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## Physics library

### Course: Physics library>Unit 13

Lesson 1: Magnets and Magnetic Force

# 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?
• reminded way back of the method for determining the poles of an electro magnet. Thumb up and the curl of the fingers Thanks • Yes, the curl of the finger is the best way in physics. When I was first doing this stuff, my school teacher made me memorize all of those Fleming's left hand rules, and right hand rules, and all those other guys who posed their fingers in front of the camera to make up a new rule :-p
But the curl of the fingers rule deals with all of them at once.
• Doesn't the right hand rule for magnetism take into account the sign of the charge? The video says the force is downwards, however force is a vector quantity so shouldn't the negative sign if the electron cause the force to be upwards not downwards? • There are many different versions of right hand (and left hand!) rules, and you should use the one you are used to. Whichever one you use will give you the right answer. Most of the rules give you the direction for a POSITIVE charge, so if you have a negative charge then you have to flip the answer. There also are some versions of the rule that tell you to change hands when you change the sign of the charge.
• So as the proton is pushed down by the force and the direction of the velocity is changed the magnetic force constantly changes on the proton, remaining perpendicular and causing the proton to go in a circular motion? Will it just continue to move in a circle upon entering the field, B? • what is the difference in left hand rule & right hand rule
(1 vote) • We know V= IR
and P = VI
so P = I^2 R and P = V^2 / R
How is this possible ?
P is both directly and inversely proportional to R 