Circuits Flashcards
Current
The flow of positive charge even though only negative charges are actually moving
What are two categories of conductivity?
metallic and electrolytic
Conductance
The reciprocal of resistance (Siemens S) sometimes S/m for conductivity
Why are metals good electrical and thermal conductors?
Metal atoms can easily lose one or more of their outer electrons to then move around amongst the larger collection of metal atoms
Metallic bonds
A sea of electrons flowing over and past a rigid lattice of metal cations
What is electrolytic conductivity dependent on?
Strength of a solution
How is conductivity in an electrolyte solution measured?
Placing the solution as a resistor in a circuit and measuring changes in voltage across the solution
In which is conductivity higher? Ionic or nonionic solutions?
Ionic
Formula for magnitude of current
I = Q/delta t
1 A = 1 C/s
The direction of a current is…
opposite of that of electron flow, so the direction is high electrical potential to low electrical potential
Two patterns of current flow
direct current DC or alternating current AC
Direct current
Charge flow unidirectional
Alternating current
Flow changes direction periodically
electromotive force
Measures in Volts V = J/C
no moving charge between cells terminals but are at different potential values
“pressure to move” if you will
Kirchhoff Junction Rule
I (into junction) = I (leaving)
Kirchhoff Loop Rule
V source = V drop
Resistance
The opposition within any material to the movement and flow of charge
Very low resistance
Conductors
Very high resistance
Insulators
Conductive materials that offer resistance between these two extremes are called
Resistors
resistance formula
R = pL/A
p resistivity
L length
A cross sectional area
Resistivity
p = ohm x meter
Cross sectional area
The wider the resistor the more current can flow
Does not follow incompressible fluid continuity Av=Av
Temperature
Most conductor have high greater resistance at high temperatures
Due to increased thermal oscillation of the atoms in the conductive material which provides greater resistance to electron flow
Ohm’s law
V = IR
The voltage drop between any two points in a circuit can be calculated with this
I is current
R is magnitude of resistance
Internal resistance
Even conductors have a small amount of resistance in them
V (voltage provided by cell) = E(cell) - ir(int)
i is current
Ecell emf of cell
rint internal resistance
Internal resistance is zero
When cell isn’t driving any current
Cell discharge
It supplies current and the current flows from the positive high potential end of the cell around the circuit to the negative lower potential end
Secondary batteries
Can be recharged
An external voltage is applied in such a way to drive the current to the positive end of the secondary battery
Power
P = W/t = deltaE/t
How is energy supplied in electric circuits?
- by the cell that houses spontaneous redox reaction which when allowed to proceed generate a flow of electrons
- electrons have electrical potential energy, convert that energy into kinetic energy as they move around a circuit driven by the emf of the cell
- current delivers the energy to various resistors that covert this energy to some other form depending on the resistor configuration
Rate at which energy is dissipated by the resistor is the power of the resistor, as displayed by this formula…
P = IV = (I^2)R = (V^2)/R
I = current through the resistor
V = voltage drop across the resistor
R = resistance of resistor
What are the two ways resistors can be connected into a circuit?
Series
Parallel
Resistors in series gen description
All currents must pass sequentially through each resistor connected in a linear arrangement
Resistors in parallel gen description
current will divide to pass through resistors separately
Resistors in series resultants
- As the electrons flow through each resistor, energy is dissipated and there is a voltage drop associated with each resistor
- voltage drops are additive
- Vs = V1 + V2 + … + Vn
- resistances of resistors are also additive
- Rs = R1 + R2 + … + Rn
^resultant resistance will always increase as resistors are added
Resistors in parallel resultants
- when in parallel, wired with a common high potential terminal and a common low potential terminal
- electrons have a “choice” regarding which path they will take, but regardless of the path voltage drop experienced by each division of current is the same because all pathways originate from a common point and end at a common point within the circuit
- waterfall analogy
- Vp = V1 = V2 = … = Vn
- but resistance in each pathway may differ
- 1/Rp = (1/R1) + (1/R2) + … + (1/Rn)
- is a circuit divided into two branches and one has twice the resistance it will have half the magnitude of current compared to the other
n number of resistors in parallel effects
- when n identical resistors are wired in parallel, the total resistance is given by R/n
- note that voltages across each of the parallel resistors is equal and that, for equal resistances, the current flowing through each of the resistors is also equal (current of [I total]/n runs through each)
Capacitators
- characterized by their ability to hold charge at a particular voltage
- defibrillator
- eventually discharge
Capacitance
- C = Q/V
- when two electrically neutral metal plates are connected to a voltage source, positive charge builds up on the plate connected to the positive high potential terminal
- negative charge build up on the plate
- SI unit is 1 F (farad) = 1 C/V
capacitance of a parallel plate capacitor formula
C = E0 (A/d)
E0 = 8.85 x 10^-12 F/m
permittivity of free space
A = area of overlap of the two plates
d = separation of 2 plates
uniform electric field between parallel plate capacitors formula
E = V/d
The direction of the electric field at any point between the plates is…
from the positive plate toward the negative plate
potential energy stores in a capacitor
U = 1/2 x CV^2
dielectric material
- insulators
- when this is introduced between the plates of a capacitor, it inc capacitance by a factor called the dielectric constant (k)
Capacitance due to dielectric material
C’ = kC
dielectrics in isolated capacitors
- when a dielectric material is introduced into a isolated capacitor, the inc in capacitance arises from a dec in voltage
when dielectric material placed on a charges capacitor…
- charge on capacitor inc
- voltage must remain constant bc must be equal to the voltage source
- inc capacitance from an inc in stored charge
- discharge can happen across plates or through some conductive material
capacitors in a series
1/Cs = 1/C1 + 1/C2+…+ 1/Cn
- total capacitance dec
- must share voltage drop in the loop and therefore cannot store as much charge
- act as one single capacitor w larger distance between its plates (smaller capacitance)
capacitors in parallel
Cp = C1 + C2 +…+ Cn
- 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
- used to measure the current at some point within a circuit
- requires circuit to be on
- ideal ammeters have zero resistance and no voltage drop across themselves
- inserted into the series where the current is being measured and use magnetic properties of a current carrying wire to cause a visible needle movement or a calibrated display of the current
voltmeters
- requires circuit to be active
- use magnetic properties of current-carrying wires
- measure voltage drops between 2 points
- wired in parallel between these two points
- ideal voltmeter has infinite resistance
ohmmeters
- does not require an active circuit
- have their own battery of known voltage
- and then functions as ammeters through another point in the circuit
