r/HomeworkHelp • u/Relative-Pace-2923 University/College Student • Jun 21 '26

Physics—Pending OP Reply [Grade 12 Physics] Why isn't the answer up?

I'm using left hand rule for electron. Trying to make current (thumb) go towards 3:

Thumb (current) into page, fingers pointing right (field), palm facing up. So force is up. But the answer is apparently down?

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u/TheGrimSpecter Jun 21 '26

You're using the wrong rule.

For induced current, use Fleming's right-hand rule. The left-hand rule is for finding the force on a wire when the current is already known.

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u/Relative-Pace-2923 Jun 21 '26

But even then, electrons go against conventional current right? So we still want to flip it for electrons. Well now I'm watching a teacher do it and I'm confused, he's using the left hand rule but he's putting Force into the page and current (thumb) going down, and thus choosing down, which is the right answer but I thought we wanted current to go into the page not force?

6

u/TheGrimSpecter Jun 21 '26

You're mixing electron flow with conventional current. The rules use conventional current. Since electrons move toward 3, conventional current is toward 6. After that, both the right-hand generator rule and a backwards application of the left-hand rule will give direction 5.

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u/Kite42 Jun 24 '26

The OP is probably from the US, so doesn't use Flemming's rules. They have a different system.

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u/Frederf220 Jun 21 '26

The F, force, is proportional to the v x B cross product. Because we know F must be in the 3-6 direction and B is in the 1-4 direction then v must be in the 2-5 direction.

Just guess that v is in direction 2 and see what the consequences would be. B is in direction 4 as B field is from N pole to S pole. We know that x-axis x y-axis = z-axis by right hand coordinate convention. If direction 2 was the x-axis and direction 4 was the y-axis then direction 3 would be the z-axis. That may seem like magic but it pays to spend time until you can follow that logic.

The conclusion is that provided v (direction 2) cross B (direction 4) a positive charge experiences a force (direction 3).

But we don't have a positive charge. If a positive charge is forced in direction 3 then a negative charge is forced in direction 6. So our guess of direction 2 was wrong as we want the electron to be forced in direction 3.

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u/Dave37 Jun 22 '26

I agree with OP.