Refrigerator Magnets

How many time has it happened that you came home and find magnets stuck to your door.  Instead of throwing them away or immediately putting them on your fridge , lets try and learn something about them. Here is a short, easy and fun-to-do scientific experiment with refrigerator's magnets. For our experiment we need two magnets and we will try to stick them together in different ways.

Watch the video to understand what we are going to do and then read the explanations

Try and repeat the experiment shown in the video. Put the magnets together and try to slide one of them up and down. There can be three possibilities:
1) The magnet will slide smoothly
2) The magnet will jump and make a noise
3) The magnets will not hold very well

If the magnets do not hold well, turn one of them 90 degrees and try again, than you will have either option 1 or 2 depending on the direction you choose to slide the magnet (up-down or left-right). Change the direction of sliding and the direction of one of the magnets until you encounter all three possibilities given above.


So why is this? A full explanation of magnetism will require to go into a detailed explanation about magnetic fields and is really unnecessary. We will focus just on remembering that a magnet has its end referred to as the north and south poles. When a north pole is attached to a south pole, the magnets will pull each other, but when attempting to attach a north side to a north side, the magnets will reject each other.
The refrigerator magnets have indeed the two poles but the poles are lined out one next to the other, like columns, within the magnets. Look at the illustration below to see how it works.

Refrigerator Magnet is built from strips of North and South next to each other

So when we attached our refrigerator magnets in a way that a north meets south, they  attach strongly. In one direction (for example: up-down) we can slide them with ease, since even if we slide them, the south pole is still on a north pole, but when we try to slide them in the other direction (left-right), it happens that the north pole meets another north pole which immediately rejects it until it finds the next south pole, this makes the non-smooth movement and the loud noise.
When the magnets are attached in the other direction, the north poles meet north and south poles all the time. This creates rejection of the magnets and attracts force which more or less eliminates each other and the magnets do not stick, or connect very loosely.

Visual Feedback

A feedback is when the output of the process become its input. When giving feedback to someone we expect him to use the feedback and change something according to it. In the following example of visual feedback, a camera is photographing a TV screen which broadcast what the camera photographs. This create an infinite closed loop with very strange results. Notice the colors, the patterns, the repetitions, the multiple clocks changing one after another. I even manage to create a galaxy shaped figure after zooming and tilting the camera. It felts like being a choreograph .
Another known example of the feedback effect is when a microphone is too close to the speaker creating an audio loop which ends in a very loud noise. this is the reason I left only the visual channel and removed the audio channel. But the amplification exists in the visual film as well, as the center of the photo get whiter and whiter.

If you are a teacher, you can repeat this little experiment in class. The kids will love it for sure.

Measuring the speed of light with microwave

in this experiment we will measure the speed of light with microwave and chocolate . This is a great experiment for kids. One way is to measure how long it takes, from opening a packet of chocolate to the moment the kids finish it. This is indeed very high speed but still less than the speed of light. The speed of light is one of the most important constants of nature science in general and astronomy in particular but measuring it was a challenge which required complicated experiments.

Today you can conduct an experiment and obtain a very accurate value for the speed of light using nothing but a domestic microwave oven and chocolate bars. The experiment is suitable for children of all ages. First we will describe the experiment and then explain the physics behind it.
Required equipment:
Microwave oven – Almost any microwave will be suitable. Microwaves in which the revolving plate cannot be removed or have more than one radiation source, do not qualify. You must check the frequency of the oven. The frequency usually appears on a sticker at the back of the oven. Most ovens work at a frequency of 2450MHz but it is worth checking anyway.
Note frequency (2450)

Note the frequency 2450Mhz

To perform the experiment you must be able to remove the revolving plate from the microwave and place the chocolate bar on a plate in parallel to the door as sjhown in the following picture:

The chocolate lying in the microwave

Start the microwave at maximum power for half a minute to let some of the chocolate melt, remove carefully and put on a flat surface.
Please note that there are places that the melting is observable (three places) and other places did not melt at all. Find the distance between two places which have not melted at all. From the photo, the distance is about 6cm (2.362 inches)

Measure between two non-melted places in the chocolate

All that's left to do is simple arithmetic:
(Distance * 2 * frequency) divided by (100,000) KM/Sec which is to be used if the distance is measured in centimeters
Or
(Distance * 2 * frequency) divided by (77,335) Miles/Sec which is used for distance in inches

In our example (sorry U.S. guys, the metric system rules!):
(6*2*2450,000,000)/100,000 = 294,000 KM/second (~182000 Miles/Sec). Amazing.
Why does it work?
Electromagnetic waves are moving at the speed of light (almost, it is little less due to the air, but it really does not matter for the experiment). Microwave structure causes the wave to be a standing wave.

Standing wave - Source: Wikipedia

Notice the red dots on the wave, which always remain at the same height. Where there is no movement there is no heat and energy. Also, there are the peaks of the wave, at which the warming will be greatest. For this reason there are rotating plates in the microwave so the food will be evenly heated.
The photo shows that the distance between two points with zero energy is exactly equal to half of the wavelength. These are the places where the chocolate has not melted at all. The Wave formula says that the wave length * number of waves (frequency) = wave speed.
So we get the frequency from the microwave manufacturer, the wavelength, we measured with the chocolate. All we need is to put them into the formula (division is to transfer from centimeters to kilometers) and get an amazing precise result for the speed of light.