ch 6 - circuits Flashcards
current
considered the flow of positive charge even though only negative charges are actually moving
metallic conductivity
solid metals and molten forms of some salts
electrolytic conductivity
seen in solutions
conductance
reciprocal of resistance, property we will examine in detail later
SI unit for conductance
siemens (S) sometimes given as siemens per meter (S/m) for conductivity
metallic bond
an equal distribution of charge density of free electrons across all of the neutral atoms within the metallic mass
how to measure conductivity of electrolyte solution
place solution as a resistor in a circuit and measure changes in voltage across the solution
electrical current
the flow of charge between two points at different electrical potentials connected by a conductor (such as copper wire)
magnitude of current
I(i) = Q/change in t; amount of charge Q passing through the conductor per unit time
SI unit of current
ampere (1 A = 1 C/s)
how would positive charge flow if it flowed (direction of current)
from higher electrical potential to lower potential
direct current
tested to exclusion of alternating current (AC) on mcat; charge flows in one direction only
potential difference (voltage)
produced by electric generator, galvanic (voltaic) cell, a group of cells wired into a battery, etc.
electromotive force (emf or epsilon)
when no charge is moving between the two terminals of a cell that are at different potential values; not actually a force but is a potential difference measured in joules per coulomb (1 V = 1 J/C)
Kirchhoff’s junction rule
at any point or junction in a circuit, the sum of currents directed into that point equals the sum of currents directed away from that point; expressed as I (sub into junction) = I (sub leaving junction)
Kirchhoff’s loop rule
around any closed circuit loop, the sum of voltage sources will always be equal to the sum of voltage (potential) drops; V (sub source) = V (sub drop)
true of closed loops and not necessarily entire circuits
Resistance
opposition within any material to the movement and flow of charge; insulators have very high resistance; conductors have very low resistance
resistors
conductive materials that offer amounts of resistance between that provided by conductors and insulators; dependent on characteristics of it like resistivity, length, cross-sectional area, and temp.
equation for resistance
R = (fancy p x L)/A fancy p = resistivity, L = length of the resister, A = cross-sectional area
resistivity
intrinsic resistance to current flow in a material represented by fancy p; SI unit is ohm-meter (omega x m)
Length of resistor
resistance is directly proportional to length of resistor; longer means electrons will have to travel greater distance through a resistant material; if resistor doubles in length, resistance will also double
Cross-sectional area of resistors
inverse proportionality to resistance; if cross-sectional area is doubled, resistance is cut in half increasing number of conduction pathways
Temperature of resistors
most conductors have greater resistance at higher temps due to increased thermal oscillation of the atoms in the conductive material which produces greater resistance to electron flow
Ohm’s Law
states that for a given magnitude of resistance, the voltage drop across the resistor will be proportional to the magnitude of the current. And for a given resistance, the magnitude of the current will be proportional to the magnitude of the emf (voltage) impressed upon the circuit. V = IR where V = voltage drop, I is the current and R is the magnitude of the resistance measured in ohms (omega symbol)
actual voltage supplied by a cell to a circuit due to internal resistance
it is less: V = E sub cell - ir sub int where V is the voltage provided by the cell, E sub cell is the emf of the cell, i is the current through the cell and r sub int is its internal resistance
internal resistance
there is a measure of internal resistance by every conductor; if the cell is not actually driving any current the internal resistance is 0 and voltage of cell is = to emf. If not zero, then voltage is less than emf
secondary batteries
certain type of power cells that can be recharged by an external voltage applied in such a way to drive current toward the positive end of the secondary battery rather than the typical of moving from the positive (higher potential) end to the negative (lower potential) end; acts as galvanic cell when discharging and electrolytic cell when recharging
Power equation
ratio of work (energy expenditure) to time: P = W/t = delta E/t
equation for rate at which energy is dissipated by a resistor
= the power of a resistor: P = IV = I^2R = V^2/R; I = current through resistor; V = voltage drop across resistor; R = resistance of resistor
series
one of two ways in which resistors can be connected into a circuit, all current must pass sequentially through each resistor connected in a linear arrangement
parallel
one of two ways in which resistors can be connected into a circuit, current will divide to pass through resistors separately
total voltage drop for series of resistors
V sub S = V sub 1 + V sub 2 + … + V sub n
resistances of resistors in series
R sub S = R sub 1 + R sub 2 + …. + R sub n
equivalent or resultant resistance
set of resistors wired in series can be treated as a single resistor with a resistance equal to the sum of the individual resistances
voltage in circuits with parallel arrangements of resistors
V sub P = V sub 1 = V sub 2 … = V sub n because all lines originate at a common high potential terminal and move to common low potential terminal
equivalent resistance of resistors in parallel
1/R sub p = 1/R sub 1 + 1/R sub 2 + 1/R sub 3 + … + 1/R sub n; total will always decrease as more resistors are added
when some number (n) of identical resistors are wired in parallel what is total resistance?
R/n note: (I (current) sub total)/n is also true for this
Four physical quantities that determine resistance of a resistor
cross-sectional area, resistivity, length, temp
Capacitors
characterized by their ability to hold charge at a particular voltage; ex defibrillator
capacitance
the ratio of the magnitude of the charge stored on one plate to the potential difference (voltage) across the capacitor; If voltage is applied across the plates of a capacitor and a charge (Q) collects on it (+Q at positive end and -Q at negative plate), then capacitance = C = Q/V
SI unit for capacitance
farad (1 F = 1 C/V) also given in microfarads (1 fancy uF = 1 x 10^-6 F) or picofarads (1 pF = 1 x 10^-12 F)
Do not confuse the farad with faraday constant:
faraday constant is amount of charge in one mole of electrons (96,485 C/mol e-)
parallel plate capacitance
C = epsilon sub 0 (A/d); epsilon sub 0 = permittivity of free space (8.85 x 10^-12 F/m); A = area of overlap of the two plates; d = distance of separation of the two plates
uniform electric field
set up by the separation of charges between plates with parallel field vectors in capacitors: E = V/d
potential energy stored in a capacitor
U = 1/2 x CV^2
dielectric materials
insulation
dielectric constant (fancy K)
this is the factor by which an added insulator increases capacitance when introduced between the plates of a capacitor; arises from a decrease in voltage and increase in stored charge
equation for capacitance due to dielectric material
C’ = fancy K x C; C’ = new capacitance with the dielectric present; C = original capacitance; fancy K = dielectric constant
What happens to total capacitance in series if more capacitors are added
it decreases (resistance increases)
equation for calculating equivalent capacitance for capacitors in series
1/C sub S = 1/C sub 1 + 1/C sub 2 + 1/C sub 3 + … + 1/C sub n
what happens to capacitance when capacitors are added in parallel
capacitance increases (resistance decreases)
ammeters
used to measure the current at some point within a circuit
voltmeters
requires circuit to be active; also use magnetic properties of current-carrying wires; used to measure voltage drop across two points in a circuit
ohmmeters
used like ammeters but with a circuit that is not active. Measures resistance through calculation using Ohm’s Law
equation for current
I = Q/change in time
