B MCAT- Physics Flashcards
mechanical advantage is the ratio of …
input force/output force.
efficiency is the ratio of …
useful work / total work performed by a system.
W=f d
V=change in position / change in time
g=
9.81 m/s^2
For MCAT, can round to 10 m/s^2.
W=mg
F=ma
U initial + K initial = K final + U final
Central dogma of energy in B’s words
Energy is the ability to do work.
Energy is conserved.
Work converts potential energy to kinetic energy (or elastic potential energy) and visa versa.
Potential energy is always = force x distance. This often comes in the form of U=mgh.
Kinetic energy is 1/2mv^2. So velocity has a much greater impact on KE than mass does.
Many problems are just asking for the delta, so remember to simplify and set the zero point where ever is most convenient.
In many frictionless ramp questions, the angles have no impact on final velocity. E.g. Given two boxes of same weight sliding off a truck down ramps of 15 vs. 45 degrees, they end with the same velocity.
Often the trig can be approximated.
Pressure vs. Force
Pressure is Force/Area.
ex. squeeze a water bottle and the “diver” goes down.
Buoyant Force
Buoyant force is the difference between the force on the top vs. bottom of an object in liquid.
BF acting on an object is equal to the density of the liquid it is in multiplied by the amount of fluid displaced by the object.
Buoyant force is equal to the weight of the fluid the object displaces. (Archimedes Principle)
Buoyant force is equal and opposite to the force of gravity on a floating object. And F=mg.
Specific Gravity/ Relative Density
The ratio of densities between an object/substance and a reference fluid.
sg=p object/p reference fluid
kg/L
Specific gravity, also called relative density, is the ratio of the density of a substance to that of a standard substance, usually water at 1.0 kg per litre (62.4 pounds per cubic foot).
Gases are commonly compared with dry air, which has a density of 1.29 grams per litre (1.29 ounces per cubic foot) under so-called standard conditions (0 °C and a pressure of 1 standard atmosphere).
If an object has a lower specific gravity than a given fluid or gas = the object will float in that fluid or gas.
Specific Gravity will also tell us what portion of object will be displaced in the fluid while it floats. Objects more dense than the fluid will have ratio > 1 and will sink.
ideal gas = theoretical = no volume = no intermolecular forces = no loss of energy from collisions (elastic collisions)
Relationship between pressure and volume
smaller volume = more pressure = more frequent collisions
Boyle’s Law:
PV=Constant, so
P and V inversely proportional
Increasing the temperature will make particles more faster. So, if you take a container and keep the number of particles and the pressure constant, then increasing the temperature will increase the volume of the container.
Relationship between volume and temperature
Charles’ Law:
V/T=Constant
So, V and T inversely proportional.
Decrease in V=Constant x increase in T.
Definition of temperature in Kelvin
Temperature is the average kinetic energy of all the molecules in a system.
At zero Kelvin–absolute zero–all particles has stopped moving (theoretical).
Absolute zero Kelvin = -273 degrees C
Moles and Volume
Avogadro’s Law:
Vol/number of moles = constant
increase in vol = constant x increase in number of moles.
Ex. Moles of gas can decrease with temperature changing to liquid.
Ideal Gas Law
PV=nRT
Pressure x Volume = number of moles x ideal gas constant x temperature
Value of R depends on units used.
UNITS!!!!!
STP for gas problems
standard temperature and pressure for gas law problems:
= 273 Kelvin (0 degrees Celsius)
1 atm
Vol of 1 mole of an ideal gas is 22.4 L
standard conditions for thermodynamics problems:
= 298 Kelvin (25 degrees Celsius)
Pascal’s Law, pressure and fluid
pressure exerted on an incompressible fluid is the same throughout the fluid. F/A is the same throughout.
So, if area is smaller in one spot, it increases the force proportional to the area. Ex. Hydraulic lifts
density = mass/ volume
kg
L
1000 L = 1 meter^3
Convert Celsius to Kelvin
degrees Celsius + 273 = Kelvin
Water boils at:
373 Kelvin
100 degrees Celsius
212 degrees F
Henry’s Law
The amount of gas dissolved in a liquid is proportional to the partial pressure of the gas above the liquid.
relationship between pressure and force
Pressure (in pascals) =
Force (in newtons) /Area (in m^2)
P=F/A
This is how hydraulic lifts work.
Under what conditions is an ideal gas LEAST likely to behave like an ideal gas?
High pressure, and low temperature.
High pressure, small volume, low temp conditions mean that gases don’t have as much space to disperse and overcome intermolecular forces.
Consider Ideal Gas Law:
PV=nRT
Speed of sound through different medium
velocity of sound = square root of the ratio of the bulk modulus / density of the medium.
The bulk modulus is measure of a medium’s resistance to compression. Larger B = less compressible.
Therefore, sound travels fastest through solids like bone, slower through liquid, and the slowest through gas.
heat capacity (mc)
The energy needed to raise the temperature of a system by 1 degree.
heat capacity = mass * specific heat
(mc)
Q=mcdeltaT
specific heat (c)
Specific heat is the ability of something to hold heat–resistance to change of heat.
specific heat = heat capacity per unit mass
(c)
(i.e. mc/m)
e.g. Water has a high specific heat; it can hold heat well.
Air has a lower specific heat than water; it does not hold heat as well.
Q=mcdeltaT
Latent heat (L)
Heat absorbed that does not change the temperature of the system.
i.e. delta H of a phase change
Q=mL
How does pressure affect phase change?
The lower the ambient pressure, the easier it is to boil something.
i.e. There is less heat energy needed to change the system.
Water boils at a slightly lower temperature at lower pressure (higher elevations).
Conduction
Exchange of kinetic energy between particles.
i.e. Heat energy transfer via particles bouncing off one another.
Thermal conductivity (k)
The ability of something to conduct heat (to transfer energy between particles).
Metals have a large k. Metal can heat and cool very quickly.
Gases, non-metals have a small k.
Convection
Energy transferred by movement of a fluid (liquid or gas).
e.g. A radiator (steam), air conditioner, convection oven
Radiation
Conversion of thermal energy into electromagnetic waves.
Only form of energy transfer that can go through a vacuum. This is why the sun can heat the earth.
bond enthalpy
the energy stored in a bond between two atoms
calculating temperature change
Q = mcdeltaT
Thermodynamic systems energy and work equation
delta E = Q - Wby
change in energy of the system (E) = heat added to the system (Q) minus work done BY the system (Wby)
Helpful to set up a table with Temp change, Work by </> 0, and delta E </> 0.
Volume of gas at STP
22.4L
Volume and buoyancy
Volume and buoyancy have a direct relationship. As volume increases, so does buoyancy, and visa versa.
Ideal gas law R =
R = 0.082 (LATM)/(Kmol)
PV=nRT
density of water
~1000 kg/m^3
relationship between velocity and area
velocity and area are inversely proportional.
v1A1=v2A2
Remember that A of a circle = pi*r^2
light energy…
C=
energy of a photon=
C = speed of light = 3*10^8
C=(freq)(wavelength)
Energy of a photon E = hf = (h*c)/wavelength
h=6.6*10^-34 Js
magnification formula for mirrors and lenses
m = (image distance)/(object distance)
|m|>1= magnified
|m|<1= minimized
negative (-) m indicates real image, inverted.
positive (+) m indicates virtual image, upright.
Hyperopia
Hyperopia is far-sightedness.
Eyeball too SHORT for radius of curvature, so image forms BEHIND RETINA.
Need a converging (convex) lens. Always create a virtual image at the ideal focal point.
Power (in diaptors) is positive +.
Use 1/f=1/do + 1/di
P=1/f
Myopia
Myopia is near-sightedness.
Eyeball is too LONG for radius of curvature. Image forms inside retina. do is infinite, so f~di.
Need a diverging (concave) lens.
Power (in diaptors) is negative - .
My lasik corrected my myopia by making my radius of curvature bend light less.
2 types of electrochemical cells:
Galvanic cells…
Electrolytic cells…
Galvanic/voltaic cells RELEASE ENERGY.
(Assume that a battery is a galvanic cell unless otherwise stated or reference to charging.) Directs the movement of electrons from a redox reaction to a wire making an electromotive force / current of electrons flowing from the anode to the cathode. The salt bridge (or in earlier battery models a porous divider) provides a means for charged particles to flow again and neutralize the solutions so that the reaction and current can continue.
Electrolytic cells STORE ENERGY. Two inert electrodes in solution. Electrons flow from an external power source toward one electrode, making it the cathode, where reduction happens. Then the anions flow to the other electrode where they are oxidized, making that the anode.
Electrochemical half reactions draw out the flow of electrons, often with reduction potential. The reduction potential of each half reaction indicates the electromotive force (related to voltage) of each half reaction. The rxn with the larger magnitude reduction potential will be the one that gets the electrons (the reduction rxn that happens at the cathode). The oxidation rxn will happen at the anode.
turbulent vs. laminar flow
Turbulent flow is rough and disorderly. Can form EDDIES–swirls of fluid on the downstream side of an obstacle.
Laminar flow is smooth and orderly. Modeled as STREAMLINES–layers of flow, sometimes moving faster near the sides of a pipe than in the middle.
Ideal fluid
An ideal fluid is one that has low viscosity (resistance to flow). Gases generally have lower viscosity than liquids.
range of frequency that a human can hear
20 - 20,000 Hz
Ultrasound, xrays, etc. are all above this range.
The wavelength and velocity of a wave changes as it moves through different mediums, but the frequency remains the same. E=hf
conservation of energy equations
conservation of energy can be used to solve all sorts of problems.
kinetic energy = 1/2 mv^2
potential energy = mgh
Related, power is change in energy over time. So, you can solve some problems subtracting initial KE or PE from final KE or PE and equating to power.
Ecell
The electromotive force of a battery cell; the cell potential. Measured in voltage; positive reduction potential if spontaneous, negative reduction potential if nonspontaneous.
The magnitude of Ecell describes the amount of voltage produced by a spontaneous reaction (positive Ecell), or consumed by a nonspontaneous reaction (negative Ecell).
Discharging a battery = spontaneous; creates a positive Ecell to power something else.
deltaG=-nFEcell
where n is the number of electrons transferred, and F is Faraday’s constant. Negative because delta G and E cell will always have opposite signs. (neg delta G = spontaneous, pos Ecell = spontanteous)
A Voltmeter will tell you the Ecell value for a galvanic cell.
In a cell, the anode is always the site of ___
The anode is always the site of oxidation.
In a cell, the cathode is always the site of _____
The cathode is always the site of reduction.
circuit strategy
V=iR
Resistance is measured in ohms.
The big battery line is the + side, small line is -. By convention, circuit problems use the flow of positive charge instead of electrons.
Resisters in PARALLEL divide the current, resulting in a net DECREASE IN RESISTANCE (smaller than any individual resister in the measurement). 1/Rt=1/R+1/R+1/R etc.
Total voltage of the circuit is fixed and resistance decreases, so current increases (i.e. more current pulled from battery).
Resister in SERIES INCREASE THE TOTAL RESISTANCE because current must pass through both/all in series (the total will be larger than any one resister in the series. Rt=R+R+R etc.
Total voltage of the system is fixed and resistance increases, so current decreases.
To calculate, start with the resisters in parallel, then move on to the resisters in series.
capacitance
A parallel place capacitors involves two plates that store charge connected to a voltage source. + charge builds up on one plate, - charge builds up on the other, so potential energy is stored via the electric field between them. The magnitude of the electric field is larger with greater voltage (V), and E is larger with a smaller distance (d) between the plates. Note that the area of the plates doesn’t matter, because we assume the plates are much greater in size than the distance between them.
E=V/d
C=Q/V
Capacitance is measured in Farads. 1 F = 1 Coulumb per Volt.
While capacitor is charging, V is constant, and Q increases to maintain voltage. Once disconnected from the voltage source, Q is constant and V decreases.
Capacitors in series DECREASE the total capacitance (the sum of d increases).
Capacitors in parallel INCREASE the the total capacitance (total A increases).
capacitance WITH a dielectric
A dielectric is an insulator (glass, plastic) that INCREASES capacitance when inserted between plates. Vacuum and air = 1.
The area of the plates matters, bigger area = bigger capacitance, since more space to store charge.
C=Q/V
(voltage constant while connected to the power source; dielectric increases C, Q goes up).
When inserting a dielectric/insulator, can use formula
C’=(dielectric constant)(C)
Relating current, charge, time
change in current = change in charge/ change in time
i=q/t
Relating charge and number or moles of electrons
Given moles of electrons, multiple:
Avagadro’s number (6 x 10^23)
by the moles of electrons
by the charge per electron (1.6 x 10^-19)
Transformers
Power=current x voltage
P=iV
Power is constant at the transformer site.
A STEP-UP transformer increases voltage (thereby decreasing current).
A STEP-DOWN transformer decreases voltage (thereby increasing current).
How do you know if a rxn took place in a galvanic vs. electrolytic cell?
If rxn is spontaneous (Positive Ecell, negative delta G) then it took place in a galvanic cell.
Opposite for electrolytic cells–nonspontaneous reactions.
power, voltage, and resistance
Power=voltage^2 / resistance
