Basic Electronics

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Basic Electronics
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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

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Electrical Parameters

• Potential Difference (V or E)

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Charges

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

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Cont.

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

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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

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• 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.

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• 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.

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The Volt

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

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Current

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

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

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• 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.

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

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• 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.

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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.

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

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Units of current flow

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

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Flow

• From Franklins 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.

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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.

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

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• 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

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Ohms 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.

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

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• That is
• E RI
• Where E is in volts
• R is in ohms
• I is in Amps

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Ohms 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)

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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

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Conductivity (s)

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

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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.

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Aqueous solutions

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

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Lipids

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

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• In resistance terms,
• Resistance (r) s (length/area)
• In conductance terms
• Conductance (siemens) (Area/length)

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Capacitance

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

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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

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Work done separates charges
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• The picture represents two plates of a capacitor.

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• 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.

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• 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.

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• Initially, the test charge is attracted by the
• negative charges on y and weakly repelled by
  the distant positive charges on x.

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• 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.

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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.

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Electrostatic force

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

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Work

• Work is force times distance or
• W f x D

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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.

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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)

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Capacitance variables

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

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• Increase in charge density increases the
  potential difference

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• 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.

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• 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

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• 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.

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• 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.

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Change in potential difference across a capacitor

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

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• 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.

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• Symbols
• Battery
• Voltage source
• Resistor
• Capacitor

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Current loop through a resistor
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• Resistance in series add, while resistances in
  parallel add reciprocally

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• An arrow designates direction of current flow
  (net movement of positive charges).
• Ohms law is
• I Vg V/R

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• 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.

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Battery and resistor in series
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Resistors in parallel current has alternative
paths
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• Ia 3A
• Ib -2A
• Ic -1A
• Ia Ib Ic 0

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Battery Capacitor circuit
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