# Magnetism/Current

#### IlyaR

Why would the bar climb? There is no external magnetic field to cause a force on it. I'm lost
I understand that the current induces a magnetic field, but I thought it would not act on itself

OP

#### IlyaR

Since parallel wires have current running through them in opposite directions, wouldn't they repel eachother? This would explain the reason that the bar is moving up.

It also seems against physical intuition. Would that also mean that circuits that have wires close to each other have a very large repelling force between them? (if current travels in opposite direction)

#### Schenker

Yes, wires a-b and e-f would repel each other, but they are fixed in place, so they won't move anywhere. It sounds like you know the right hand rule.

Let's assume the current flows from a to f. In between wires a-b and e-f, in what direction does the magnetic field (B) point? Up. In what direction does the current through the bar (I) point? Into the page. The magnetic field will act on the current to produce a force (F), called the Lorentz force, which will be in the direction of the cross product I x B. You can use that other right hand rule (the one that looks like a gun with your middle finger pointing out) to figure out the direction of that cross product.

Here's a picture.

OP

#### IlyaR

Yes, wires a-b and e-f would repel each other, but they are fixed in place, so they won't move anywhere. It sounds like you know the right hand rule.

Let's assume the current flows from a to f. In between wires a-b and e-f, in what direction does the magnetic field (B) point? Up. In what direction does the current through the bar (I) point? Into the page. The magnetic field will act on the current to produce a force (F), called the Lorentz force, which will be in the direction of the cross product I x B. You can use that other right hand rule (the one that looks like a gun with your middle finger pointing out) to figure out the direction of that cross product.

Here's a picture.

This is exactly what I needed. Thanks very much!

#### phattestlewt

Yes, wires a-b and e-f would repel each other, but they are fixed in place, so they won't move anywhere. It sounds like you know the right hand rule.

Let's assume the current flows from a to f. In between wires a-b and e-f, in what direction does the magnetic field (B) point? Up. In what direction does the current through the bar (I) point? Into the page. The magnetic field will act on the current to produce a force (F), called the Lorentz force, which will be in the direction of the cross product I x B. You can use that other right hand rule (the one that looks like a gun with your middle finger pointing out) to figure out the direction of that cross product.

Here's a picture.

WOuldn't the magnetic field in between the two wires be pointing out of the page as current flows from a-f?

#### Schenker

No problem, glad that helped.

WOuldn't the magnetic field in between the two wires be pointing out of the page as current flows from a-f?
It'll be pointing upwardly perpendicular to the aXY plane.