Magnetic fields and moving point charges. the RH and LH rules

[Noodles141]

When do you use your right hand for the RH rule and when to use your left hand for the LH rule. I'm just havinf second thoughts about this one. I cant remember which one to use in certain cases. when a point charge is moving through a magnetic field depending on wether it is positively or negatively charge which hand do you use to get the direction of the force on the charge if you have the direction of the magnetic field and the velocity of the charge. say they charge is + and the magnetic field is into the screen and the point charge is moving down the page. which hand rule do you use. and the same question but for a -(negatively) charged point charge.

 

[Pommie]

Charge? When I learnt this, many years ago, it was motion, field and current (in a conductor). You used the left hand rule for motors and the right hand rule for generators. I remembered the LH rule due to the local football team, Manchester United Football Club - MuFC. The u was the gap between thumb and finger.

Or did they introduce some weird and wonderful new thing since I did it.

Edit, A question, if it was a point charge in a field - how will it know which way is up, down, left or right?

Mike.

 

[Bob Scott]

Hi Noodles,

I get confused with these hand rules too because I use them so little. Check out this link:

https://en.wikipedia.org/wiki/Right_hand_rule

It explains both left and right.
Note that conventional current vectors and fingers point in the direction of + current (charge) flow. The vector's arrowhead points in the opposite direction as - charged electron flow.

 

[crutschow]

To determine the direction of the north magnetic field in a coil, you use the RH rule for current (positive charge) flow, and the LH rule for electron flow.

 

[MrAl]

When do you use your right hand for the RH rule and when to use your left hand for the LH rule. I'm just havinf second thoughts about this one. I cant remember which one to use in certain cases. when a point charge is moving through a magnetic field depending on wether it is positively or negatively charge which hand do you use to get the direction of the force on the charge if you have the direction of the magnetic field and the velocity of the charge. say they charge is + and the magnetic field is into the screen and the point charge is moving down the page. which hand rule do you use. and the same question but for a -(negatively) charged point charge.

Hi,

This is mostly a matter of orienting the right hand properly aligning the index finger, middle finger, and thumb.

If you hold your right hand out and point with the index finger and curl the other fingers, then let the middle finger extend part way then let the thumb point up, you'll have the thumb, index finger, and middle finger all at right angles to each other. In this drawing the middle finger points out of the page and is in the direction of the B field, and the index finger is in the direction of the velocity vector v, and then the thumb points in the direction of the force. Note the direction of the velocity and force may change after the particle enters the field. Also note that the paths shown are idealized in that the magnetic field would not actually be present until the particle reached either the top (left hand side drawing) or the bottom (right hand side drawing) and then it would be turned on and kept on (if the magnetic field were always present the particle would start to turn as soon as it entered the field). This also means that if the field was only present on the right hand side of each circular path the particle would eject once it did 1/2 turn.

 

[Noodles141]

Thank you all for the help, i only really used the right hand rule cause i didn't know when to apply the left hand rule but i don't think i needed it anyway. was for my exam donno how i did and it was pretty hard so i'm holding thumbs.

 

[MrAl]

Hi again,

If you look at the diagram in my previous post, the force in the direction of the thumb is the force that a positive charge would experience. If you look at the particle traces at the bottom you'll see that a negative charge experiences the same force but it is in the opposite direction.
So changing only the direction of the force, if we orient the left hand the same way as the diagram shows the right hand then the only difference will be that the thumb points up instead of down, and that would mean the thumb would point in the direction of the force for a negative charge.

So use the left hand rule (if you care to) for negative charges and the right hand rule for positive charges. It's probably simpler just to use the right hand rule and just reverse the direction of the force for negative charges.

It's also possible to draw the three axises x, y, and z, in some standard position you are comfortable with and assign the velocity to one, the B field to another, and the force to the other. If we draw the three axises with x to the right, y up, and z out of the page, then x is the velocity, y is the force for a negative charge, and z is the B field. This is a good idea for people who have had double amputations too.

 

[Noodles141]

yes ok thats exactly what i was confused about with regards to negative and positively charged particles. Thank you mrAl

 

[MrAl]

yes ok thats exactly what i was confused about with regards to negative and positively charged particles. Thank you mrAl

Hi,

You're welcome and good luck with your other assignments.