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# THE MCAT-PHYSICS 6

### ELECTROSTATICS/MAGNETISM & ELECTRIC CIRCUITS

Question | Answer |
---|---|

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 |