Ch. 6: Circuits Flashcards
defn: current
the flow of positive charge (by historical convention), even though only negative
what are the two types of conductivity?
metallic conductivity –> seen in solid metals and the molten forms of some salts
electrolytic conductivity –> seen in solutions
defn + SI unit: conductance
the reciprocal of resistance
SI unit: siemens (S), sometimes given as Siemens per meter for conductivity
defn: electrical conductors
materials that allow free flow of electric charge within them
why are most metals good electrical and thermal conductors?
because metal atoms can easily lose one or more of their outer electrons, which are then free to move around in the larger collection of metal atoms
defn: metallic bond
an equal distribution of the charge density of free electrons across all of the neutral atoms within the metallic mass
what does electrolytic conductivity depend on?
the strength of the solution
is conductivity in ionic or nonionic solutions lower?
nonionic
SI unit: current
ampere
how is charge transmitted?
by a flow of electrons in a conductor
do electrons move from high to low electrical potential or vice versa? why?
from lower electrical potential to higher (and in doing so, reduce their electrical potential energy)
why? because they are negatively charged
the direction of current is opposite to the direction of actual electron flow
defn + ex: direct current DC vs. alternating current AC
direct current: the charge flows in one direction only
- household batteries
- assume this on the MCAT
alternating current: the flow changes direction periodically
- current supplied over long distances to homes and other buildings
defn: electromotive force (emf)
when no charge is moving between the two terminals of a cell that are at different potential values, the voltage is called the electromotive force
it is not actually a force (think of it more as a pressure to move that results in current)
defn: electric circuit
a conducting path that usually has one or more voltage sources (like a battery) connected to one or more passive circuit elements (like resistors)
defn: Kirchhoff’s laws
2 rules that deal with the conservation of charge and energy within a circuit
defn: 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
defn: 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
defn: resistance
the opposition within any material to the movement and flow of charge (think of it like friction, air resistance, viscous drag –> motion is opposed)
defn: insulator vs. conductor (in terms of resistance)
conductor: materials that offer almost no resistance
insulator: materials that offer very high resistance
defn: resistor
conductive materials that offer amounts of resistance between the two extremes of insulators and conductors
what are the four characteristics of a resistor that affect the resistance of a resistor?
- resistivity
- length
- cross-sectional area
- temperature
defn + SI unit: resistivity
the number that characterizes the intrinsic resistance to current flow in a material
SI unit: ohm-meter
if a resistor doubles in length, what happens to the resistance?
it doubles
if a resistor’s cross-sectional area is doubled, what happens to the resistance?
it will be halved
why does doubling a resistor’s cross-sectional area halve its resistance?
the increase in cross-sectional area increases the number of pathways through the resistor (conduction pathways)
do most conductors have greater resistance at higher or lower temperatures? why?
greater resistance at higher temperatures
why? due to increased thermal oscillation of the atoms in the conductive material, which produces a greater resistance to electron flow
what are the three materials or items that do not follow the rule that resistivity is a function of temperature?
- glass
- pure silicon
- most semiconductors
func + defn: Ohm’s law
how to calculate the voltage drop between any two points in a circuit
the basic law of electricity
it states that for a given magnitude of resistance, the voltage drop across the resistor will be proportional to the magnitude of the current
likewise, for a given resistance, the magnitude of the current will be proportional to the magnitude of the emf (voltage) impressed upon the circuit
where does the voltage drop come from?
it is an energy loss resulted from electrical resistance
is charged gained or lost through a resistor?
no
does voltage drop across each resistor?
yes
func: internal resistance
as a result of internal resistance, the voltage supplied to a circuit is reduced from its theoretical emf value by some small amount
do conductive materials provide any resistance?
yes! they act as weak resistors themselves, offering some magnitude of resistance to current and causing a drop in electrical potential (voltage)
what is the internal resistance and voltage of a cell when a cell is not actually driving any current (such as when it is open)?
internal resistance = 0
cell voltage = emf
is the actual voltage of a cell always less than the emf of the cell?
yes (if current is not 0 and the internal resistance is not negligible)
what happens to current when a cell is discharging?
it supplies current that flows from the positive, higher potential end of the cell around the circuit to the negative, lower potential end
defn: secondary batteries
batteries that can be recharged
explain how secondary batteries are recharged
also explain it from an electrochemical perspective
an external voltage is applied in such a way to drive current toward the positive end
electrochemical: cell acts as a galvanic (voltaic) cell when it discharges and as an electrolytic cell when it recharges
defn: resistors in series vs. in parallel
IN SERIES = all current must pass sequentially through each resistor connected in a linear arrangement
IN PARALLEL = the current will divide to pass through resistors separately
for resistors in parallel, do all electrons pass through all the resistors?
no, electrons have a “choice” regarding which path they will take (some will choose one pathway, others will choose a different pathway)
for resistors in parallel, is the voltage drop the same or different across each resistor? why?
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 resistance of each path the same or different? how does the current flow?
they may be different across each path
electrons prefer the path of least resistance (the current will be largest through the pathways with the lowest resistance)
for resistors in parallel, is the equivalent resistance greater than or less than the individual resistances? why?
less than: we could replace all resistors in parallel with a single resistor that has a resistance thatf is less than the resistance of the smallest resistor in the circuit
why? there is an inverse relationship between the cross-sectional area of a resistor and the resistance of that resistor
for resistors in series, why are the voltage drops additive?
as electrons flow through each resistor, energy is dissipated and there is a voltage drop associated with each resistor
for resistors in series, what is the equivalent resistance in relationship to the individual resistances?
resistances are also additive
the set of resistors wired in series can be treated as a single resistor with a resistance equal to the sum of the individual resistances = the equivalent (or resultant) resistor
defn + func: capacitor
are characterized by their ability to hold charge at a particular voltage
all MCAT: parallel plate capacitor
func; store an amount of energy in the form of charge separation at a particular voltage
what is a real-life example to help us understand a capacitor? explain how it works.
defibrillator
while charging: a high-pitched electronic tone sounds as electrons build up on the capacitor
when fully charged: that charge can be released in one surge of power (discharging)
explain how a parallel plate capacitor works
when two electrically neutral metal plates are connected to a voltage source, positive charge builds up on the plate connected to the positive (higher potential) terminal and negative charge builds up on the plate connected to the negative (lower potential) terminal
it is a capacitor because it can store a particular amount of charge at a particular voltage
defn + SI unit: capacitance
the ratio of the magnitude of the charge stored on one plate to the potential difference (voltage) across a capacitor
SI unit: farad
if a voltage V is applied to the plates of a capacitor, and a charge Q collects on it, what is the charge on the positive plate? on the negative plate?
positive: +Q
negative: -Q
there is a uniform electric field between the plates of a parallel plate capacitor with parallel field vectors, what is the direction of the electric field at any point between the plates?
from positive toward negative
defn: dielectric material
another way of saying insulation
what are 5 examples of dielectric materials?
- air
- glass
- plastic
- ceramic
- certain metal oxides
defn: dielectric constant
when a dielectric material is introduced between the plates of a capacitor, it increases the capacitance by this factor
a measure of its insulating ability
what is the dielectric constant of a vacuum by definition?
1
what happens when a dielectric material is placed in an isolated, charged capacitor (a charged capacitor disconnected from any circuit)?
the voltage across the capacitor decreases
because the dielectric material shields the opposite charges from each other
thus the capacitance has been increased
what happens when a dielectric material is placed in a charged capacitor within a circuit (still connected to a voltage source)?
the charge on the capacitor increases
voltage must remain constant because it must be equal to that of the voltage source
by increasing the amount of charge stored, the capacitance has been increased
what is the difference in WHY capacitance increases when a dielectric material is placed in an isolated charged capacitor vs. a charged capacitor within a circuit?
ISOLATED: increase arises from a decrease in voltage
IN A CIRCUIT: increase arises from an increase in stored charge
the stored energy in a capacitor is only useful if it is allowed to discharge, what are the two ways the charge can be released from the plates?
- discharging across the plates
- discharging through some conductive material with which the plates are in contact
defn: failure vs. normal function of a capacitor
failure: large rapid discharge across the plates of a capacitor
normal: creating a current through attached wires
for capacitors in series, does the total capacitance increase or decrease? why? + what does this translate to functionally
decreases
because the capacitors must share the voltage drop in the loop and therefore cannot store as much charge
functionally: a group of capacitors in series acts like one equivalent capacitor with a much larger distance between its plates (with a distance equal to those of each of the series capacitors added together)
what is the total voltage for capacitors in series?
sum of the individual voltages
what is the total capacitance and total voltage for capacitors in series?
capacitance = sum of the individual capacitances
voltage across each capacitor is the same and equal to the voltage across the source
defn: meters
devices that are used to measure circuit quantities in the real world
func: ammeter
used to measure the current at some point within a circuit
how does an ammeter work/what does it require? (4)
- requires circuit to be on
- inserted in series where the current is being measured
- use the magnetic properties of a current-carrying wire to cause a visible needle movement or a calibrated display of the current
- ideal ammeters have zero resistance and no voltage drop across themselves so as not to interfere with circuit mathematics
what happens to an ammeter if there is a particularly high current?
the ammeter will be overwhelmed and a special low resistance shunt is used in parallel with the ammeter to allow a reading
func: voltmeter
used to measure the voltage drop across two points in a circuit
char: voltmeter (4)
- requires a circuit to be active
- use magnetic properties of current-carrying wires
- wired in parallel to the two points in the circuit they are trying to measure the drop between
- ideal voltmeter has infinite resistance
func: ohmmeter
used to calculate the resistance by knowing the ohmmeter’s voltage and the current created through another point in the circuit
char + needs: ohmmeter
- does not require a circuit to be active (in fact some give false readings or get damaged by an active circuit)
- have their own battery of known voltage
- function as ammeters through another point in the circuit
