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# Magnetic force on a proton example (part 1)

Sal does an example problem to determine the size and direction of the magnetic force on a proton moving through a magnetic field. Created by Sal Khan.

## Want to join the conversation?

• does the thumb really point downwards? it pointed upwards when i tried the right hand rule... • Make sure your index finger is pointing in the direction the particle is moving and your middle finger is pointing away from your computer screen (towards you). Your thumb should be pointing down. Sometimes you have to a little hand-and-arm gymnastics.
• i was told that while using the right hand rule.. the thumb indicates the force.. the forefinger the magnetic field and the middle finger the direction of motion..
so which is the correct rule? • There are many correct rules. The diference between them (and the reason because there are more than one) is because the sense of the vector-force depends on the charge of the particle, whether it is positive or negative. So, let's see some examples:

Right Hand Rule of Sal (positive):
Index = Velocity,
Middle = Field,
Thumb = Force.
Sal concludes in the video that the force is pointed south. This is true only if the charge is positive. If it is negative, we must invert the sense of the vector.

Another Right Hand Rule (negative)
Index = Field,
Middle = Velocity,
Thumb = Force.
If we use this rule we will find out that the force is pointing north, but that is not true. As you may have guessed, this is true only if the charge is negative. However, just like the other rule, we just have to invert the sense of the vector to find out if the charge is positive. So we discover the same result: South.

You don't have to learn them all, you will be fine if you remember one of them and with which charge it works.
Hope it helps.
• Will this concept work with a group of protons or groups of atoms? • is there any relation between magnetism and gravity • Magnetism is part of a broader force called the electromagnetic force. Currently gravity and electromagnetism have no relations however many scientists are searching for a theory of everything (TOE) which will incorporate gravity and electromagnetism and all other laws of physics. Look up String Theory on Wikipedia, for an example of a proposed TOE.
• does this mean that once an electron or a proton enters a magnetic field, it will always start moving in a circle?
like,will it never be able to escape from the magnetic field (even if the intensity of the magnetic field is very very small) • great question.

Yes. if the particle is moving at a constant speed in a uniform mag field, then it will move in a circular path, assuming that it does not lose energy say...
Now, if the particle approached perpendicular to the field, then , you can see by diagram that the particle would leave the field 180 degree later.
So your idea is right but it is difficult to draw a circular path in which the particle does remain in the field. Try it.
Can you think of a situaiton in which the particle can remain in the field?
• in the right hand rule, the thumb gives you the direction of the conventional current in a wire (if you use it to figure out the direction of the current being induced in a moving wire)or it depicts the direction a positive charge will go. to find where a negative charge will go, it will be the opposite direction of the positive charge due to the fact that a negative charge is basically opposite to a positive charge.

Is my reasoning correct?
Also, if I missed something, tell me.(further explain this thing) • Why does the particle go down and not up? Isn't it symmetrical, even if the right-hand rule seems to dictate that it goes down? • How is it that the magnetic field applies a force on a moving charge in a direction perpendicular to both the field and the direction of motion of the charge (or charged body)? This appears weird to me (maybe because when we think about force, the conventional image of something being pushed or pulled comes in). • What do you mean "how is it"? It just is: charge that is stationary in a magnetic field experiences no force, and charge that is moving in a mag field experience a force that is perpendicular to its direction of motion and to the field.

What direction do you think it should be, if this seems weird to you? What wouldn't be weird? Think about it carefully and ask yourself what would justify other "choices" of directions.  