User:Arianna Pregenzer-Wenzler/Notebook/Junior Lab/2008/10/01

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Plank's Constant

Set Up

SJK 01:49, 22 October 2008 (EDT)
01:49, 22 October 2008 (EDT)
You don't give the model numbers, etc. of equipment! Nor a link or mention of the lab manual. Remember to do this by habit--you will forget what stuff you use easily over time. A picture is very useful too.

We are using a mecury light source shining through a lens that focuses and splits the 'white light' into it's spectrum. By moving the lens on its support rods (all the way out) we focused the first maxima on the slit in the h/e apperatus. We decided to measure the bands of light to the right of the first maxima. The first three bands are shades of blue that we will call Blue 1,2,3 then a yellow/green band and a orange band. The first part of this experiment is compareing the time to reach the stopping potential (Vmax) for the five different band varying the intensity of the light using filters that allow between 100% down to 20% of the light to pass. For the 3 blue bands we just use the filter to change the intensity, for the orange and yellow bands we need to add the apropriate colored filter.


For each intensity we wait until our volt meter has reached a steady(ish) max value that we call the max for that intensity. Then we make 5 timed trials of the time needed to reach this Vmax, making sure we reset the apperatus between trials. We need to do this for only two bands of color!!

  • in the Lab manuel it says it is important to take the time to reach the same Vmax, but Vmax seems over all to decrease as the intensity decreases. This has caused some real inconsistancy in the data. It might be good to start with the max filter and increase amount of light up to 100% passing, that way all trials would be the time required to reach the same V, even if this V did not necessarily equal Vmax.

Data, Experiment 1

part A note pushing reset button can result in moving the h/e apperatus.

moving from center maxmia to the right we start w/ blue 1 , then blue 2, blue 3 , yellow/green, orange

note all readings V/20,this is not correct, the 20 on the voltmeter corresponds to how precisely the voltmeter is mesuring the voltage, ie how many decimal places are being reported.

Blue 1

Intensity Max Voltage time 1 time 2 time 3 time 4 time 5
100% 1.971V 31.06s 30.4s 25.56s 29.65s 23.62s
80% 1.961V 48.22s 40.31s 43.91s 42.06s 48.44s
60% 1.944V 46.66s 47.41s 40.31s 37.72s 41.16s
40% 1.953V 64.33s 51.25s 49.41s 51.88s 49.16s
20% 1.921V 68.28s 72.1s 67.25s 60.72s 60.32s

Blue 2

Intensity Max Voltage time 1 time 2 time 3 time 4 time 5
100% 1.626V 36.63s 33.69s 29.5s 35.31 29.03
80% 1.598V 43.12s 52.82s 34.50s 62.5s 57.62s
60% 1.617V 50.22s 42.66s 52.82s 51.19s 48.59s
40% 1.627V 55.81s 54.71s 54.41s 52.47s 43.56
20% 1.611V 81.22s 70.72s 75.75s 73.81s 62.75s

Blue 3

Intensity Max Voltage time 1 time 2 time 3 time 4 time 5


This time I am going to start with the intensity at 20%, measure the Max stopping potential, then measure the time it takes to get to that stopping potential for all the other intensities. Ok this was frustrating, I started here with the idea of taking another set of measurments on the time it takes to reach a stoping potential set by taking the stoping potential of the green band at its lowest intensity. I made the assumption that if I could reach a given potential at 20% intensity, then I could reach that same potential at 40% and greater intensities. Wrong assumption, the potential I reached at 20% intensity, was greater than what seemed to be the Max potential at 40% so just to check I moved to 60% and 100% I should have recorded the potential's I did reach but I didn't think to, but they weren't easily comming up on the potential reached at 20%.

Intensity Max Voltage time 1 time 2 time 3 time 4 time 5
60% 0.853
40% 0.853
20% 0.853 0.41 0.44 0.53


Intensity Max Voltage time 1 time 2 time 3 time 4 time 5


we hooked up the oscilliscope so we could see the patter of the voltage. We thought we should see the voltage go from zero to some max voltage in an exponitial fassion. We spent quite awhile with the oacilliscope set to AC coupling (should have been set to DC). On AC coupling we saw a voltage occilating around zero with a spike right after we zeroed the insterment, decaying exponetially back to zero, not what we wanted to see (this didn't make any sense). With the coupling se to DC, you see a voltage of zero (as the insturment zeros), once you release the button there is a spike of positive voltage toward (appearing to reach) the max voltage then the voltage drops and then approaches Vmax in the expected exponitial fassion. The voltage reaches its max value very quickly (a second or so) so trying to measure the time to reach Vmax in the first experiment was pretty much pointless.

Part B

record DVM voltage for each band

I am going to assume this is the (aprox Max) voltage for each color

remember to divide Voltage by 20 (our Voltmeter is set on 20)

I am recording the values the voltmeter fluxuates between

making sure to zero out the instrument using the discharge, and waiting to record my value until the voltmeter reading is fluxuating in a constant manner

1st Blue:2.059-2.060 Volts this is a medium blue

2nd Blue:1.720-1.721 Volts this is a purple blue

3rd Blue:1.497-1.498 Volts this is a bright blue

making sure to use the yellow filter, I adjusted to light so that it fell on the slit/photodiod then put the filter in front of the slit then zeroed the insturment

Green:0.839-0.840 Volts (with yellow filter) this is a bright yellow/green,

I have a yellow and a green filter and I'm thinking that this value shoud be intrupreted as green so....

Green:0.852-0.853 Volts (with green filter)

Orange:0.714-0.715 Volts (with yellow filter) this is just orange

Data Analysis Experiment I

Data Analysis Experiment II

Media:Planck's Constant IIfinal.xls

accepted value for Planck's Constant

Data, Experiment 2

For each band of color in the first order measure stoping potential, then move to second order and repeat

Starting at the center maxima (a 'white' band) moved to the lst order to the right. Note these values should be the same as the values I recorded for Part B of experiment B, if they are different this points to systematic error (our equiptment is giving different values for what should be the same reading). I am noticing that this time through the values given by the voltmeter are not fluxuating between values.

1st Order:

Blue 1: 2.053 Volts

Blue 2: 1.713 Volts

Blue 3: 1.487 Volts

Green: 0.850 Volts (using green filter)

Orange: 0.716 Volts ( using yellow filter)

use results of part B experiment 1 for a second trial

2nd Order (trial 1):

Blue 1: 2.049-2.050 Volts

Blue 2: 1.715-1.716 Volts

Blue 3: 1.510-1.511 Volts

Green: 1.235-1.236 Volts (using green filter)

Orange: 0.801-0.802 Volts ( using yellow filter)

note I did not ecpect these values for 2nd order, I expected that the stopping potential would decrease, of course there are plenty of variables, like the amount of light in the room. But I am taking these measurments one after another so the conditions should be similar for all trials.

2nd Order(trial 2):

Blue 1: 2.039-2.040 Volts

Blue 2: 1.708-1.709 Volts

Blue 3: 1.510-1.511 Volts

Green: 1.232-1.233 Volts (using green filter)

Orange: 0.803-0.804Volts ( using yellow filter)

note: there is a lot of variation here, I will go to record a value that I think has stablized only to re-check myself and find that my voltmeter is fluxuating between a slightly different set of values.