Welcome to Advanced Physics Lab
Advanced Physics Lab, PHYS 3600
Don Heiman, Northeastern University, 2017
This Week – Introduction
Today – Intro to Course
Motivation, boiler plate (syllabus/labs)
Fermi Questions
Tuesday – Errors and Noise, Optics
Roundoff, propagation of errors
Optics lecture
Wednesday – Semiconductors, Acoustics
Thursday – Intro to Experiments
Visit 230 DA
Tour my research lab
WHY Physics Labs
What are the common features in physics departments
that students need for future work in the STEM workforce
(science, technology, engineering, and math).
Study by the
American Institute of Physics
WHY Physics Labs
The Career Pathways Project (CPP)
American Institute of Physics
Summary of Findings
Ten features were identified by CPP as common
among departments that are effective in preparing
students to enter the STEM workforce.
The four Curricular Features
 Varied and high-quality lab courses
 Research opportunities for undergrads
 Curricular flexibility
 Building communication skills (writing)
WHY Physics Labs
The Career Pathways Project (CPP)
American Institute of Physics
Varied and high-quality lab courses
“Through taking varied lab courses, students conduct experiments and collect data
on a broad set of physics theories and gain practical experience with a variety of
laboratory techniques, types of equipment, and software packages.“
“These experiences are valued by employers because they teach students valuable
skills like
problem solving, troubleshooting,
persistence, attention to detail,
equipment operation, error analysis, data reduction, and teamwork.”
“Varied and high-quality lab courses offer powerful preparation for a diverse range
of career pathways, including graduate school, employment in the STEM workforce,
and teaching science at the high school level.”
WHY Physics Labs
Physics Today, April 2017
Introduction to Course
Adv Phys Lab website
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Physics Department Website
Scientists Driven by Intellectual Curiosity
Some Quotations
Doc is ˝interested in knowing something about
everything˝
- Steinbeck's Cannery Row
Doc, ˝His mind had no horizon˝
˝All life is an experiment. The more experiments you
make the better.˝
- Ralph Waldo Emerson
˝Even theorists have an advantage when they
thoroughly understand experiments.˝
- Don Heiman
“I learned very early the difference between knowing
the name of something and knowing something.”
- Richard P. Feynman
Feynman was also famous for persuading scientists
and mathematicians to explain complex ideas using
only simple terminology.
Scientists Driven by Intellectual Curiosity
Intellectual Curiosity
Turn every experience into a question.
Can you analyze it?
If you can, you’ll learn something.
If not, you’ll also learn something.
Enrico Fermi
Enrico Fermi
Who was Enrico Fermi?
What is a “Fermi Question”?
What is a Fermi Solution?
Why are Fermi Questions Useful?
Enrico Fermi – Who and Why
Who was Enrico Fermi?
ENRICO FERMI (1901-1954)
was an Italian physicist who was
awarded the 1938 Nobel Prize his
contributions to nuclear physics
and quantum theory. He was also
a noted experimentalist.
Fermi was forced to flee Italy and in 1942 he
achieved the first controlled nuclear chain
reaction at the University of Chicago. During
World War II he was a leading member of the
Manhattan Project that developed the atomic
bomb in Los Alamos, New Mexico.
Overpass; bomb test; slow neutrons; lunch walk
Why - Fermi Questions
What is a Fermi Question?
How many piano tuners are there in Chicago?
How many jelly beans are in a container?
Why are Fermi Questions useful?
Fermi questions challenge you to ask more questions,
not just provide "an answer".
What is a Fermi Estimation
A solution requires estimation of physical quantities.
Fermi was legendary for being able to figure out things in his head, using information that
initially seems too meager for a quantitative result. He used a process of "zeroing in" on
problems by saying that the value in question was certainly larger than one number and less
than some other amount. He would proceed through a problem in that fashion and, in the
end, have a quantified answer within identified limits. Bracket the answer.
In a Fermi question, the goal is to get an answer to an order of magnitude (typically better
than a power of ten) by making reasonable assumptions about the situation, not necessarily
relying upon definite knowledge for an "exact" answer.
It is like a “back-of-the-envelope” solution.
 Fermi question is posed with limited information given.
o How many buckets of water or balloons would fill a swimming pool?
 A Fermi question requires that you ask many more questions.
o How big is a bucket or water balloon?
o What are the approximate dimensions of the pool?
o What measurement must be estimated using the dimensions of the pool?
o ... and the list goes on.
 A Fermi question utilizes estimation, done over and over
 Fermi estimations emphasize process
The Classic Fermi Question
HOW MANY PIANO TUNERS ARE IN CHICAGO?
How might one figure out such a thing? Surely the number of piano tuners
in some way depends on the number of pianos. The number of pianos must
connect in some way to the number of people in the area.
Approximately how many people are in Chicago? -- 3,000,000
Does every individual own a piano? -- No
Would it be reasonable to assert that "individuals” don't tend to own pianos? – Yes. Households do.
About how many individuals in a household. Guess an average of 3 per household.
That says that there are 1,000,000 households in Chicago.
Does every household own a piano? -- No. Perhaps one out of every ten does.
That would mean there are about 100,000 pianos in Chicago.
How many piano tuners are needed for 100,000 pianos?
Some people never get around to tuning their piano; some people tune their piano every month.
If we assume that "on the average" every piano gets tuned once a year,
then there are 100,000 "piano tunings" every year.
How many piano tunings can one piano tuner do?
Let's assume that the average piano tuner can tune four pianos a day. Also assume that there are 200
working days per year. That means that every tuner can tune about 800 pianos per year.
How many piano tuners are needed in Chicago?
The number of tuners is approximately 100,000/800 or 100 piano tuners; between 500 and 50.
Why did I round off 125 to 100 piano tuners? Does that seem plausible? Why not?
Examples Fermi Questions
1. How many jelly beans fill a one-liter jar?
2. How many golf balls will fill a suitcase?
3. What is the height of $1M in $20 bills? What is the weight?
4. How many gallons of gasoline are used by cars each year in the US?
5. What fraction of the area of the United States is covered by automobiles?
6. What is the weight of solid garbage thrown away by Americans every year?
7. What is your average hourly wage over your lifetime?
8. How many cells are there in the human body?
Back of the Envelope Calculation
Simple Measuring Instrument
Using only a sheet of paper,
measure the height and width of the white board.
Q: What is the Height in inches?
Q: What is the width in inches?
Let’s do the experiment, then analyze the data.
The best measurement receives a prize.
Back of the Envelope Calculation
Paper Folding Problem
What would be the thickness of a standard piece of paper
folded in half 50 times?
Q: What is the thickness of a piece paper?
(everyone guess, unless you already know the answer)
Q: Can you relate the thickness to a know quantity?
Q: How do you deal with exponents?
Let’s try to do the experiment, then find an approximate answer.
Exercise – EasyPlot
Print Name ________________________________
PHYS 3600 AdvPhyLab, 5/15/2017
Many momentous scientific discoveries have first appeared to simply look like random noise.
This exercise points out how seemingly bad data can be crucial. Here you will use EasyPlot, a
simple plotting and analysis software program, to graphically examine minimal data.
(1) Use the y(x) data set on the right.
x
y
(2) Enter and plot the (x,y) values in EasyPlot.
Use the following underlined command buttons.
File, New, Enter Data,
then enter (x,y) data in (a,b) columns,
Plot, Plot
1
5
2
4
3
8
4
5
(3) Find the mean and standard deviation of the y-values with EasyPlot
by double clicking on a data point, then press Statistics (σ).
5
7
6
6
What is the mean:
________________
What is the standard deviation: _____________
(4) Curve fit the data to a linear function.
Double click on a data point, press Curve Fit,
highlight “1 - ax+b”,
OK
What is the curve fit value for the slope (dy/dx) and uncertainty in slope?
(You may need to go to “preferences” and click on “show uncertainties”.)
dy/dx = ____________ ± ____________
10
slope dy/dx = 0.31  0.36
8
(5) Question: Can you give a reasonable magnitude for
the slope? __________________
y
6
4
(6) Question: Can you assign a reasonable sign to the
slope (positive or negative)? ___________
(7) Question: If y(x) represents the price of the stock
over a year, would you buy or sell the stock for
$4/share?_______________
0
0
2
0.8
0.6
0.4
0.2
0
-0.2
2
4
x
slope dy/dx
At first, you might say that you cannot give a reasonable
magnitude for the slope when the uncertainty is large.
However, suppose you are offered 1,000 shares of stock
well below the current $6 listed on the stock exchange
and the offered price is only $4/share.
1
y = +0.3143x +4.733, r =0.1596
a=0.3606, b=1.405
2
6
Tomorrow, Tuesday
Tomorrow
(1) Read Notes on Errors
(see website)
(2) Example on Rounding Off
(3) Lecture on Optics
Experiments:
Fuel Cell (Si photvoltaicsolar cell)
Ruby Spectroscopy (laser, photocell)
Speed of Light (nsec pulses, photodetector)
Faraday Rotation (polarizers, laser, photodetector)