THE MAGNETIC FIELD SLIDES BY ZIL E HUMA

OBJECTIVES MAGNET THE MAGNETIC FIELD B MAGNETIC FIELD DUE TO A CURENT

THE MAGNETIC FIELD B DEF: “ The space or region around a magnet where the effects of magnetism can be detected by a compass needle is called a magnetism field.”

MAGNETIC FIELD DUE TO A CURENT If we place a compass needle near a current carrying wire, its negative needle is deflected. As we know that the magnetic needle is only deflected under the influence of a magnetic field, so the current in the wire must have a magnetic field around it. The magnetic field due to the current is stronger near the wire and gets weaker as we go away from the wire

RIGHT HAND RULE. Def: If the right hand thumb is pointed in the direction of conventional current and the fingers are allowed to curl the wire the finger tips will point in the direction of the lines of force i.e., magnetic field.

A MAGNETIC FORCE ON A MOVING CHARGE 1) We first test for the presence of an electric force by placing a small test charge at rest at various locations. Later we subtract the electric force from the total force, and we left with only the magnetic force. 2) Next we project the test charge q through a particular point P with a velocity v.

We find that the magnetic force F, if it is present always acts side ways, I.e., at right angles to the direction of v. We can repeat the experiment by projecting the charge through P in different directions, we find that no matter what the direction of v, the magnetic force is always at right angles to that direction.

F B V +  q

3) As we vary the direction of v, through point P. we also find that the magnitude of f changes from zero when we has a certain direction to a maximum when it is at right angles to that direction. At intermediate angles, the magnitude of f varies as the sign of the angle  that the velocity of vector makes with that particular direction. Note: Actually there are two directions of p for which F is zero, these directions are opposite to each other, that is  = 0  or 180 .

4) As we vary the magnitude of the velocity, we find that the magnitude of F varies in direction proportion. 5) We also find that F is proportional to the magnitude of the test charge q, and that F reverses direction when q changes sign.

DERIVATION. On the above observations The direction of B at point P is the same as one the direction of v. In which the forces zero and the magnitude of B is determine from the magnitude F (perpendicular) of the maximum force exerted when the test charge is projected perpendicular to the direction of B.

i.e. B = F/qv At arbitrary angles, our observations are summarized by the formula F=qvB sin , Where  is the smaller angle between v and B. Because F, v, and b are vectors. F=qv * B (Vector Product) By writing v * B instead of B * v in the above eq., we have specified which of the two possible directions of B that we want to use.

The above figure shows the geometrical relationship among the vectors F, v and B. F is perpendicular to the plane formed by v and B. F is always perpendicular to v and the magnetic force is always side ways deflecting force. F vanishes when v is either parallel or anti parallel to the direction of B (  = 0  or 180  and v * B =0 ). F has its maximum magnitude = qvB, when v is at right angles to B.

SI UNIT of B. The SI Unit of B is the Tesla (abbreviation T) 1 Tesla = 1 (Newton/(Coulomb.meter/second)) =1 (Newton/(ampere.meter))

NON SI Unit of B The non SI unit for B which is still in common use is the gauss. Relation between Tesla and gauss. 1 Tesla = 10 4 gauss