## Iconic Voices from MIT: Opening a New Window into the Universe with Dr Nergis Mavalvala

This is a free public lecture by Dr Nergis Mavalvala (an astrophysicist from MIT) on how her team detected gravitational waves generated from colliding black holes and neutron stars at the Laser Interferometer Gravitational-wave Observatory (LIGO).  Held on this coming Friday 26 Jul 2019 from 5 to 6 pm, the venue is at the Singapore University of Technology and Design (SUTD)'s Auditorium, along 8 Somapah Road, Singapore 487372.

## Why is Glass Transparent?

This video relates a phenomenon that we have taken for granted to the study of quantum physics (more specifically, photon absorption) and atomic structure.

## Phase Difference GeoGebra Apps

I created a series of GeoGebra apps for the JC topics of Waves and Superposition, mainly on the concept of Phase Difference. The sizes of these GeoGebra apps are optimised for embedding into SLS. When I have time, I will create detailed instructions on how to create such apps. Meanwhile, feel free to use them.

Instructions on how to embed the apps into SLS can be found at this staging environment of the SLS user guide.

Phase difference between two particles on a progressive wave. Move the particles along the wave to see the value.

Phase difference between two particles on a stationary wave. Move the particles along the wave to observe how their velocities are different or similar.

Observe velocity vectors of multiple particles on a progressive wave.

## Javascript Game to Learn How to Count Money

Trying to brush up my Javascript skills after being inspired by one of the senior specialists in ETD, I created this simple Javascript Game to teach kids how to count money using Singapore coins.

To play this game, click or press the "Play Button". Click on the coins to make up the targeted amount. Be careful as the coins will move over one another.

This is meant for children entering primary one soon so that they can learn how to pay for food at the canteen.

## How to Understand the Image of the Black Hole

Here's a good explanation by Veritasium on why the image of the black hole looks the way it does.

## GeoGebra in SLS

1. update on 2 Jul 2019: The SLS lesson shared during IPSG 2019 can now be found in the SLS Community Gallery.
2. Join the local community of GeoGebra users at: https://www.geogebra.org/group/stream/id/VFX2EG8xa
3. GeoGebra tutorials at: https://www.geogebra.org/m/Ebm5wBW5  (Start with Geometry and Functions & Graphing)
4. GeoGebra apps curated for A-level Physics: https://www.geogebra.org/m/dgedzmz3
5. GeoGebra apps curated for O-level Physics: https://www.geogebra.org/m/z5nfs8qd
6. IPSG Poster on "An SLS Learning Experience with GeoGebra Apps on the First Law of Thermodynamics"
7. Instructions on how to embed GeoGebra into SLS.
8. Let us know if you have used or adapted the SLS lesson, or if you have ideas for new GeoGebra apps in the comment section below.

## Idealized Stirling Cycle

I created a new GeoGebra app based on an ideal Stirling Cycle (A. Romanelli Alternative thermodynamic cycle for the Stirling machine, American Journal of Physics 85, 926 (2017)) which includes two isothermal and two isochoric processes. The Stirling engine is a very good example to apply the First Law of Thermodynamics to, as the amount of gas is fixed so the macro-variables are only pressure, temperature and volume. Simplifying the cycle makes it even easier for first time learners to understand how the engine works.

For those who prefer to be impressed by an actual working model, it can be bought for less than S\$30 on Lazada. All you need for it to run is a little hot water or some ice. Here's a video of the one I bought:

The parts of the Stirling engine are labelled here:

My simulation may not look identical to the engine shown but it does have the same power piston (to do work on the flywheel) and displacer piston (to shunt the air to and fro for more efficient heat exchange).

## Embedding GeoGebra Apps into SLS

Teachers in Singapore have been provided with the Student Learning Space (SLS) - an online platform meant for students' self-directed learning using MOE's curriculum-aligned resources as well as for teachers to create engaging technology-assisted learning experiences for their charges.

One of the features that many science and math teachers find useful is the ability to create links out to simulations and other interactive apps. Those who know how can also embed html5 packages into their "Lesson" so as to provide a more seamless experience for their students. These packages must be stand-alone packages that do not require external sources such as sound files or images. In other words, every media file that is needed for the html5 package to run must be all zipped up into a folder before uploading.

Here, I will go through step-by-step how to embed a GeoGebra app into SLS.

Step 1: Find a suitable GeoGebra App (https://www.geogebra.org)

You might like to check out the list of GeoGebra apps that I've curated for Singapore's O-level and A-level syllabi.

Step 2: Open the App and click to see Details.

Step 4: Unzip the package and rename the html file to index.html.

What the file name looks like before:

What the file name should look like after:

Step 4: Zip up the files again. Select all the files including the index.html file and zip it up. Do not zip the main folder. Select only the index.html and the folder with the title GeoGebra. The index.html file must be in the root directory of the new zip folder.

The new zip folder could be given any name.

Step 5: In SLS, select the Lesson and Activity in which you want your GeoGebra app to appear

Step 6: Select a new "Media" object

Step 8: Browse to find the zip folder

You should see this message. That is a sign that your package is correctly packaged.

Finally, you can try out the app in the student view.

If you need any assistance, feel free to leave a comment below. I will try my best to support you. In the worst case scenario, I can create the SLS lesson with the desired GeoGebra app and share it with you.

NOTE (ON 2 JUL) : With the new SLS embed function, you can also choose to embed the live GeoGebra App using an iframe following the method found here.

## Geogebra App on Maximum Power Theorem

This simulation demonstrates the power dissipated in a variable resistor given that the battery has an internal resistance (made variable in this app as well).

Since the power dissipated by the resistor is given by

$P=I^2R$

and the current is given by

$I=E(R+r)$,

$P= E^2\times\frac{R}{(R+r)^2}=\frac{E^2}{\frac{r^2}{R}+R+2r}$

This power will be a maximum if the expression for the denominator

$\frac{r^2}{R}+R+2r$

is a minimum.

Differentiating the expression with respect to R, we get

$\frac{d(r^2/R+R+2r)}{dR}=-\frac{r^2}{R^2}+1$

When the denominator is a minimum,

$-\frac{r^2}{R^2}+1=0$, so

r = R.

## Geogebra Simulation of a Potentiometer

Some of the more challenging problems in the topic of electricity in the A-level syllabus are those involving a potentiometer. The solution involves the concept of potential divider and the setup can be used to measure emf or potential difference across a variety of circuits components. Basically, students need to understand the rule - that the potential difference across a device is simply a fraction of the circuit's emf, and that fraction is equal to the resistance of the device over the total resistance of the circuit.

$V_{device}=\frac{R_{device}}{R_{total}}*emf$

The intention of this Geogebra app is for students to practise working on their calculations, as well as to reinforce their understanding of the principle by which the potentiometer works.