142 Lab Final Flashcards
Where does the measured value go?
on the y-axis
Where does the control value go?
on the x-axis
How do we transform the power law into a linear relationship?
take the log of each side
How did we simplify Coloumb’s law in the first lab?
use the same magnitude of charge so the charge can be simplified to Q^2
then, we took the log of each side of the equation to make a linear relationship
and since Ke and radius was held constant, we could combine them to get log(F)=log(Q^2) +b
can take pull the ^2 outside the log(Q) as well
Relationship between Force and radius in Coloumb’s law
inverse square relationship
What did we vary in the first lab?
first, varied the amount of charge on each sphere and then, varied the distance between the charges while charge charges
What do you have to do when using the Coulomb’s law simulation?
make sure to convert the charge from microcoulombs to coulombs
Why did we get smaller force values than expected in the Coulomb’s law simulation?
in reality, the R distance would be bigger from polarization. The positive charges would have repelled each other and spread out further, but this does not happen in the simulation
since F varies inversely with R, the bigger R would give a smaller F than what we got
Why can we not make the same assumptions about electric force that we can with gravitational force?
we can neglect the radius of the earth in the simplified formula F=mg
can’t neglect the radius between two charges since we are discussing relatively smaller values
Is the electric force always pointed in the same x-direction or same y-direction in the field hockey simulation?
No. As the puck rotates around the negative charge the x and y components of the electric force are changing
Is there anything constant about the electric force in the field hockey simulation?
As the puck rotates in a circle, we know that the radius remains constant and the magnitude of the charges remain constant
therefore, the magnitude of the electric force is constant
Can you split the electric force into components?
yes
Electric fields
deal with the forces a test charge will feel due to the source charge
Electric potential
deal with the energy changes a test charge will undergo as a result of the source charge
How can you find the magnitude of an electric field at a specific point?
Find the x and y components from each source and vector sum. Make sure to include the direction/angle of field in answer
Draw all the electric fields on a specific point in space. Lines move away from + charges and towards - charges
Do you include charge signs when calculating electric field at a specific point?
no
What to watch out for when calculating electric field?
make sure coordinate system lines up and add negative signs according to vectors
What are the units on electric fields?
N/c or V/m
How to determine angle of net electric field?
use tan(Enetx/Enety)
make sure to do a drawing to determine the correct quadrant
What can you do on charges and fields phet simulation?
Can add gridlines and measurement of the net electric field at a specific point in space
When using the gridlines, make sure to check the key at the bottom to see what one block on the grid corresponds to
Why does the sensor in the middle of a symmetric square of positive charges feel no net electric field?
The sensor records the net electric field which includes magnitude and direction
The positive charges opposite each other in the square cancel each other out
What happens if you would remove one charge from a symmetric square of positive charges to the net electric field at the square’s center?
if you removed a charge from the top, a charge on the bottom of the square would have no charge to counteract it
since positive charges create electric fields away from themselves, the charge at the bottom would produce a net electric field upwards
Volt units
Joule / c
*voltage describes the effects of potential energy so makes sense that Joules are included
Equipotential curve
a circle around a source charge where every point in the circle has the same voltage
Geometric relationship between equipotential lines and electric field lines?
Electric field lines are perpendicular to equipotential lines
What changes with voltage as you move across equally spaced equipotential lines?
Potential energy changes as voltage changes
PE=qV
Why do the equipotential lines spread out more the further you get from the charge?
the electric field strength weakens the further you are from the charge
also: V=kq/r so a bigger r will mean a smaller V
How to calculate the total electric potential at a certain point?
don’t need to break down vectors
just sum V=kq/r from different charges using signs
use signs in calculation
for r just use the straight line distance between two points. don’t need to break into components
Which way does electric field point in a parallel plate capacitor?
electric field points from + to - charges
points in the direction of decreasing electric potential
What is the unit for potential energy?
joules
DC circuit
the voltage supply is not changing over time
constant voltage supply
positive and negative charge on the DC power supply
positive charge is the orange terminal
negative charge is the black terminal
Rheostat
the type of resistor we use
big clunky metal looking thing
Multimeter
can measure either voltage or current depending on setting
Voltmeter
measures voltage at a location
to use multimeter as a voltmeter, it must be connected to the VΩ jack
voltmeter must be in parallel with element of interest
What is true of items in parallel?
they have the same voltage
Ammeter
measures current at a location
to use multimeter as an ammeter, it must be connected to the 10A jack
ammeter must be in series with element of interest
Ohmeter
measures resistance
to use multimeter as an ohmeter, it must be connected to the VΩ jack
does not have to be in series or in parallel
What does the “m” on the voltmeter dial stand for?
Stands for milivolts. It is the maximum amount of milivolts the voltmeter can read
might purposefully set to a lower setting in order to get greater precision
What should the sig figs be for a number you get from a graph on Excel?
3 sig figs
When calculating percent error when do you use negative sign?
Use a negative sign if the experimental value was smaller than the actual value
Ohmic device
it is a device with constant resistance
How can you measure if something is an Ohmic device?
look at the slope of the current versus voltage plot
if the resistance is constant, the slope of the current versus voltage plot will be linear
Is a diode an Ohmic device?
No
the diode does not allow current to flow in the reverse direction
And since V=IR and V is being held constant, we know that R must be changing to accommodate the changing I from the diode
Resistors in series
Resistors in series add
R1+R2… = Req
Capacitors in series
Capacitors in series add by inverse
1/C1 + 1/C2… = 1/Ceq
Resistors in parallel
Resistors in parallel add by inverse
1/R1 + 1/R2 = 1/Req
Capacitors in parallel
Capacitors in parallel add
C1+C2 … = Ceq
What is true in series?
Current is the same everywhere
What does a light bulb behave like?
a resistor
Resistance of an ammeter
In order to accurately measure current, and not change the current of a circuit, an ammeter must have low resistance
Resistance of a voltmeter
In order to accurately measure voltage, the voltmeter must have high resistance
The high resistance will make almost no current flow through the voltmeter in order to accurately measure voltage (V=IR)
Voltmeter just measures voltage without effecting the current (has almost none flowing through it)
Slide-wire
high resistance wire (ran along meter stick in experiment)
high resistance allows for changes in voltage with very little current
can measure noticeable voltage drops
How do you measure voltage drops in a slide-wire?
use a slide-key
Slide-key
slide key runs along the slide-wire
when pushed down it makes electrical contact with the wire and measures the ∆V
allows us to measure voltage drops in the slide-wire at any length we want
Resistivity
a measure of the resistance of a given size of a specific material due to electrical conduction
symbol: p
units: Ω*m
some known resistivities can be found on reference chart
What is resistance measured in?
Ω
What is current measured in?
amps (A)
How did we find resistivity in lab 3?
We measured voltage across a varying length
Then, we knew current, so we converted voltage into resistance by dividing by current (V/I=R)
Then, we found the slope of resistance by length which gave us Ω/m
Then, we rearranged the resistivity formula to solve for p using our value of Ω/m from the graph
Purpose of Wheatstone bridge
allows us to measure an unknown resistance using a second, known resistance
Set up of Wheatstone bridge
place the slide-wire in parallel with Rx (unknown resistance) and Ry (known resistance) and an aluminum bar of negligible resistance
Rx, the bar, and Ry are in series, this series is in parallel with the slide-wire
then, connect the middle of the aluminum bar to the slide-wire to create a “bridge”
Galvanometer
measures current along the bridge pathway in the Wheatstone bridge
Under what circumstances will there be no current through the Galvanometer?
there will be no current through the Galvanometer when there is no potential difference between the two sides of the parallel circuit
potential differences create an electric field that generates a current to move articles along. Without a potential difference, there will be no current through the Galvanometer
Does current move across things?
No! Moves through
How did we use the galvanometer to determine the unknown resistance of Rx?
We measured current versus position and made a graph of µA/m
Then, we used the y-intercept (µA) and the slope (µA/m) to find Lq
Then, we subtract Lq from 1m to find Lp
Then, we used the formula Rx=Ry(Lp/Lq) to solve for Rx
Where is the Earth’s magnetic north pole?
at the south pole
What direction do magnetic field lines travel in?
from north pole to south pole
Where does the colored end of a compass point towards?
the magnetic north pole
earth’s south pole
What component of Earth’s magnetic field does a normal compass show you?
the horizontal component in the xy plane
How did we set up the Earth’s magnetic field lab?
used a coil that produced a magnetic field with the same magnitude as Earth’s magnetic field
compass feels the combination of the horizontal component of Earth’s magnetic field and the coil’s magnetic field
compass points in direction of combined Earth’s magnetic field and coil’s magnetic field
What happens at the center of the coil (Earth’s magnetic field lab)?
resultant vector is a combination of Earth’s magnetic field and coil’s magnetic field
tan(theta)= Bcoil / Bearth
How did we generate the current in the coil that then creates the coil’s magnetic field (Earth’s magnetic field lab)?
Used a switch
when the switch is flipped, the current flows in an opposite direction through the coil
What to pay attention to when collecting measurements?
the distance between lines
need to check the scaling
What did we have to do when calculating the theta deflection from the coil’s magnetic field?
had to average the left and right deflections together for each current level
What are the units for magnetic field?
Telsa (T)
How did we find Beh using the coil?
found Tan(deflection angle) versus Current
the graph’s slope gave us 1/A
then, use the formula for Bcoil with 1/slope for current A
Bcoil=Beh since they have the same magnitude in our set up
Dip angle
Angle between the horizontal and the overall direction of Earth’s magnetic field
How to find magnetic field of current loop given only diameter, voltage, and length?
Use length and area to calculate resistance
Use V=IR to calculate current
Then, use magnetic field of circular loop formula with the current to find the magnetic field
How will a magnetic field create a Fb on a moving particle?
only creates an Fb if some component is perpendicular to the motion
use RHR to determine the direction that Fb will point
If a particle starts from rest and E and B fields are in the same direction what will happen?
The E field will accelerate the particle in the direction of the E field
This will also be in parallel to the direction of the B field
Therefore, the particle will not feel any force from the B field
What does Faraday’s Law refer to?
the phenomena that relative motion between a conductor and a magnetic field will create an induced emf
Lenz’s Law
emf will oppose the change which produces it
In our set up of the coil with the induced emf what does a negative reading indicate?
tells us that current is flowing out of the VΩ jack and in through the COM jack
When coil is set up to voltmeter how do we determine direction of the magnetic field?
First, use the voltage to determine which direction the current flows (neg. is out of VΩ jack and pos. is out of COM jack)
Then, use RHR to determine which direction B points within the coil (left or right)
Next, draw magnetic field lines. Inside the coil, the magnetic lines look like going south to north. Draw the circles to help
Why does removing the magnetic bar create a counterclockwise current in the coil?
as the magnet moves away, the flux decreases
Lentz’s Law wants to keep this decreasing flux, so you produce a magnetic field in the same direction as the magnet’s
The magnet has a leftward field (down pole of magnetic)
Make a leftward magnetic field down the middle of the coil which by the RHR gives us a counterclockwise current in the coil
How does flipping the switch closed in the electromagnet produce an induced field in the coil of the sensor?
when the circuit is closed, a clockwise current forms in the electromagnetic, which gives it a rightwards magnetic field
the coil with the sensor wants to oppose this increase in flux (Lentz’s law) so will produce a leftward magnetic field which will give a counterclockwise current
What are the three ways to change magnetic flux?
changing surface area
changing strength of B
changing angle of plane
How does flipping the switch open in the electromagnet produce an induced field in the coil of the sensor?
when the circuit is opened, the clockwise current stays in the electromagnetic with a rightwards magnetic field
however, now the flux is decreasing so the other coil wants to add to this decrease and put a magnetic field in the rightwards direction as well
will make a clockwise current from the rightwards magnetic field in the coil attached to the sensor
Is higher electric potential where protons or electrons are in a rod?
protons
Which direction on a compass does Earth’s magnetic field point?
in the -z direction
into the page
What does the trigger level and the trigger scope show in the oscilloscope?
the trigger level allows us to set the voltage point at which the graph will actually start
when set to positive, the trigger slope starts the trace on the positive slope of the wave
when set to negative, the trigger slope starts the trace on the negative slope of the wave
oscilloscope
graphically displays varying voltage levels
How do you find period from the oscilloscope?
multiple the number of divisions in one period by the TIME/DIV
How do you get the amplitude of volts from the oscilloscope?
multiple the number of divisions to one peak by the VOLTS/DIV
What happens when voltage is positive in a RC circuit?
When voltage is positive, charge is storing on the capacitor
What happens when voltage is negative in a RC circuit?
When voltage is negative, charge is building up on the opposite plate of the capacitor
in this way, the plate is discharging
Half life of the oscilloscope
the amount of time it takes the capacitor to discharge 1/2 of its initial voltage
How does a capacitor discharge when connected to a resistor?
will decay exponentially over time
What is the index of refraction?
shows how much slower light moves in a material compared to in a vacuum
Refraction
change in direction of light causes by moving from one material to another
What do you need to be aware of when working with Snell’s law?
that Excel does trig in radians
need to convert degrees to radians
What do you have to do when using arc trig functions in excel?
convert the answer back into degrees
When collecting data what should you do if possible?
run multiple trials to be able to average the results together
When using the semicircle on the Bending Light simulation what do you need to consider?
you are measuring light going from glass to air
have to make sure you’re measuring the incidence angle and retracted angle from the normal
What critical point are we looking for in the bending light simulation?
Where light transactions go from not being able to refract at all (reflecting completely) to being able to partially refract
Angle of deviation
difference between the light’s direction of travel before and after hitting the prism
When is the deviation of the light in the prism at a minimum?
when from the critical angle you rotate the prism clockwise and the light moves to the right
the moment where the light is the furthest to the right before it starts moving left is where the deviation of the light is at a minimum
What is the triangle angle, alpha, in the prism?
60 degrees
If light bends more towards the normal does this indicate a higher or lower n?
This indicates a higher n
What does object look like in optical imaging lab?
two sticks drawn
What does image look like in optical imaging lab?
hooded screen where the images form
What should you read the meter stick to in the optical imaging lab?
to 1/10 of a centimeter
What should every numerical answer have in the exam?
UNITS and SIG FIGS
What is something to make sure of when doing measurements on the exam?
make sure you are taking measurements to the proper decimal place
protractor=nearest degree
meter stick = 1/10 of a centimeter
When using excel and making a graph what should you include in your explanation on webassign?
the equation found for the trendline
What is the syntax for graphing?
y vs x
How many sig figs can you get from a graph?
3
Does log() give you units?
no
logs are unitless quantities
Which is a scalar and which is a vector quantity for electric fields and electric potentials?
Electric field = vector
Electric potential = scalar
What is an Ampere equivalent to?
C/s
shows how charge is moving overtime
What are the units of a Hz?
cycles / seconds
s-1
What is the unit of capacitance?
Farads
How do you know if your index of refraction is incorrect?
if it is less than 1
Angle of incidence
Angle between incoming light and normal
