replace demos 21-30

book/demos/demo21/demo21.md

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+
 
 # Making Einstein young again with an overhead projector
 Author:     Maarten van Woerkom\

book/demos/demo22/demo22.md

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+
 
 # Infra-red investigations and surprises
 
 
-Author:     Unknown\
+Author: Norbert van Veen\
 Time:	  	10 minutes\
 Age group:	12 - 18\
 Concepts:	infrared, visible light, absorption, transparency
 
+```{figure} demo22_figure1.jpg
+---
+width: 50%
+align: center
+---
+CAPTION
+```
+
 ## Introduction
 Infrared radiation is a well-known subject in physics. Students get acquainted with it in junior secondary and learn about the properties of this kind of radiation in senior secondary. This demonstration is suitable for both junior and senior secondary physics. With the help of the IR- camera (FLIR) which can take photos and video, we can show the world in visible light and IR on an interactive white board or screen.
 
@@ -30,6 +27,15 @@ Infrared radiation is a well-known subject in physics. Students get acquainted w
 ## Preparation
 Provide a garbage bag and a glass plate. Connect the IR-camera to the computer and set it to streaming mode so that you can use the live streaming feature of the software. Show the video on a digibord or screen.
 
+
+```{figure} demo22_figure2.jpg
+---
+width: 50%
+align: center
+---
+CAPTION
+```
+
 ## Procedure
 1.	Ask students to mention differences between IR and visible light.
 2.	What similarities do students know between IR and visible light?
@@ -40,6 +46,13 @@ Provide a garbage bag and a glass plate. Connect the IR-camera to the computer a
 7.	Use this experiment to discuss transparency, opacity or absorption of objects for electromagnetic radiation. For example, UV radiation and glass.
 
 
+```{figure} demo22_figure3.jpg
+---
+width: 50%
+align: center
+---
+CAPTION
+```
 ## Physics background
 Visible light is not absorbed by the glass, but is absorbed by the black garbage bag. The opposite is true for IR radiation. Glass reflects infrared like a mirror.
 

book/demos/demo23/demo23.md

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+
 
 # LEDs and Photons
 
 
-Author:     Leo te Brinke\
-Time:	  	10 minutes\
-Age group:	15 - 18\
+Author: Leo te Brinke\
+Time:	10 minutes\
+Age group: 15 - 18\
 Concepts:	particle-wave duality of light, quantization, threshold frequency, photons
 
 ```{figure} demo23_figure1.jpg
@@ -36,7 +25,7 @@ It is not generally known that a light emitting diode (LED) can be used in rever
 * A red laser pointer
 * A white light
 * A green laser pointer
-* A voltmeter with a very high internal resistance (10 M$\Ohm$ or higher)
+* A voltmeter with a very high internal resistance (10 M$\Omega$ or higher)
 
 ```{figure} demo23_figure2.jpg
 ---
@@ -76,7 +65,7 @@ The main principles have been mentioned above. After this demonstration it will
 A practical application of the same phenomenon is the fact that one cannot get a brown skin from visible light (behind a glass window), regardless of the intensity. UV energy packages are needed and they are absorbed by glass. Apparently pigment cells in our skin need energy packages with at least the energy of UV. The teacher can also refer to the fact that traditional photo paper is not sensitive for red light .... but who still knows that?
 
 ```{tip}
-* As the internal resistance of the Voltmeter is very high, the LED cannot produce a current and retains a fixed voltage. One needs an internal resistance of at least 10 $M\Ohm$ in order to measure the voltage; with 1 $M\Ohm$ the LED will already discharge.
+* As the internal resistance of the Voltmeter is very high, the LED cannot produce a current and retains a fixed voltage. One needs an internal resistance of at least 10 $M\Omega$ in order to measure the voltage; with 1 $M\Omega$ the LED will already discharge.
 * LEDs in a colored holder may not work in this experiment as there could be light absorption of certain colors in the holder. 
 * In our experiments small LEDs reacted better than big ones. It is unclear why.
 * The extra resistance is only needed to limit the current when LEDs are on. The magnitude of the resistance depends on the power source and the maximum power of the LEDs. Most LEDs can stand a current of several tens of mA and this is sufficient for well visible light.

book/demos/demo24/demo24.md

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+
 
 # Writing with a Laser using Phosphorescence
 
@@ -21,7 +10,7 @@ Concepts:	Phosphorescence, laser, photon energy, excitation, emission
 ## Introduction
 Phosphorescence (luminescence) is a familiar phenomenon for many students. They recognize this from various "Glow in the Dark" stickers and objects. What they do not know is that a threshold energy is needed to make phosphorescence possible. We do this experiment using several lasers, each with different wavelengths. The intensity (or output power) of the laser makes no difference whether or not the surface will illuminate. We show this by using a He-Ne laser and by spreading the beam using a concave lens.
 
-```{figure} demo24_figure1.jpg
+```{figure} demo24_figure1.JPG
 ---
 width: 50%
 align: center

book/demos/demo25/demo25.md

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+
 
 # Center of mass demo’s, feeling Physics with your own body!
 
@@ -18,6 +7,15 @@ Time:	  	10 minutes\
 Age group:	6 and up (elementary and secondary)\
 Concepts:	center of mass, distribution of mass, stability
 
+
+```{figure} demo25_figure1.jpg
+---
+width: 50%
+align: center
+---
+CAPTION
+```
+
 ## Introduction
 Learning to recognize Physics in the everyday environment, visualizing Physics and “feeling” it with your own body, and then thinking back-and-forth between phenomena and concepts, that is the object of this series of demonstrations. That is possible with a class of 30 students, but also with a conference room of 500 students and parents. And it is fun! 
 
@@ -31,6 +29,14 @@ Learning to recognize Physics in the everyday environment, visualizing Physics a
 ## Preparation
 At the location (classroom or auditorium), think briefly about maximizing visibility and how to create just enough movement space for the participating audience. 
 
+```{figure} demo25_figure2.png
+---
+width: 50%
+align: center
+---
+CAPTION
+```
+
 ## Procedure
 The description is for a demo during an event with a large audience. For a demo in the classroom the teacher will include more interaction. 
 
@@ -45,10 +51,25 @@ The description is for a demo during an event with a large audience. For a demo
 9. A Powerpoint is available with some spectacular slides such as a [bicycling bird](https://www.youtube.com/watch?v=nrbvx17Ql-c), a flying donkey, a truck, and a tractor missing a wheel and ask for each slide how the center of mass concept can be used to explain the situation. 
 10. Show the photo of the child and the man on the see-saw. *Is this possible? Is something wrong? Using the center of mass concept or the law of moments of force, how can you explain?*
 
+```{figure} demo25_figure3.tif
+---
+width: 50%
+align: center
+---
+CAPTION
+```
 
 ## Physics background
 For definitions and background, see the physics textbooks. 
 
+```{figure} demo25_figure4.png
+---
+width: 50%
+align: center
+---
+CAPTION
+```
+
 ## Follow-up
 There are many interesting phenomena around us which illustrate the power of the center of mass concept. Students could collect such examples like: 
 * A fishing heron has to stand such that the center of mass is above the toes. The long toes of some birds prevent them from tipping over. 

book/demos/demo26/demo26.md

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-# Chapter title
 
+# Upward and downward force
 
-Author:     \
-Time:	  	\
-Age group:	14 - 18\
-Concepts:	
-
-## Introduction
 
-## Equipment
+Author:     \
+Time:	15 minutes  	\
+Age group:	Grade 10\
+Concepts:	weight, buoyant force, action/reaction, pressure
 
-## Preparation
+The demo is derived from the demo "the-bolt-in-the-beaker" from the York Science Education Center.
 
-## Procedure
+## Introduction
+This is a typical example of a Predict-Explain-Observe-Explain demonstration about weight and buoyant force. Students predict what happens to the mass indicated on the scale when a weight is hung in water and explain their prediction. This demonstration can lead to a lot of confusion, even amongst physicists that see it for the first time. 
 
 ```{figure} demo26_figure1.jpg
 ---
 width: 50%
 align: center
 ---
-some caption
+A cup of water on an electronic balance. We hang a substantial screw in the water or another metal object.
 ```
 
-## Physics background
+## Equipment
+* Electronic scale (accuracy to 0.1 g)
+* Beaker or drinking glass
+* String
+* Substantial screw or other solid metal object 
+* Spring balance (e.g., 10 g)
+
+## Preparation
+Set up the needed materials on the table.
+
+## Procedure
+1. Present the setup (figures 1a and 2a). Explain that you will lower the screw into the water and ask for individual predictions in multiple-choice format, as follows: \
+What will happen to the mass indicated on the balance?
+   1. Will increase
+   2. Will remain the same
+   3. Will decrease 
+
+   How do you explain this?
+   1. The weight of the screw is fully supported by the string.
+   2. There is an buoyant force on the screw and therefore an equal force downward on the cup.
+   3. The screw displaces some water by pushing it upward.
+   4. Other, namely ...... (fill in).
+
+2. Allow students to discuss their predictions with each other.
+3. Plenary: Survey the predictions and the arguments for them.
+4. Conduct the experiment and write down the results on the board.
+5. Have students write down a complete explanation before the discussion starts. *If we rest the screw on the bottom, what will the scale indicate? Why?*
+6. The teacher writes down some student explanations on the board, after which the discussion begins. Don't forget to conclude the discussion with a brief but complete explanation on the board including a force diagram.
+7. Repeat with string attached to the spring balance instead of the stand.
+
 
-## Follow-up
 
-## References
-```{bibliography}
-:filter: docname in docnames
-```
+## Physics background
+The screw experiences an buoyant force equal to the weight of the displaced liquid. Therefore, there is an buoyant force from the water on the screw ($F_{\text{water on screw}}$) and hence, according to Newton's 3^{rd}^ law, a downward force from the screw on the water ($F_{\text{screw on water}}$). The latter force acts on the scale. An alternative explanation is that the water level rises, increasing the pressure on the bottom ($p = F/A = ρgh$) and thus increasing the force on the scale.<br>
+If the string is connected to a spring balance, the increase on the scale will be exactly equal to the decrease in force on the spring balance. This can be demonstrated at the end. Use the setup without the spring balance first.
+
+```{tip}
+There will be clever students who still get it wrong. Console them with the information that many physicists still make mistakes here.
+```