# Thread: V x b ...

1. in another thread the formula of lorentz force was mentioned.

now i know the the pythagorean theorem is used to show the resulting displacement i.e. i walk 10 m up and 10 m right = 45 degree to east right?

i don't know if i'm right, but doesn't a magnetic field act with the same force from all directions on an object? so if an object travels through a magnetic field, is only its speed affected? or the direction too?

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3. Originally Posted by curious mind
in another thread the formula of lorentz force was mentioned.

now i know the the pythagorean theorem is used to show the resulting displacement i.e. i walk 10 m up and 10 m right = 45 degree to east right?

i don't know if i'm right, but doesn't a magnetic field act with the same force from all directions on an object? so if an object travels through a magnetic field, is only its speed affected? or the direction too?
Magnetism is an interesting force because it acts at right angles to what you might otherwise expect. So if I spew some electrons past a magnetic field, the electrons experience a force that is at right angles to the direction of motion of the electrons, and also at right angles to the magnetic field.

Because the force is at right angles to the motion, no work is actually done on the electrons, so their energy stays constant. They certainly deflect, though, as there is a force acting on the charges.

If you have ever watched a CRT TV, you've seen the results of this force. A powerful electromagnet is used to deflect a pencil-thin electron beam from left to right, and from top to bottom, to produce each visible frame.

Generators need to take this "right angle" business into account correctly so that electrons get pushed along the length of the wire, etc.

4. Ok so to answer your question magnetic fields are produced by charged particles in motion and are polar in nature meaning they have a north and south polarity. They create lines of force that act in accordance with the polarity of the poles. They will seek the path of least resistance between opposite magnetic poles. They are considered to have direction as if flowing, though no actual movement occurs. They flow from the south pole to the north pole within a material and north pole to south pole in air.

The image below shows a 2 dimesional representation of magnetic lines of force flowing from are bar magnet creating a magenetic field, what you can clearly see is that the lines of force interact differently within different parts of the field. Now imagine taking your object an placing it within this field, do you now think that this object is being subjected to equal amounts of magnetic force being exerted on every direction of the object?

5. what's a crt tv and you have a simple mathematical explanation?

6. Originally Posted by Ascended
Ok so to answer your question magnetic fields are produced by charged particles in motion and are polar in nature meaning they have a north and south polarity. They create lines of force that act in accordance with the polarity of the poles. They will seek the path of least resistance between opposite magnetic poles. They are considered to have direction as if flowing, though no actual movement occurs. They flow from the south pole to the north pole within a material and north pole to south pole in air.

The image below shows a 2 dimesional representation of magnetic lines of force flowing from are bar magnet creating a magenetic field, what you can clearly see is that the lines of force interact differently within different parts of the field. Now imagine taking your object an placing it within this field, do you now think that this object is being subjected to equal amounts of magnetic force being exerted on every direction of the object?

aaah i see, so if a particle with a constant speed enters a magnetic field it goes through different phases of changed speed/direction? so that formular only accounts for the change upon entering until leaving the magnetic field?

7. Originally Posted by curious mind
what's a crt tv and you have a simple mathematical explanation?
A crt (cathode-ray tube) TV is the type of TV that everyone had until about 20 years ago (before projection TVs and flat-panel plasma and LCD TVs took over our homes).

You'll want to look up CRT online for more info, but here's a quick description. A cathode provides a stream of electrons within a vacuum bulb. The screen of the CRT is coated on the inside with "phosphors," which are chemicals designed to glow different colors (e.g., RGB) when struck by electrons. Electrons are accelerated to the screen by an electric field produced by a voltage in the neighborhood of 30,000 volts (for a large-ish CRT). Without the electromagnet I described earlier, a CRT would just produce a bright dot in the center. To make a picture, the dot has to sweep from left to right, and from top to bottom, quickly enough so that your eye doesn't realize that such sweeping is going on. That sweeping is the job of the electromagnet. And that's how TV worked from its birth in the 1920s, up until the advent of flat-panel sets.

8. Originally Posted by tk421
Originally Posted by curious mind
what's a crt tv and you have a simple mathematical explanation?
A crt (cathode-ray tube) TV is the type of TV that everyone had until about 20 years ago (before projection TVs and flat-panel plasma and LCD TVs took over our homes).

You'll want to look up CRT online for more info, but here's a quick description. A cathode provides a stream of electrons within a vacuum bulb. The screen of the CRT is coated on the inside with "phosphors," which are chemicals designed to glow different colors (e.g., RGB) when struck by electrons. Electrons are accelerated to the screen by an electric field produced by a voltage in the neighborhood of 30,000 volts (for a large-ish CRT). Without the electromagnet I described earlier, a CRT would just produce a bright dot in the center. To make a picture, the dot has to sweep from left to right, and from top to bottom, quickly enough so that your eye doesn't realize that such sweeping is going on. That sweeping is the job of the electromagnet. And that's how TV worked from its birth in the 1920s, up until the advent of flat-panel sets.
is that why old movies have a bad quality?

9. Originally Posted by tk421
A crt (cathode-ray tube) TV is the type of TV that everyone had until about 20 years ago (before projection TVs and flat-panel plasma and LCD TVs took over our homes).
Makes me feel old, reading this...

Originally Posted by curious mind
is that why old movies have a bad quality?
Hey! Watch it Sonny. They had "Technicolor!"

10. dude, i'm no sonny, answer #5 now pls will ya.

11. Originally Posted by curious mind
is that why old movies have a bad quality?
The original TV standards were certainly "low def" relative to what we enjoy today. Older TVs were hard-pressed to resolve much more than 300 or so vertical lines. DVDs came along and upped the ante, forcing TVs to evolve.

That said, some movies look better to me on old CRT displays. I suspect the reason has to do with the ability of low-def displays to hide certain kinds of defects. But anything with a lot of fine detail (small text, eg) definitely looks better on modern displays.

12. Originally Posted by curious mind
dude, i'm no sonny, answer #5 now pls will ya.
The Lorentz formula tells you the force acting on some charge at any given place. You need to do extra work to figure out the full implications for motion.

If the magnetic field is uniform, the force will be constant (and still at right angles). That means that an electron will traverse a circular trajectory. If the field is not uniform, the motion will be different.

13. Originally Posted by curious mind
dude, i'm no sonny, answer #5 now pls will ya.
tk421 seems to have handled that... But, Sonny, I'm no dude!

14. i a field is uniform, wouldn't that be like refracting? and not uniform like zig-zag(which could give a result like its uniform when it isn't?)

15. Originally Posted by Neverfly
Originally Posted by curious mind
dude, i'm no sonny, answer #5 now pls will ya.
tk421 seems to have handled that... But, Sonny, I'm no dude!
dude don't call me sonny then

16. Originally Posted by curious mind
i a field is uniform, wouldn't that be like refracting?
Huh?

and not uniform like zig-zag(which could give a result like its uniform when it isn't?)
That hurt my brain. Can you try again, with more detail? I have no clue what you are trying to say, sorry.

17. i don't know why i typed refraction, i meant friction.

18. Originally Posted by curious mind
i don't know why i typed refraction, i meant friction.
Oh, ok.

As far as friction is concerned, the answer is no. Friction is dissipative (i.e., energy-losing). There's no energy loss in the magnetic scenario we've been discussing, so friction doesn't quite apply as an analogy.

19. is that why old movies have a bad quality?[/QUOTE]
No. Old movies were not made with cathode ray tubes. They were made with cameras and photographic film. The early photographic film was not very good.

20. ok, this is what i mean:

the red particle passing through an area of equal magnetic force would only affect velocity?
the yellow one passing through different areas of a magnetic force field would affect velocity and direction?
with the result being, that the yellow particle leaving the magnetic field in an angle that gives the impression it only passed through an area of equal forces?

v x b.jpg

21. Originally Posted by curious mind
ok, this is what i mean:

the red particle passing through an area of equal magnetic force would only affect velocity?
the yellow one passing through different areas of a magnetic force field would affect velocity and direction?
with the result being, that the yellow particle leaving the magnetic field in an angle that gives the impression it only passed through an area of equal forces?
Please re-read what I wrote about the right-angle property. I also gave you a specific example explaining what happens in a uniform field. The trajectory you drew for the particles is not what happens, so asking questions based on the incorrect trajectories is meaningless at best, misleading at worst.

Again, the force acting on the electron at any point is at right angles to both the field and the velocity at that point. The magnitude of the force is the product of the magnitudes of q, v, and B, all multiplied by the sine of the angle between the velocity v and the field B.

Because of the right-angle property, there is NEVER any work done on the electron, so its energy never changes. Only the direction changes.

[footnote for the nitpickers]: I am, of course, neglecting radiation in this explanation. One step at a time.

22. Also, neither your red nor your yellow lines pass through a uniform magnetic field. Those field lines are a bit like contour lines on a map and the fact they are there, and curved, means the field is not uniform.

23. ah i see, i thought the area between 2 lines would mean that the magnetic force is the same, then increases i.e. stronger between line 3 and 2 than between 1 and 2. ty.

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