Basic Electronics

Need to know

• Definition of basic electrical paramater

• A set of rules for elementary circuit analysis

• The means of current flow in circuits with capacitance

Electrical Parameters

• Potential Difference (V or E)

Charges

• Electrical charges exert an electrostatic force on one another.– Like charges are repelled from one another.– Unlike charges are attracted to one another.

Cont.

• As the distance between two charges increases, the force exerted is reduced.

cont

• Work is done when two charges that were initially separated are brought together.– Negative work is done if the polarities are

opposite– Positive work is done if the polarities are the

same

• The greater the values of the charges and the greater their initial separation the greater will be the work that is done.

• Work = ∫r0f(r)dr, where

• f is electrostatic force• r is the initial distance between the two charge.

• Potential difference is a measure of the wok done.

• The potential difference is the work done to move a unit of positive charge (1 coulomb) from one point to the other.

The Volt

• One volt is the energy required to move one coulomb a distance of 1 meter against a force of 1 newton

Current

• A potential difference exists in a system whenever positive and negative charges are separated.

• Charge separation may be generated by a chemical reaction (Volta’s battery) or by diffusion between two electrolyte solutions with different concentrations across a selectively permeable barrier such as a cell plasma membrane.

• To say a membrane is permeable is to say the membrane has a whole through which a give ion or ions can pass and the two chambers are in contact.

• If a region of charge separation exists within a conducting medium, then charges move between areas of potential difference

• The direction of these charge movements is: positive charges are attracted to the the region with the more negative potential, and negative charges are attracted to the regions of positive potential.

• The movement of charges is current flow.

Current Flow

• Current flow is defined as the movement of positive charges per unit time.

• In metal electrodes current is carried by electrons which move the opposite direction of current flow.

• In nerve and muscle cells current can be carried by positive and negative ions in solution

Units of current flow

• One ampere represents the movement of one coulomb per second.

Flow

• From Franklin’s view one can think of current flow in terms of bulk flow of a liguid due to hydorstatic pressure, flow of a solute in response to a concentration gradient or flow of heat in response to a temperature gradient.

Ease of flow vs. retardation of flow

• One can think of the flow being retarded or eased dependent on many different physical properties of the liquid, of the material the flow is moving through or the frictional forces operating on the material flowing.

• In so far as electrical flow is characterized there are two properties that will be used;

• CONDUCTIVITY OR RESISTIVITY

• It should be intuitively obvious that Resistivity and Conductivity are the reciprocal concepts: The ease of flow conductivity and the retardation of flow resistivity or:

• Resistance = (Conductance)-1

Ohm’s LAw• Ohms law says:

• There is a relationship between the driving force and the flow of current; they should be proportional.

• The proportionality constant is resistance.

• According to Ohm’s law; current, I, that flows through a conductor is directly proportional the potential difference imposed on it.

• That is:

• E = RI

• Where E is in volts

• R is in ohms

• I is in Amps

Ohm’s law variation

• If conductance (g) is the reciprocal of resistance then it follows

• E/R = I or rewritten, E x g = I where

• E is volts

• I is in amps

• g is in semens (was mhos)

Conductor

• The object through which an electric current flows is a conductor.

• As charges move through a conductor some of the energy is lost through the conversion to heat.

• This loss is called entropy

Conductivity (σ)

• Each type of material has an intrinsic property called conductivity (σ)

Metallic conductors

• Metallic conductors have very high conductivities – current moves easily.

• Different metals have different abilities to faithfully represent the current pulse implied on them.

Aqueous solutions

• Aqueous solutions containing high ionized salt concentrations have somewhat lower values of σ

Lipids

• Lipids have low σ and thus they are poor conductors but are good insulators.

• In resistance terms,

• Resistance (r) = σ (length/area)

• In conductance terms

• Conductance (siemens) = (Area/length)

Capacitance

• Capacitance is the ability to hold a charge of opposite sign: positive charges on one side, negative charges on the other side.

Capacitors

• Capacitors consists of plates separated by an insulating layer.

• The Leyden jar is a capacitor.

• The lipid portion of a plasma membrane can act like a capacitor.

• Symbolized as: Farads

Work done separates charges

• The picture represents two plates of a capacitor.

• One can measure this potential difference by determining how much work is required to move a positive “test” charge from the surface of y to that of x.

• There is a a net excess of positive charges on plate x and an equal number of negative charges on plate y, resulting in a potential difference between two plates.

• Initially, the test charge is attracted by the

negative charges on y and weakly repelled by the distant positive charges on x.

• As the test charge is moved to the left across the gap the attraction by the negative charges on y diminishes, but the repulsion by the positive charges on x increases.

Capacitance Electrostatic forces

• The results of the above electrostatic interactions is a force that opposes the movement of the tests charge from y to x.

Electrostatic force

• The net electrostatic force on the test charge is constant everywhere between x and y.

Work

• Work is force times distance or:

W = f x D

Farads (F)

• Capacitance is measured in farads (F)

• The greater the density of charges on the capacitor plates the greater the force acting on the test charge, and the greater is the resulting potential difference across the capacitor.

Capacitance charge and potential difference.

• A linear relationship exists between the amount of charge Q stored on a capacitors plates and the potential difference across the plates:

• Q (coulombs) = C (farads) x E (volts)

Capacitance variables

Capacitance of a parallel-plate capacitor is determined by: the area (A) of the two plates and the distance between them.

• Increase in charge density increases the potential difference

Increasing the area (A) of the plates increases capacitance because a greater amount of charge must be deposited on each side to produce the same charge density, which is what determines the electrostatic force operating on the test charge.

• Increasing the distance (D) between the two plates does not change the charge density but does increase the work to be done because the test charge must move a longer distance.

• See next slide

• The potential difference across a capacitor is determined by the excess of positive charges and negative charges on its plates. In order for the potential across a capacitor to change, the amount of electrical charges stored on the conductor must change.

• Given the above rules, to charge a capacitor from 0 to some higher value, the moment-to-moment change in potential difference of a given capacitor will be an increasing change of force due to and increase change in charge density.

Change in potential difference across a capacitor

• The plotted curve of this change in force will be an exponential curve.

• Thus the capacitor will charge in an instantaneous manner.

• We will see latter that resistors in parallel with capacitors will respond quite a bit differently.

• Symbols

• Battery

• Voltage source

• Resistor

• Capacitor

Current loop through a resistor

• Resistance in series add, while resistances in parallel add reciprocally

• An arrow designates direction of current flow (net movement of positive charges).

• Ohm’s law is:

– I = Vg = V/R

• The algebraic sum of all currents entering or leaving a junction is zero.

• We arbitrarily define current approaching a junction as positive and current leaving a junction as negative.

Battery and resistor in series

Resistors in parallel – current has alternative paths

• Ia = +3A

• Ib = -2A

• Ic = -1A

• Ia + Ib + Ic = 0

Battery – Capacitor circuit