# Increasing flux density increases magnetic force?

1. Apr 12, 2013

### Wiz700

Hi!

When calculating the attraction/repulsion force generated by a magnetic field should I consider the flux density of both and add them to the calculation?
By increasing that vector would it increase the total force generated or it has no relevance at all?

2. Apr 13, 2013

### Wiz700

3. Apr 13, 2013

### tiny-tim

Welcome to PF!

Hi Wiz700! Welcome to PF!

Sorry, but I don't understand the question …

what is "both", and what vector?

4. Apr 13, 2013

### Wiz700

Hey Tiny!

Well, I want to try to calculate the attraction/repulsion force between two magnetic fields.
And I was curious... Should I consider the flux density of both magnetic fields? Is it important to determine the magnetic "forces"?

5. Apr 14, 2013

### tiny-tim

Hey Wiz700!

(just got up :zzz:)

A magnetic field has no charge, so it doesn't feel any magnetic (or electric) force.

If you mean the force from one magnet on another magnet, that would be caused by the effect of the first magnet's magnetic field on the charges in the other magnet (and not on its field).

(and yes, you combine magnetic fields just by adding them)

6. Apr 14, 2013

### Wiz700

Sorry, I made the "magnetic field" sound differently that what it really is.
Yes, interms of the force... Its from a magnet-to-magnet interaction or electromagnetic interactions of multiple kinds.

"from one magnet on another magnet"
Hmm Tim, isn't the force due to both of them?
One applies a force on the other and the other does the same :P?

Btw,thank you for the effort!

7. Apr 14, 2013

### Wiz700

Since the force of attraction/repulsion is based on "two" charges or "two" magnets etc...
They both apply a force on each other.

However, I noticed that force is really a pain to calculate :s

8. Apr 14, 2013

### tiny-tim

assuming the magnetic field from a magnet has no effect on that magnet itself, the only force is the outside force from the other magnet
yes, i've never seen a calculation of it … i believe it's very complicated, and depends on exactly how the magnet is composed

it's not like the simple coulomb's law for electric charges

my recommendation is to forget about it (unless your professor sets it as a question)

9. Apr 14, 2013

### mikeph

I don't quite understand the question. A magnet produces a magnetic field, which then interacts with the magnetic dipoles of other magnets. The force on a magnet due to its own magnetic field is zero.

10. Apr 14, 2013

### Wiz700

Wait, could you explain this point in detail! This is the first time I've ever heard this :s
I thought that in any situation where there are two magnetic fields, the force of attraction/repulsion is due to the objects that have those fields. Hence the force is due to both of them acting on each other. An analogy is needed here!

Magnet(1) and Magnet(2) , are = in mass, strength,type,etc...
They are separated by a distance (d), they are opposite poles.
M(1) applies a force on M(2) and viceversa.

So far so good?

But what I do not understand is ... How can the magnetic field effect the magnet itself? Could you please add an example?

I believe the best way to figure this problem out is to do an experiment!
Thankfully this question is not an important on, but for future references it nice to know how to deal with a problem like this.

11. Apr 14, 2013

### Wiz700

I can not understand this point at all... I never knew that piece of information.

12. Apr 14, 2013

### tiny-tim

i'm saying the opposite … i don't expect the magnetic field of a magnet to affect the magnet itself

13. Apr 14, 2013

### Wiz700

This is interesting! What cases will the field effect the object itself? Could you give me some example so I can investigate more about the subject!

Aside from what's said above, the attraction/repulsion force is indeed due to the two poles applying a force on each other correct?

14. Apr 14, 2013

### Wiz700

When I imagine two magnets that can move freely on a table, I can only seen two of them moving closer towards a certain point due to the force of attraction.
When I see both of them moving, that clearly meant that the force is due to both poles. The only explanation was that each magnet applies a force on the other, in conclusion the force's of attraction or repulsion is based on the two magnets.

Interesting subject indeed.

I remember back in school when the idea of magnetism was firstly introduced, it was very fascinating and our teacher explained it so simply at that time, but its one of the most complicated ideas ever... When studying E&M and relating magnets to it.

15. Apr 17, 2013

### pumila

The main effect of a magnet on itself is that when one has a strong magnet that is not well sintered it can explode from the internal magnetic stresses.

The force between the magnets must be the same but reversed for each magnet. There. Is no easy answer to evaluating the force because every magnet geometry is different and every magnetic material has different characteristics.

The strength of both magnets must be taken into account. The forces at a given separation are related indirectly to the product of the field strengths.

16. Apr 18, 2013

### Wiz700

That barley made sense to me!(Complicated stuff here!)

What I understood is that some magnet's would have strong magnetic stresses and would case them to break/explode, I never heard of this before.

I'm just interested more in magnet to magnet interaction or electromagnet to magnet or electromagnet to electromagnet.

Agreed. Each magnet's would have a strength(I assume you mean each pole's strength?). I don't rely on field strength because in some weird cases you would have two magnets of the same type. But one with a magnetic field weaker than the other, yet has a higher ability to lift objects because it's a larger size. I assume its because of the higher rate of the aligned electron spins.

But man ow man you guys have me a lot of info. on things I had no IDEA about.

I discovered the calculation is very very difficult to do. Thus, created experiments to measure the force between them.

17. May 4, 2013

### Wiz700

If the magnetic flux increases what would be the result?
MF(magnetic flux) = BA
If the area increased or the field(B) increased, what would happen?

18. May 4, 2013

### pumila

The ability to lift objects comes from the force between the magnets. This in turn comes from the rate of change of magnetic field energy with separation via the equation F=dW/dl, W being the energy, 'l' being the separation.

Two overlapping magnetic fields will interact, the energy in the composite fields being in some cases weaker than the sum of the energies of the individual fields, thus creating attractive forces.

It is easy to see that a simple scaling up in 3d of the geometry results in the force rising as the cube of the linear dimension. Other geometry changes are more complex to determine, being dependent on the geometry. A very flat magnet whose area is increased might just have the force increase linearly with linear dimension.

19. May 4, 2013

### Wiz700

Sorry Pumila
But I didn't really understand what you ment.
However, I'd like to ask by increasing the magnetic flux ( i.e the area or magnetic field is MF = BA) Would the magnetic force change?

20. May 28, 2013

### Wiz700

Correct!
But, a magnet will react to the outside force. The magnet will either attract/repel itself towards the outside "force".
The amount of attraction/repulsion would depend on that magnet's pole strength and separation.

I hope that's right!