The protons are in the steady magnetic field now,

which was randomly distributed without, when there is no external market field.

And then when there is external market field, which it typically denoted as a B0,

and then they are tend to align along the direction of parallel or

anti-parallel to the field.

So the B0 unit is typically in the unit of Tesla.

So 1 Tesla is 10,000 Gauss, which is about 20,000

times stronger than the Earth's magnetic field.

So the proton magnetic moments tend to align with B0, it's energy dependent.

The parallel to the field has a lower energy state, and

the anti-parallel to the field has higher energy state.

Number of protons parallel to and anti-parallel to B0 are very similar.

There are huge number of protons that are aligned along parallel or

anti-parallel to the field.

And most of them, they cancel each other.

But there are very small number of spins a little bit higher for

those spins, parallel to the field is a little bit higher than the number for

the anti-parallel to the field.

So this is called spin excess.

So this is number of protons parallel to the B0 is

slightly higher than the number of protons anti-parallel to the field.

Summation is not 0 anymore.

And there is some very small number net magnitation is not 0, and there is

persistent amount of net magnetation, and we will talk about that later.

So the number spins parallel to the field and

the number of spin anti-parallel to the field.

A subtraction between those two generates a non-zero component, and

that is divided by the number of total protons.

And that number is very small, so each

about tenth minus five order

in the room temperature at three Tesla, and which is given in this equation.