Find direction of current in a magnetic field

[moenste]

When the current is increased, you need to also increase r if you want to keep the B constant.

But we have two wires to worry about. We're trying to find the point where the combined field from the wires cancels the uniform field. It used to be point P, but then they went and increased the current on us.

But you said that B doesn't equal to zero, and now you say that the combined field from the wires cancels the uniform field.

Good! So which direction will the new 'neutral point' be?

Closer to R?

 

[Doc Al]

But you said that B doesn't equal to zero

The B from the two wires doesn't equal zero.

and now you say that the combined field from the wires cancels the uniform field.

That's right.

Closer to R?

Just the opposite. The field from the wires is proportional to I * (1/r_Q - 1/r_R). Since I is increasing, and we want B to remain the same, we need (1/r_Q - 1/r_R) to decrease.

 

[moenste]

Just the opposite. The field from the wires is proportional to I * (1/rQ - 1/rR). Since I is increasing, and we want B to remain the same, we need (1/rQ - 1/rR) to decrease.

1 / 0.8 - 1 / 0.2 = - 3.75

1 / 0.9 - 1 / 0.1 = -8.89

1 / 0.7 - 1 / 0.3 = -1.9.

If we want it to increase, we need to increase the distance from Q. Therefore P goes closer tto R.

 

[Doc Al]

If we want it to increase, we need to increase the distance from Q. Therefore P goes closer tto R.

That's true, but we want that factor to decrease, not increase.

 

[moenste]

That's true, but we want that factor to decrease, not increase.

Thank you, I think somehow I got it.