Mustard Seeds

This is a simple packet of mustard seeds. Do you notice that some of the seeds are clinging the side of the packet? Simply open and pour a packet of Mustard seeds and then you will see this happening.

Why are these seeds hanging around!? What stops them from falling to the bottom like the rest of the seeds? What gives them repulsive force to oppose the force of gravity?
It doesn't happen like this, for example, for a salt or a sugar packet.

In case of mustard seeds, what is happening is, the seeds are getting charged up, and the charge stays on them for a while, after which they become neutral. The charging is due to stripping off of an electron or two from the lattice of the mustard seed.

Can you estimate how much is the force generated because of charges separated in the case of mustard seeds shown in packet?

Assume that two seeds are 1 cm apart, and 1 seed's force on the other is balancing its weight.

mg=k(q^2)/(r^2)

m= mass = density * volume = 0.426902 gm/cm^3 * volume from this link.

g= 980 cm/s^2

k= 1 in case of CGS units

q= charge to be found out

r=1cm

Rad= radius or mustard seed = 1 mm say = 0.1 cm

hence q^2= density*volume*g* r^2

hence q= sqrt(0.426902 * (4/3)*3.14 * 10^(-3) * 980 * 1^2)

= 1.132346 statCoulomb

and using 1 StatCoulomb = 0.1 Am/c ≈ 3.3364×10¹⁰ C

and charge on one electron , e = 1.6 * 10 ^ (-19) C, we get

q = 1.132346 * 3.3364 * (10 ^ 10 ) C = 3.77796e+10 C which is a huge charge! so the mustard seeds are not hanging on the sides because of repulsive forces. They are sharing particular positions on the plastic cover. What must be happening is that the electrons are stripped off the mustard seed, and deposited on the plastic cover. The same equation with r = 1 0 Angstrom (Typical lattice planes separation distances are in this range, e.g. from this ref.) then becomes

r= 10 Angstrom = 10 * 10^(-10) m = 10 * 10 ^ (-8) cm = 10^(-7) cm

q = sqrt ( 0.426902 * (4/3)*3.14 * 10^(-3) * 980 * 10^(-14) )

= 1.32 * 10 ^(-7) stat coulomb

= 1.32 * 10 ^(-7)* 3.3364 * (10 ^ 10 ) C

= 4415 C= 3806 * 10^19 e

which is also huge.

So what must be happening is, even at smaller level, say 1 Angstrom , the electrons are stripped off and deposited to the plastic cover. Then some kind of temporary chemical bondings must be happening, since we approach molecular distances at that level.

Notice how in this simple physical situation in front, we could calculate and figure out what's happening in there. Thus physics helps you to be a Sherlock Holmes in the nature's mysterious ways of functioning!

Electrostatics Lab demo

Mysteries in Sound Phenomena

One friend asked, why in the place that is an empty square covered at a sixth floor ceiling and surrounded by classrooms in a college, echos a lot? 

After some discussions, we came to a simple conclusion: they had used that place after college hours, when there is absolutely no-one in the college. During the day-time, when it is flooded with students, it doesn't echo at all! The absence of students here, is giving rise to a lack of any absorbing medium for the sound energy, and it finds it easy to bounce back and forth. This effect can be seen in any empty place, flat, hall, most famous being the hilltops, as the sound seems to come back from distant hills.

Our staircase has hollow pipes as hand railings. I was having fun striking the metal with my metal keys, and it would create quite a sound! I checked striking on different sized pipes, and the sound is so very different! When the metals being stroked the same, why did the sound differ?

The energy of the stroke gives rise to an impulse of vibration in the solid of the pipe. This impulse is a superposition of a huge number of frequencies. These all frequencies get transmitted to the air within the pipe. Due to the size of the pipe, the back and forth oscillations are sustained only at the natural frequencies of that length and radius of the pipe. So the standing waves are dependent on the radius of the pipe, thus giving different sound in different sized pipes.

Physics of Flame

Deep Physics lies hidden in simple phenomena around. This week, let's see what a candle flame can teach us:

 

When I take photos of the candle flame through my Kodak Easy Share C533 camera, I get different photos on different settings. This Kodak Digital camera is a simple point and shoot camera. Same candle showed up as follows in another picture:

Q.1: Compared to the first picture, why is there so much sideways glow in this picture?

 

 

 

Q.2: Why is the shape of the candle flame like a tapering upwards always?

Q.3: What would be the shape of candle flame, in place where there is no gravity?

I took further close up photographs of candle flame with my Kodak Digital Camera. Here is the picture:

Q.4: Do you see the color distribution between blue and yellow? What gives rise to the two colors? What decides the position of these two colors in the flame?

 

 

 

 

To verify whether its a feature only of wax candle flame, I took photograph of a glowing matchstick:

It also shows the similar features, indicating the same Physics governing.

Answers below[can you spot them? :) ]  :-

Ans.1. the glow captured in the camera depends on the exposure setting. The molecules are exciting and de-exciting at a rapid rate. The de-excitation is very fast, 10^(-8)sec, but the excitation depends on the temperature in that region. In the second photo, wider region is captured, indicating larger exposure time.

Ans.2.Fluid dynamics of simple laminar flow... Its same to the flow of water when you open it at very slow speeds. This is flow below the Reynolds number. Above Reynolds number, the flow is chaotic, as can be seen in the case of smoke from an incense. 

Ans.3.Sphere !? :) that's my guess... 

Ans.4.higher temperatures give higher excitation and de-excitation, resulting in blue. Only two colors are seen corresponding to energy levels available. the position shows the temperature profile.

Properties of light - 2

I almost can't show to you how my fan is rotating. Here is a photo of a rotating fan:
It looks stationary!
So I changed the setting on my camera from auto to night mode:
Night mode has lot of light gathering due to large aperture and longer exposure. So you see the blurred image, and because of flash reflection, you see the 3 pans too. Two bright lights are tubelights.
And when I put it on fireworks mode:
Its a very long exposure mode. Its more or less what my eye also sees. So what our eyes see and what a camera sees are different worlds! Here is a video of the same fan. Which way is it rotating?

In the video, it apears to rotate in the opposite way than in reality! This can be seen by switching off the fan:

!!! ??? So how come this illusion!!! ???
You may have taken photos like this one:
Now as a traveller, your eyes won't see such an image, wherever you look, you would see things pretty much clear only( Unless you are travelling in some super fast train!).

Whether eye or camera, each observing instrument has a characteristic exposure time, the time for which the instrument gathers light information, stores it, and then takes next information to process.
This also shows how versatile our eyes are! They are very powerful cameras!
The opposite motion of fan is due to the frequecy of camera capturing the frames of video. If it was a single pan rotating, the video would be different. It would maintain the same sense of rotation. Here, three pans replace each others' positions during the frame re-capture time, and it is done with a lag, so gives a sense of opposite motion than real.

The time taken by eye to take next frame is called Persistance of Vision. Long exposure photos often show nice slow motion, as people observe in motion of stars:

Of course, the stars appear to rotate because we are rotating!

Anytime the exposure is longer enough than the motion, we get a streak photo, as seen in the fireworks here: