell, is of fundamental importance in understanding neuronal function. I.A. Basic quantities CHARGE : this reflects a fundamental property of an atom or molecule: atoms carry a net charge if the number of positively and negatively charged subatomic particles are not equal in the atom. The units of charges which are commonly used are Coulombs (C), and the abbreviation for charge is commonly Q or q. In terms of moles of a monovalent ion, 1 coulomb of charge = 6.242x10 18 charged particles. There are 6.023x10 23 particles in a mole of substance, so 1 mole of particles = 9.648x10 4 coulombs The number of coulombs in a mole of univalent charged particles is the Faraday constant (F). To convert an electrical charge measurement to the number of moles of charged particles present: Moles charge = Q/F (coulombs)/(coulombs/mole) CURRENT : an electrical current is an expression of the number of charges moving through some substance in a given time period: current (I) = # charges moved/time (Q/t). Usually this is expressed as a differential equation: I(t) = dQ(t)/dt The units of I are Amperes (A), where 1 A = 1 coulomb/second. To determine how many charges were moved during a given time by a given current, integrate I(t) over the time interval. Currents are given a sign because they have a direction; the usual convention is that the current between two points is positive if positive charges move from the first point towards the second point. VOLTAGE
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CN2011 Handout 1 Electrical circuits - BIO5571 Electrical...

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