Place the movable conductor Mezhdu the poles of a horseshoe magnet, as shown in figure. 22.13. Under the action of the magnetic force F_A (it is called the ampere force) this conduit is drawn into the gap between the poles. At change of a direction of the current I a conductor is moving in the opposite direction.

The direction of the force F_A, acting on a straight conductor with current in magnetic field, is determined by the left-hand rule (rice. 22.14): if you place your left hand along the conductor so, to four fingers pointing to the direction of current in it, and the lines of magnetic induction was included in the palm, then bent the thumb will indicate the direction of the force, acting on a conductor with current (ampere).

Amps showed, what force F_A is directly proportional to. the conductor length l and current I in it. It also depends on the angle α between the current direction and the direction of the lines of induction in the wrong place, where is the Explorer (rice. 22.15). Was, what force F_A proportional to sinα and has a maximum value F_Amax, when the conductor is perpendicular to the induction lines of the field.

Thus, ampere force is expressed by the formula:

F_A = BIlsinα, (22.6)

F_Amax = ВIl. (22.6and)

The multiplier In the formulas (22.6) and (22.6and) expresses the dependence of the ampere force from the magnetic field, in which is placed the conductor. The movement of the conductor under the action of the ampere force is remarkable in, what happens is the transformation of electrical energy into mechanical energy. This phenomenon underlies the principle of operation of electric motors.

Find out the physical meaning of the multiplier In the formula (22.6and). Putting the same conductor with current I in magnetic field of different, easy to install, what force F_Amax changes in magnitude and direction. Since I and l is a constant, it, so, changes In. From (22.6and) see, the greater the value of F_Amax will be in the field, which would be more In. The same can detect when you move a conductor in various areas of the same field. As with the increase In growing and F_Amax, In the multiplier it is convenient to take the power characteristic of the field, so once it is changed when you change fields in the area, where is the conductor. From (22.6and) get:

IN = F_Amax /Il. (22.66)

This formula Bern, if the field along the conductor is not changed.

However, in the General case of an inhomogeneous field, you can take the Explorer, length Δl is so small, field along it considerably does not change. Then the value In to field will be characterized to a certain point:

IN = F_Amax /IΔl. (22.6in)

The Value In, strength characteristics of the magnetic field at a given point, called magnetic induction. Magnetic induction in any point of the field is measured by, the current per unit length of the conductor, located at that point perpendicular to the lines of induction, with a current in it, equal to one.

It should be borne in mind, that magnetic induction b is the vector, the direction of which is determined by the position of the magnetic needle (formula (22.66) gives only the numeric value of the magnetic induction). Vector In any point of the magnetic field is directed along the tangent to the flux lines at this point and in the direction, in which the North pole of a magnetic needle.

We derive from (22.66) the unit of magnetic induction In:

IN=1 N/(1 And * 1 m)=1 N/(And * m)=1 kg/(with² * And)=1 T.

The SI unit for magnetic induction Tesla, was adopted (TL) — magnetic induction of such a homogeneous field, in which on a conductor with current in 1 And, placed perpendicular to the lines of induction, the force in 1 N on every meter of length.

Arranged to pass through the unit area of the surface, placed perpendicular to the lines of induction, this number, which is proportional to the value In in the wrong place, where is the surface. This means, what in the drawings induction line feature there more often, where there are large magnetic forces.