Tag Archives: phd

2016: looking back

It has been a while since I had a chance to blog. Between classes, work, and life, it’s really hard to keep the promise I made to myself about writing regularly.

Anyway, here’s what had happened between my last post and now.

The Qualifying Exam aka The Ph.D. Lottery:

This was perhaps the biggest achievements for this year. My friends and I took the Ph.D. qualifying exams – a daunting, haunting, four-hour long exam that determines whether you are qualified to pursue a Ph.D. in Purdue.
Most of us, including myself, passed by a respectable margin.

The Soham Saha Library:

The West Lafayette Public Library was having a fundraising sale last month. You could get a bag of books For just three dollars. I always dreamed of having my own library. And thus, the Soham Saha Library was born.
I have never been an ardent reader of non-fiction and thought this would be a good time to start reading them. I ended up buying about thirty non-fiction books, and am currently reading whenever I have free time.
Some of the notable ones among the books:
– The Nobel Duel, by Nicholas Wade – a true story about the rivalry between two Nobel Laureates in Medicine. This is for inspiration.
– The Idea Factory, by Jon Gertner – It’s about the inception of Bell Labs and the brilliant innovations that took place there. It’s an interesting book on the research dynamics of one of the best Labs the world has ever seen.
– Writing Science: How to write papers that get cited and proposals that get funded, by Joshua Schimel – for obvious reasons.
If you happen to have an office at the Birck Nanotechnology Center and are walking past Room 1238, drop by and take a look. You might find something you like.


Made some new friends, learned how to kickbox, the usual random things I do.

Oh yeah, also, the Presidential election happened. But that is too much for one post.

Where Newton’s Third Law doesn’t work

In the last post, I talked about some of the basics of Newton’s three laws of motion.
Reiterating them.
1. The first law is about a body’s reluctance to change its state of motion – If it’s not acted upon by an external force, a body undergoing uniform motion will moving, and a body at rest will remain at rest.
2. The second law is about how a body reacts to a force – The rate of change of momentum of a body is proportional to the force acting on it.
3. The third law states that for every action, there is an equal and opposite reaction.
Here’s an instance, however, where the third law does not apply.
For this bit, you’ll need a little bit of background in electromagnetism.
A moving charge creates a magnetic field around it. The direction of the field is given by the right hand rule. If the thumb of your right hand points along the velocity of a positive charge, your fingers curl along the direction of the field.
Right hand rule
A charge moving in a magnetic field experiences a force that is proportional to the charge of the particle, its velocity, and the magnetic field.
F = qv x B.
The direction of the force experienced by the particle can be given by Fleming’s Left Hand Rule, depicted below. If your forefinger points along the direction of the field a positively charged particle is moving through, and your middle finger in the direction of motion , your thumb points along the direction of the force experienced by the charged particle.
Fleming's left hand rule
Now look at this figure where two positive charges are moving in directions perpendicular to each other.

2016-02-20 06_54_02

The red charged particle (Particle 1) is producing a field B1. The blue particle (Particle 2) is moving through the field upwards. As it does so, it experiences F21, which pushes it to the right side, as shown by the red arrow.

According to Newton’s third law, Particle 1 should also feel a magnetic force F12 to the left, created by Particle 2. However, since the field produced by Particle 2 (B2) is zero at point 1, Particle 1 feels no force acting on it when it is directly underneath Particle 2.

So, F12 = 0.

Newton’s Third Law does not apply.

If you want to dig deeper and understand why momentum is still being conserved in this scenario, you can mull over reference 3. It’s explored there in great detail.

[Note: I am omitting the Coulomb forces the particles are exerting on each other. They are equal and opposite. It’s the magnetic forces that aren’t obeying Newton’s Third Law.]


  1. Fig1:https://www.physics.rutgers.edu/ugrad/227/L15%20Magnetic%20Field%20of%20Currents%20Biot-Savart.pdf
  2. Fig2: http://www.bbc.co.uk/schools/gcsebitesize/science/triple_aqa/keeping_things_moving/the_motor_effect/revision/3/
  3. http://physics.stackexchange.com/questions/138095/newtons-third-law-exceptions