(1) Definition: The ratio of the magnetic field force F to the product IL of the current intensity I and the length L of the wire perpendicular to the magnetic field direction in the magnetic field is called the magnetic induction intensity at the position of the wire, which is represented by B. ..

(2) formula: b = f/(I l)

(3) Vector: the direction of B is the same as that of the magnetic field, that is, the stress direction of the N pole of the small magnetic needle.

(4) unit: Tesla (t)1t = 1N/(a m), that is, a conductor with a length of 1m and a current of 1A, if the magnetic field force is1n,

Generally, the magnetic induction intensity near the magnetic pole of a permanent magnet is about 0.4T-0.7T；; ; In the iron cores of motors and transformers, the magnetic induction intensity is 0.8T~ 1.4T, and the magnetic induction intensity of near-surface geomagnetic field is about 0.5× 10-4T.

Uniform magnetic field

In order to know not only the direction but also the magnitude of the magnetic induction intensity from the magnetic induction lines, we can stipulate that the number of magnetic induction lines is proportional to the magnetic induction intensity-in the area of 1 m2 perpendicular to the magnetic field direction, the number of magnetic induction lines is the same as the magnetic induction intensity.

(1) The direction of magnetic induction line reflects the direction of magnetic induction intensity, and the density of magnetic induction line reflects the magnitude of magnetic induction intensity.

(2) The area where the magnitude and direction of magnetic induction intensity are the same everywhere is called uniform magnetic field. Its magnetic induction lines are parallel and equidistant.

For example, the magnetic field inside a long energized solenoid and the magnetic field between two very close different poles are both uniform magnetic fields.

(3) If b = f/(I L) is used to measure the magnetic induction intensity of uneven magnetic field, the wire should be short enough to reflect the magnetic field at this position.

The magnitude of magnetic induction intensity (physical quantity representing magnetic field intensity)

(1) Definition: The ratio of the force (ampere force) f to the product IL of the current I and the length L of the wire perpendicular to the magnetic field is called the magnetic induction intensity. Symbol: b

Note: If the conductor is short, B is the magnetic induction intensity of the conductor. Among them, the product IL of I and wire length l is called current element.

(2) Definition formula: ②

(3) unit: in the international system of units, it is tester, abbreviated as Te, and the symbol is t.1t = n/a.m.

(4) Physical meaning: the magnetic induction intensity B is a physical quantity representing the magnetic field intensity.

Understanding of the definition of b;

① Let students understand that the ratio F/IL is the position function of each point in the magnetic field. In other words, the ratio F/IL changes point by point in a non-uniform magnetic field, that is, at a certain position in the magnetic field, no matter how I and L change, F changes in direct proportion to the product of IL, and the ratio F/IL has nothing to do with the product of IL. Therefore, the ratio F/IL reflects the intensity of the magnetic field at different positions, so people use it to define the magnetic induction intensity of the magnetic field. It should also be noted that f refers to the magnetic field force when the current direction of the electrified wire is perpendicular to the magnetic field direction where it is located, and the magnetic field force on the electrified wire is the largest at this time.

② Some students often get the wrong conclusion that B at a certain point in the magnetic field is directly proportional to F and inversely proportional to IL from the mathematical point of view.

(3) It should be emphasized that for a certain position in the magnetic field, b does not change with the change of detection current and line segment length (current element), but is determined by the magnetic field itself; The fact that the ratio F/IL remains unchanged reflects that the magnetic field strength at the measuring position is certain.

For example, if a charged wire perpendicular to the magnetic field is placed in a magnetic field, its current intensity is 2.5 A, the length of the wire is 1 cm, and its ampere force is 5× 10-2 N, then what is the magnetic induction intensity at this position?

Answer:

Introduce the magnetic induction intensity of some magnetic fields. (Table 3 on page 89. 2- 1)

(3) Summary: The analogy of magnetic field and electrostatic field can be continued, and the following two aspects can be summarized:

First, the electric field force and the magnetic field force have different directions. The direction of electric field force is always the same as or opposite to the direction of electric field intensity e; The direction of magnetic field force is always perpendicular to the direction of magnetic induction intensity B.

Second, there are differences between E and B in introduction mode. The introduction of electric field strength e takes into account the ratio of the force F of the test charge in the electric field to the electric quantity Q of the test charge. However, when the magnetic induction intensity b is introduced, the ratio of the force f to the product IL of the current element in the magnetic field is considered.