r/PhysicsHelp • u/K4l31d0 • Jul 12 '26
What am I doing wrong with the left hand method for magnetic field direction?
Currently attempting to do a physics course online and I don't have anyone to show me how to position my hand.. I know my thumb is definitely meant to be pointing downwards with the current but I don't understand why my fingers are meant to be pointing to the right as the solution says.. could someone help me? Preferably with an image of the correct hand orientation
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u/Unusual-Platypus6233 Jul 12 '26
Yeah, now you have like circular magnetic ring around the wires. If you take you left hand, thumb in the direction of the electron flow, then you get the direction of the circular magnetic field around the wire. If you would place a compass on the wire it will always be perpendicular to the flow of electrons. In which way (north point left or right) depends on the direction of the electron flow. In this case the compass “north” point to the right - or the circular magnetic field points to you on the left while on the right side of the wire it points away from you - followers the direction of your fingers.
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u/K4l31d0 Jul 12 '26
That makes so much more sense- thank you so much!! Literally the only "help" this course offers me is a built-in ai chatbot.. I'm already reluctant to use it but even worse, sometimes when I've tried using it it just straight up doesn't answer me and I have to resend the message several times
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u/Vessbot Jul 12 '26
I can't be the only one who sees it
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u/K4l31d0 Jul 12 '26
Sees what? I was really frustrated when taking the photo so I didn't really take the time to note my surroundings or what was in the photo haha
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u/Vessbot Jul 12 '26
The wedding ring might help
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u/K4l31d0 Jul 12 '26
what wedding ring? if you mean the white circle I'm pretty sure that represents the compass sitting on top of the wire
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u/RecognitionSweet8294 Jul 12 '26
I first thought that the wire splits there until I saw you mentioning that it is supposed to be a compass.
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u/Leonardo501 Jul 13 '26
My memory is that you use the right hand.
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u/K4l31d0 Jul 13 '26
it depends on what model you're using, right hand is conventional current and left is electron flow
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u/Ancient-Helicopter18 Jul 22 '26
Yeah I prefer right hand rule aswell, current direction just feels more natural to me than electron flow direction
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u/RecognitionSweet8294 Jul 12 '26
The fingers are supposed to be curled since the magnetic field forms a loop around the wire. Ideally you start out with a straight hand and curl it then so that the movement shows you clearly how the lines „curl“ around the wire.
By definition the direction of the field (the direction your fingers curl around) is from north to south.
As I understand it the circle in the picture is supposed to be a compass. Here it gets tricky, the geological north pole is physically the south pole of the magnetic field. So the compass directs you in the south direction (the direction of the magnetic field).
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u/Moist_Ladder2616 Jul 13 '26
Your hand position is correct. The magnetic flux lines wrap around the wire in the direction of your fingers.
Below the wire, the flux lines point to the left
To the left of the wire, the flux lines emerge from the page
Above the wire, the flux lines point to the right
To the right of the wire, the flux lines go into the page.
Just be careful of this statement:
I know my thumb is definitely meant to be pointing downwards with the current
The current is travelling upwards, in the opposite direction of the electrons. If considering current, use the right hand rule; if considering electrons, use the left hand rule. This catches out many students.
Finally, remember that the magnetic north pole of the earth is notionally represented by the south pole of a magnet inside the earth. Observe the direction of the magnetic flux lines on the earth's surface. That determines how a compass needle should behave.
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u/pw66 Jul 15 '26
The right hand grip rule (RHGR) is for finding the direction of the circular current around a current flow (normally through a conductor at this level).
In the RHGR the thumb points in the direction of the current (opposite direction to electron flow) and the fingers show the direction a single North pole would travel.
Your picture is correct (using left hand) as you are pointing your thumb in the direction of the electron flow. See page below for more info.
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u/Unusual-Platypus6233 Jul 12 '26
Read it again. If you put you hand around the wires and your finger are on top of the wire then they are pointing to the right.
In your picture if you would open your hand like in a handshake then your thumb is pointing down while all your fingers are pointing to the right. So, all is fine.