Today we will take a look at 4 Conservation Laws : Energy , Momentum , Angular Momentum and Charge . Most of us have studied these laws in our school days . But we will also be answering the important question ” Why is the Electron stable ? “
a) Energy : How do you define Energy ? I dont know ! How about this :
” It is the ability to do work . ” Not very convincing . Anyway we
all kinda know what energy is . The formula is E = m (c ^ 2 ) .
Mass is a kind of Energy because it can be converted into Kinetic
Energy . This is a conserved quantity .
b) Momentum : Now this is trickier . Momentum is defined by mass times
velocity . This is a vector quantity and has to be added vectorically.
c) Angular Momentum : This is the amount of “circular” motion contained
in a body . There are two kinds of angular momentum :
i) Orbital Angular Mom : This is the momentum of a body that revolves
around another body .
ii) Spin : This is the momentum of a body that spins or rotates about
its axis .
The sum total of Orbital and Spin Angular Mom has to remain constant .
elementary particles( electrons , protons etc ) also have spin . But
this is not the normal spin . No one has ever seen an electron
rotating ! In elemantary particles spin is defined as some sort
of intrinsic angular momentum . This is nothing like we can ever imagine
But it is there .
d) Charge : The total amount of charge that a system contains is always
conserved . The smallest unit of charge is that contained by the
electron . We call it -1 .
We can have reactions like :
neutron(charge 0) -> proton( ch +1) + electron(ch -1) + antineutrino( ch 0)
The total charge on both sides of the equation is 0 .
These kinds of reactions are called decay . In the above reaction
the neutron has decayed into three particles .
The electron is one of the very rare particles that never decays !
The reason is that it is the lightest charged particle . So if
it decays into a set of smaller particles their charge will have to
add upto the electron’s charge . But there are no lighter charged particles
than the electron . So the electron NEVER decays . One experiment
estimated the life of an electron to be 10 ^ 25 years . which is :
10000000000000000000000000 years . Now this is huge !!!
Tomorrow we will do the more exotic conservation laws : Baryon family no
electron family no and the muon family no .