# Day II : The “simple” laws

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 .