Magnetic induction refers to the descriptionmagnetic fieldThe physical quantities of strength and direction arevector, common symbolsBIndicates that the international unit is Tesla (the symbol is T).Magnetic induction intensity is also called magnetic flux density orMagnetic flux density。In physics, the strength of the magnetic field is expressed by the magnetic induction intensity. The larger the magnetic induction intensity is, the stronger the magnetic induction is.The smaller the magnetic induction intensity, the weaker the magnetic induction[1]。
Current (movementcharge）Exists aroundmagnetic fieldIts important external performance is that it has the effect of magnetic force on the motion test charge, current carrying conductor or permanent magnet in the introduced field. Therefore, the effect of magnetic field on the motion test charge can be used to describe the magnetic field, and thus the magnetic induction strength can be introducedBAs a quantitative description of the characteristics of each point in the magnetic fieldBasic physical quantity, its position and the electric field strength in the electric fieldEEquivalent.[2]
This physical quantity is called magnetic induction, notmagnetic field intensity, because the word "magnetic field strength" has been used to indicate anotherphysical quantityThe difference is that the magnetic induction intensity reflects the interaction force and the stress relationship between the two reference points A and B, while the magnetic field intensity is a single quantity of the main body. This quantity is unchanged whether Party B participates or not.
definition
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Charge atelectric fieldThe electric field force in is certain, and the direction is the same as that of the pointElectric field directionSame or opposite.The current received somewhere in the magnetic fieldMagnetic force（Amberi）, which is related to the direction of the current placed in the magnetic field. When the current direction is parallel to the magnetic field direction, the current receivesampereForce is the smallest, equal to zero;When the current direction is perpendicular to the magnetic field direction, the ampere force on the current is the largest.
Point chargeqAt speedvForce when moving in magnetic fieldfRole of.Under the given magnetic field,fThe magnitude of is related to the direction of charge movement.WhenvIn a special direction or opposite direction, the force is zero;WhenvWhen perpendicular to this special direction, the maximum force isFm。FmAnd|q|AndvIt is proportional, and the ratio is independent of the moving charge. It reflects the nature of the magnetic field itself, and is defined as the magnitude of the magnetic induction, namely.The direction of B is defined as the maximum force exerted by the positive chargeFmThe direction of is turned to the direction of charge movementvThe direction in which the right-hand spiral moves.DefinedBAfter that, the moving charge in the magnetic fieldBThe force in can be expressed asF=QVB, this isLorentz forceformula。
Definition of division by Lorentz forceBIn addition, according to the current elementIdlAmpere force d in magnetic fieldf=Idl×BTo defineB, or according tomagnetic momentmIn magnetic fieldmomentM=m×BTo defineBThe three definitions are completely equivalent.
dimension
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In the International System of Units (SI), the unit of magnetic induction intensity is Tesla (T) for short, and the dimension is [M] [T] - 2A-1. 1 Tesla=1 N/(Ampere meter).[5]
In the Gauss unit system, the unit of magnetic induction intensity is Gauss (Gs), 1T=10KGs, that is, 10 quartic Gauss.Due to historical reasons, the basic physical quantity describing the magnetic field corresponding to the electric field strength E in vacuum is called magnetic induction strengthBIn the medium, another auxiliary quantity is called magnetic field strengthHThe name is inconsistent with the reality and is easy to be confused.The so-called magnetic field usually refers toB。
B is numerically equal toverticalThe magnitude of the magnetic force on the straight wire with a length of 1m and a current of 1A in the direction of the magnetic field.
B=F/IL, (byF=BILAnd come).
Note: magnetic induction intensity of a point in the magnetic fieldBIt exists objectively and has nothing to do with whether the electrified wire is placedF=BILIt is required that a small section of energized wire should be placed perpendicular to the magnetic field. If it is placed parallel to the magnetic field, the forceFIs zero[4]。
example(Unit: T)
The nuclear surface is about 10 ^ 12;
The neutron star surface is about 10 ^ 8;
Interstellar space 10 ^ (- 10);
Human body surface 3 * 10 ^ (- 10).
The direction of the magnetic field is the direction of the magnetic induction intensity. The determination method is to put in the direction of the magnetic force applied to the north pole of the small magnetic needle, which is also the direction of the small magnetic needle when it is stable and balanced.
The left hand rule can be used for the direction of the ampere force on the electrified conductor: let the magnetic induction line vertically pass through the left hand palm, the four fingers point to the current direction, and make the thumb perpendicular to the four fingers. The direction pointed by the thumb is the direction of the magnetic field force (ampere force) on the electrified conductor.If the magnetic induction line is not perpendicular to the current direction, the magnetic induction intensity shall be divided into two parts: one is perpendicular to the current and the other is parallel to the current direction. The left hand rule can be applied to the component perpendicular to the current. If it is parallel, it is not subject to the ampere force.It can be seen that the ampere force is perpendicular to the plane determined by the magnetic induction and current.The currents in the same direction attract each other, while the currents in the opposite direction repel each other.
Calculation formula
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B=F/IL=F/qv=Φ/S
F: Lorenz force or Amber force;
q: amount of charge;
v: speed;
E: electric field strength;
Φ：=ΔBSorBΔS，BIs the magnetic induction intensity,SIs area, magnetic flux;
S: area;
L: The length of the conductor in the magnetic field.
Definition formula:F=ILB。
expression:B=F/IL。
computing method
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Outside the infinite length current carrying straight conductor:
Among them,
, forVacuum permeability。rIs the distance from the point to the straight wire.
At the center of circle current:
Among them,rIs the circle radius.
Outside the infinite uniform current carrying plane:
Among them,αIs the current flowing through a unit length.
A current carrying arc is at the center of the circle:
Among them,φIs corresponding to the arcCentral angle, in radians.
IdlA current element representing constant current,rIt indicates that the current element points to a certain field pointPRadial vector of.WhereB、dl、rAre vectors,eIs the unit vector, the direction andridentical[3]。
