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R2 Physics 16 – Specific Heat

For class, students were asked to read Chapter 7.1 Woodstoves in How Things Work: the Physics of Everyday Life and watch the following videos.

And lastly, Doc Schuster with heat capacity and specific heat:

I started class with a lecture on woodstoves and did a few demonstrations.  I had a small candle and we talked about what was required for it to burn – oxygen, fuel (wick & wax) and watched it go out when I put a glass jar over it, depriving it of oxygen.  I also put some food coloring in a beaker of cold water and showed the students how the food coloring just sank to the bottom of the cold water. You can also drop some food coloring into some hot water and see that it mixes much faster, demonstrating convection.  Lastly, I had some magnesium ribbon left over from chemistry last year so I burned a small piece of that by placing it over a propane burner – you can see photos of it in last year’s post.  I burned the magnesium ribbon to demonstrate that burning produces light as well as heat and to show that chemical bonds require a bit of energy (activation energy) to break them.  The magnesium ribbon needed to be over a hot flame for 20 seconds or so before it started to burn.

I found a lab for specific heat online at physicslab.  The link will take you to the lab handout and you can click on a printable version.  Students measure the mass of an empty styrofoam cup and again with approximately 50ml of cold water in the cup.  The difference in the masses gives them the mass of just the water.  IMG_3682They also measure the mass of a metal cylinder and then place it in a beaker of boiling water.  After a minute or two its a safe assumption that the metal cylinder is now at the same temperature as the boiling water, roughly 100 Celsius (ours was measured to be 99.9 Celsius).  Students then put the Vernier Go Direct Temperature Probe in the cold water and set up an iPad with the Graphical Analysis app to collect temperature data for 2 minutes.  When they were ready they lifted the hot metal cylinder out of the boiling water, letting excess water drip off, or touching it lightly to a paper towel before putting it in the cold water.  Students stirred the water gently with the temperature sensor making an effort to keep it from touching the metal cylinder.  The temperature of the water rose approximately 6 degrees Celsius in just a minute or so while the temperature of the metal cylinder dropped over 70 degrees Celsius.

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Students were given the specific heat of water of 4.184 J/gC and were able to calculate the heat gained by the water,  Qwater = mwatercwaterΔTwater. We assume that the heat energy gained by the water is equal to the heat energy lost by the metal cylinder (Qwater = Qmetal) so we can use the same equation but solve for the specific heat of the metal, cmetal = Q/(mmetalΔTmetal). All the groups got specific heats very close to the expected values for the metals we were using (copper, brass and aluminum).

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Temperature of water as hot metal cylinder is placed in the cup. Graph from Graphical Analysis App using Go Direct Temperature Probe by Vernier.

I’ve done labs like this before using regular thermometers and it works just fine, but using a temperature probe just makes it a little bit easier and produces a beautiful graph of temperature as a function of time.

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SF Physics 13 – Mini Motors

We did a few different activities today.  I had out the snap circuits and had students build a series circuit with 2 light bulbs and then use the same light bulbs in a parallel circuit.  A series circuit is bascially one big loop. series parallelThe current flows from the battery and through each lamp in turn before returning to other end of the battery.  In a parallel circuit, the current splits up and some goes to each lamp separately.  If one of the lights fails, in the parallel circuit the other light will still light up because you still have a closed circuit. But in the series circuit, if one bulb breaks, then the circuit is open and currently can’t flow through the loop.

To show students that an electric field can also make a light bulb glow, I put an old fluorescent bulb near a plasma toy and parts of it would light up, as seen in the photo below.IMG_3672

Lastly we made mini motors with 28 gauge copper magnet wire.   Wind 15 to 20 loops around the end of a fat marker to make a coil, leaving about 10 cm of wire on either end of the loop.  Take the ends of the wire and wrap them around the sides of the loop once or twice to hold it together, then take sandpaper and rub off the insulation on the wire sticking out from the coil.

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You need to get the insulation off so the wire can make a good electrical connection with the paper clips.  Paper clips are bent so they can support the coil and are taped withelectrical tape, to each end of a battery (C or AA work fine).  Place a magnet under the coil loop and then give the coil a flick and if you have everything positioned just right it will keep spinning – see the video below.   I got this lab from Teaching Physics with Toys.  I used this book a lot when my boys were younger.

It can take some fussing to get this to work right.  You have to make sure the insulation is off the ends of the wire and you want it to be pretty close to the magnet.  If you have a variety of magnets you might test to see which ones work best and how close the coil loop needs to be to the magnet to keep it spinning.  This class is small enough that I let them take home the mini motors if they wanted to.

This is the last class til after winter break.

R2 Physics 15 – Ideal Gas Law

The only new physics ‘toys’ I bought for teaching this year were some Go Direct sensors by Vernier.  They work directly with an iPad via bluetooth and their cost was fairly reasonable ($59 & $79 for the ones used in this lab) for a small class.   For today’s lab the students needed to measure the pressure of a gas as they changed its volume (at constant temperature) and as the temperature changed (with constant volume).

The first set up is very simple, as seen in the photo below, its just a syringe attached to the Go Direct pressure sensor.  The syringe actually screws and locks onto the sensor.  Students started with 10 ml of air at atmospheric pressure and room temperature in the

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syringe and then attached it to the sensor.  One student in each group connected their iPad to the sensor via bluetooth and the Graphical Analysis app.  They then changed the volume of the gas sample by pushing the plunger in on the syringe and recording the pressure and volume.  They also took data at lower pressures by increasing the volume, pulling the syringe out.  The volume is easily read off the syringe.

P vs V graph

The data came out great.  Some students graphed the results by hand, some used the Data Analysis app or the Graphical Analysis app, did a fit to the data and found that pressure did indeed depend on 1/Volume. Solving the ideal gas law for P (pressure), you get P = nRT/V and in this experiment nRT are all constant (n = number of moles, R = ideal gas constant, T = temperature).

Since I only had one pressure sensor the students had to wait and take turns doing the experiment, but it only took about 10 minutes to take the data you see in the graph on the right.  I put the next homework assignment on the board and had students work on that until it was their turn to take data.

This worked sooooo much better then when we did this lab in the past using a syringe and balancing books on it, calculating the pressure on the syringe from the weight of the books and then measuring the volume. It just never came out very well so I’m very glad I bought this sensor, it made this lab a lot easier and it actually worked!  This is one of the few labs you end up doing in both chemistry and physics class so I should get plenty of use out the pressure sensor.

For the second lab, we used two Go Direct sensors, the pressure sensor and a Go Direct temperature sensor.  We could have just used a thermometer, but I bought the temperature sensor to use with some other labs we’l be doing in the spring and figured I might as well use it. I’m pretty sure the iPads will only connect to one sensor at a

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time so we had one student measuring the pressure and another student measuring the temperature.  The pressure sensor came with the stopper and tubing seen in the photo to the left.  It fit a flask I had and that became our gas sample for the second experiment. We did this experiment as one big group.  The first data point was at room temperature, then we placed the flask in an ice water bath to get a lower temperature data point.  Then we put the flask in a warm water bath and a hot water bath for a total of 4 data points. You can see the results in the graph below,  pressure depends linearly on the temperature of the gas when the volume is held constant. From the fit you can also see that the y-intercept, b, is pretty much zero, which is what you would expect. Pressure should be zero at T= 0 K, absolute zero.  P vs T graph

From the ideal gas law, P = nRT/V, in this experiment n (number of moles) and V, the volume were held constant and we see pressure equals a constant times temperature.  I’m very happy with these sensors, this lab was always a chore and now its so easy.  I’m going to have the students write up the results of this lab in a formal lab report.

At the end of class I brought out a windbag  – basically a 2 meter long thin bag, roughly 6 inches in diameter.  I laid it out on the table and asked my students how many breaths they thought it would take for me to blow it up.  Most immediately said a LOT of breaths, but one already knew the trick and said I could fill it with one breath – which is the right answer!  By holding the end of the bag wide open and having the bag laid out on the table, I had my mouth about a foot from the opening of the bag and blew into it.  Blowing into the bag at a distance lowered the pressure around the mouth of the bag causing the air around it to rush into the bag – hence filling it with one breath.  I had a box of these bags and each student got to take one home. These were more fun than I expected and a nice demonstration of Bernouilli’s principle. IMG_4996

 

 

SF Physics 12 – Holiday Circuits

Instead of doing the usual circuits lab I decided to have the kids make light up holiday cards.  Fortunately I found this great website on Sparkfun with templates you can download for FREE and I still had some LEDs left over from the last time we did this years ago.  You can order all the stuff to do these from Sparkfun, but I just happened to have the lights and button batteries.  I did not have copper tape but that got me thinking… can’t we just use aluminum foil?  So a little searching on the internet and yes, people use foil for this as well.

The lights I used were from 15 piece “Gumdrop” LED assortment from Evil Mad Scientist, that I had picked up at the Exploratium.  Amazon also sells LED, and you can probably pick them up at Radio Shack if you’re lucky enough to still have one near you.

Here’s my card and the aluminum foil circuit inside.IMG_3545IMG_3581

I didn’t have the button to turn it off or on or the fancy button holder.  I just put a little piece of scotch tape around the side of the battery to keep the loose piece of foil from touching both the top and bottom of the button battery (short circuiting).  When you squeeze the card over the button battery it closes the circuit and lights it up.

A couple of the kids tried using the conductive paint but it was really hard to get a nice line of paint and it didn’t actually seem very conductive at all.  The foil was easy and you don’t have to wait for it to dry.  There was also a window template and light up gingerbread house.  You can also find robot templates or have the kids make up their own.

R2 Physics 14 – Buoyancy

Students were asked to watch the following videos and/or read 5.1 Balloons in How Things Work: The Physics of Everyday Life.

And lastly Smarter Every Day, does a cool experiment with a balloon in his car.

While we waited for everyone to arrive for class I gave students large syringes with mini marshmallows inside.  By plugging the syringe they then had a contained volume of gas. By compressing the plunger and decreasing the volume in the syringe, they increased the pressure and the marshmallow gets smaller (kind of looks like a raisen).  As they pull out the plunger, increasing the volume of the gas in the syringe, decreases the pressure on the marshmallow and it expands.  Make sure students keep a finger over the plug to keep it from shooting across the room.

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I also crushed a few soda cans with air pressure – you can see a description and video in my previous post on buoyancy and pressure.    For the main activity we tried to measure the buoyant force on a floating object.  I had two large plastic containers with overflow spouts in them, and we filled them up until water started pouring out the spouts.  Once the water stopped coming out, students placed their objects in the water, which caused the water level to rise and overflow.  The overflow was captured in a container and IMG_3475students either measured the mass of the water that was displaced or they measured the volume and calculated the mass from the density.  They then compared the weight of the object floating in the bath to the weight of the water displaced.  They should have been the same and were in a few cases but many were off by 30-50 grams.  It was pretty evident that surface tension was a problem in our overflow spout, so cutting down the pipe we were using, or putting a little bit of dish soap in the bath would help with that.  having a smaller bath – smaller area on top might have made it more accurate as well. One group tried placing a 45 gram boat in the water and no overflow occurred, so we definitely had a problem with the set up.  I may have to try again with just a regular bowl that I can fill all the way up to the top and collect the overflow in a basin and see if I get better results.

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