Magnetic field strength, its basic characteristics. Examples of tasks
Probably, we all faced in childhood with the wonderful properties of ordinary magnets. A small piece of metal attracted some pieces of iron to itself and repelled others.The amazing properties of the magnet are not limited to this. For example, a magnet suspended from a string is always located in space in a certain way — this property formed the basis of the invention of the compass. The end points of the magnet are the strongest. They are called "poles". The specific properties of a magnet are due to its magnetic fields, which are not a substance, but behave very tangibly. One of the most important characteristics is the magnetic field strength.
Magnetic field characteristics
Any magnetic field has energy that manifests itself when interacting with other bodies.Under the influence of magnetic forces, moving particles change the direction of their flow. The magnetic field appears only around those electric charges that are in motion. Any change in the electric field entails the appearance of magnetic fields.The converse is also true: a change in the magnetic field is a prerequisite for the appearance of an electric one. Such close interaction led to the creation of the theory of electromagnetic forces, with the help of which various physical phenomena are successfully explained today.
Magnetic field image
The magnetic field can be drawn on a sheet of paper with the help of lines of force. They are drawn in such a way that the real direction of the field forces at each point coincides with the drawn ones. The directions of the force fields can be determined using a compass needle, the north pole of which is always directed tangentially to the line of force. The North Pole is usually denoted by the place where the magnetic field lines come from, and the south pole by the place of their entry. It should be remembered that such a division is very conditional, and is taken into account only because of its clarity.
What is magnetic tension
Iron filings lining up along magnetic fields prove that the magnetic field has two important indicators — magnitude and direction. At any point in space, the magnetic field propagates at a speed equal to the speed of light in a vacuum — 300,000 kilometers per second.To define the characteristics of the magnetic field, scientists have introduced the value of "intensity". This is a vector quantity indicating the direction of the magnetic field and the number of its lines of force. According to its characteristics, the magnetic field is similar to the concept of "force" in mechanics. This indicator does not depend on the parameters of the environment in which the experiments are conducted, but only on the magnetic flux strength and the distance to the source producing the field. In various cases, such a source can be a single magnet, a magnetic coil, an electrical wire. In each of these cases, a magnetic field arises with certain characteristics.
Electromagnetic field strength in experiments
Consider a single wire through which an electric current moves. When this wire moves around it, a magnetic field arises.Its characteristics can be expressed in terms of intensity, which is determined by the measure of the magnetic field effect on the body under study.
You can explore the magnetic field inside the coil. In this case, the intensity will directly depend on the number of turns of the coil and the distance between it and the body under study.Combining these two conclusions, we can sum up: the magnetic field strength at any point in space is inversely proportional to the length of the magnetic line and directly proportional to the product of the number of turns of the coil and the current strength.
The determination of the magnetic field strength would be incomplete without the concept of “magnetic induction”. This value explains what kind of work a given magnetic field can do. The stronger the magnetic field, the more work it can produce, the greater the value of its magnetic induction.
In physics, magnetic induction is denoted by. It can be visually represented in the form of the density of magnetic field lines per unit surface area, which is perpendicular to the measured magnetic field. Currently, magnetic induction is measured in Teslah.
Another value, capacitively characterizing the magnetic field. Magnetic flux determines how many lines of force permeate a certain unit of area. In a uniform magnetic field, the value of the magnetic flux will be calculated by the formula:
F = Ḇ / S, where:
Ф - magnetic flux;
Ḇ is the value of magnetic induction;
S is the area through which the magnetic field lines pass.
In the system of SI units, the magnetic flux is measured in Weber.
The physical meaning of this quantity can be expressed by the formula: H = I × ω / L, where:
H is the magnetic field strength;
L is the distance between the body and the source of the magnetic field;
ω is the number of turns of the coil;
I - current in the electrical circuit.
From this equation we can conclude that the intensity is measured in [A / m], because the turns in the coil is a quantity.
The product of H × I in this formula is nothing more than an analogy of the voltage of the electric field. If this parameter is applied to the entire length of the magnetic induction line, then the resulting product will be called the magnetizing force (ns). This physical quantity is measured in amperes,but experts prefer the term "amp-turn", emphasizing the direct dependence of force on the number of turns of the coil.
To determine the direction of the magnetic field of the coil or wire, experts apply the gimlet rule. If the “twisting” movement of an imaginary gimlet is parallel to the direction of the current in the circuit, then the “grip” of the gimlet shows how the magnetic field lines will be located.
Examples of the determination of the magnetic field
Example 1There is a coil with the number of turns 100 and having a length of 10 cm. It is necessary to provide the specified value of the magnetic field strength in 5000A / m. What is the strength of the current to flow through the coil?
Solution: according to the definition, the magnetizing force of the coil is H = I × ω / L. And the product H × I gives a magnetizing force. From here you can derive the value of the current strength, which is equal to: 5000A / m * 0.1m = current strength * number of turns. Solving a simple proportion, we find that the current in this problem should be equal to 5A.
Example 2In the coil 2000 turns, through it flows a current of 5 Amps. What is the magnetising force of the coil?
Solution: a simple formula gives the answer: ns = I × ω.Thus, ns = 2000 × 5 = 10,000 ampere turns.
How to determine the magnetic field of a direct electric wire at a distance of 5 cm? The current flowing through the wire is 30 A.
In this example, we also need the formula
H ∙ l = I ∙ ω.
In the case of a direct wire, the number of turns of the coil will be 1, and the length l = 2 π ∙ r.
From here we can deduce that
H = 30 / (2 * 3.14 * 0.02) = 238.85 A / m.
These and similar problems can be easily solved with the help of a basic course of school physics. The solution of such simple examples will help to understand the qualitative essence of electromagnetic processes in the nature around us.