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# THE MCAT-PHYSICS 6

### ELECTROSTATICS/MAGNETISM & ELECTRIC CIRCUITS

How do charged particles interact with each other? 1.) direct interaction of charges 2.) Magnetic field when charges are in motion
Conductor promotes movement of charge (i.e. metals)
Insulator impedes movement of charge (i.e. glass)
Coulomb's Law F = Kq1q2 / r^2
What is current? movement of positive charge from high --> low voltage -or mvmt. from high electric potential --> low
How do electrons flow? electrons flow from low --> high voltage
What is an electric field? region of space a test charge would feel an electrostatic force if added (can be made of single or multiple charges)
Which way do E-field lines point? + ----> - -point in direction net force acts on + charge -starts on + and ends on - -MUST point from positive charge and end on negative charge -the way a + charge would move
Why do the electric-field lines curve towards the edges? horizontal components of E-field cancel out till edge
A test charge on equipotential line experiences same ____? force
Does E-field do work to move test charge within same equipotential line? NO
When a test charge moves in direction of a field line, to what potential does it move to? higher --> lower potential (test charge is positive)
What happens to EPE when like charges are brought close to each other? EPE increases --> must do work
What happens to EPE when opposite charges are brought close to each other? EPE decreases --> no work, charged particles move naturally
General rule about potential energy? PE always decreases as equilibrium is reached
What is work for charges? Work = q*E*delta(d) = q*delta(V) (V = voltage)
What is energy conservation? E(initial) + W(on object) = E(final)
What is electric force? F = E * q
Relate voltage, electric field, and distance V = E * d
Every time a negative charge is created, what else is created? positive charge
What is one major difference between gravitational and electrical forces? gravitational are always attractive while electrical forces can be attractive or repulsive
What forces can act at a distance? gravity, electricity, magnetism
Where do force lines point to for electric fields? positive to negative
Where do force lines point to for gravitational fields? towards mass creating the field
Electric field equation E = k*q / r^2
Potential energy of a charge related to electric field? PE = E * q * d
Potential energy of a charge related to Coloumb's Law PE = k*q1*q2 / r
What is voltage? potential for work by an electric field in moving any charge from one point to another
Voltage due to a point charge V = k * q1 / r
Work for an electrostatic charge? W = q * E * d
Is work done by electrostatic field dependent on path? NO --> conservative force that conserves mechanical energy
Point charges create electric fields that change with...? radius
Name the basic equations for force, electric field, PE, and voltage F = kq1q2/r^2 E=kq1/r^2 U=kq1q2/r V=k q1/r
How does a dipole orient itself within an E-field? positive end of dipole is in direction of electric field
Dipole not perfectly aligned with E-field will have PE of what? PE = -pEcostheta
Equation for electrostatic force F = kq1q2 / r^2
A uniformly charged sphere has an electric field in the middle of? 0
Relate electric potential energy, q, and V delta PE = q * V
What are equations for potential energy and Voltage in relation to electric field? U = E * q * d V = E * d
What gravitational component is voltage analogous to? "gh"
What is voltage? potential for work by an electric field in moving any charge from one point to another
Is electric field a vector? YES --> must use vector addition
Is voltage a vector? NO --> can addidrectly
Is work done by gravity / electrostatic field dependent on path? NO --> both are conservative forces
Relate voltage, EPE, and work -Voltage is EPE diff. between 2 points -Equal to work which would have to be done against static E-field to move charge between 2 points
Dielectric Non-insulating/nonconducting material that acts to resist creation of E-field so capacitor can store more charge
How does dielectric allow capacitor to store more charge? -each dielectric creates its own E-field that reduces overall E-field within capacitor -more charge required to build E-field --> more energy stored within capacitor -increases Q if V is constant, decreases V if Q is constant
What is dielectric constant of vacuum and air? 1
Why must dielectric be insulator? Would conduct charge otherwise
What happens during dielectric breakdown? large enough E-field that current conduction can occur
Which has bigger "k" value --> polar or nonpolar? -polar -better able to reorient itself to reduce E-field
----------> How does dipole orient itself? - ------ +
Calculate electric dipole moment p = q * d q = charge on one
Torque and dipole t = p * E sin theta
PE and dipole PE = -(p * E cos theta)
Which direction do B-field lines go? N --> S
Magnetic force equation F = q v B sintheta
Can a magnetic force do work? NEVER --> magnetic force is always perpendicular
What can a magnetic force change? Change direction but never magnitude
How does a magnetic force change direction if it can do no work? moving charge --> B-field --> E-field -B-field does indirect work through E-field
Lenz's Law When conducting loop experiences change in magnetic flux, current will be induced in loop to create opposing magnetic flux -induced current creates B-field to oppose inducing B-field
When magnetic flux changes, what is induced? What can that then create? -current -it's own B-field
What creates flux? change in number of B-field lines in loop
DC Current Net movement of electrons is in one direction around circuit
AC Current oscillating electrons back and forth in SHM ( one used in U.S.)
What happens when AC current is at max? electrons are moving at max. velocity
What is V(max) and I(max)? V(max) = square root (2) * V (rms) I(max) = square root (2) * I (rms)
How to calculate rms value? square all terms, take average, and then take square root
Electric force in relation to electric field? F = q * E
Particle moving through E-field and B-field, what happens when particle goes straight? (B-force up, E-force down) magnetic force = electric force
Particle moving through E-field and B-field, what happens when deflected up? (B-force up, E-force down) Magnetic force > Electric force ( v is too big)
Particle moving through E-field and B-field, what happens when deflected down? (B-force up, E-force down) Electric Force > Magnetic Force (v is too small)
What is a B-field generated by? Moving charge
What does a moving charge experience when moving through a B-field? Forces
What 2 conditions creates a B-field? 1.) moving charge 2.) changing electric field
How to create an E-field from a moving charge? moving charge --> B-field --> E-field
How is B-field created E-field different from E-field of stationary charge? -B-field created E-field is non-conservative and ME is dissipated as heat in charged object (PE has no meaning) -E-field from stationary charge has conserved ME and gains PE
What happens when you move a charged particle against a field for an electric field created from a stationary charge? gains potential energy
What happens when a loop is pulled out of a B-field? B-field around wire changes --> creates electric field --> current created -current then creates its own B-field
Formula for Resistance R = p * L / A p = resistivity
Ohm's Law (delta) V = I R
Why do electrons flow? Flow in response to chang ein voltage
Voltage can be related to? Potential energy (gh)
Kirchoff's 2 rules? 1.) amount of current flowing into any node must be same as amount that flows out 2.) voltage around any path in closed circle must = 0
What does a battery do? adds energy to a circuit by increasing voltage from one point to another
How does positive charge flow? Flows from high --> low potential + ----> - in battery
What is a capacitor? temporarily stores energy
What does separation in parallel plates capacitor create? E-field
Capacitance formula C = Q / V = e A / d
How to store more charge on capacitor (relate to SA, distance, thickness)? -taller and wider plate -thickness makes no difference -smaller "d" -Greater charge on plates
EPE stored on plates Formula for capacitor U = 1/2 Q * delta V PE = 1/2 C* V^2
What does a stronger dielectric allow capacitor to do? store more charge
What happens to current flow as charge accumulates on capacitor? reduces
Once capacitor is completely charged, what happens to current? current can no longer flow back to - terminal
Perfect conductors have _____ resistivity. zero
Perfect insulators have _____ resistivity. infinite
In metals, when temperature increases, what happens to conductivity? decreases
What is superconductivity? virtually no resistance to current flow at extremely low temperatures
Power equation P = I * V P (avg) = I (rms) * V (rms)
What is power? rate of energy transfer
What are 2 equations for voltage (related to resistance and capacitance)? V = I * R C = Q / V
Sum of voltage across a resistor and capacitor must equal? sum of voltage across terminals of emf source
Why do larger resistors increase charging and discharging time? reduce current flow
Ohm's Law for Circuit V (circuit) = I (total) * R (equivalent)
Current: Series vs. Parallel Series - same current Parallel - different current (split)
Voltage: series vs. parallel series - different voltage parallel - same voltage
Do larger or smaller resistors drain more power for series? larger resistors
Do smaller or larger resistors drain more power for parallel? smaller
For series, larger resistor has _____ voltage drop? larger
For parallel, larger resistor has ______ current? smaller
_____ is the same for series capacitor. charge
____ is the same for parallel capacitor voltage
For series, larger capacitance has _____ voltage gain and stores ____ energy. less / less
For parallel, larger capacitance can store _____ charge and _____ energy. more / more
For series, ______ equivalent resistance and _____ equivalent capacitance increased / decreased
For parallel, ______ equivalent resistance and ______ equivalent capacitance. decreased / increased
Identical voltage sources connected in parallel produce ____ output voltage as single source. same
Why can't electrons move easily from one atom to another in insulators? valence electrons are tightly bound to atoms so it takes a lot of energy to free them
Formula for resistance resistance (R) = (resistivity(p)*length) / Area
What is current for circuit? Electrochemical cells? flow of positive charges / flow of negative charges
What is emf? potential difference --> if there is resistance, subtract from emf
Voltage across capacitor equation? V = E * d
Why do capacitors in series have the same charge? charge is result of accumulated current flow into capacitors --> series always has same current
Conductivity is affected by? electrolyte concentration and temperature
Thinner wires create higher or lower resistance? higher
What is aresistor? anything that drains energy from electrical flow and converts it to some other form of energy, often light, heat, or work
Adding a resistor in parallel will decrease / increase equivalent resistance? decrease
Adding resistor in series will decrease / increase equivalent resistance? increase
A voltmeter should have a larger or small internal resistance? larger
Parallel arrangement of resistors means that all bulbs have the same...? potential difference across them
Adding another resistor in parallel will reduce / increase overall resistance? decrease
Adding another resistor in series will reduce / increase overall resistance? increase
If the voltage increases on a capacitor, how will charge be affected? it will also increase C = Q / V
Calculate heat dissipated by a resistor. Heat = power * time (seconds)
What do resistors do in terms of power drain? oppose flow of current and drain energy from an electric circuit
How to calculate electrical energy? P = I * V * time = Joules
breakdown voltage minimal voltage that causes portion of an insulator to stop charging up and become electrically conductive
A capacitor with a greater Q has stronger / weaker dielectric? stronger
With current in a parallel arrangement, higher resistance means higher / lower current? lower
In series, a bigger resistor has a smaller / bigger V and P? larger
In parallel, a bigger resistor has smaller / bigger I and P? smaller
Capacitors in parallel, same ___ and bigger C has smaller / bigger Q and PE? voltage gain / larger
Capacitors in series, same _____ and bigger C has smaller / bigger V and PE? charge / smaller
Charge and mass of an electron? 1.6E-19 / 9.1E-31
What direction do E-field lines emanate from and point towards? emanate from positive charge and go towards negative charge
EPE for single point charge? delta PE = q * V
Do nonpolar / polar materials have stronger dielectric? polar
What does ionization transform gas into? How does this affect is properties as dielectric? cation --> no longer has dipole
Do solids or liquids have higher polarizability? liquids because molecules can orient themselves
As dielectric constant increases, how is force between two charged particles affected? decreases --> dielectric can orient itself to reduce strength of E-field between 2 charged particles
Something that is more stable has more / less potential energy? less
What orientation of the B-field results in the smallest B-force acting on electron? F = qvB(perpendicular) --> least perpendicular B-field lines
RHR is ONLY used for positive / negative charges? positive --> opposite orientation for negative charges
What generates B-fields? charged particles in motion
Spinning loops in an external B-field generate what type of current? induced, alternating current
When does an induced current occur with Lenz's Law? entering or exiting a field
Describe the E-field lines for a wire? run radially perpendicular to wire
Describe the E-field lines for a cylinder? run radially perpendicular to cylinder and is zero inside the cylinder
Formula for calculating potential? V = k*q / r
How to calculate potential from a dipole? calculate individual potential from positive charge and negative charge and add them together
Gauss' Law Electric flux = EAcostheta
Faraday cage E-field inside a closed conducting cage is zero --> external charges on conducting cage will rearrange to cancel out any external field
When do B-fields exist? moving charge experiences a force due to its motion
How is the B-force oriented in relation to B-field and velocity of charge? force is perpendicular to both B-field and to velocity of charge
Magnetic force for current-carrying wire? F = I * L * B sin theta
Two wires will attract each other if current is in opposite / same direction? same
Two wires will repel each other if current is in opposite / same direction? opposite
In what medium does light travel fastest? vacuum
2 Formulas to use for mass spectrometry? mv^2 / r = qVB
Which way do electrons flow in a circuit? anode to cathode - --> +
Which way do electrons flow in an Electric field / gel electrophoresis? anode to cathode + --> -
Doubling voltage increases / decreases KE? increases
Does Electric-field do work? magnetic field? yes / no
Moving charges: E-force vs. B-force -E-force exists regardless of moving charge -B-force requires charges be in motion
Perpendicular vs. parallel: E-force vs. B-force -E-force is parallel with E-field -B-force is perpendicular with B-field
Moving a charged particle: E-force vs. B-force Both can move a charged particle
If a charged particle is stationary, will it experience B-force or E-force? -E-force only since stationary
Potential lines are always in what orientation relative to field lines? perpendicular
With series, as more bulbs are added, does current increase or decrease? decrease
With parallel, as more bulbs are added, does current increase or decrease? decrease
If a bulb is removed from series, what happens to rest? all go out
If a bulb is removed from parallel, what happens to rest? remain lit
What influences how bright light bulbs are? voltage (potential difference) and how that relates to power
Lightbulbs that are brighter dissipate more or less power? more power
In series, what happens to voltage when resistance increases? decreases (split between more lightbulbs)
In parallel, what happens to voltage when more resistors are added? increases
What always remains constant, current supplied by power source or current going through each resistor? current going through each resistor
Why do energy-saving light bulbs have lower power than regular light bulbs? convert greater proportion of power consumed to light rather than heat
Which is brighter: 2 bulbs in parallel or 2 bulbs in series? 2 bulbs in parallel --> greater voltage since don't have to split --> greater power
Add extra bulbs to parallel circuit, what happens to brightness? stays the same since receives the same voltage
Add extra bulbs to series circuit, what happens to brightness? decreases
In a parallel circuit, does the number of light bulbs affect the brightness? no
Equation for energy stored in a capacitor Energy = 1/2QV
E-field of an infinitely large charged plate is? uniform
Force felt by a charge in an infinitely large charged plate is? constant
How does voltage relate to work and charge V = W/q
Is the E-field constant? YES --> as long as E-field lines remain parallel
Do the E-field lines for a close charge or a far away charge better represent parallel lines? far away charg
What is the E-field for a very large, single charged plate? parallel
Are electrostatic force and gravitational force equal at all points? YES
What is the formula for the velocity of a projectile upon impact? v = sqrt (2gh)
What does mass of a projectile represent? inertia and its tendency to continue moving upward
What defines the voltage? What is the voltage a function of? battery / circuit!!
How long does a capacitor take to discharge / charge? fraction of a second
When voltage is max, what energy forms are max/min? EPE is max and KE is zero
T/F: The forces of attraction/repulsion between 2 magnetic poles or between 2 electric charges are of equal strength TRUE
Will breaking a magnet in half create 2 distinct poles? yes
Will a current create a B-field? YES --> moving charge
2 wires: Which wire does the B-field from Wire 1 affect? Wire 2
If a bar is slid at constant velocity through a constant B-field, will the current change? NO --> constant since flux increases at a constant rate
Created by: 507935299

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