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MCAT Physics > Circuits > Flashcards

Circuits Flashcards

1
Q

Current

A

The flow of positive charge even though only negative charges are actually moving

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2
Q

What are two categories of conductivity?

A

metallic and electrolytic

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3
Q

Conductance

A

The reciprocal of resistance (Siemens S) sometimes S/m for conductivity

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4
Q

Why are metals good electrical and thermal conductors?

A

Metal atoms can easily lose one or more of their outer electrons to then move around amongst the larger collection of metal atoms

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5
Q

Metallic bonds

A

A sea of electrons flowing over and past a rigid lattice of metal cations

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6
Q

What is electrolytic conductivity dependent on?

A

Strength of a solution

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7
Q

How is conductivity in an electrolyte solution measured?

A

Placing the solution as a resistor in a circuit and measuring changes in voltage across the solution

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8
Q

In which is conductivity higher? Ionic or nonionic solutions?

A

Ionic

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9
Q

Formula for magnitude of current

A

I = Q/delta t
1 A = 1 C/s

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10
Q

The direction of a current is…

A

opposite of that of electron flow, so the direction is high electrical potential to low electrical potential

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11
Q

Two patterns of current flow

A

direct current DC or alternating current AC

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12
Q

Direct current

A

Charge flow unidirectional

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13
Q

Alternating current

A

Flow changes direction periodically

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14
Q

electromotive force

A

Measures in Volts V = J/C

no moving charge between cells terminals but are at different potential values

“pressure to move” if you will

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15
Q

Kirchhoff Junction Rule

A

I (into junction) = I (leaving)

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16
Q

Kirchhoff Loop Rule

A

V source = V drop

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17
Q

Resistance

A

The opposition within any material to the movement and flow of charge

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18
Q

Very low resistance

A

Conductors

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19
Q

Very high resistance

A

Insulators

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20
Q

Conductive materials that offer resistance between these two extremes are called

A

Resistors

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21
Q

resistance formula

A

R = pL/A

p resistivity
L length
A cross sectional area

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22
Q

Resistivity

A

p = ohm x meter

23
Q

Cross sectional area

A

The wider the resistor the more current can flow

Does not follow incompressible fluid continuity Av=Av

24
Q

Temperature

A

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

25
Q

Ohm’s law

A

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

26
Q

Internal resistance

A

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

27
Q

Internal resistance is zero

A

When cell isn’t driving any current

28
Q

Cell discharge

A

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

29
Q

Secondary batteries

A

Can be recharged
An external voltage is applied in such a way to drive the current to the positive end of the secondary battery

30
Q

Power

A

P = W/t = deltaE/t

31
Q

How is energy supplied in electric circuits?

A
  • 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
32
Q

Rate at which energy is dissipated by the resistor is the power of the resistor, as displayed by this formula…

A

P = IV = (I^2)R = (V^2)/R

I = current through the resistor
V = voltage drop across the resistor
R = resistance of resistor

33
Q

What are the two ways resistors can be connected into a circuit?

A

Series
Parallel

34
Q

Resistors in series gen description

A

All currents must pass sequentially through each resistor connected in a linear arrangement

35
Q

Resistors in parallel gen description

A

current will divide to pass through resistors separately

36
Q

Resistors in series resultants

A
  • 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
37
Q

Resistors in parallel resultants

A
  • 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
38
Q

n number of resistors in parallel effects

A
  • 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)
39
Q

Capacitators

A
  • characterized by their ability to hold charge at a particular voltage
  • defibrillator
  • eventually discharge
40
Q

Capacitance

A
  • 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
41
Q

capacitance of a parallel plate capacitor formula

A

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

42
Q

uniform electric field between parallel plate capacitors formula

A

E = V/d

43
Q

The direction of the electric field at any point between the plates is…

A

from the positive plate toward the negative plate

44
Q

potential energy stores in a capacitor

A

U = 1/2 x CV^2

45
Q

dielectric material

A
  • 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

46
Q

dielectrics in isolated capacitors

A
  • when a dielectric material is introduced into a isolated capacitor, the inc in capacitance arises from a dec in voltage
47
Q

when dielectric material placed on a charges capacitor…

A
  • 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
48
Q

capacitors in a series

A

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)

49
Q

capacitors in parallel

A

Cp = C1 + C2 +…+ Cn
- voltage across each parallel capacitor is the same and is equal to the voltage across the source

50
Q

meters

A

devices that are used to measure circuit quantities in the real world

51
Q

ammeters

A
  • 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
52
Q

voltmeters

A
  • 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
53
Q

ohmmeters

A
  • does not require an active circuit
  • have their own battery of known voltage
  • and then functions as ammeters through another point in the circuit

MCAT Physics (10 decks)

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