Transcript Basic Electronics - Western Washington University

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
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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
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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:
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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=fxD
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
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Ia = +3A
Ib = -2A
Ic = -1A
Ia + Ib + Ic = 0
Battery – Capacitor circuit