Month: August 2011

How to Write Lab Reports

If I have seen further it is only by standing on the shoulders of giants
— Isaac Newton (1676) via Wikiquotes.

Science advances when scientists share their results. If someone tests an hypothesis and finds that it’s wrong, if they share their results, others won’t have to waste time by repeating the same experiments. If someone makes a breakthrough and publishes what they found, then scientists all around the world can use that information to develop new experiments and new applications of that newly discovered principle. Sharing is essential, so it’s important for students to learn how to share well.

Scientists usually communicate their results by giving presentations to other scientists at conferences and publishing articles in scientific journals. Often these presentations are full of the specialized language different types of scientists use with each other, so sometimes science journalists will translate that into regular English news articles that everyone can read and understand. The New York Times and the BBC have good science sections, but what they present comes first from scientists’ formal presentations and articles.

As a result, good presentations and good lab reports are a great way to start learning how to communicate like a scientist.

Lab Reports

A good way to figure out what should go into a lab report is to look at a published article. We have a bunch of copies of Science, which has research articles toward the middle and the back. Articles in Science tend to be brief and fairly dense because it’s one of the premiere journals, so the outlines are not as explicit as you’d find in other places; an Open Access Journal might provide better examples, especially if you’re looking them up online.

Based on our observations, we decided on the following parts for a good lab report:

  • Title: Be short, but unique to give a good idea of what your project is about. Since my classes seldom all do the same experiment, this is very useful. Answer the questions: What did you do? Why did you do it? and What did you find?
  • Authors: Who gets the credit for the work. Usually authors are listed by who did the most work first, but since everyone’s expected to work equally on their group projects you can choose some random or arbitrary order.
  • Abstract: A brief summary of the work, include: what is the problem you’re trying to solve; what you did to solve the problem; and what results you came up with. The abstract should contain all the spoilers.
  • Introduction: Go into some more detail about what the problem is you’re working on, and why it’s important. State your hypothesis and how you’re going to test it. Overview previous work your project is based on.
  • Procedure/Methods: Describe, in detail, what you did, what apparatus you used. Both words and diagrams are useful here.
  • Results: Tell us what you found. Graphs, charts and tables will be very useful here.
    • Figure 1. An example of a diagram. In this case labels have been placed on a photograph of the apparatus. Notice also the caption, which you are reading at this very moment, that goes with the figure.
  • Note on Figures: You should have figures, charts, diagrams and tables in your Procedure and Results sections, but you can have them anywhere they’re appropriate. Each figure needs to have a caption explaining the figure. A useful approach to figures and captions is to try to write them so that someone could understand the entire article by only looking at the figures and reading their captions. One of my students says that popular magazines, like People, are written that way (or at least that’s how they’re read).
  • Analysis and Discussion: To paraphrase a student, “Explain why you think you got those results.” Even if the results are unexpected, or especially if they’re unexpected, you need to explain them. This is also your chance to explain why all of your critics are wrong and you were right all along. If you do that though, it should be written in scientific, passive-aggressive language.
  • Conclusions: Summarize. In the abstract you’re telling them what you’re going to tell them. In the Introduction, Procedure, Results and Discussion sections you’re telling them. In the Conclusion, you’re telling them what you told them. Hopefully by that time they’ll have had enough chances to figure out what you were trying to tell them.
    • Figure 2. An example of a citation for a website.
  • References: Be sure to include a list of the references you used to do your work. This is how you give credit to the people who’s work you are building on. The Yale Library has an excellent page on citing sources. There are a different citation styles you can use but remember the purpose: to give credit where it’s due, and to allow others to be able to find those references easily. All citations should have the author, the date published (or when you accessed it if it is a website), the title, and a way to track down the work.

Note that scientific magazines, like Science and Nature, are very different from a popular magazine like People, for one thing, as was pointed out to me today, the pages don’t smell like perfume (instead they smell like science).

Updates

This paper, on how to bend a soccer ball, is a good example of a student research paper.

The Thermal Difference Between Land and Water

The change in temperatures over the course of the year. Click image to enlarge. Images from 1987 via NOAA's GLOBE Earth System Poster.

The continents heat up faster than the oceans, and they cool down faster too. You can see this quite clearly in the animation above: notice how cold North America gets in the winter compared to the North Atlantic. It’s why London has an average January low temperature of 2˚C while Winnepeg’s is closer to -20˚C, even though they’re at almost the same latitude. There are a few reasons for the land-ocean cooling differences, and they all have to do with how heat is absorbed and transported.

(1) Specific Heat Capacity. Water has a higher heat capacity than land. So it takes more heat to raise the temperature of one gram of water by one degree than it does to raise the temperature of land. 1 calorie of solar energy (any type of energy really) will warm one gram of water by 1 degree Celcius, while the same calorie would raise the temperature of a gram of granite by more than 5 degrees C. The Engineering Toolbox has specific heat capacities of common materials.

(2) Transparency. The heat absorbed by the ocean is spread out over a greater volume because the oceans are transparent (to some degree). Since light can penetrate the surface of the water the heat from the sun is dispersed over a greater depth.

(3) Evaporation. The oceans loose a lot of heat from evaporation. In the evaporative heat loss experiment, While there is some evaporation from wet soils and transpiration by plants, the land does not have anywhere near as much available moisture to cool it down.

(4) Currents. Not only do the oceans absorb heat over a greater depth, but they can also move that energy around with their currents. The solar energy absorbed at the equator gets transported towards the poles, while the colder polar water gets transported the other way. Currents help average out ocean temperatures.

The Freezing Core Keeps the Earth Warm

The internal structure of the Earth.

The inner core of the Earth is made of solid metal, mostly iron. The outer core is also made of metal, but it’s liquid. Since it formed from the solar nebula, our planet has been cooling down, and the outer core has been freezing onto the inner core. Somewhat counter-intuitively, the freezing process is a phase change that releases energy – after all, if you think about it, it takes energy to melt ice.

The energy released from the freezing core is transported upward through the Earth’s mantle by convection currents, much like the way water (or jam) circulates in a boiling pot. These circulating currents are powerful enough to move the tectonic plates that make up the crust of the earth, making them responsible for the shape and locations of the mountain ranges and ocean basins on the Earth’s surface, as well as the earthquakes and volcanics that occur at plate boundaries.

Conceptual drawing of assumed convection cells in the mantle. (via The Dynamic Earth from the USGS).

Eventually, the entire inside of the earth will solidify, the latent heat of fusion will stop being released, and tectonics at the surface will slow to a stop.

The topic came up when we were talking about the what heats the Earth. Although most of the energy at the surface comes from solar radiation, students often think first of the heat from volcanoes.

Note: An interesting study recently published showed that although the core outer core is mostly melting, in some places it’s freezing at the same time. Unsurprising given the convective circulation in the mantle.

Model of convection in the Earth's mantle. Notice that some areas on the mantle are hotter, creating hot plumes, and some are cooler (image from Wikipedia).

Note 2: Convection in the liquid outer core is what’s responsible for the Earth’s magnetic field, and explains why the magnetic polarity (north-south) switches occasionally. We’ll revisit this when we talk about electricity and dynamos.

Learning from Multiple Perspectives Works Better

In fact scientists have found that variety boosts both attention and retention.

–Patti Neighmond on NPR’s Morning Edition (2011): Think You’re An Auditory Or Visual Learner? Scientists Say It’s Unlikely

Morning Edition has an excellent piece that points out that there is little or no actual experimental data supporting the idea that teaching should be individually tailored for different learning styles.

So presenting primarily visual information for visual learners has no proven benefit.

This is something we’ve seen before, however, this article points out that providing each student with the same information in different ways makes it much more interesting for them, increasing their motivation to learn and their retention of what was taught.

Which is fortunate because it means that if you were trying to teach in multiple ways, hoping that the more vocal stuff benefits the auditory learners and the pretty diagrams resonate more with the visual learners, even if this principle is all wrong, all of your students would still have gotten the benefits of variety.

Another key point is that:

Recent studies find our brains retain information better when we spread learning over a period of time versus cramming it into a few days or weeks.
–Patti Neighmond on NPR’s Morning Edition (2011): Think You’re An Auditory Or Visual Learner? Scientists Say It’s Unlikely

So the educational psychologist, Doug Rohrer, recommends giving less math problems at a time but spreading the work out over a longer time. Our block schedule, with three weeks on and three weeks off, ought to work well for this, since students will be studying math intensely on the on-blocks and doing revision assignments on the off-blocks.

The article is below:

The (almost) Perfect Teacup

It’s a glass really. Double walled, liquid suspended in air, beautiful to look at. But it really becomes a wondrous artifact of engineering when its combined with the heavy, rubber and stainless steel lid. The beauty and thermal efficacy of this tea-making system is … elegant. It’s certainly a worthy starting point for our discussion of heat, temperature and thermodynamics in general, and generates interesting questions about heat transport (convective, evaporative, conductive and radiative) and the greenhouse effect, that can be tested with relatively simple experiments.

The first, and most obvious thing my students observed was the fact that the lid prevented heat escaping. The weight of the lid confines the head space, which reduces convective heat loss above the cup and increases the vapor pressure, which reduces the amount of tea that evaporates. Evaporation is the primary way heat is lost from hot liquids, since each gram that evaporates takes 540 calories of heat with it. A simple evaporative heat loss experiment showed that about 70% of the cooling of a cup of water came from evaporation.

The second thing the students pointed out is that the double walled glass insulates, because it reduces conductive heat loss. Solid glass has a thermal conductivity of about 0.24 cal/(s.m.K) (Engineering Toolbox.com; 1 J = 0.24 calories). The conductivity of the air in the space between the walls is two orders of magnitude less at 0.0057 cal/(s.m.K). Of course, having a vacuum in the space would be even better, but it would test the strength of the glass.

Thermally, the glass falls short when it comes to radiative heat loss. A silvery coating would reflect radiated heat back into the cup much better than transparent glass. However, silica glass is relatively opaque to infra-red, which should reduce radiated heat emission. A simple experiment, comparing the cooling rates of water a glass flask wrapped in aluminum foil to one without the foil should give some indication if radiative heat loss is significant.

Finally, the glass does have a thermal advantage though, via the greenhouse effect. Because it is transparent to short, high-energy wavelengths of light, like that of sunlight, but blocks the longer wavelengths of heat energy, the glass should be able to capture some heat from sunlight. This can also be experimentally tested with a couple flasks in the sun. It would be interesting to find out how any greenhouse warming compares to the radiative heat loss through the glass walls.

Last week my students did some basic observations and came up with their own experiments. Then they learned a little about thermodyamics from reading the textbook. This week, we’ll try to get a little more quantitative with the experiments and applications of what they know, and it should be interesting to see if what they’ve learned has changed the way they observe the common objects around them.

Evaporative Heat Loss from a Cup Experiment

This simple experiment was devised to estimate just how much heat is lost from a teacup due to evaporation as compared to the other types of heat loss (conduction and convection).

Experimental setup for measuring evaporative heat loss.

The idea is that if we can measure the mass of water that evaporates over a short period of time, we can calculate the evaporative heat loss because we know that the amount of heat it takes to evaporate one gram of water (its latent heat of evaporation) is 540 cal/g. So we’ll take some hot, almost boiling, water and weigh and take its temperature as it cools down.

Materials

Apparatus.

It requires:

  • A thermometer (Celcius up to 100 degrees)
  • A styrofoam cup (because it’s light)
  • A digital scale (to take quick measurements to tenths of a gram)
  • A 100 ml graduated cylinder (optional)
  • A beaker (100 ml) or cup that can go in the microwave
  • water

Procedure

Our scale has a capacity of about 120 g so we need to make sure that the combined weight of our apparatus that will go on the scale is less. The plan is to have the styrofoam cup, with a thermometer and some water on the scale. Since we can be somewhat flexible with how much water is in the cup we’ll first weigh the cup and thermometer.

(my measurement, not necessarily yours)
Mass of styrofoam cup and thermometer = 29.6 g

So it should be safe to use 70 g of water, which is approximately equal to 70 ml since the density of water is 1 g/ml.

1. Measure the 70 ml of water in the graduated cylinder and put it into the beaker (or microwavable cup). The exact volume is not crucial here since we’ll be using the scale to measure the mass of water more precisely.

2. Microwave the water for about 40 seconds. Again you do not have to be too precise here, you just want the water to be close to boiling. The length of time you need to microwave the water will depend on the strength of your microwave. 40 seconds raised the temperature of my 70 ml of water from 22˚C to 82˚C. If you like you can calculate the heat absorbed by the water, and the effective power of the microwave from these numbers, but it is not necessary for this experiment.

3. Quickly place the hot water into the styrofoam cup with the thermometer on the scale and measure the mass and the temperature of the water.

4. Measure the mass and temperature of the water every 2 minutes for the next 10 minutes.

Calculations

1. Every time you took a measurement, the temperature and the mass should have dropped. The change in mass is due to evaporation. Every time one gram evaporated, 540 calories are lost. Calculate the amount of heat lost due to evaporation at every time measurement.

Hint: Evaporative Heat Loss = mass evaporated × latent heat of evaporation
QE = mE LE

2. Now that you know how much heat was lost, you can figure out how much of the temperature drop was caused by evaporation. Since the specific heat capacity of water is 1 cal/g/˚C, each calorie lost due to evaporation should have reduced the temperature of one gram of the water by one degree Celsius.

Hint: Evaporative Temperature Change = Evaporative Heat Loss × mass of water in container × specific heat capacity of water
∆TE = QE / (m Cw)

You should also the temperature drop caused by evaporation as a percentage of the total temperature drop. Hopefully, your result is less than 100%.

For comparison, here is my data: Evaporative Heat Loss Results and Calcualtions

Discussion and Conclusion

There are quite a number of things that might come up in discussion here, for example: just how large are the potential for measurement errors; and are the results comparable to an actual teacup.

My trial of this experiment indicated that about 69% of the heat loss was due to evaporation. It should be possible to also calculate the amount of heat loss from conduction through the walls of the cup; the thermal conductivity of styrofoam is 0.033 W/mK (via the Engineering Toolbox). The radiative heat loss can be estimated using Stefan’s Law, which can be used to account for all the different methods of heat loss.

Finally, there is no control described in this experiment. A useful thing to try would be to use a styrofoam cup with a lid.

Additional Notes

When my students tried this experiment they use a small (50ml) beaker and 25g of water. Their evaporative heat loss was only 44% of the total, probably due to the smaller volume of water, which as a larger surface-area to volume ratio, and the thinner, more conductive glass walls of the beaker.

Talking Themselves into Depression

When it comes to approaches to solving problems, boys tend to think that talking isn’t particularly useful, while girls, who do prefer to talk things out, run the risk of talking themselves into depression.

When girls talk, they spend so much time dwelling on problems that:

it probably makes them feel sad and more hopeless about the problems because those problems are in the forefront of their minds [and]…makes them feel more worried about the problems, including about their consequences.

…In general, talking about problems and getting social support is linked with being healthy. [But it can be] too much of a good thing.

— Amanda Rose (2007) from the University of Missouri, Columbia, in Girls Who Complain About Their Problems At Greater Risk Of Developing Anxiety And Depression

Rose recommends that they, “engage in other activities, like sports, which can help them take their minds off their problems, especially problems that they can’t control.”

Mobile Classroom

The furniture is starting to move. So far it has just been the couch, which also happens to be the heaviest piece of furniture. Yesterday I helped a couple students rotate it 180 degrees to face the wall, to make quieter, less distracting space. Today we rotated it toward the whiteboard and about half a dozen kids piled onto it for a lesson. I’ve always favored giving students as much control of their environment, and allowing the flexibility of movement, so I’m glad to see that they’re starting to take advantage of that freedom.

Rotating couch.

While I’m not quite sure why the couch has been the first thing to move there are probably a couple of reasons. One is that, compared to the rest of the furniture, the couch is relatively informal. This, in and of itself might have lead the students to consider it a good candidate for rearrangement, but I think it’s also that the couch’s informality meant that no one sat on it on the first day of class; everyone was at one of the desks (bright eyed, bushy tailed and eager to learn). As a result, no one specifically “owned” that space, and negotiating its movement did not involve a large group of people.

The couch also has the space around it so it’s easy to move without having to rearrange a lot of other furniture. It’s not the only piece like that though.

Now, with everyone piling on for today’s lesson, the couch-space has a much more communal feel. Students are becoming more attracted to it when they feel the need for a break, but they tend to go back to the desks when they need to work. Its population shifts over the course of the class.

It’s nice to see that, so far, the students are using the space responsibly. We’ll see how it develops.