Circuits Flashcards
metallic conductivity
in solid metals and the molten forms of some salt; free flow of electric charge due to metal atoms losing their outer electrons
electrolyte conductivity
seen in solutions; depends on the strength of a solution and can be used to determine the ionic concentrations in solutions
conductance
the reciprocal of resistance, a measure of permissiveness to current flow; units are siemens (S)
current (I)
the flow of charge between two points at different electrical potentials connected by a conductor; unit is ampere (1 A= 1 C/s)
I= Q/ delta t
Q= charge
t=time
how is charge transmitted?
through the flow of electrons in a conductor, moving from a point of lower electrical potential to a point of higher electrical potential
what is the direction of current?
in the direction in which positive charge would flow; from higher potential to lower potential. Current flows in the opposite direction of actual electron flow (charge)
direct current (DC)
the charge flows in one direction only
alternating current (AC)
the charge flow changes direction periodically
electromotive force (emf or e)
measured voltage when no charge is moving between the two terminals of a circuit that are at different potential values; it is not a force, it is the potential difference (voltage); units are J/C=V
what is an electric circuit?
a conducting path that usually has one or more voltage sources (battery) connected to one or more passive circuit elements (such as resistors)
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
I into junction= I 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
Vsource=Vdrop
resistance
the opposition within any material to the movement and flow of charge; motion is being opposed
R= pL/A
R=resistance
p=resistivity
L=length of the resistor
A=cross sectional area
conductors
give almost no resistance
insulators
give very high resistance
resistivity
number that characterizes the intrinsic resistance to current flow in a material; unit is Ohm-meter (omega x m)
length of the resistor
directly proportional to the resistance of a resistor; if a resistor doubles its length, the resistance will also be doubled
conduction pathways
the number of pathways that are available for charge to move through; this is the idea behind cross sectional areas affect on resistance, if you double the area, the resistance is cut in half
temperatures affect on resistance
most conductors have greater resistance at higher temperatures due to increased thermal oscillation of the atoms in the conductive material which produces a greater resistance to electron flow
Ohm’s law
states that for a given resistance, the magnitude of the current through a resistor is proportional to the voltage drop across the resistor
V=IR
V=voltage drop
I=current
R=magnitude of the resistance (ohms)
the voltage supplied by a cell to a circuit
V=Ecell - ir(int)
V=voltage provided the cell
Ecell= emf of the cell
i= current through the cell
r(int)= internal resistance
if no internal resistance is present, then the voltage is equal to the emf
galvanic (voltaic) cell vs electrolytic cell
a galvanic cell discharges, meaning that it supplies a current; while an electrolytic cell (secondary battery) recharges, meaning an external voltage is applied in such a way to drive current toward the positive end of the battery
power of a resistor
rate at which energy is dissipated by a resistor
P=IV=(I^2)R= (V^2)/R
I=current through the resistor
V=voltage drop across the resistor
R=resistance of the resistor
resistors in series
current has to travel through each resistor in order to return to the cell; energy is dissipated as electrons flow through each resistor, so there is a voltage drop through each resistor
resultant resistance in series
total resistance is just the sum of all the resistors
Rs= R1+R2+R3….
resistors in parallel
in this case, resistors are connected in parallel with a common high potential terminal and a common low potential terminal; the voltage drop is the same because all pathways originate from a common point and end at a common point within the circuit
for resistors in parallel, is the voltage and resistance the same for each pathway?
the voltage is the same for all parallel pathways, but the resistance of each pathway may differ; so electrons will prefer the path of least resistance, so current will be largest through the pathways with the lowest resistance
resultant resistance in parallel
there is a net reduction in resistance; could replace all resistors in parallel with a single resistor that has a resistance that is less than the resistance of the smallest resistor in the circuit
1/Rp=1/R1 + 1/R2 + 1/R3….
Rp will always decrease as more resistors are added
capacitors
have the ability to store and discharge electrical potential energy; they store an amount of energy in the form of charge separation at a particular voltage
capacitance
defined as the ratio of the magnitude of the charge stored on one plate to the potential difference (voltage) across the capacitor; unit is farad (1 F= 1 C/V)
C=Q/V
capacitance of parallel plate capacitor
C= eo (A/d)
eo= is the permittivity of free space (8.85x 10^-12 F/m)
A=area of overlap of the two plates
d= separation of the two plates
uniform electric field
created by the separation of charges
E= V/d
d=separation of the two plates
direction of the electric field is from the positive to the negative plate
potential energy in a capacitor
U= (1/2)CV^2
dielectric constant (k)
the value of the increase in capacitance when you stick a dielectric material (insulator) in between the plates of a capacitor; measure of the insulating ability of that material
capacitance due to a dielectric material
C’ = kC
C’= the new capacitance with the dielectric material present
what happens when a dielectric material is placed in a isolated, charged capacitor?
the voltage across the charged capacitor will decrease, thus increasing the capacitance
what happens when a dielectric material is placed in a charged capacitor within a circuit?
the charge on the capacitor increases because the voltage must remain constant in circuits. thus it still increases the capacitance
capacitors in series
the total capacitance decreases in similar fashion to the decrease in resistance in parallel resistors; this due to capacitors having to share the voltage drop n the loop so they cannot store as much charge
1/Cs= 1/C1 + 1/C2 + 1/C3….
Cs decreases as more capacitors are added, the voltage is the sum of the individual voltages
capacitors in parallel
produce a resultant capacitance that is equal to the sum of the individual capacitances
Cp= C1+C2+C3…
the voltage across each parallel capacitor is the same and is equal to the voltage across the source
meters
devices that are used to measure circuit quantities in the real world
ammeters
are inserted in series in a circuit to, used to measure the current at some point within a circuit; ideal ones have zero resistance and no voltage drop
voltmeter
are inserted in parallel in a circuit to measure a voltage drop (potential difference); have very large resistances
ohmmeters
are inserted at two points in series with a circuit element of interest, used to measure resistance; they are self powered and have negligible resistance, ideal resistance of zero
