Question 37

[McatCrusher]

Hi,
I underestand the concept behind this question, but what i do not underestand is how i should figure out the magnetic field direction? The wire is N --> S so the force must be perp. to both the velocity N --> S and mag. field. But magnetic field is a circle here around the wire so thats how i got confused...

[admin]

The magnetic field direction is determined by the Right Hand Rule: PHY 9.2.2/3 in The Gold Standard; or look at http://www.physics.montana.edu/demonstrations/video/5_electricityandmagnetism/demos/righthandrule.html

[MC]

Hi,
Thanks for your feedback. Well, i have already read the Gold Standard and your explanation there was really comprehensive about finding the mag. field. However, what i dont understand is how magnetic Force could be perpendicular to the circular magnetic field.

Thank you

[admin]

OK, this should be clearer for you but you'll still need to use a bit of imagination.

Look at PHY 9.1.5 and examine Fig III.B.9.4. The circles are a bit distorted because there are 2 charges next to each other but nonetheless, you'll get the idea.

Begin by imagining the circles as the magnetic field lines. Imagine the electric field lines as the lines of Force (which in terms of direction is true since F = qE thus the Force is in the same direction is the Electric field).

Notice that the lines of Force are perpendicular to the Magnetic field lines.

What about current? Well imagine the charge +Q is moving toward you, out of the paper. Now the direction of the current is perpendicular to the Force which is also perpendicular to the Magnetic field. Does that sound clearer?

[M.C.]

Hmm...interesting. You made it really clear. It couldnt get any clearer.

Thank a lot for your helpful feedbacks.