# B Force strength

1. Dec 15, 2015

Staff Emeritus
Mod note: After merging posts from a duplicate thread, the time stamps put the posts in the incorrect order.

The original question was,
Field strengths aren't measured in joules, so I don't think your question can be answered.

PS If your name isn't "Brian Green" you might find another one. It's not so nice to appropriate the name of a living person.

Last edited by a moderator: Dec 15, 2015
2. Dec 15, 2015

### brian.green

But are these two field type comparable?

Sorry but I don't know how you mean. Many people have this name, this name is not so rare or unique. This is my real name translated to english.

3. Dec 15, 2015

### Khashishi

Electric and magnetic forces are generally treated as a unified force, the electromagnetic force. An electric field is a combination of electric and magnetic fields viewed in any other frame of reference.

That said, if you look at the Lorentz force in Gaussian units,
$F=q\left(\mathbf{E} + \frac{\mathbf{v}}{c} \times \mathbf{B}\right)$,
it suggest that the electric force is stronger for objects moving at less than the speed of light. The strengths become equal at the speed of light.

For most calculations involving interaction of light with atoms, the electric dipole effect is the dominant term and the magnetic effects are much weaker. Only when the electric dipole term is "forbidden" do we need to use the magnetic term.

Last edited: Dec 15, 2015
4. Dec 15, 2015

### Khashishi

When you are dealing with ferromagnets, you probably notice the magnetic field more than the electric field because most of the macroscopic objects in your everyday life are very close to electrically neutral, while a magnet is prepared such that the magnetic dipoles mostly line up and add together. But the internal forces between the electrons and nuclei are mostly electric force.

5. Dec 15, 2015

### Staff: Mentor

In specific setups, you can compare them. In general, there is no way to say "X is stronger". What is faster, a car or a pedestrian? A car certainly has a higher top speed on a highway, but if the car is standing around it is not faster.

If you want to deflect moving charges, magnetic fields are often easier to get than electric fields with the same effect. If you want to influence charges at rest, or increase the speed of charged particles, magnetic fields don't help and you have to use electric fields.

6. Dec 18, 2015

### brian.green

But one more question: Are you sure?
A rest particle also have magnetic moment because an e- for example have spin moment and therefore it is never totally rest. That's why permanent magnets work and produce constant magnetic field. A neutron doesn't have charge but have magnetic moment.
If I want to accelerate an e- why I cannot use magnetic field?

7. Dec 18, 2015

### Staff: Mentor

The spin of elementary particles has nothing to do with motion.
Yes, theoretically you have a force acting on those spins from inhomogeneous magnetic fields. Practically the force is so tiny that a repetition of the Stern-Gerlach experiment with free electrons instead of ions has not been achieved yet. You cannot even see a deflection - making electrons notably faster would need orders of magnitude stronger field gradients. Moving from 0 to 100 Tesla an electron can gain about 5 milli-electronvolts. It is extremely challenging to shield all electric fields at a level better than 5 meV, while having (electro)magnets powerful enough to produce a 100 T field, the current record for long-term magnetic field strengths.

8. Dec 18, 2015

### brian.green

OK, my last question: Why I can produce significant EMF by moving a bar magnet in a coil of copper wire? In this case a changing magnetic field produce moving electrons and it is strong enough to make significant electricity. There is three important component of this action (angel is ideally perpendicular): the number of loops, the strength of the magnet and the velocity of the magnet/coil. According to Faraday's law these are equally important. Conclusion: magnetic field significantly affect - we can say - rest e- particles.

9. Dec 18, 2015

### Staff: Mentor

Moving a bar magnet (=changing magnetic fields) produces electric fields. Those lead to the induced voltage, not the magnetic field itself.

10. Dec 18, 2015

### brian.green

OK, let's see something else:
When I'm magnetizing a piece of iron the electrons in this material going to allign to the direction of the magnetic field. Those electrons are moving or more precisely rotating.

11. Dec 18, 2015

### Staff: Mentor

Electrons are point-like, they cannot rotate.
Their spin direction can change. That is neither motion nor rotation in the classical sense.

12. Dec 20, 2015

### David Lewis

In the context of useful applications, considering the current state of electric motor technology, the force of magnetism seems to be more powerful and practical than electrostatic force. The problem with harnessing the forces between electric charges is that it takes high voltage to create relatively small charges, and the resulting force is weak.

13. Aug 9, 2016

### Stephanus

I think what you meant is Brian Greene not Brian Green.