Explore Cellular Neurophysiology at UCL with a focus on Bioelectricity, Action Potentials & Ion Channels. Uncover the intricacies of action potentials, diffusion, equilibrium, and membrane potentials. Please note that these materials are intended for personal use only and should be used in accordan...
Introduction to Electricity
Introduction to Electricity
Electricity
o The force that drives out brain
Allows communication between neurons
o Concepts
Electricity = movement of charges
Transfer of charges from one place to another
Electrons are the carriers of charge in electrical circuits
In biological systems – ions carry charge across cell membranes
o Information process in the CNS – results from the operation of channels + transporter proteins =
enable + regulate the movement of ions across membranes
Conductors and insulators
o Electrons in an atom are confined in orbits around the nucleus
o Conductors
Electrons from outer cells can move easily from atom to atom
Cannot store charges
Whenever a potential difference is applied to a conductor electrons move in one
direction = impossible for conductor to store charges
o Insulators
Electrons tend to stay in their own orbital – not free to move
Can store charge
Usually molecules – not elements
Charge and its movement
o Charge (Q) – measured in coulombs (C)
o Charge of one electron or univalent ion = 1.6 x 10 -19C
o One coulomb = 6.35 x 1018 electrons
o Faradays constant – charge of one mole of univalent ions
Faradays constant = avogadro’s number x charge on each ion
F = Faradays constant = 96500 coulombs per mole
Avagadro’s number = NA = 6.022 x 10 23
Charge on each ion = e0 = 1.602 x 10-19
Current electricity
o Electric current (I) = flow of electrons or ions from one place to another
Rate of change of charge per unit time
I = current
Q = charge
t = time
Measured in amperes (A)
1 ampere = the flow of 1 coulomb of charge in 1 second
Potential difference
o Charge only moves if there is a potential difference between
two points = driving force of charge
o Measured in volts (V)
o Voltmeter – measures the potential difference
o Water analogy
,Introduction to Electricity
2 reservoirs – at different heights
Big height difference strong flow = high current
Small height difference weak flow = low current
Current = transfer of water from one reservoir to another
Potential difference = difference i n height between two containers
Voltmeter = measures difference in height between two reservoirs
Resistance – Ohms law
o For any given potential difference – the current that flows through an element of a circuit =
determined by its resistance
o Ease with which a charge moves through a conductor
Impedes the movement of charge
o Unit of resistance = ohm (Ω)
o Equation
V = IR
V = voltage
R = resistance
I = current
o Conductance = reciprocal of resistance
G=1/R
I = GV
o Unit of conductance = siemen (S)
o Water analogy
Constriction in pipe
Smaller constriction = less flow of water through resistance
Circuits topology
o Elements in an electrical circuit can be arranged in series / parallel / combination of both
o Water analogy
Pump = battery – provides potential difference
Water = electrons
Flow of water = electric current
Resistance = narrowing of pipes
Series circuit
Parallel circuit
Flow of water splits – F1 + F2
o F1 goes down one pipe
Encounters one resistance
o F2 goes down another pipe
,Introduction to Electricity
Encounters another resistance
2 flows of water re-join after resistances
o Resistances in series and parallel
Series
Current is same throughout the circuit
o V = IR1 + IR2 = I(R1 + R2)
Rtotal = R1 + R2
Resistance in series add
Voltage divider
o I = V / (R1 + R2)
o Voltage difference across resistances
V1 = IR1
V2 = IR2
o Battery voltage = addition of voltage across resistances
Parallel
I = I1 + I2 = V/R1 + V/R2 = V(1/R1 + 1/R2)
o 1/Rtotal = 1/R1 + 1/R2
Resistances in parallel add as their reciprocal
Capacitance
o An insulator can store charges
o Capacitor
Arrangement of an insulator between two conductors = allows storage of
charges in an electrical circuit
o Capacitor connected to a battery electrons build up on one plate – repelling
electrons from the other plate resulting in one negative and one positive plate
, Introduction to Electricity
Once each plate is fully charged up electron flow stops charge is stored on plates
o Charge stored in a capacitor = proportional to the applied voltage
Q = CV
Q = charge stored in capacitor
C = capacitance – how much charge can be stored for a given voltage
o Higher capacitance = more charge can be stored
V = voltage across 2 plates
o Units of capacitance = farad (F)
1 F = capacitance of an element that can store 1C of charge given a 1V potential difference
o Factors affecting capacitance
Plate area
Larger plates = more capacitance
Plate spacing
Closer plates = more capacitance
Dielectric material – insulator between the plates
Good insulator between plates = more capacitance
C = capacitance in Farads
ε = absolute permittivity of dielectric
A =area of plate overlap in m2
D = distance between plates in meters
o Can be connected in series of in parallel
In parallel – total capacitances = sum of all capacitances
Ctotal = C1 + C2 + C3 + … + Cn
In series – 1/total capacitances = sum of reciprocal capacitances
1/Ctotal = 1/C1 + 1/C2 + 1/C3 + … + 1/Cn
o Water analogy
Capacitor = elastic membrane in pipe
Blow pushes water in one direction
Elastic membrane expands until reaching equilibrium with strength of water
stops resulting in no flow of water
Current in a circuit containing a capacitor – depends on the rate of change of voltage
o Faster change in voltage = more current flows
Direct and alternating current
o Direct current (DC) sources
Direct flow of ions in one direction – without changes in polarity
Generated by a battery
o Alternating current (AC) sources
Periodically reverse their polarity
UK mains – sinusoidal – frequency of 50Hz + voltage of 240V
Time dependent circuits – the RC circuit
o Battery = generats a potential difference
o Resistor
o Capacitor
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