Vcu Phis 501 Lecture 5

Front 1

Coulomb’s Law

Back 1

q= electrical point charges

r=distance

**FORCE IS STRONGLY EFFECTS BY DISTANCE.

Front 2

One coulomb is N0 /F charges

Back 2

where N0 is Avogadro’s number and F is the Faraday: 6.02 x 1023 charges mol-1 / 96,485 coulombs mol-1 = 6.24 x 10^18 elementary charges.

The charge on the electron = 1.6 x 10^-19 coulomb.

Front 3

potential energy = U

Back 3

We define the potential at any place A in space as being the work to bring a positive unit charge from infinite separation to point A.

Front 4

Because it takes work to bring a positive charge towards a positive fixed charge, the potential surrounding a positive charge is positive

Back 4

Conversely, since a positive test charge is accelerated towards a negative fixed charge, the potential around a negative charge is negative.

Front 5

Separation of charge produces a potential proportional to the charge. The potential around a positive charge is positive; the potential around a negative charge is negative.

Back 5

True

Front 6

Ohm’s Law relates current to potential

Back 6

I is given in amperes = coulomb/s

I= Q/ driving force

1/resistance = Q (conductance)

Front 7

Current is defined as the movement of positive charge

Back 7

so that movement of cations (positive ions) constitutes a current in the direction of the flow; movement of anions (negative ions) makes a current in the opposite direction to the flow.

Front 8

The current due to an ion is related to its flow by

Back 8

where current (I) is in coulombs s-1, flow (Q) is in mol s-1 and zF converts mol to coulombs. The sign of z shows that current is in the same direction as flow for cations, opposite for anions.

Front 9

conductors

Back 9

materials that offer little resistance

Front 10

insulators

Back 10

offer high resistance- examples are membrane lipids and the myelin sheath that surrounds nerve axons

Front 11

ohms law

Back 11

where I is the current, (fork thingy) is the potential difference, often symbolized as E or V, and R is the resistance.

where g = 1 / R is the conductance. The SI unit for conductance is the siemen = amp / volt

Front 12

for an axon interior

Back 12

d is the distance or length, A is the cross-sectional area and p is the specific resistance of the electrolyte fluid in the axon interior.

R has the units of ohm

Front 13

resistance of a patch of membrane

Back 13

R(m) / Area

Rm= specific resistance

Front 14

dielectric

Back 14

insulating material between 2 plates

Front 15

capacitive current- there is no flow across the dielectric but there is a flow in the circuit

Back 15

The flow of positive ions onto the top plate of the capacitor produces an electric field (a force per unit charge) that repels positive charges from the bottom plate. This movement of charges away from the bottom plate is the capacitive current. As the charges move away, there is a separation of charges on the capacitor and it now has a potential difference. Charges continue to move until the potential across the capacitor is equal but opposite to the potential across the battery

Front 16

Capictance *KNOW THIS!

Back 16

C is the capacitance, q is the charge separated and V is the voltage across the capacitor

Front 17

The ability to store charge depends on the physical characteristics of the capacitor

KNOW THE RLTSP between AREA and DISTANCE

Back 17

k= dielectric constant

x= distance that seperates plates

E= electrical permittivity

Front 18

t=RC

t= time

RC= TIME CONSTANT because it describes the time taken to charge the capacitor

Back 18

If t = RC, then q = qinfitnity (1 - 1/e)

q= the full charge (EC)

1/e = .37

Front 19

ions are subjected to two kinds of forces

Back 19

concentration gradient

potential gradient

* The forces due to these gradients exactly balance each other at the equilibrium potential given by the Nernst equation*

Front 20

T=RC defines how fast the capicator is charging

Back 20

charge=.63

current = .37 ?

Front 21

equilibrium potential is reached when the system is at equilibrium ∆G=0

Back 21

true

Front 22

NERST equation

Back 22

Front 23

For a solute with z=+1 and conversting ln to log, the Nerst equation is often written as

If z is negative, like CL-, then it would be -61.5log...!!

(when volts is in mV)***

Back 23

Front 24

The equilibrium potentials are those at which flux is zero for the indicated ion.

Back 24

True

Front 25

The resting potential is given by the Goldman-Hodgkin-Katz equation, also called the constant field equation because its derivation assumes that the electric field in the membrane is constant.

Back 25

Here Px is the permeability of ion X. Note that in this equation [K+]o and [Na+]o appear in the numerator, while [Cl-]I appears there. This is a consequence of their opposite signs of electric charge.

Front 26

The current carried by an ion is directly proportional to the flow of the ion

Back 26

true

Front 27

If the concentration of the ion is higher, there is a higher flow of ions, a greater current, in response to a potential difference than if the concentration is lower.

Back 27

True

Front 28

As Em moves away from the equilibrium potential, the current can be described as

Back 28

gNa and gK are the chord conductances for the respective ions.

ENa and EK are the equilibrium potentials. Em is the membrane potential.

Front 29

chord conductance

Back 29

slope of the chord connecting the curve to its reversal potential.

The chord conductance is the slope of the line connecting the GHK current to its equilibrium potential.

THEY ARE ALWAYS POSITIVE

Front 30

At rest the total current across the membrane is zero. It doesnt mean each current is 0, but they sum up to 0.

Itotal = 0

Back 30

If it were not, then the membrane would be building up charge or discharging and the membrane potential would not be stable - it would not be the resting condition.

Front 31

chord conductance equation. It basically says the resting membrane potential is the chord conductance weighted average of the equilibrium potentials for each of the ions

Back 31

Front 32

There is some small leak of ions, K+ out of the cell and Na+ into the cell, during rest.

Back 32

These leaks are kept in check by the Na-K-ATPase that pumps K+ back into the cell and Na+ out of the cell. This pump is electrogenic ( it carries a current because the stoichiometry of K+ is 2 and Na+ is 3.) Thus it carries an outward current that polarizes the cell.

At rest this makes a small contribution: inhibition of the pump by ouabain depolarizes the cell by about 2mV.