difference between magnetic field and B-field? What 's the difference between those two?
why the direction of magnetic field of a loop is a circle and the professor use the word" B field" to refer to the direction at a certain point which is tegant to the circle?
Magnetic field and "B-field" are the same. Since we usually use "B" to denote magnetic field, we sometimes refer to the magnetic field as the "B-field". Consider it physics slang. As confusing to the uninitiated as OMG, LOL. ;)
Now, for you second question, if we consider the magnetic field around a current carrying wire, we know that the magnetic field forms a closed loop around the wire. But the magnetic field is a vector, right? So those vectors are tangential to these magnetic field lines and represent the vector field. Those lines just make it a little easier to visualize (without too many crazy arrows all over the place, because I drew 3 and that doesn't look too bad, but to truly represent it we need one for every point along that circle as finely as we are interested in).
to clarify the meaning of "perpendicular distance" when talking about B-fields.
perpendicular distance is the distance from the wire to the point in the magnetic field we are interested in (the angle between this line and the wire does not have to be 90).
Please correct if needed.
The perpendicular distance does indeed need to be perpendicular. I think you are still referring to the magnetic field from a straight segment of wire. In this case, the magnetic field produced is uniform along the wire, and varies radially away from it (~1/r). Since the only variation is radially, you only need this radial distance (which is always perpendicular to the wire).
Please let me know if you are concerned about a different situation.
Force of B-Field of one current-carrying wire on another.
Example: one horizontal wire (I1) has a current flowing to the right. Another, perpendicular to it, is above it with a current flowing upwards. The direction of the force of I1 is downward, by RHR. But, the direction of the force acting on I2 is to the right, because, though the direction of B1 is taken into consideration, the direction of the force is based on the direction of I2 and not I1. Why?
Ahh! But you have already taken into account the direction of I1 when determining the direction of the magnetic field (B1) produced by it. You then consider the force exerted by this magnetic field on the second wire, so you must then consider the direction of I2.
I hope that clears up some confusion concerning magnetic fields.
