a. Write a paragraph describing the particle model. What is it, and why is it important? b. Give two examples of situations, different from those described in the text, for which the particle model is appropriate. c. Give an example of a situation, different from those described in the text, for which it would be inappropriate.
ANSWER: a) There are two model by which we understand matter, particles and waves. These two models are like the maps or manifestations of matter but still unable to explain it completely. Particle model was the first model described by physicists, and by using it most of the phenomenons have been explained. First of all they give the idea of rigid body which is not accepted by special relativity, one of the best and testified theory of 21st century. Quantum mechanics also has so many problems with the particle model which we are not about to discuss now. But apart from these particle model is an well accepted and generalized model for our day to day life. All the macroscopic particles which are moving very slow as compared to the speed of light, can be described properly with certain degrees of accuracy. Classical mechanics, newtonian mechanics, many aspects of statistical mechanics, electrodynamics and quantum mechanics also fit with this model. The basic principle of it is, everything in the universe can be assumed to be particles whether small or large and we can write the equations of motion for it. It’s fine for a large range of phenomenons but not true certainly for everything. It’s an approximation to quantum mechanics and relativity both and works pretty well in it’s range. b) Example 1: The simplest example is the parabolic motion of a bowling ball. Once the ball is fired at some angle, it can be considered as a point particle with no internal and substructure, then we can write the equation of motion for it. The equation has the power to predict almost everything what the ball will do through the motion, how it’s trajectory will be, at a given instant of time where exactly the ball will be. This model gave us the power to predict all these things, for which it is named as a deterministic theory. Example 2: Another example is the motion of ideal gas molecules inside a closed system. This tells us that the gas molecules are particles and we can get a lot of thermodynamic information from it without solving the equation of motion of every individual particle. This could have been a terrific job and probably a zoo, but by assuming them as particles we can get more information about the system. For example the average kinetic energy of all the molecules will decide the temperature of the system, volume, pressure and many more also can be calculated by such assumption, c) There are thousands of examples for which this model will be inappropriate. One example will be the interference of electrons in the famous two slit experiment. As described by the particle model, if we fire electrons towards a slab with two small holes in it and next to it one detector will be there, then we should not expect an interference pattern there. Because electrons are particles and interference is purely a wave property. This anomaly lead us to think about the wave model and forced us to think that they are wave as well. Here is a diagram representing the interference pattern of electrons. Particle model can never explain this phenomena.