Physics 115B Lab 1: Energy, Temperature, Heat, and Power

Physics 115B
Lab 1: Energy, Temperature, Heat, and Power
Learning goals
“Technique”: looking for lab practices that make a
measurement reliable.
Distinguish between heat and temperature.
Measure energy in different forms and demonstrate that it is
conserved.
Apply these experiences to everyday life.
Part I: Thermometers from many lands
There is a collection of thermometers on the table, including an
unusual Star-Trekky one (the “grey instrument”, “g.i.”) and several
digital thermometers, some hooked up to computers.
1. Pick up the g.i., study it, play with it, and see if you can figure
out how to use it.
2. Use it on yourself (or a partner), on the wall, on a computer
screen, and at least four other interesting items. Make a table in
your notebook showing the results. If you find especially
interesting uses or outcomes, record them.
3. Shine the red lamp on the mirror at about a 45º angle. Use the
g.i. to measure the bulb temperature. Then use it to measure the
temperature of the mirror on the reflection of the bulb. Is the
mirror really that hot? Discuss how the g.i. might work, and put
some speculations in your notebook.
4. Fill a beaker of water and use each of the thermometers
(including the g.i. and a computer one) on the water. Record
your results in a table in your notebook. Does the temperature
change if you gently stir the water?

5. Put your hands around the beaker and repeat Step 4 with the
computer thermometer. Is the energy of the water changing?
Why? Does the temperature change if you stir gently? Why?
6. Describe in one paragraph some real-world situations where the
g.i. would be useful. Describe cases in which it would be
inaccurate.
Part II: Liquid Nitrogen Demos
The AI’s will show you cool things with serious cold!!

Part III: Transforming Kinetic Energy to Heat Energy
1. Carefully open the bottle and see what is inside. Measure the
temperature inside using a digital thermometer. Make sure the
thermometer doesn’t touch the bottom of the plastic bottle.
Leave the thermometer in place to see how long the temperature
takes to stabilize.
2. Describe (to your lab partners and in your notebook) what
would happen if you shook the bottle vigorously and then set it
down. We want a description in terms of energy. (Energy in
what form? From where?) Would the energy content of the
bottle change? What about the temperature? What if you
waited awhile afterward?
3. Put the lid on the bottle and shake it 40 times vigorously.
Repeat your temperature measurement. Watch it for a while.
Record what you observe, then graph the result. Compare what
you observe to your expectation in Step 2. Try repeating the
experiment several times. (Everyone should do it at least once.)
Why might such repetition be useful? Describe several reasons.
4. There is a big brass object attached to the table. On top of it,
there is a hole where you can insert a digital thermometer and
measure its temperature. Using friction with the belt, try to
increase its temperature by 2°C. Do you have more respect for
heat energy now? Note: please don’t let the belt slip off the
object – this could damage the thermometer probe.

Part IV: Heat and Temperature
Using the materials available on the table, we’ll take a look at
how objects store heat. In order to get this accurately, you’ll need
to make some careful measurements and record the data in your
notebook. You will follow the steps listed below. Read the steps
now, but, before you do them, read the rest of this section. You
should understand exactly how the steps will allow you to
determine the specific heat of your sample before you start making
the measurements.
1. Get a metal block from an AI and measure its mass (M metal ).
Also measure the mass of an empty styrofoam cup.
2. Fill a beaker with cold water and ice. You’ll find those in the
fridge/freezer.
3. Put tap water in a styrofoam cup and measure its total mass.
From that determine the mass of the water (M H20 ). There should
be just enough water in the cup just to have your block
completely submerged. Measure its temperature without the
block (T H20,intial ). You can use two probes on the same
computer, one for the ice water, one for the tap water.
4. Attach a string to the metal sample and put it in the ice water.
Measure the temperature of the water (T ice water =T metal,initial ).
5. Then, carefully place the metal sample in the styrofoam cup of
water. Stir and track the temperature on the computer. When it
reaches a constant value, record it: (T H20,final =T metal,final ).
We can connect the temperature change of the water to energy.
Recall from your textbook that the heat (energy) required to raise
the temperature of 1 kg of water by 1°C is 1 Calorie ≈ 4200 J, so it
takes about 4.2 J to raise 1 g of water by 1°C. How much did the
temperature of the water in the styrofoam cup change when the
block was added? How many Joules of heat energy did the water
lose? What principle allows you to determine the change in the

energy of the block? By how much did its energy change? Did its
temperature change by the same amount as the water?
Temperature and heat are related, but not the same! Each
substance has a “specific heat,” the amount of energy needed to
raise 1 g of the substance by 1°C. From the above discussion, the
specific heat of water is C H2O ≈ 4.2 J/g-°C. (It can also be quoted
in other units, for example, 1 Cal/kg-°C.) C does not depend on
the mass, shape, or temperature of the material – it just depends on
what the object is made of.
Use this definition and your measurements to figure out the
specific heat, Cmetal, for your block. Report your result to one of
the AI’s – we will compare the values measured by the different
groups in all the labs. Does your metal or water have the higher
specific heat?
Discuss and list some real-world situations when having a
material with high specific heat or low specific heat can be
important.

Part V: Electrical Energy and Power – a Watt is a Watt
The 100-Watt light bulb has been set up for safe immersion in
water. (Don’t try this at home!)
1. First try plugging it into the Kill-A-Watt meter. Try the various
buttons. How much electrical power is being used? We will try
to measure this ourselves.
2. Put enough tap water in the plastic water jug to cover the bulb.
500 ml should do it. Figure out the mass of the water. (Why is
it easy if you used 500 ml?) Put the probe of a digital
thermometer in the water. It should not touch the wall of the
jug or the bulb. Use a plastic ruler to stir the water gently.
Measure the temperature of the water with the bulb off.
3. Turn on the bulb. Record the temperature at fixed time
intervals, say every 30 seconds, for several minutes. Stir the
water gently and continuously with the ruler. Plot the
temperature vs. time.
4. We can connect the temperature change of the water to energy.
Recall from your textbook that the heat (energy) required to
raise the temperature of 1 kg of water by 1°C is 1 Calorie ≈
4200 J, so it takes about 4.2 J to raise 1 g of water by 1°C. At
what rate is energy being added to the water (J/s = Watts) while
the bulb is on?

Physics 115B
Lab 2: Atoms, molecules, and measuring the
very small
In this lab we will make a series of measurements using simple
equipment (rulers, beakers, lab balances) that will give us
estimates of molecular-scale quantities such as the size of a
molecule, the wavelength of light, and Avogadro’s number.
Learning goals
Understand the relative and absolute sizes of atoms,
molecules, the wavelength of light, the average molecular
spacing in a gas.
Apply mathematical and geometrical concepts to the physical
world.
Understand some techniques for making realistic
measurements of things that are invisibly small.
Warm-up 1: Guesses
Do this exercise individually, without consulting lab partners or
other sources. You won’t be judged (or graded) on the accuracy of
your results – we really mean guess. This is for later comparison
with what you’ll measure in today’s lab.
1. In your notebook, rank in order of increasing size the
following items:
The wavelength of visible light
The diameter of an atom
The length of a molecule. We’ll be using an oil (oleic acid)
with chemical formula C 18 H 34 O 2 , that forms a long chain of
atoms, so use that in your thinking. Here’s a model:
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(Wikipedia, Oleic Acid)
The average distance between molecules in air
2. Guess numbers for the sizes, indicating both a guess for the
size of each item and the ratios between adjacent sizes in your list.
(Example: you might guess that an atom is 25 times larger than the
wavelength of light.)
Warm-up 2: Slick
You know that oil floats on water, and, at least since the Gulf
oil spill, you know that oil on water forms a “slick” – a very thin
layer or film. Under controlled circumstances, this film can be one
molecule thick! (So an oil spill spreads very far indeed.) It isn’t
easy to prove that the layer is one molecule thick, but, by assuming
this, we can measure the size (it turns out to be roughly the length)
of an oil molecule, something Lord Rayleigh did in 1890. Though
there are some practical issues, the concept couldn’t be simpler.
Figure out how to determine the thickness t of the film
formed by a volume of oil V by measuring the area of the
film A.
We make the patch of oil visible by dusting a bit of the surface
of the water with baby powder. The AIs will demonstrate this.
They will also suggest some assumptions we are making and
maybe test them.
Parts I, II, and III can be done in any order – the AIs will get
your group started on one after the demo, then move on to the
others. Don’t forget Part IV at the end.
Both Parts I and II will need you to measure the volume of a
drop from an eyedropper. We will assume that all the droppers are
the same (we tried a few, and they are at least similar), so your
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group need only measure one dropper for each part. We will not
assume that a drop of oil-water mix (Part I) and a drop of
turpentine (Part II) are the same size. 1 Do this as part of Part I and
II, as the graduated cylinders are available. For now, think how
you would do it.
Using oil-water or turpentine and one of the 25-ml graduated
cylinders, determine the volume of one drop from your
eyedropper. Each group member should try this, using the
same dropper. Record all the measurements for your group.
Part I: Measuring the size of a molecule with a ruler
The trick now is to make this quantitative, and what makes it
hard is that V must be known and must be quite small to keep the
film smaller than our pans. (Rayleigh used a very large pan; Ben
Franklin, who started all this, publishing an account in 1774, used
a pond.) As part of the Warm-up 2 demo, the AIs will make a
1:1000 mixture of oil:water. Oil and water, famously, do not mix,
but we can do well enough by making a suspension of tiny oil
droplets. When you put a drop of the mix in your pan of water, the
water in the mix will just enter the water in the pan, leaving the oil
droplets to join and form the film.
Measure the volume of a drop of the oil-water mix, as
described in Warm-up 2.
Make several measurements of areas of films, each from a
single drop, in the pans, recording the results. (You may
only get to do one trial per pan. Let the AIs know if you
need to reuse a pan – they can help dump it out and clean it
for another trial.)
Estimate the size of a molecule of the oil, assuming that the
film is one molecule thick. Remember that your drop was
not all oil!
1 We used to assume this, but then we tried it – it isn’t true!
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Assuming that all the measurements you have made are
accurate, discuss what effects could make your estimate of
the molecular size too big? too small?
From the size of a molecule, estimate the number of oil
molecules in a cubic centimeter of oil. This is called the
number density, n. To do this, you need to make
assumptions about the shape of an oil molecule. Assume
that the molecules are cubes that are packed together. (This
turns out to be a poor assumption – see the picture on page 1
– but it will do for now.) You can (and should) calculate the
number of oil molecules in your film. It is surprising that
you can measure such a number (even roughly) so simply!
For the next steps, we will need the density (mass per cubic
centimeter) of the oil. The AI’s have some oil you can use
to measure this. Give the oil back to them when you are
done.
The oil we are using is oleic acid, the major component of olive
oil. (Rayleigh and Franklin just used olive oil.) Oleic acid is
described by the chemical formulas
C 18 H 34 O 2 or CH 3 (CH 2 ) 7 CH = CH (CH 2 ) 7 COOH .
The important constant Avogadro’s number, N A , is the number of
atoms or molecules of a substance in a mole of that substance. The
atomic or molecular weight is the mass in grams of a mole of
atoms or molecules of a given type. The atomic weights of
hydrogen, carbon, and oxygen are 1, 12, and 16, respectively, so,
for example, a mole of carbon atoms has a mass of 12 g.
From this information, what is the molecular weight of oleic
acid?
Using this molecular weight and your measurements,
calculate an estimate of N A , Avogadro’s number.
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Discuss factors that could cause your estimate to vary from
the accepted value, N A = 6.02×10 23 molecules/mole.
Note from the right-hand chemical formula above and the
picture on page 1 that oleic acid is a chain of carbon and oxygen
about 20 atoms long. (The structure is a bit bent, but we’ll ignore
that.)
From your measurements, what is the diameter of an atom?
Part II: The wavelength of light
As we will study later in the course, light is an electromagnetic
wave. No matter is moving in this wave – what is oscillating as the
wave propagates is the strength of electric and magnetic fields.
Though this is very abstract, our usual picture of a wave applies,
but with a different meaning of the graph.
whole pattern
moves
wavelength
We often ask questions like this, and it is worth noting what we are looking for.
We are not looking for “I could have forgotten a term in my equation.” or “I could have
multiplied wrong.” or “I could have weighed the sample wrong.” Even though we call
this exercise “understanding the errors,” that does not mean mistakes. We are looking for
specific effects or inaccurate assumptions (explicit or hidden) that could have distorted a
measurement or its interpretation. An example from this lab would be assumptions about
the shape of an oil molecule. An example from last week would be the assumption that
no heat was lost through the wall of a beaker. You should not only list such effects, but
indicate, if possible, the direction they would change your result. For example: “Not
taking into account the heat entering through the wall of the beaker during the
measurement would make our measured specific heat too small.”
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Instead of the graph representing, say, the height of the water
surface in an ocean wave, it can be the strength of the electric field.
A stationary observer would measure the electric field getting
stronger and weaker as the wave went by, just as a person in the
ocean would bob up and down as a water wave passed.
Like any sine wave, this wave has a wavelength, the distance
between two adjacent crests or troughs, for which we use the
Greek letter lambda, . We can ask, If light is a wave, what is its
wavelength? You may know that the wavelength of light depends
on its color. Since we just want an order-of-magnitude estimate,
this won’t matter to us (though it will make the patterns we
observe quite beautiful.)
Dip a wire-loop in the bubble solution. Go ahead – blow a
bubble or two! Look specifically at the reflections of the
room lights, noting that the reflections are pretty colors, not
the white of the lights themselves. We can make the pattern
of colors more regular. Dip the loop again. This time hold it
so the loop is vertical and look at the reflections in the flat
bubble spanning the loop. Note that there are horizontal
bands of alternating green and red. The pattern moves as the
water drains downward, making the film thinner.
Eventually, there is no reflection at all from the top, even
though the bubble hasn’t popped yet (though it is about to.)
Each member of the group should make and observe this
pattern.
As we’ll learn when we study waves later in the course, the
bands of color appear when the film is about a wavelength thick,
via a process called interference. In fact, for a color with
wavelength , when the film is ¼×, ¾×, 5/4× thick you see a
bright reflection of that color. Because gravity drains the water
downward, the bubble in the loop is thinner at the top and thicker
at the bottom, and there are horizontal bands of constant thickness.
This means the green and red reflections alternate as each color
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goes through the multiples of its . As the water drains, the pattern
shifts downward. Eventually, the top of the bubble is less than ¼-
wavelength thick and there is no color that reflects, that is, no
reflection at all. For our purposes, we just need the fact that when
there are a few bands present, the film is about a wavelength thick.
To make this quantitative, we will use a variant of the same
trick used in Part I. In this part, we use turpentine. We use
turpentine because, for some reason, it forms films that show
interference bands. This tells us the film is about a wavelength
thick, even if we don’t know why that happens. Note that, unlike
the oil we use in Part I, the turpentine evaporates quite rapidly.
This means that there is some uncertainty about the volume of
turpentine remaining. On the plus side, it means we can try several
drops in succession in the same pan without dumping it and
restarting.
To make our measurements of the wavelength of light, we will
drop one drop of turpentine into a tray of water. To know the film
thickness is about a wavelength, we need to see the colored
reflection bands. We have found that in the black pans we are
using, it is easy to see both the bands and the edge of the slick
without using any baby powder.
Measure the volume of a drop of turpentine, as described in
Warm-up 2.
Make observations of what happens when you put a drop of
turpentine on the water. Make sketches of what you see or
describe it in words. We have always seen one oddity: the
first drop spreads to cover the whole tray, but subsequent
drops make near-circular films of reasonable size. Measure
the diameters of all films after the first. Note roughly how
long after the drop you see the colors appear, when they
vanish, and when the film itself vanishes. We found that you
can do five or six drops in the pan before it gets too messy.
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Estimate the wavelength of light using your observations and
measurements.
Part III: Weighing air
It seems odd, but it turns out to be easy to weigh air. Your
equipment: a flask with a valve, a balance, and a vacuum pump.
The pump can suck just about all the air out of the flask in about a
minute. (The flasks have a protective coating and are glued inside
the boxes for safety.) Note the valve with the black handle on the
flask. It is open when the handle points straight out from the stem.
Figure out how to find the mass of air in the flask and do it –
record the result. Pretty easy. Do it again (have each group
member try it) to see if you get the same result. What would
happen if you tried to weigh helium this way?
Use your result to find the density (g/cm 3 ) of air. What do
you need to do this? Figure this out with the tools at hand.
Air is mostly nitrogen molecules, N 2 . (For simplicity, we’ll
assume it is all nitrogen. The oxygen molecules have about the
same mass, so this is a fine approximation.) The molecular weight
of nitrogen is 28, so one mole of N 2 has a mass of 28 g. In Part I,
you determine the number of molecules in a mole, called
Avogadro’s number, from measurements you made. Don’t worry
if you haven’t done Part I yet – here we’ll use the actual value, N A
= 6.02×10 23 molecules/mole.
From your measured density of air, the molecular weight of
N 2 , and the value of N A , find the number density, that is, the
number of molecules per cm 3 of air.
Now figure out the average distance between air molecules.
This will take some thinking and discussion in your group.
Indicate your reasoning in your notebook.
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Part IV: Summary
Remake the same list from Warm-up 1, using the results of this
lab, including the sizes and ratios from your measurements.
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Physics 115B
Lab 3: Motion
Learning goals
Perform an experiment that demonstrates the predictive
power of Newton’s laws.
Apply the laws of motion in two dimensions.
Determine and observe the constraints placed on spinning
objects by conservation laws (energy and angular
momentum.)
Warm-up 1
As director of research for a toy manufacturer, you regularly
receive proposals for new toys. Indicate which of these seem
possible for an inexpensive toy with no internal power source
(battery, etc.):
a) A plastic top that spins on a plastic base for weeks.
b) A simple top that, while spinning, flips over so that its larger,
heavier end is on top.
c) A specially weighted wheel that rolls back and forth on a V-
shaped ramp for weeks.
d) A specially weighted wheel that rolls back and forth on a V-
shaped ramp, reaching a higher point on the ramp on each
pass.
e) The situation in c) and d) if you know there are magnets in
the wheel and the base.
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Part I: Catch the ball
On the table is a track on which a cart can roll with very little
friction. The top section of the cart is a launcher that shoots a ball
straight up. Ask an AI to show you how to activate the launcher.
Never stand with your face directly above the launcher – it can
fire without warning!!! There is also an adjustable spring that
can give the cart a reproducible velocity along the track.
First: read through all of Part I so you’ll understand the overall
goal of these steps.
Determine how high the ball goes when launched with the
cart at rest. This takes at least two people: one to trigger the
launcher and hold the meter stick, and one or more to watch
how high the ball goes. Do quite a few trials with each
member of your group launching and watching the height.
Everyone should have a record of all the launches. Look for
reproducibility in two forms: precision, how close in height
are the launches? are there outliers, occasional misfires that
go to a quite different height? Also, be sure to decide what
you should define as the initial height of the ball.
At this point, ask the AI to take the ball away. Ask your AI to
demonstrate the cart-launching spring and how to adjust it.
You will use a light gate (an LED light source and light
sensor that straddle the track and a timer) to measure the
cart’s speed just before it launches the ball. You shouldn’t
have to do anything to the timer other than turning it on
(power switch on the back) and hitting the reset button on
the front. Figure out how this system works. (Try passing
your hand between the LED and the sensor. What happens?)
How can you use this setup to measure the speed of the cart?
Note the setting of the cart-launching spring (see the scale on
the side of the launcher) and record it in your notebook.
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Without launching the ball, launch the cart several times,
again looking for reproducibility in both forms.
Knowing the velocity of the cart and the height the ball
reaches, compute the location where the ball will land (see
formula below). Place the cup at that location, making sure
the bumper is between the ball-launch point and the cup.
Call the AI over. He’ll give the ball back and observe.
Launch the cart with the ball-launcher armed. If the ball is
caught in the cup, congratulations! Try it again. If not,
check the calculations and the velocity and try again.
Now remove the cup, move the bumper to the far end of the
track. Discuss where the ball (launched straight up from the
cart) will land – ahead of the cart? behind the cart? in the
cart? What if the cart is launched faster? Slower? Write
your predictions in your notebook. Now try a few more
launches. Vary the cart speed by just giving it a push with
your hand and recording the velocity without adjusting the
launcher. What do you observe? (No numbers needed, just
watch.) Why does it make sense?
Useful formula:
The time it takes an object to fall a height h starting at rest is
2h
t fall
,
g
where g is the gravitational acceleration, g = 9.8 m/s 2 = 980 cm/s 2 .
(See pp. 11-12 of Chapter 3 in your textbook. If you know how to
derive this result, show your lab partners, but for this lab we will
take it as given.) Figure out how to use this formula in your
calculation of where to put the cup. Show your reasoning and all
calculations in your notebook.
The Big Picture: this is the essence of Newtonian determinism,
discussed in lecture. Given the initial position and velocity of an
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object and knowledge of the forces, the object’s future position is
completely predictable. Where in your calculation did you express
the knowledge of the forces? The understanding of situations like
those in your last measurements (without cup or bumper) were
essential to Galileo’s realization that the Earth could, in fact, be
moving.
Part II: Spinning toys
This lab will be our only look at things that spin. We’ll mostly
just ask that you observe, but we do want you to realize that
conservation of energy and, as we will see in Part III, a new
quantity called angular momentum, are universal.
The toys in this section fall into three categories.
Category 1: spin the “Top secret top,” the little silver top on
the round black stand. Start the “Space wheel,” the little
satellite thingy, by putting it on its rails (the two vertical
transparent plates on a black stand) near one end and letting
go from rest. Just start each one once and let them do their
thing while you go on to the other toys.
Category 2: Before the first spins, guess how long each toy
will spin. Take turns spinning the “Quark top” and the
“Euler disk” on their mirrors. (The mirrors are just smooth
surfaces.) Time how long each spins. To set the Euler disk
going, you place on edge at a slight angle (tilted on the edge
that that is rounder; the sharper edge tends to scratch the
surface) on its mirror (the slightly concave one). If you give
it a good spin it will spin like a coin. Record in your
notebook interesting things you observe with the Euler disk
as it comes to rest.
Category 3: There are three similar tops, two green wooden
ones and one orange/green plastic one, that do something
odd. (Have an AI show you how to work the button on the
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plastic top.) Why is this flipping behavior unexpected?
What must be true about the motion for this to be consistent
with conservation of energy? Though the sippy birds are not
spinning, their iconic behavior is odd. What might provide
the energy needed for them to keep going despite friction?
Now look back at the Category 1 toys. Can they actually be
frictionless? (Hint: no.) What if we told you (truthfully)
that they’d keep going for days or weeks? What would you
suspect was going on? (Hint: aren’t the bases a bit heavy?)
Try starting the satellite thingy at a point about halfway to
one end.
Compare your understanding with your responses to the
Warm-up.
Part III: Angular momentum (and more toys)
If you are uncomfortable trying this, do it very slowly
and/or ask an AI to help you. You must be careful – if
you spin too fast you can hurt yourself by falling off or
someone else by hitting them, especially when you get
dizzy! Sit on the comfy chair. Sit upright (don’t lean in any
direction), with you arms extended horizontally to the sides.
Push yourself with your feet, or have a lab partner push your
arm so you turn slowly. When you are turning steadily,
bring you hands in to your chest. What happens to your rate
of spin? Put your arms back out. What happens? Everyone
should try this – if you are nervous, do it in the reverse order,
starting with your arms in. This will be quite tame. You can
try this with the dumbbells in your hands. If you do, start
with arms in.
Sit on the stool again. Have a partner spin up the bicycle
wheel as fast as possible (note that this one is meant to turn
only in one direction). Have him/her hand the wheel to you
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with the axle oriented vertically. With the stool at rest and
with you holding the axle firmly at both ends, turn the wheel
over, so that the axle that was on the bottom is now on the
top. What happens?
Stand on the floor (no chair this time) and spin the wheel
with the axle horizontal. Try to turn the wheel as if you
were steering it for a left turn, keeping it upright. Hold the
axles tightly, but let your arms respond to what the wheel
does.
Find an AI and describe your observations to him. We will try
to extract the properties of a new quantity called angular
momentum from your observations.
Find the colored plastic oblongs. Try spinning them on a
table, first counter-clockwise then clockwise. Why is this
behavior odd? Does it violate the law of conservation of
angular momentum? Why or why not?
Just for fun: turn over the pair of connected bottles to see the
tornado. Spin the water-filled gyroscope.
Find the book held closed by the rubber band. Try flipping it
in the air about each of its three perpendicular axes. What
do you observe? This odd behavior is universal and well
understood from Newton’s Laws.
Part IV: Air
An AI will do a demonstration in which the big vacuum cleaner
can support a ball on its air jet. What must the direction of the net
force of the air on the ball be?
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Physics 115B
Lab 4: Radioactivity
In this lab, we will study radioactivity. Chapter 4 of your
textbook is a good introduction to the physics and the terminology,
but also to the effects of radiation. You probably know a bit, and
you will learn more in this course, about the technological,
political, and health impacts of radioactivity. From a scientific and
philosophical point of view, however, radioactivity and radiation
are a direct, observable connection to the atomic and sub-atomic
world.
Though some of the sources of radioactivity we will use are
created for scientific purposes, several are from the grocery store
or the hardware store and are found in almost every home. We
will get the radioactive atoms for one part of the lab from the air.
None of the sources used in this lab are dangerous. Still, we
will apply sensible rules that should always be applied to
radioactive sources.
1. Minimize contact. Hold the needle sources by the stopper
only. Keep them in their tubes when not in use. Hold the
plastic disk sources by the edge. Don’t touch the actual
source in the smoke detector. Decide what you are going
to do with a source before you pick it up, and then do it.
2. Keep track. If your group is using one or more sources,
each member should know where each source is. Never
put the source in your pocket or somewhere hidden where
it can’t be seen. Return the source to an AI when done.
3. No eating (or food) or drinking in the lab. Wash your
hands before leaving the lab.
4. When in doubt, ask an AI.
To start this lab, look at the Learning Goals and the questions in
Part III; use them to guide your investigations in Parts I and II.
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Learning goals
Perform measurements of things you cannot see.
Distinguish different types of radiation and different types of
radioactive materials by measuring their penetration power,
half-life, and tracks in the cloud chamber.
State evidence that radiation is made of particles.
Explain how half-life is a property of exponential decay.
Measure radiation from everyday objects.
Part I: Geiger-Mueller counter
The Geiger-Mueller counter is a very sensitive device that can
detect the passage of individual particles (beta and gamma and
maybe alpha rays) from radioactive decays. Because it can detect
individual decays, it allows you to count them, hence “counter.”
Your AI can describe how it works. Our G-M counters (GM-45s)
are connected to computers, so the results can be stored and
plotted. The computers are trained to make the traditional “clicks”
that let you get a sense of how many decays are detected without
plotting. We will find it most useful to measure the rate of
detected decays. The computer does this by counting for a minute,
plotting the result, then doing the same thing for the next minute,
etc. The displayed rate is the counts per minute, or CPM.
The instructors will demonstrate how to use the GM-45
counters. They will also introduce some of the concepts we
need to study radioactivity.
Note that the counting is a statistical process. Governed
directly by quantum mechanics – it is fundamentally random.
Even with measurements of perfect precision, we do not expect to
get the same number of counts in each minute! This means that we
must do some averaging, either numerically or by eye. It also
means that we must be patient and let the average unfold. During
the longer measurements, you can work on Part II.
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First make sure the computer is operating and recording
data. The first thing to measure is the room background, the
rate of counts with no source present. This may be partly an
occasional misfire of the GM-45, but it is mostly
radioactivity in the materials used to make the device itself
and in the air and other surroundings (including you).
Record this number – you will want to subtract it from all
measurements of other sources.
The AIs will give each group a source. Write down what
your source is. Place it gently on the GM-45 (the mesh is
fragile). Record the rate. Try some other sources.
Use the ringstand and clamp to hold the Co-60 or Cs-137
source at various heights above the GM-45, say 4, 8, 16,…
cm. Before you take the measurements, what do you expect
to see? (We are looking for something more specific that “It
will decrease.”) Plot the count rate vs. distance. Discuss the
result.
We can learn a bit about the radiation from the source by
trying to block it. There are various sheets of materials from
paper to metal in various thicknesses. Carefully remove the
plastic cover from the smoke detector and the metal cover
from the source itself, then put it face-down on the GM-45,
as the AI did in the demo. Try the various materials between
your source and the GM-45. Start with one, then 2, then
more sheets of paper, then go to the heavier materials. What
do you conclude about the source?
Try this shielding test with another source if there is time.
In a radioactive material, individual nuclei decay to other
nuclei, emitting particles that we call radiation (the old name was
rays). We now know what these rays are: alpha rays are helium
nuclei (these are made of two protons and two neutrons, an
especially stable configuration that can be emitted as a unit from a
larger, less stable nucleus); beta rays are electrons, and gamma
3

ays are photons, the quantum-mechanical particles of light, but
with energies much higher than visible light. X-rays are just lessenergetic
gamma rays.
Note how the particle model is “obvious” here: we count them.
(We’ll address this again in a later lab.) The particles have specific
properties, notably mass and electric charge. The most intuitive
property, the particles’ sizes, turns out to be unimportant here.
They are all so small compared to the size of atoms that none of
our measurements reveal any information about particle size.
If the radioactive nuclei are decaying, should they eventually be
gone entirely? Yes! For the sources we’ve been using so far, this
takes years (Co-60), or tens of years (Cs-137, Sr-90), or hundreds
of years (Am-241), or even billions of years (K-40), so you haven’t
seen it in your measurements.
When radioactive nuclei decay, they do so in an odd way. Each
type of unstable nucleus has its own half-life: the time it takes for
half of the individual nuclei of that type in a sample to decay.
Example: the half-life of Americum-241 is 432 years. The odd
thing is that after 432 years, when half the original Americium is
gone, it is not the case that the other half is about to kick. In fact,
half of the remainder will live another 432 years, exactly as if the
whole surviving half had just been born. Your text calls this
“dying, but not aging.” Mathematically, this is exponential decay,
which, along with exponential growth is observed in many natural
phenomena (see Chapter 5 in your text).
The problem with doing experiments with radioactive nuclei
with a half-life short enough to measure in a 3-hour lab is that if
you buy some on Monday, it’s mostly gone by Tuesday. Instead,
we need something that is made continuously that we can harvest
and observe. Fortunately (in a sense, see below for the downside),
uranium and thorium in the Earth’s crust is slowly decaying, and
one of the resulting nuclei, Radon-222 (Rn-222), is a chemically
inert but radioactive gas. It has a long enough half-life (about 4
4

days) that it can seep out of the ground. When it seeps into a
basement, it can collect there, and, if levels are high, can become a
health problem. The nuclei that Rn-222 decays to (see Figure 1)
are metals. They stick to dust particles in the air, and we can
collect and concentrate them onto a paper filter by using the yellow
air samplers that have been running in the adjacent classroom.
Note from the figure that lead-210 ( 210 Pb) has a long half-life. It is
thus a bottleneck, and what we will see is the decays that precede
this. We will be most sensitive to the beta-decays of lead-214 and
bismuth-214. Because we are seeing a chain of linked decays, we
will not observe a single half-life. For example, for awhile, as long
as there is 218 Po (polonium-218), it will replenish the lead-214, and
likewise, the decay of the lead-214 replenishes the bismuth-214.
Still, the nuclei will decay in tens of minutes, and we can find
when the number of decaying nuclei has dropped to about half of
its initial value.
Note that we will be measuring the rate of decays (counts per
minute), while our discussion of half-life above was in terms
of surviving radioactive particles. Based on your
understanding of half-life, predict the shape of the measured
CPM vs. time you will see if you observe a sample with a
single type of radioactive nucleus for several half-lives.
Make a sketch, labeling the location of one half-life.
5

Figure 1. The radioactive decay sequence of uranium-238. The portions
relevant to this lab are highlighted. The half-life of each nucleus is in its box
(note the different units!), with the type of decay (, ) between it and the next
box. (Note: a + is a positive electron: a bit of anti-matter!)
6

When you’ve read the above description, make another room
background measurement by leaving your GM-45 with no
source on it for a few minutes. Then place an unused paper
filter on the GM-45 to see if it changes the count rate.
Go with an AI to get a sample from the air samplers.
Choose one and shut it off, noting the time and the time on
the blackboard indicating when the sampling started.
Carefully remove the paper ring holding the filter paper and
take the filter back and place it on your GM-45, noting the
time again. (Note that holding the filter is no more
dangerous than cleaning the filter in a clothes drier, which is
doing the same thing that the air sampler did.)
You will leave this running for much of the rest of the lab. This
would be a good time to do Parts II and III.
When you have enough data: does the shape of the graph of
CPM vs. time match your prediction? Use the graph made
by the computer to estimate the half-life of this decay chain.
(Call over the AI to help you print copies of the graph for
your notebook.) Look back at Fig. 1 and think about
whether your result for the half-life makes sense.
Part II: Cloud chamber – seeing the rays
Set up:
In its operating state, the chamber should have isopropyl
alcohol soaked all the way up the blue filter paper on the
side, with a couple mm of liquid on the bottom. If the whole
thing is dry, add about 30 ml of isopropyl.
Make sure there is ice in the water cooling bath (a tub or
Styrofoam cooler). Make sure the rubber hoses from the
pump (small square black plastic thing) to the chamber and
from the chamber to the bath are connected and the pump is
submerged in the ice water. Plug in the pump and make sure
the water is circulating.
7

After the water has circulated for a few minutes, plug in the
chamber. This does several things. It turns on a Peltier
refrigerator in the base, cooling the bottom of the chamber.
It applies a voltage to the yellow wire, which, if connected to
the metal needle through the stopper, is intended to help
clear away old tracks. It also turns on lights inside to
illuminate the tracks.
If starting from scratch, it takes about 20 minutes for the
chamber to show tracks. Check periodically that there is still ice in
the bath.
How it works:
The alcohol wicks up the sides of the chamber and evaporates
near the top, forming a clear vapor filling the chamber. The vapor
near the bottom is cooled by the fridge in the base. In fact, it
supercools, falling below the temperature at which it would
normally be liquid. This state is unstable, needing only a
triggering event to cause condensation. When ionizing radiation,
that is, a charged particle such as a beta or alpha ray (but not a
gamma ray), passes through the vapor, it can remove electrons
from the atoms, leaving a trail of positive ions along the path of the
ray. The ions trigger the condensation of the supercooled alcohol
vapor along the path of the ray, and the resulting tiny droplets form
a visible track. Neutral particles such as gamma rays do not make
tracks. However, a gamma ray can strike an electron in an alcohol
molecule, making the electron travel through the vapor. The
electron (now identical to a beta ray) then leaves a track.
When the chamber is working, you will start seeing little
blobs of fog near the bottom. You may also see the
occasional longer tracks. These are due to traces of
radioactivity in the lab and also perhaps to cosmic rays,
which are due to high energy radiation from space hitting the
atmosphere. Watch them for awhile, noting where in the
chamber the tracks form.
8

Have the AI bring you a radioactive source. The “thoria”
and Po-210 sources produce alphas, the Sr-90 produces
betas. These two needle sources can replace the stopper at
the top of the chamber. Record notes on what you see in
your notebook. Some sketches would be appropriate. The
Co-60 and Cs-137 produce betas and gammas. These
sources have to stay outside the chamber. Explore moving
these sources around until you see the best tracks. Try
blocking the radiation with the same materials used in Part I
or other things lying around the lab. Again, record some
notes on what you observe.
Part III: Some conclusions
Discuss these questions within your group and briefly note your
conclusions in your notebook.
How might you distinguish between alpha, beta, and gamma
radiation with measurements, like those you’ve made today
and others?
What evidence have you seen today that “radiation” is
particles?
List at least two ways to protect yourself from a source of
radiation. (Let’s say it was small enough to carry, and you
had to move it.)
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