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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

 

 

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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.

R2 Physics 13 – Kepler’s Laws

I started class with a slide show on Kepler’s Laws and gravity.  We talked about Copernicus, Galileo, Tycho Brahe and Kepler and their contributions.  Here’s Doc Schuster’s video on Kepler’s Laws:

For the lab activity I had the students draw ellipses using a loop of string and two tacks, as illustrated in the first video above. Students labeled one of the foci as the sun and then labeled the perihelion (point on the ellipse closest to the sun) and the aphelion (point on the ellipse furthest from the sun).  They then measured the semi-major axis (distance ellipse_eccentricity_1

from the center of the ellipse to the aphelion and c, the distance from the center of the ellipse to a foci.  The ratio of c/a is a measure of the eccentricity of the ellipse.

For the second part of the lab, I had plotted the orbit of Mercury, which is one of the more elliptical planetary orbits.  Students were to show that Kepler’s Second Law – a planet sweeps out equal areas in equal time holds for Mercury’s orbit.  At two locations, when Mercury is closest to the sun and furthest from the sun, they had to measure the radius of the orbit and the angle swept out in that time period.  From that they could estimate the area of triangular section of the orbit and found the two sections to be approximately the same.IMG_3287

For more detailed descriptions of these and related activities you can look at an earlier blog post.

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High School Physics Round 2 (R2)

UnknownThis class is in progress, 2017-2018, most of the students are freshmen and we’re using How Things Work: The Physics of Everyday Life by Bloomfield.

Round 2 – High School Physics 01 – homeschoolsciencegeek

R2 Physics 02 – Velocity – homeschoolsciencegeek

R2 Physics 03 – Falling objects – homeschoolsciencegeek

R2 Physics 04 – Archery & Ramps – homeschoolsciencegeek

R2 Physics 05 – Energy – homeschoolsciencegeek

R2 Physics 06 – Circular Motion – homeschoolsciencegeek

R2 Physics 07 – Friction – homeschoolsciencegeek

R2 Physics 08 – Conservation of Momentum – homeschoolsciencegeek

R2 Physics 09 – Hooke’s Law – homeschoolsciencegeek

R2 Physics 10 – Bouncing Balls – homeschoolsciencegeek

R2 Physics 11 – Paper Rollercoasters – homeschoolsciencegeek

R2 Physics 12 – Force of Gravity – homeschoolsciencegeek

R2 Physics 13 – Kepler’s Laws – homeschoolsciencegeek

R2 Physics 14 – Buoyancy – homeschoolsciencegeek

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R2 Physics 12 – Force of Gravity

For Chapter 4: Mechanical Objects in How Things Work: The Physics of Everyday Life, I’m skipping over the bicycles and going to spend two class covering the material in 4.2 Rockets and Space Travel.  I had students watch the following videos before class.

It’s Rocket Science! with Professor Chris Bishop by the Royal Institute is an hour long but his videos are amazing and full of great demos that I can’t do in my house.

We did a virtual lab today since its difficult to measure the tiny force of gravity between two objects unless one of those objects is the earth.  I found this website, thephysicsaviary.com which has a virtual lab where you have two objects and you can change the masses of the objects and the distances between them.  When you have it the way you want you click a button and it ‘measures’ the gravitational force between the objects.   I didn’t lecture on this topic at all before doing this lab. If they had read the book they might already know that the force is going to depend on both masses and depend strongly on the distance between the masses, otherwise they will discover the dependence when they plot their data.

gravity labStudents picked two masses and then measured the force at 10 different distances, which they have to read off the ruler on the screen.  They plotted the data by hand or in the Graphical Analysis App.  If they used an app they could actually do a fit to the data and find that the force depends on one over the distance squared.

force vs distance

For the second part of the lab, they picked a distance and kept that constant and only changed the mass of object 1 and recorded the force between the two masses.  Finally they repeated this changing the mass of object 2 and keeping mass 1 constant.

The graphs of force vs mass were linear for both mass 1 and mass 2.  So the students  concluded that the force of gravity between two objects depends on the mass of both objects and one over the distance between them squared.

Next week we’l cover Kepler’s Laws.

 

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