Physics Flashcards
Which of the following is true about heat?
It can only be transferred by direct contact between objects.
It is proportional to temperature.
It is a type of energy transferred between objects.
All of these statements are true.
I DON’T KNOW YET
It is a type of energy transferred between objects.
Water flowing out of a hose:
is subjected to atmospheric pressure.
is subjected to greater pressure the faster it is moving.
is subjected to less pressure the faster it is moving.
must have a constant flow speed.
is subjected to atmospheric pressure.
A dam is built to create an artificial lake. The pressure at the bottom of the dam:
depends upon the total weight of the water held back by the dam and the area of the dam.
depends only upon the depth of the water at that point.
depends only upon the total weight of the water held back by the dam.
depends upon the depth of the water and the area of the dam.
depends only upon the depth of the water at that point.
An ideal gas undergoes a closed cycle of processes on a PV diagram. Which of the following must be true?
The total heat exchanged with the environment is zero.
All of these statements are true.
The total work done by the gas is zero.
The change in internal energy is zero.
The change in internal energy is zero.
A block of wood is floating on water. If you push down on the top of the block until it’s completely submerged, the buoyant force on it:
increases.
decreases.
depends upon the density of the wood.
remains constant.
increases
Bernoulli’s Law should not be applied to which of the following cases?
A tiny leak in the side of a large swimming pool.
Water flowing rapidly out of a hose.
Standing blood pressure difference between arms and legs.
Air flowing smoothly over an airfoil.
Standing blood pressure difference between arms and legs.
Which of the following are true about heat and temperature?
Heat flows from a higher temperature material to a lower temperature material and in the process always raises the temperature of the second material.
The temperature of a material is directly proportional to the kinetic energy of its constituent particles.
Both heat and temperature can be measured in kelvins.
A hotter object always contains more thermal energy than does a cooler object.
The temperature of a material is directly proportional to the kinetic energy of its constituent particles.
How is an atom held together as a single unit?
Due to the fact that protons and electrons have a special property- they carry electric charge which gives rise to an attractive force between them
Elementary Charge
e = 1.6 x 10^-19
Coulomb’s Law
If two charged particles are a distance r apart
FE = k ((q1q2)/ r^2)
Coulomb’s Law Proportionality Constant
Depends on the material between the particles. If separated by air, the constant is named k0 and called Coulomb’s constant and = 9 x 10^9 N*m^2/C^2
Principle of superposition
The net electric force on a charge (q) due to a collection of other charges (Q’s) is equal to the sum of the individual forces that each of the Q’s alone exerts on q
Periodic or harmonic motion
Any motion that regularl repeats
Ex: object undergoing uniform circular motion, mass oscillating on a spring, pendulum (characterized by period or frequency)
Period
Time it takes an object to move through one full cycle of motion
Frequenc
number of cycles that occur in one second
Hooke’s Law
F= - kx
Spring Constant
Tells us how strong the spring is; the greater the value of k the stiffer and stronger the spring
Units: N/m
Elastic Potential Energ
PEelastic = 1/2 kx^2
W of spring and potential energy
Wby spring = -Delta PEelastic and
Wagainst spring = Delta PEelastic
Amplitude
Maximum displacement of the block from equilibrium. This positive number tells us how far to the left and right of equilibrium the block will travel
Block on spring vmax
vmax = A square root k/m
Frequency and Period Spring
f = 1/2pi (square root k/m) and T = 2pi (square root m/k)
Pendulum Frequency and Period
f = 1/2pi (spring root g/l) and T= 2pi (square root l/g)
Mechanical Wave
is a series of disturbances (oscillations) within a medium that transfers energy from one place to another
vibrating string or sound
cannot exist without a medium
wave speed
v = wavlength x frequency
Wave Rules
- The speed of a wave is determined by the type of wave and the characteristics of the medium, not by the frequency
- When a wave passes into another medium, its speed changes, but its frequency does not
Interference
When two or more waves are superimposed on each other, they will combine to form a single resultant wave. The amplitude of the resultant wave will depend on the amplitudes of the combining waves and on how these waves travel relative to each other
In phase
If crest meets crest and trough meets trough. Their amplitudes will add and we say the waves interfere constructively
Out of phase
Crest of one wave coincides with the trough of the other; amplitudes subtract and we say that the waves interfere destructively
Insulator
Will not easily distribute a charge over its surface and will not transfer that charge to another neutral object very well- especially to another insulator. Electrons of insulators tend to be closely linked with their respective nuclei (most nonmetals)
Conductor
The charges will distribute approximately evenly upon the surface of the conductor Conductors are able to transfer and transport charges and are often used in circuit or electrochemical cells. Nuclei surrounded by a sea of free electrons that are able to move rapidly throughout the material and are only loosely associated with the positive charges (metals)
Which has a greater mass? Proton or electron?
Proton
Coulomb’s Law
Fc = kq1q2/r^2
k is Coulomb’s constant
Electric field
Makes their presence known by exerting forces on other charges that move into the space of the field. Whether the force exerted through the electric field is attractive or repulsive depends on whether the stationary test charge q (charge place in electric field) and stationary source charge Q (creates the electric field) are opposite charges or like charges
E= Fe/q = kQ/r^2
Direction of the electric field vector
The direction that a positive test charge would move in the presence of the source charge. If the source charge is positive then the test charge would experience a repulsive force and would accelerate away from the positive source charge. If the source charge is negative, then the test charge would experience an attractive force and would accelerate toward the negative source charge. Positive charges have electric field vectors that radiate outward from the charge negative is opposite
Field lines
Represent how a positive test charge would move in the presence of the source charge. Field lines are drawn in the direction of the actual electric field vectors and also indicate the relative strength of the electric field. Lines are closer together near the source charge and spread out at distances farther from the charge so the field is stronger but when lines are farther apart the field is weaker
Electrical potential energy
This is a form of potential energy that is dependent on the relative position of one charge with respect to another charge or to a collection of charges.
U = kQq/r
Opposite charges will have negative potential energy and this energy will become increasingly negative as the charges are brought closer and closer together. The closer the more stable they will be
When will potential electrical energy be positive or negative?
If the charges are like (both positive or both negative) then the potential energy will be positive. If the charges are unlike (one positive and the other negative) then the potential energy will be negative
What if the two charges are positive (electrical potential energy)
These will exert repulsive forces and the potential energy of the system will be positive. Because like charges repel each other the closer they are to each other the less stable they will be. The like charges become more stable the father apart they move
How does electrical potential energy change between two particles as the distance between them increases?
If both particles have the same charge the electrical potential energy decreases as distance increases. If the two particles have opposite charges then the electrical potential energy increases as distance increases
Electrical potential
Defined as the ratio of the magnitude of a charge’s electrical potential energy to the magnitude of the charge itself
V= U/q
V: electrical potential measured in volts
V= kQ/r
positive source charge: V is positive
Negative source charge: V is negative
Potential difference
Vb-Va = Wab/q
Wab is the work needed to move a test charge q through an electric field from point a to point b
Negative test charge
A negative test charge will spontaneously move from a position of lower electrical potential to a position of higher electrical potential. Vb-Va will be positive because q is negative , Wab must also be negative which represents a decrease in electrical potential energy. V= kQ/r
Which way will positive and negative potentials spontaneously move?
Positive charges will spontaneously move in the direction that decreases their electrical potential (negative voltage) whereas negative charges will spontaneously move in the direction that increases their electrical p
v= Wab/q
For a positive test charge, this means moving from a position of higher electrical potential to a position of lower electrical potential. So V is negative and q is positive so Wab has to be negative which represents a decrease in electrical potential energy
Equipotential line
A line on which the potential at every point is the same. The potential difference between any two points on a equipotential is zero.
Electric dipole
Results from two equal and opposite charges being separated a small distance d from each other (can be transient like London Dispersion forces) or permanent. Similar to a barbell: the equal weights on either end of the bar represent the equal and opposite charges separated by a small distance represented by the length of the bar
For a collection of charges, the electrical potential P is
V= kq/r1 - kq/r2 = kq(r2-r1)/r1r2 V= (kqd/r^2)costheta
Dipole moment
p=qd
Vector points from the negative charge towards the positive charge
Perpendicular bisector of the dipole
Plane that lies halfway between +q and -q. The angle between this plane and the dipole axis is 90 degrees and cos 90 =0 so the electrical potential at any point along this plain is 0. The magnitude of the electric field on the perpendicular bisector of the dipole
E= 1/4pie0 x p/r^3
The electric field vectors at the points along the perpendicular bisector will point in direction opposite to p
What does the dipole do in an absence of an electric field?
The dipole axis can assume any random orientation
Net torque on a dipole
T= pEsintheta
What is the behavior of an electric dipole when exposed to an external electric field?
A dipole will rotate within an external electric field such that its dipole moment aligns with the field
Magnetic field
created by any moving charge; Si unit is the tesla (T)
Diamagnetic materials
made of atoms with no unpaired electrons and that have no net magnetic field. These materials are slightly repelled by a magnet and so can be called weakly antimagnetic. ex; wood plastics, water, glass, and skin
Paramagnetic and Ferromagnetic
atoms have unpaired electrons so these atoms do not have a net magnetic dipole moment but the atoms in these materials are usually random oriented so that the material itself creates no net magnetic field
Paramagnetic materials
Will become weakly magnetized in the presence of an external magnetic field aligning magnetic dipoles of the material with the external field. Upon removal of the external field, thermal energy of the individual atoms will cause the individual magnetic dipoles to reorient randomly
Ex: aluminum, copper, gold
Ferromagnetic materials
Unpaired electrons and permanent atomic magnetic dipoles that are normally oriented randomly so that the material has no net magnetic dipole. But will become strongly magnetized when exposed to a magnetic field or under certain temperatures
Ex: Iron, nickel, and cobalt
For an infinitely long and straight current-carrying wire, the magnitude of the magnetic field?
B= u0I/2pir
B: magnetic field at a distance r from the wire
u0: permeability of free space (4pi x 10^-7)
I is the current
Equation demonstrates an inverse relationship between the magnitude of the magnetic field and the distance from the current
Straight wires create magnetic fields in the shape of concentric rings
Magnitude of the magnetic field at the center of the circular loop
B= uoI/2r
Lorentz force
Charges often have both electrostatic and magnetic forces acting on them at the same time; the sum of these electrostatic and magnetic forces
Force on a moving charge
FB= qvBsintheta q: charge v: magnitude of velocity B: magnitude of the magnetic field The magnetic force is a function of the sine of the angle so the charge must have a perpendicular component of velocity in order to experience a magnetic force
Right hand rule for magnetic force
Thumb: velocity (indicates direction of movement)
Fingers;field lines (fingers are parallel)
Palm: force on a positive charge
Back of hand: force on a negative charge
Force on a current carrying wire
FB= ILBsintheta
Current
Considered flow of positive charge even though only negative charges are actually moving
Metallic conductivity
- seen in solid metals and molten forms of some salts
- Some materials allow free flow of electric charge within them (electrical conductors)
Electrolytic Conductivity
- Depends on strength of solution
- Distilled deionized water has such a low ion concentration that it may be considered an insulator while sea water and orange juice are excellent conductors
- Conductivity in an electrolyte solution is measured by placing the solution as a resistor in a circuit and measuring changes in voltage across the solution
- Conductivity in nonionic solutions is always lower than in ionic solutions
Conductance
The reciprocal of resistance
SI unit: siemens
Magnitude of Current
I = Q/ Delta t
Direction of current
The direction in which positive charge would flow (higher to lower potential) so its opposite to the direction of the actual electron flow
Direct current
Charge flows in one direction only (household batteries)
Alternating Current
The flow changes direction periodically (current supplied over long distances to homes and other buildings)
Potential Difference (voltage)
Produced by an electrical generator (galvanic cell)
Electromotive force
When no charge is moving between the two terminals of a cell that are at different potential values this is the voltage. Not a force but a potential difference
Kirchhoff’s Junction Rule
At any point or junction in a circuit, the sum of currents directed into that point equals the sum of currents directed away from that point. This is an expression of conservation of electrical charge and can be expressed as
I into junction = I leaving junction
Kirchhoff’s Loop Rule
Around any closed circuit loop, the sum of voltage sources will always be equal to the sum of voltage (potential) drops. This is a consequence of the conservation of energy. All the electrical energy supplied by a source gets fully used up by the other elements within that loop. NO excess energy appears and no energy disappears that cannot be accounted for.
Vsource= Vdrop
resistance
opposition within any material to the movement and flow of charge
Materials that offer no resistance
conductors
materials that offer very high resistance
insulators
Resistors
conductive materials that offer amounts of resistance between these two extremes (conductors and insulators)
Resistance
R=DL/A
D= resistivity
L= length of the resistor
A= cross sectional area
resistivity
characterizes the intrinsic resistance to current flow in a material
SI unit: ohm-meter
Length-resistance equation
The resistance of a resistor is directly proportional to its length. A longer resistor means that electrons will have to travel a greater distance through a resistant material. If a resistor doubles its length it will also double its resistance
Cross Sectional Area-resistance equation
If a resistor’s cross sectional area doubled its resistance will be cut in half. this is because an increase in cross sectional area increases the number of pathways through the resistor, called conduction pathways. the wider the resistor the more current that can flow
temperature-resistance equation
Most conductors have greater resistance at higher temperatures. This is due to increased thermal oscillation of the atoms in the conductive material which produces a greater resistance to electron flow
Ohm’s Law
Electrical resistance results in an energy loss which reflects a drop in electrical potential. The voltage drop between any two points in a circuit can be calculated
V= IR
I= current
R= magnitude of the resistance
-Basic law of electricity because it states that for a given magnitude of resistance, the voltage drop across the resistor will be proportional to the magnitude of the current. As current moves through a set of resistors in a circuit, the voltage drops some amount in each resistor
voltage supplied by a cell to a circuit
V= Ecell - ir int
Ecell is the emf of the cell
i is the current through the cell
r int is the internal resistance
The rate at which energy is dissipated by a resistor
P=IV=I^2R = V^2/R
Series
Resistors can be connected this way in which all current must pass sequentially through each resistor connected in a linear arrangement
Parallel
Current will divide to pass through resistors separately
Resistors in series
The current has no choice but to travel through each resistor in order to return to the cell. As the electrons flow through each resistor energy is dissipated and there is a voltage drop associated with each resistor.
Vs = V1 + V2 + V3 + ….. + Vn
Resistors in parallel
When resistors are connected in parallel, they are wired with a common high-potential terminal and a common low-potential terminal. This configuration allows charge to follow different parallel paths between high potential terminal and low potential terminal. Electrons have a choice regarding which path they will take. No matter the path the voltage drop will be the same because all pathways originate from a common point and end at a common point within the circuit
Vp= V1 = V2 = …Vn
While the voltage is the same for all parallel pathways, the resistance of each pathway may differ. Electrons prefer the path of least resistance and the current will be the largest through pathways with lowers resistance
Capacitors
Characterized by their ability to hold charge at a particular voltage.
Ex; defibrillator
Capacitance
Defined as the ratio of the magnitude of the charge stored on one plate to the potential difference across the capacitor
C=Q/V
SI Unit: Farad (1F= 1 C/V
Dependent upon the geometry of the two conduction surfaces. For the simple case of the parallel plate capacitor
C= Eo (A/d)
Eo is the permittivity of free space (8.85 x 10^-12)
A is the area of overlap of two plates
d is the separation of the two plates
Uniform electric field
E = V/d
Potential energy stored in a capacitor
U = (1/2)CV^2
Dielectric material
Another way of saying insulation
Ex: air, glass, ceramin
Dielectric constant
Measure of its insulating ability
Capacitance due to a dielectric material
C’= kC
C’ is the new capacitance with the dielectric present
C is the original capacitance
Dielectrics in Isolated Capacitors
When a dielectric material is placed in a isolated charged capacitor, the voltage across the capacitor decreases because of the dielectric material shielding the opposite charges from each other. By lowering the voltage across a charged capacitor, the dielectric has increased the capacitance of the capacitor by a factor of the dielectric constant
Dielectrics in Circuit Capacitors
The charge on the capacitor increases. The voltage must remain constant because it is equal to that of the voltage source. B increasing the amount of charge stored on the capacitor, the dielectric has increased the capacitance of the capacitor by a factor of the dielectric constant. When a dielectric material is introduced into a circuit capacitor, the increase in capacitance arises from an increase in stored charge. The stored energy is only useful if it is allowed to discharge. The charge can be released from the plates either by discharging across the plates or through some conductive material with which the plates are in contact.
Capacitors in Series
When capacitors are connected in series the total capacitance decreases because the capacitors must share the voltage drop in the loop and therefore cannot store as much charge. Group of capacitors acts like one equivalent capacitor with a much larger distance between its plates. this increase in distance means a much smaller capacitance
1/Cs = 1/C1 + 1/C2 + ….. 1/Cn
Cs decreases as more capacitors are added
Capacitors in parallel
Produce a resultant capacitance that is equal to the sum of the individual capacitances
Cp increases as more capacitors are added
Cp = C1 + C2 + C3 +… Cn
Annmeters
Used to measure the current at some point within a circuit. Circuit must be on. Ammeters are inserted in series where the current is being measured and use the magnetic properties of a current carrying wire to cause a visible needle movement or a calibrated display of the current. Ideally ammeter will not change circuit mathematics when it is inserted into the circuit. To do so it must have an extremely low resistance. Ideal have zero resistance and no voltage drops across themselves
Voltmeter
Requires a circuit to be active. Use magnetic properties of current-carrying wires. Used to measure the voltage drop across two points in a circuit. Ideal one has infinite resistance.
Ohmmeters
Does not require a circuit to be active and will often have their own battery of known voltage and then function as ammeters through another point in the circuit. Can be used to calculate resistance by knowing the ohmmeters voltage and the current created through another point in the circuit
Sinusoidal waves
Waves may be transverse or longitudinal. The individual particles may oscillate back and forth with a displacement that follows a sinusoidal pattern
Transverse waves
Those in which the direction of particle oscillation is perpendicular to the propagation (movement) of the wave (electromagnetic waves, visible light microwave, x-rays)
Longitudinal waves
Ones in which the particles of the wave oscillate parallel to the direction of the propagation. Wave particles are oscillating in the direction of energy transfer.
period
T= 1/f
If frequency defines the number of cycles per second then its inverse is the number of seconds per cycle
Angular frequency
Measured in radians per second and is often used consideration of simple harmonic springs and pendula
w = 2pif = 2pi/T
timbre
quality of the sound determined by the natural frequency or frequencies of the object. Some objects vibrate at a single frequency, producing a pure tone. Others vibrate at multiple frequencies that have no relation to one another. (Hitting a chair- unpleasant)
Forced oscillation
If a periodically varying force is applied to a system the system will then be driven at a frequency equal to the frequency of the force. If the frequency of the applied force is close to that of the natural frequency of the system, then the amplitude of oscillation becomes much larger
Resonating
If the frequency of the periodic force is equal to a natural (resonant) frequency, the system is resonating and the amplitude of the oscillation is at a maximum. If the oscillating system were frictionless the periodically varying force would continually add energy to the system and the amplitude would increase indefinitely
Dampening or attenuation
No system is completely frictionless so dampening is a decrease in amplitude of a wave caused by an applied or non conservative force. Many objects cannot withstand the large amplitude of oscillation and will break (shattering of a glass by loudly singing frequency of glass)
Sound
Longitudinal wave transmitted by the oscillation of particles in a deformable medium. Sound can travel through solids liquids, and gases but cannot travel through a vacuum. Speed of sound:
V = square root B/D
B- bulk modulus : measure of the medium’s resistance to compression (B increases from gas to liquid to solid)
D; density of medium
Sound travels fastest through a solid and slowest through a gas
Production of sound
Sound is produced by the mechanical disturbance of particles in a material along the sound wave’s direction of propagation. Although the particles themselves do not travel along with the wave they do vibrate or oscillate about an equilibrium position which causes small regions of compression to alternate with small regions of depression. These alternating regions travel through the material allowing sound wave to move
Pitch
Frequency of sound
Lower frequency sounds have lower pitch
Higher frequency sounds have higher pitch
Normal range of human hearing
20 Hz to 20,000 Hz
Infrasonic waves
Sound waves with frequencies below 20 Hz
Ultrasonic
Sound waves with frequencies above 20,000 Hz
Doppler effect
Phenomenon affecting frequency which describes the difference between the actual frequency of a sound and its perceived frequency when the source of the sound and the sound’s detector are moving relative to one another. If the source and detector are moving toward each other, the perceived frequency f’ is greater than the actual frequency, f. If the source and detector are moving away from each other the perceived frequency is less than the actual frequency.
f’ = f (v +/- VD)/ (v -/+ Vs)
f’
perceived frequency
f
actual emitted frequency
vD
speed of the detector
Vs
speed of the source
Sign Convention of Doppler Effect
Upper sign should be used when the detector or source is moving toward the other object. The lower sign should be used when the detector or source is moving away from the other object
Ex: You’re driving down the street and hear an ambulance approaching from behind. You are the detector and the ambulance is the source sound. You are driving away from the ambulance but the ambulance is moving faster toward you. The lower sign (-) should be used in the numerator which relates to the detector. The driver of the ambulance is moving toward
you so the top sign (-) should be used in the denominator
Echolocation
The animal emitting the sound serves as both the source and the detector of the sound. The sound bounces off a surface and is reflected back to the animal. How long it takes for the sound to return, the change in frequency of the sound can be used to determine the position of objects in the environment an the speed at which they are moving
Shock wave
An object that is producing sound while traveling at or above the speed of sound allows wave fronts to build upon one another at the front of the object. This creates a much larger amplitude at that point Because amplitude for sound waves is related to the degree of compression of the medium, this creates a large pressure differential or pressure gradient. Shock wave can cause physical disturbances as it passes through other objects. The passing of a shock wave creates a very high pressure followed by a very low pressure which is responsible for the phenomenon known as a sonic boom
Sonic boom
Can be heard at any time that an object traveling at or faster than the speed of sound passes a detector not just at the point that the speed of sound is exceeded. Once an object moves faster than the speed of sound, some of the effects of the shock wave are mitigated because all the wave fronts will trail behind the object destructively interfering with each other
Loudness or volume
The way in which we perceive its intensity. I
Intensity
Average rate of energy transfer per area across a surface that is perpendicular to the wave. It is the power transported per unit area
SI units: W/m^2
I = P/A
Also related to the distance from the source of the sound wave. Sound waves transmit their power over larger and larger areas the farther from the source they travel
Intensity is inversely proportional to the square of the distance fro the source
Amplitude and intensity relationship
Intensity is proportional to the square of the amplitude. Doubling the amplitude produces a sound wave that has four times the intensity
Sound level
B = 10 log I/Io
I: intensity of the sound wave
Io; threshold of hearing (1 x 10^-12) which is used as a reference intensity
When intensity of sound changes you can calculate the new sound level by: Bf = Bi + 10 log If/Ii
Standing wave
Produced b the constructive and destructive interference of a traveling wave and its reflected wave. Will form whenever two waves of the same frequency traveling in opposite directions interfere with one another as they travel through the same medium. they appear to be standings till because the interference of the wave and its reflected wave produce a resultant that fluctuates only in amplitude. As the waves move in opposite directions they interfere to produce a new wave pattern characterized by alternating points of maximum displacement and points of no displacement. Nodes no fluctuation in displacement
Maximum fluctuation are antinodes
Wavlength of a standing wave and the length of a string that supports it is:
wavelength = 2L/n
Frequency of standing wave
f = nv/2L
Fundamental frequency
lowest frequency (longest wavelength) of a standing wave that can be supported in a given length of string n=2 has one half the wavelength and twice the frequency of the first harmonic
Open pipe
Has antinodes at both ends; open at both ends
Closed pipe
Will correspond to a node and the open end will correspond to an antinode. There can only be odd harmonics
Wavelength of a standing wave and the length of a closed pipe that supports it
wavelength = 4L/n (n can only be odd integers)
Frequency of the standing wave in a closed pipe
f = nv/4L
Ultrasound
Use high frequency sound waves outside the range of human hearing to compare the relative densities of tissues in the body. Consists of a transmitter that generates a pressure gradient which also functions as a receiver that processes the reflected sound. Because speed of the wave and travel time is known, the machine can generate a graphical representation of borders and edges within the body by calculating the traversed distance
Doppler ultrasound
Used to determine the flow of blood within the body by detecting the frequency shift that is associated with movement toward or away from the receiver
J in 1 calorie
4.184
J in 1 electron-volt (eV)
1.602 x 10^-19
Oz in 1 lb
33.8 oz
N in 1 lb
4.45 N
kg in 1 atomic mass unit
1.661 x 10^-27 kg
Farenheit and Celsius Formulas
F = 9/5C + 32 K = C + 273
FINER method
For evaluating a research question is a method to determine whether the answer to one’s question will add to the body of scientific knowledge in a practical way and within a reasonable time period
- Is study feasible
- Do other scientists find it interesting?
- Is this question novel?
- Would study obey ethical principles
- Is this question relevant to scientific community
Positive controls
Those that ensure a change in the dependent variable when it is expected. In the development of a new assay for detection of HIV, for example, administering the test to a group of blood samples known to contain HIV could constitute a positive control
Negative controls
Ensure no change in the dependent variable when no change is expected
Accuracy
Also called validity is the ability of an instrument to measure a true value. For ex: an accurate scale should register a 170 lb persons weight as 170 pounds
Precision
Also called reliability is the ability of the instrument to read consistently or within a narrow range
Cohort studies
Those in which subjects are sorted into two groups based on differences in risk factors (exposures) and then assessed at various intervals to determine how many subjects in each group had a certain outcome
Ex: Form of a longitudinal study
Case-Control study
Identifying the number of subjects with or without a particular outcome and then look backwards to assess how many subjects in each group had exposure to a particular risk factor
Hill’s Criteria
Describe the components of an observed relationship that increase the likelihood of causality in the relationship. While only the first criterion is necessary for the relationship to be causal, it is not sufficient. The more criteria that are satisfied by a relationship, the likelier it is that the relationship is causal. Relationship should be described as a correlation
- Temporality: exposure independent variable must occur before outcome dependent variable
- Strength
- Dose-response relationship: as study or independent variable increases, proportional increase in response
- Consistency
- Plausability
- Consideration of alternative explanations
- Experiment
- Specificity
- Coherence
Detection bias
Educated professionals using their knowledge in an inconsistent way
Hawthorne Effect or observation bias
Posits that the behavior of study participants is altered simply because they recognize that they are being studied. patients in a study for a given weight loss drug may begin exercising more frequently or make healthier choices
Beneficence
Obligation to act in the patient’s best interest
Nonmaleficence
The obligation to avoid treatments or interventions in which the potential for harm outweighs the potential for benefit
Respect for persons
Includes the need for honesty between the subject and the researcher and generally but not always prohibits deception Also includes informed consent
Morally relevant differences
Those differences between individuals that are considered an appropriate reason to treat them differently.
Skewed Distributions
One that contains a tail one one side or the other of the data set. A negatively skewed distribution has a tail on the left or negative side and a positively skewed distribution has a tail on the right or positive side. Mean of a negatively skewed distribution will be lower than the median while the mean of a positively skewed distribution will be higher than the median
Insulator
Will not easily distribute a charge over its surface and will not transfer that charge to another neutral object very well especially not to another insulator. The electrons of insulators tend to be closely linked with their respective nuclei. Most nonmetals are insulators
Conductor
When given a charge, the charges will distribute evenly upon the surface of the conductor. Conductors are able to transfer and transport charges and are often used in circuits or electrochemical cells. Conductors are often seen as the nuclei surrounded by a sea of free electrons that are able to move rapidly throughout the material that are only loosely associated with the positive charges. Conductors are generally metals although ionic Solutions are also effective conductors.
Coulomb’s Law
Quantifies electrostatic force between two charges Fe = kq1q2/r^2 Fe: magnitude of the electrostatic force k: Coulomb's constant q1 and q2 are magnitude of cahrges r is the distance between the charges k= 8.99 x 10^9 Nx m^2/C^2
Coulomb’s Law and the Equation for Gravitational force
In the electrostatic force equation the force magnitude proportional to the charge magnitude in the gravitational equation, gravitational force is proportional to the mass. In both equations the force magnitude is inversely proportional to the square of the distance of separation. In both equations the force magnitude is inversely proportional to the square of the distance of separation.
Electric field
Every electric charge sets up a surrouding electric field just like every mass creates a gravitational field. Electric fields make their presence known by exerting forces on other charges that move into the space of the field. Whether force exerted through the electric field is attractive or repulsive depends on the whether the stationary test charge q (charge placed in the electric field) and the stationary source charge Q (which actually creates the electric field) are opposite charges or like charges
Magnitude of electric charge calculated how?
E = Fe/q = kQ/r^2 E is the electric field magnitude Fe is the magnitude of the force felt by the test charge q k is the electrostatic constant Q is the source charge magnitude
Fe/q method
Place a test charge q at some point within the electric field, measure the fore exerted on that test charge, and define the electric field at that point in space as the ratio of the force magnitude to test charge magnitude
Disadvantage: a test charge must actually be present for force to be measured
kQ/r^2 method
Does not require the presence of a test charge. Only need to know the magnitude of the source charge and the distance between the source charge and point in space at which we want to measure the electric field
Field lines
Imaginary lines that represent how a positive test charge would move in the presence of the source charge. The field lines are drawn in the direction of the actual electric field electric field vectors and also indicate the relative strength of the electric field at a given point. The lines are closer together near the source charge and spread out at distances further from the charge. When the field lines are closer together, the field is stronger; where the lines are further apart the field is weaker
Conventions for directions electric fields
If the test charge within a field is positive , then the force will be in the same direction as the electric field vector . If the test charge is negative, then the force will be in the direction opposite to the field vector of the source charge
Electrical potential energy
This is a form of potential energy that is dependent on the relative position of one charge with respect to another charge or to a collection of charges
U = kQq/r
Electrical potential energy
-This is a form of potential energy that is dependent on the relative position of one charge with respect to another charge or to a collection of charges
U = kQq/r
-If the charges are like charges (both positive or negative) then the potential energy will be positive. If the charges are unlike (one pos and the other neg) then the potential energy will be negative
How can you define electrical potential energy for a charge
At a point in space in an electric field as the amount of work necessary to bring the charge from infinitely far away to that point
Consider two charges: a stationary negative source charge and a positive test charge that can be moved?
Because these two charge are unlike, they will exert attractive forces between them. Therefore, the closer they are to each other, the more stable they will be. Opposite charges have negative potential energy and this energy will become increasingly negative as the charges are brought closer and closer together This decrease in energy represents an increase in stability. Increasingly negative numbers are actually decreasing values because they are moving further to the left of 0 and this decrease in energy represents an increase in stability
Electrical potential
Ration of the magnitude of the charge’s electrical potential energy to the magnitude of the charge itself
V = U/q where V is the electrical potential measure in volts. Even if there is no test charge q, we can still calculate V because U = kQq/r so V = kQ/r
Electrical potential is scalar so it sign is determined by the sign of the source charge Q. For a positive source charge V is positive but for a negative source charge V is negative
Potential difference
A potential difference will exist between two points that are not at different distances from the source charge
Delta V = Vb - Va = Wab/q
Wab: work needed to move a test charge q through an electric field from point a to point b
-Electrostatic force equals conservative
-Charges will move spontaneously in a direction that results in a decrease in electric potential energy
Electrical potential: negative and positive test charge
Pos: Moving from higher to lower electrical potential (negative voltage)
Neg: moving from lower to higher electrical potential (positive voltage), q is negative so Wab must also be negative. yet in both cases the electrical potential energy is decreasing
Electrical potential vs voltage
Electrical potential is the ration of a charge’s electrical potential energy to the magnitude of the charge itself. Voltage or potential difference is a measure of the change in electrical potential between two points which provides an indication of the tendency toward movement in one direction or the other
Equipotential lines
A line on which the potential energy at every point is the same. The potential difference between any two points on an equipotential line is zero. From the equation for electrical potential, we can see that no work is done when moving a test charge q from one point on an equipotential line to another. Work depends on the potential difference and not on the path so any of the paths would require the same amount of work.
Electric potential equation for points in space distant from the dipole
V = kqd /r^2 (cos theta)
dipole moment
p = qd
perpendicular bisector of the dipole
equipotential line that lies halfway between +q and -q. because the angle between this plane and the dipole axis is 90 (cos 90 =0) the electrical potential at any point along this plane is 0. Magnitude of electric field calculated as:
E = 1/4piEo x p/r^3
Electric field vectors at the points along perpendicular bisector will point in direction opposite to p
Dipole and torque
In the absence of an electric field, the dipole axis can assume any random orientation. When the electric dipole is placed in a uniform external electric field, each of the equal and opposite charges of the dipole will experience a force exerted on it by the field. Because the charges are equal and opposite, the forces acting on the charges will also be equal in magnitude resulting in a situation of translational equilibrium. There will be a net torque about the center of the dipole axis
Torque = pEsin theta
p is the magnitude of the dipole moment
E is the magnitude of the uniform external electric field
theta is the angle the dipole moment makes with the electric field
Magnetic field
any moving charge makes a magnetic field. Si unit is the tesla
Diamagnetic materials
made of atoms with no unpaired electrons and that have no net magnetic field
- slightly repelled by magnet and so can be called weakly antimagnetic
- Ex; wood, plastic, water, glass, skin
Paramagnetic
Will become weakly magnetized in the presence of an external magnetic field aligning the magnetic dipoles of the material with the external field. Upn removal of the external field, thermal energy of the individual atoms will cause the individual magnetic dipoles to reorient randomly
Aluminum, copper, gold
Ferromagnetic materials
have unpaired electrons and permanent atomic magnetic dipoles that are normally oriented randomly so that the material has no net magnetic dipole. But they will become strongly magnetized when exposed to a magnetic field or under certain temperatures
Iron, nickel, cobalt
Bar magnets are ferromagnetic materials with a north and south pole field lines exit north pole and enter south pole
Configuration of the magnetic field lines
Surrounding a current carrying wire will depend on the shape of the wire
1. long straight wire
B = uoI/2pir
B is magnetic field at a distance r from the wire. uo is the permeability of free space (4pi x 10^-7) I is the current
Equation represents an inverse relationship between the magnitude of the magnetic field and the distance from the current. Straight wires create magnetic fields in the shape of concentric rings
2. circular loop of wire
B = uoI/2r
-First equation gives the magnitude of the magnetic field at any perpendicular distance r from the current carrying wire while the second expression gives the magnitude of the magnetic field only at the center of the circular loop of current carying wire with radius r
Magnetic forces
Exert forces only on other moving charges. Charges do not sense their own fields, they only sense the field established by some external charge or collection of charges. Charges often have both electrostatic and magneti forces acting on them at the same time (Lorentz force)
Magnetic force
Fb = qvB sin theta
B is the magnitude of the magnetic field
The magnitude of the force created by an external magnetic field for a straight wire
Fb = ILBsintheta I is the current L is the length of the wire in the field B is the magnitude of the magnetic field theta is the angle between L and B
Current
-Flow of positive charge even though only negative charges are actually moving. Any conductive substance may act as a medium through which current can pass
-Magnitude of the amount of charge Q passing through the conductor per unit time
I = Q/Delta T
-Charge transmitted by flow of electrons in a conductor
-Because electrons are negatively charged they move from a point of lower electrical potential to high electrical potential and so they reduce their electrical potential energy but the direction of current is opposite to the direction of actual current flow
Conductivity
Metallic and Electrolytic
Metallic conductivity
Some materials allow free flow of electric charge within them; these materials are called electrical conductors. Metal atoms can easily lose one or more of their outer electrons which are then free to move around in larger collection of metal atoms. This makes most metals good electrical and thermal conductors. Metallic bond has been seen as sea of electrons flowing over and past a rigid lattice of metal cations.
Electrolytic Conductivity
Depends on the strength of a solution. Distilled deionized water has such a low ion concentration that it may be considered an insulator while sea water and orange juice are excellent conductors. Conductivity is measured by placing the solution as a resistor in a circuit and measuring changes in voltage across the solution. Conductivity in nonionic solutions is always lower than in ionic solutions
Conductance
Reciprocal of resistance
SI unit: Siemens (S/m)
Two patterns of current flow
Direct current: charge flows in one direction only
Alternating current: In which the flow changes direction periodically
electromotive force
- When no charge is moving between the two terminals of a cell that are at different potential values
- Not actually a force but a potential difference
- Think of emf as a pressure to move that results in current in much the same way that a pressure difference between two points in a fluid filled tube causes the fluid to flow
Kirchhoff’s Junction Rule
At any point or junction in a circuit, the sum of currents directed into that point equals the sum of currents directed away from that point. This is an expression of conservation of electrical charge and can be expressed as
I into junction = I leaving junction
Kirchhoff’s Loop Rule
Around any closed circuit loop, the sum of voltage sources will always be equal to the sum of voltage potential drops. Consequence of the conservation of energy. All the electrical energy supplied by a source gets fully used up by the other elements within that loop. No excess energy appears and no energy disappears that cannot be accounted for. Energy can be changed from one form to another so KE of electrons can be converted to thermal light or sound energy. Vsource = Vdrop
resistance
opposition within any material to the movement and flow of charge. Materials that offer no resistance are called conductors and those materials that offer very high resistance are insulators
Properties of Resistors
Resistivity, Length, Cross sectional area, temperature
R = pL/A
Voltage
driving force kind of like pressure measured in volts
Resistivity
Number that characterizes the intrinsic resistance to current flow in a material is called the resistivity in ohm-meter
Length
A longer resistor means that electrons will have to travel a greater distance through a resistant material
Cross sectional area
The increase in cross sectional area increases the number of pathways through the resistor called the conduction pathways. The wider the resistor the more current that can flow
Temperature Resistors
Most conductors have greater resistance at higher temperatures. This is due to increased thermal oscillation of the atoms in the conductive material which produces a greater resistance to electron flow
Ohm’s Law
Electrical resistance results in an energy loss which reflects a dro pin electrical potential
V = IR
V is the voltage drop
I is current and R is the magnitude of the resistance.
-Law states that for a given magnitude of resistance, the voltage drop across the resistor will be proportional to the magnitude of the current. Likewise for a given resistance the magnitude of the current will be proportional to the magnitude of the emf
As current moves through a set of resistors in a circuit, the voltage drops some amount in each resistor but the current or sum of currents is constant. No charge is gained or lost through a resistor. Thus if resistors are connected in series all of the current must pass through each resistor
Actual voltage supplied by a cell to a circuit
C = Ecell - ir int V is the voltage provided by the cell Ecell is the emf of the cell i is the current through the cell rint is its internal resistance
Power
The rate at which energy is transferred or transformed. Measured as the ratio of work (energy expenditure) to time and can be
P = W/t = Delta E/t
emf
is not a force but is better thought of as a pressure to move exerted by the cell on the electrons
Rate at which energy is dissipated by a resistor
Is the power of the resistor and calculated from:
P = IV = I^2R = V^2/R
Resistors in series
The current has no choice but to travel through each resistor in order to return to the cell. As the electrons flow through each resistor, energy is dissipated and there is a voltage drop associated with each restrictor. The voltage drops are additive that is for a series of resistors R1, R2, R3 the total voltage drop will be Vs = V2 + V2 + ….Vn
The set of resistors can be treated as a single resistor with a resistance equal to the sum of the individual resistances termed the equivalent or resultant resistance
resistors in parallel
They are wired with a common high potential terminal and a common low potential terminal. This configuration allows charge to follow different parallel path. No matter which path is taken the 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. Vp = V2 = V3 = ….Vn
While the voltage is the same for all parallel pathways, the resistance of each pathway may differ. Electrons prefer the path of least resistance so the current will be largest through the pathways with the lowest resistance
Inverse relationship between area of a resistor and resistance
The effect of connecting resistors in parallel is a reduction in the equivalent resistance
1/Rp = 1/R1 + 1/R2 + 1/R3 + … 1/Rn
Rp will always decrease as more resistors are added
Capacitors
Characterized by their ability to hold charge at a particular voltage. Ex; defibrillator which while it is charging, a high pitched electronic tone sounds as electrons build up on the capacitor. When the defibrillator is fully charged, that charge can be released in one surge of power
Capacitance of a capacitor
Defined as the ratio of the magnitude of the charge stored on one plate to the potential difference across the capacitor
C = Q/V
The capacitance of a parallel plate capacitor is dependent upon the geometry of the two conduction surfaces
C = E0 (A/D)
Eo is the permittivity of free space (8.85 x 10^-12)
A area of overlap of two plates and D is the separation of the two plates
Separation of charges sets up a uniform electric field between the plates with parallel field vectors calculated as E = V/d
Direction of electric field at any point between the plates
From the positive plate toward the negative plate
Potential energy stored in a capacitor is
U = 1/2CV^2
Dielectric material
Just another way of saying insulation. It increases the capacitance by a factor called the dielectric constant
Dielectric constant
A measure of its insulating ability
Capacitance due to a dilectric material is
C’ = kC
C’ is the new capacitance
C is the original
Dielectrics in Isolated Capacitors
- Charged capacitor disconnected from any circuit the voltage across decreases
- The dielectric material shielding the opposite charges from each other. By lowering the voltage across a charged capacitor, the dielectric has increased the capacitance of the capacitor by a factor of dielectric constant
Dielectrics in Circuit Capacitors
When a dielectric material is still connected to a voltage source, the charge on the capacitor increases. The voltage must remain constant because it must be equal to that of the voltage source. By increasing the amount of charge stored on the capacitor, the dielectric material has increased the capacitance of the capacitor by a factor of the dielectric constant. The stored energy is only useful if it is allowed to discharge.
Capacitors in Series
The total capacitance will decrease in similar fashion to the decreases in resistance seen in parallel resistors. This is because capacitors must share the voltage drop in the loop and cannot store as much charge.
1/Cs = 1/C1 + 1/C2 + …. 1/Cn
As more capacitors are added Cs decreases
Capacitors in Parallel
Produce a resultant capacitance that is equal to sum of individual capacitances
Electromagnetic waves
Transverse waves because oscillating electric and magnetic field vectors are perpendicular to the direction of propagation
Rectilinear propagation
when light travels through a homogeneous medium it travels in a straight line
Reflection
Rebounding of incident light waves at the boundary of a medium.
Real and Virtual Images
Image is real if the light actually converges at the position of the image. An image is virtual if the light only appears to be coming from the position of the image but does not actually converge there
Negative and Positive Magnification
Pos: Upright image
Neg: inverted image
Negative i or Positive i
i > 0; positive distance and it is a real image. image is infront of the mirror
i < 0; negative distance; it is virtual and located behind the mirror
Refraction
The bending of light as it passes from one medium to another and changes speed. SPeed of any light through any medium is always less than its speed through a vacuum
What to use for speed of light in air
3 x 10^8
Snell’s Law
When light is in any medium besides a vacuum its speed is less than c. For a given medium, n = c/v
n
index of refraction; for a vacuum its 1; for all other materials it will be greater than 1. For air, n is essentially equal to 1
Refracted rays of light obey Snell’s law and as the pass from one medium to another
n1sintheta1 = n2sintheta2
Higher and Lower index of refraction
When light enters a medium with a higher index of refraction it bends towards the normal but when light enters a medium that has a lower index of refraction it bends away from the normal
Critical angle
When light travels from a medium with a higher index of refraction (such as water) to a medium with a lower index of refraction (such as air) the refracted angle is larger than the incident angle. (theta2 > theta1). As the incident angle is increased, the refracted angle also increases and a special incident angle called the critical angle is reached (thetac) for which the refracted angle theta2 is 90 degrees
Total internal reflection
A phenomenon in which all the light incident on a boundary is reflected back into the original material results with any angle of incidence greater than the critical angle
Power negative and positive
Pos: for converging lens
Neg: diverging lens
reciprocal of focal length
P = 1/f
Spherical aberration
Blurring of the periphery of an image as a result of inadequate reflection of parallel beams at the edge of a mirror or inadequate refraction of parallel beams at the edge of a lens. This creates an area of multiple images with very slightly different image distances at the edge of the image, which appears blurry
Index of refraction
When light travels through a medium, different wavelengths travel at different speeds which implies that the index of refraction affects wavelength of light passing through the medium because IR is related to speed of the wave by n = c/v.
Wavelength changes but frequency does not
Dispersion
When various wavelengths of light separate from each other (splitting of white light into its component colors using a prism)
Violet light has a smaller wavelength than red light and so is bent to a greater extent. Because red experiences the least amount of refraction, it is always on top of the spectrum
