# B What causes mass defect in the nucleus?

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1. Sep 12, 2017

### Staff: Mentor

The potential energy of a bound system like a nucleus is negative. This reduces the total energy of the nucleus, and therefore its mass also.

2. Sep 14, 2017

### Karagoz

One need to put energy (e.g. hit it hard) to break a table, but the sum of the masses of broken parts of the table is not more than the mass of the whole table. The mass of the broken parts don't increase.

And that negative energy is the "energy needed to split the nuclei"?

So in physically the mass of nucleons doesn't change (they are same both in a bound system and as separated), but it's just only in calculation that the "negative energy" is deducted from the total energy (and also from the total mass) of the nucleus?

3. Sep 14, 2017

### Staff: Mentor

Are you sure? What would be the predicted increase in mass, and do you think you can measure it that precisely?

How would you measure the mass of a single bound nucleon?

4. Sep 15, 2017

### Karagoz

But jtbell said the individual masses of the nucleons do not change. That means the mass of nucleons are the same both in a bound system and as separated?

5. Sep 15, 2017

### Staff: Mentor

@jtbell may know of a way to measure the mass of a bound nucleon, but I don't. I am not as thoroughly versed in this literature as he is.

6. Sep 15, 2017

### Biker

Isn't this some kind of random? depending on the nucleon and it is position? You may average the energy loss across them and calculate the mass but that isnt accurate.

You could do it though for simple atoms right?

7. Sep 15, 2017

### jbriggs444

Does a nucleon actually have a position in a nucleus?

8. Sep 15, 2017

### Biker

Uhh probably something with quantum mechanics..

All the animations shows them vibrating around which doesn't give a definite position and most likely that they are wrong on some level(Yes, I know it is wrong to think of them as little balls). My question should be doesn't each nucleon contribute differently than others? and does it even make sense to talk about mass loss of a single nucleon?

9. Sep 15, 2017

### jartsa

Well, if a nucleon does any work, then its energy and its mass decrease. Conversely, if nucleon's mass never changes, then we can guess that the nucleon never does any work.

Now let us consider macroscopic objects: Two metal spheres, one is positively charged, other one is negatively charged.

The spheres are floating in viscous oil and are being pulled towards each other by the coulomb force. The energy and the mass of the sphere-pair is decreasing, so the sphere-pair is doing work.

But the sphere on the right is not losing mass, so it is not doing any work, and same is true for the sphere on the left.

The sphere-pair is doing work - to be more specific the electric field of the sphere-pair is doing work, but not the electric field of the sphere on the left or the electric field of the sphere on the right.

As the sphere-pair loses mass, it floats a little bit higher in the oil. The sphere on the right has not lost any mass, but it floats higher.

That last sentence seems problematic to me. Did I make some error??

10. Sep 17, 2017

### jartsa

Well I have thought about it, and the solution to the problem seems to be this: We do not say that the metal sphere on the left has a reduced mass, instead we say the left side of the system has a reduced mass. Somehow that seems to make the problem disappear.

We have the word nucleon, so how about if we do not call a free proton or neutron a nucleon, but instead we call them a proton and a neutron?

And those nucleon particles that can be thought to exist inside nuclei, how about if we do not say that they are protons and neutrons and have the same masses as protons and neutrons? Is it not a naive idea that nucleons would have the same masses as protons and neutrons?