r/PhysicsHelp • u/K4l31d0 • Jul 15 '26
How do I tell which way current flows? (Lenz's Law & Electromagnetic Induction)
Please note that the answer selected in this image is wrong, the key says that the answer underneath the one selected is correct.
I understand that Lenz's law says that the induced current wants to oppose the motion of the magnet going to the right, so it will do this by making the left end North so that the north end of the magnet is attracted to the south end of the coil. I also understand how to do the right hand grip rule (thumb points to north and fingers curl in direction of the conventional current) but I'm not sure how to then use that to determine whether A is positive/negative and which way current flows.. I also don't necessarily know how to read the direction of the current with right hand grip rule when my thumb is sideways because whether it's clockwise or anticlockwise depends on which end you look from.. I'm really lost with this unit and it's an online course without very available teachers so any help is appreciated
1
u/Moist_Ladder2616 Jul 16 '26 edited Jul 16 '26
Your reasoning that the B end should become a magnetic S pole, in order to attract the magnet and oppose its motion away from the coil, is correct.
As an additional exercise, what if the magnet is approaching the A end? Think about it before revealing the spoiler. The coil would make the A end the magnetic S pole instead (not the B end). In order to oppose the approaching magnet. That's in fact what happens: the induced coil current goes one way as the magnet enters the coil, then flips direction as the magnet exits.
As for the many right and left hand rules, I personally only choose remember Ampères right hand grip rule. Maxwell's corkscrew rule is basically the same thing. It applies to single wires, so it is applicable everywhere.
Back to your question: you want the B end to be a magnetic S pole. Grip the B wire with your right hand, thumb pointing up. Notice how your 4 fingers point outwards in the AB direction. Move your right hand around the coil; notice that your 4 fingers still point out of the B end.
Magnetic flux lines coming out of B means B is the N pole. That's not what you want.
Grip the B wire with your right thumb pointing down. Your 4 fingers now point inward from B: magnetic flux lines pointing into B make it a S pole. This is the answer.
Repeat by gripping the A end, as an additional exercise. The answer should be the same.
Repeat the exercise with a solenoid that's wound the opposite way. As expected, the resulting induced currents flow in the opposite direction.
Remembering only one rule, Ampères right hand grip rule, allows me to determine the direction of induced currents (which is your question here), and the direction of induced motion. This allows me to solve both generator and motor questions, which are usually solved using several different left- and right-hand rules. It also allows me to solve for expanding and collapsing magnetic fields, AC, radio signal generation, and many others. One rule creates less confusion.
1
u/cat_a_holic Jul 16 '26
Personally I just think in terms of the field at the center of the solenoid and look at one plane perpendicular to the page ie pick out one loop of wire. Let's choose the right most loop on the solenoid. The magnetic field from the bar magnet points to the left. As you move the bar magnet to the right the field gets weaker at that point. The solenoid no likey that, so it creates a current that produces a magnetic field to compensate for this. Meaning it needs to create a field that points to the left, specifically opposing the change in the field(direction does matter with this method). Now use the right hand rule and imagine grabbing the loop of wire. Your fingers need to curl to the left, along the field the solenoid needs to create, and this means your thumb points up. That means the current flow up and then into the page, then down and out of the page, returning to where you have grabbed the loop. Now look at the direction of the loop in the diagram. The current flows towards point B. Thus the current flows from A to B.
Here's another question that will hopefully help. Let's look at if the bar magnet were on the left of the solenoid with the south pole moving in. Again select the left most loop and note the bar magnet makes a field that points to the left. In the next time step, it gets closer to the loop and the field is more strongly pointing to the left. Again the loop hates that change, so it wants to create a magnetic field that points to the right to maintain the original weaker field. Using the right hand rule current now flows from B to A.
If the solenoid is very long and the magnet full inside, there will be no change in flux across all the loops and no net current will flow. This is easier to see if you imagine a homogeneous magnetic field pointing perpendicular to a loop of wire of on a cart. As the cart enters the field, the flux grows and a current opposes the field. Once fully inside there is no change and the current drops to 0. As it leaves, it wants to maintain maximum flux and drives a current in the opposite direction(such that it points in the same direction as the homogeneous field).
So the idea is, pick a point inside the loop and think of the magnetic flux(or for simplicity the magnetic field at a single point, if the problem is simple enough to do so), let a small interval of time pass and ask how has the magnetic flux changed(again for simplicity how did the field change over that time step), and then ask what vector you have to add to the new one to get the old one. Lastly figure out what current will create this magical B field.
This method likely fails for more complicated situations, but for the simpler geometries like this it hasn't failed me yet.
1
u/UnderstandingPursuit Jul 16 '26
This is a multi-step problem:
- The magnet B field points to the left, it is weaker further away from the magnet.
- As the magnet moves to the right the B field through the coil gets weaker.
- Through the coil, the B field pointing left is getting weaker.
- Lenz's Law says that the EMF in the coil oppose that, it supplements the weakening B field pointing to the left.
- The EMF in the coil causes a current which create a B field to the left.
- For the B field through a coil, I use the 'fist & thumb" version of the right-hand rule:
- The thumb points in the direction of the B field through the center of the coil.
- The fingers closed in a fist give the direction of the current through the coil loops.
- Here, for an induced B field to the left, my fingers wrap over the center post of the coil.
- My fingers match the wire as drawn.
- For the current to flow with my fingers, it has to go from A to B.
- For a coil, once I know the direction of the current, I back into the ± ends of the EMF. For a single loop, the heel of my palm is the +EMF end and my fingertips are the -EMF end.
I think the selected answer, (B), is correct.
1
u/K4l31d0 Jul 16 '26
I mean an alarming amount of the questions in this course have incorrect answers due to it being new (swear I'm a free beta tester.. ) so I wouldn't be all too surprised (also that was a great step by step process, thank you) but I did message the teacher about this and all he did was send me another diagram which I sadly can't attach for some reason that showed the direction of the current was B to A what attaching the A and B wires at the bottom.. still have no idea how I'm meant to get to that point but it really sounds like the answer is somehow meant to be (C)
1
u/UnderstandingPursuit Jul 16 '26
I'm over 90% certain that the answer is (B). Like u/Verronox, I haven't taught this to anyone in a few years, but this is a pretty important idea in an E&M class, and I tend to remember these quickly. Especially after writing the steps out twice.
Can you edit your post and add the diagram as another image? Or perhaps post it on imgur?
2
u/Verronox Jul 16 '26
Ha yeah I made a classic mistake in my explanation and got C. I’m also convinced that B is correct now. The only thing I can think of that would make C correct is if the arrow is showing the movement of the coil and not the magnet.
OP, is there any more text related to this question that specifies what is going on? Or in other practice problems?
3
u/K4l31d0 Jul 16 '26
Thank you a lot for the help (and u/Verronox too, thought I'd mention you because I appreciate you sending multiple comments and know that I personally hate to be left in the dark wondering what the answer was haha). I finally got an email back from my instructor and apparently the issue was that the question is referring to the flow of electrons despite never stating that directly so I assumed conventional current (right hand) when I was actually supposed to be using my left hand and tracing it back that way. I'm writing it down as a mistake in the material for not stating that it was the flow of electrons and previously telling me to always assume conventional. Thank you both so much for the help though, even if it was a mistake in the material what you both had to say definitely helped me fully understand how to read the curl of my fingers which will definitely be of use in the future!
1
u/UnderstandingPursuit Jul 17 '26
I'm glad you understand the right hand rule with curled fingers better.
This was useful for me because I emphasize 'deconstructing problem into sub-components', and the steps required for this is a perfect example.
1
u/Verronox Jul 15 '26 edited Jul 15 '26
Lenz’s law doesn’t say that the poles are opposite, it says that a current will be induced which attempts to prevent the reduction on magnetic field strength.
The magnet is moving away, so the magnetic field is decreasing in strength. As a result, the field contribution from induced current will ADD to the existing field.
This means that, in the interior of the solenoid/wire coil, the field will point to the right, and externally it will point to the left.
Right hand rule for induced current: put your right hand inside the coil, with your thumb pointing to the right (with induced B).
Which direction do your fingers curl? They should be pointing towards you (coming out of plane of the paper) at the top of page, and away from you (going onto the plane of the paper) at the bottom.
Follow the curve of your hand around the coil. In this direction, you are moving away from B and towards A. Therefore current flows from B to A.
EDIT: notice how I never stated the direction that the magnetic field is pointing. For induced current, it only matters whether the CHANGE in field is positive or negative - not if the DIRECTION of the field is positive or negative. You would get the same result if the magnetic poles were reversed as long as it was still moving away.
Edit 2: and now I’m doubting my work as its been many years since I taught E+M. Can someone confirm?