Laboratory work 230DETERMINATION OF THE HORIZONTAL COMPONENT OF THE EARTH'S MAGNETIC FIELD STRENGTH Theoretical partI. Elements of terrestrial magnetism. The earth is a huge spherical magnet. At any point in the space surrounding the Earth and on its surface, the action of magnetic forces is detected, i.e. a magnetic field is created, which is similar to the field magnetic dipole“av” placed in the center of the Earth (Fig. I). The Earth's magnetic poles lie near the geographic poles: near the north geographic pole C there is a southern magnetic S, and near the southern geographic U "northern magnetic N. The Earth's magnetic field at the magnetic equator is directed horizontally (point B), and at the magnetic poles it is directed vertically (point A ). At other points of the earth's surface, the earth's magnetic field is corrected at a certain angle to the surface (point K). magnetic field The earth can be traced using a magnetic needle. If you hang the arrow on a thread so that the point of suspension coincides with the center of gravity, then it will be installed in the direction of the tangent to power line Earth's magnetic field. Get acquainted with the basics of Maxwell's theory, properties electromagnetic waves and the mechanism of propagation of electromagnetic waves in a two-wire line Magnetism is a branch of physics that studies the interaction between electric currents, between currents and magnets (bodies with a magnetic moment) and between magnets. Interaction of two parallel conductors with current. The Biot-Savart-Laplace and Ampere laws are used to determine the force of interaction between two parallel conductors with current. Magnetic induction vector flux. Gauss's theorem for magnetic field. Magnetic moments of atoms. For full description atom knowledge is needed quantum mechanics, which we will study later. However magnetic properties substances are well explained using simple and clear planetary model atom, proposed by E. Rutherford. Magnetization of a substance. Previously, we assumed that wires carrying current and creating a magnetic field are in a vacuum. If the wires are in any environment, then the magnitude of the magnetic field they create will change. Types of magnets. Let's conduct an experiment with a strong magnetic field created, for example, by a solenoid. A solenoid (a cylinder with a wire wound around it through which current flows) can create a magnetic field within itself that is 100,000 times greater than the Earth's magnetic field. We will place in such a magnetic field various substances and observe how the force of the magnetic field acts on them. Qualitative results similar experiences turn out to be quite varied. Domain structure of ferromagnets. Classical theory ferromagnetism was developed French physicist P. Weissom (1907). According to this theory, the entire volume of a ferromagnetic sample, located at a temperature below the Curie point, is divided into small areas - domains - which are spontaneously magnetized to saturation. Basic law of electromagnetic induction. Greatest Physicist XIX century Michael Faraday believed that between electrical and magnetic phenomena there is a close relationship. Ampere, Biot and other scientists figured out one side of this relationship with which we are already familiar, namely - magnetic action current The phenomenon of mutual induction Maxwell's theory for the electromagnetic field. In the 60s years XIX centenary D.K. Maxwell, having become acquainted with the works of Faraday, decided to give the theory of electricity and magnetism a mathematical form. Summarizing the laws established experimentally– law of total current, law electromagnetic induction and the Ostrogradsky-Gauss theorem, - Maxwell gave full picture electromagnetic field Maxwell's second equation. Maxwell introduced the concept of total current. Total current density The vertical plane in which the arrow is located is called the plane of the magnetic meridian. All planes of magnetic meridians intersect along the straight line NS, and the traces of magnetic meridians on the Earth’s surface converge at the magnetic poles N and S. The angle formed by the planes of the magnetic and geographic meridians is called the declination angle (in Fig. 1 - angle β). The angle formed by the direction of the Earth's magnetic field and horizontal plane, is called the inclination angle (in Fig. 2 - angle α). The Earth's magnetic field strength vector can be decomposed into two components: horizontal and vertical. Figure 2 shows the position of the magnetic needle NS suspended on a thread L in the Earth's magnetic field. The direction of the northern end N of the arrow coincides with the direction of the Earth's magnetic field strength. The plane of the drawing coincides with the plane of the magnetic meridian. Knowledge of declination angles and declination, as well as the horizontal component, makes it possible to determine the magnitude and direction of the strength of the Earth's magnetic field at a certain point on the surface. The horizontal component, declination angle β and inclination angle α are the main elements of terrestrial magnetism. Over time, all elements of the earth's magnetism, as well as the position of the magnetic poles, change. The origin of terrestrial magnetism is currently not fully understood. According to the latest hypotheses, the Earth's magnetic field is associated with currents circulating along the surface of the Earth's core, as well as with magnetization rocks. 2. Tangent galvanometer method. If a magnetic needle can only rotate around a vertical axis, then it will be positioned under the influence of the horizontal component of the Earth’s magnetic field in the plane of the magnetic meridian. This property of a magnetic needle is used in a tangent galvanometer. Let us consider a circular conductor of N turns, tightly adjacent to each other, which are located vertically in the plane of the magnetic meridian. In the center of the conductor we place a magnetic needle that can rotate around vertical axis. If a current I is passed through the coil, then a magnetic field appears with a intensity perpendicular to the plane of the coil turns (Fig. 3). In this case, two mutually perpendicular magnetic fields will act on the magnetic needle N1 S1: the horizontal component of the Earth's magnetic field and the current magnetic field. Figure 3 shows sections of a coil turn (A and B) in a horizontal plane. In section A, the current is directed “outside” the drawing plane, perpendicular to it. In combination, the current is directed beyond the drawing plane and perpendicular to it. The dotted curves express the magnetic field lines of the current. The arrow NS shows the direction of the magnetic meridian. Fig.3

The main characteristics of the Earth's magnetic field, which are called elements of terrestrial magnetism, include: intensity (Нт), horizontal (Н) and vertical (Z) components of the total intensity vector Нт, magnetic declination(D) and inclination (I). The direction of the total tension vector determines the direction of the magnetic lines of force, i.e. lines at each point of which the vector Нт is directed tangentially to them. Magnetic declination is the angle between the direction of the geographic meridian and the vector H (or the direction of the magnetic meridian). If the magnetic needle deviates to the right from the geographic meridian, then the declination is called eastern (or positive), if to the left, then the declination will be western (negative). Inclination is the angle between the horizontal plane and the total tension vector N t. The value of I varies from –90 0 ( Southern Hemisphere) to +90 0 ( Northern Hemisphere Thus, when the vector Нt is directed towards the Earth's surface, the inclination is considered positive, and from the Earth upward - negative.

Elements of terrestrial magnetism are measured at various points on the globe during magnetic surveys on land, in the seas, oceans, and atmosphere. The first magnetic survey in Russia was carried out in 1586 at the mouth of the Pechora River. By 1917 there were already 8,000 surveys; in the period 1931 – 1936 A general magnetic survey was carried out, during which 12,000 measurements were taken. By 1950, the number of magnetometric points reached 26,000. The measurement results are presented in the form of magnetic maps, which reflect the spatial distribution of any one element (H, Z, D, I) in isolines. The first map was built by Halley (1700). Maps are built for regions and the globe as a whole at a certain point in time, the middle of the year (July 1) was chosen as such a moment - this is the so-called magnetic epoch. World maps are built by England, Russia, and the USA. In addition to maps, a catalog of magnetic data is being compiled.

Isolines of D values ​​are called isogons. The isogon map resembles the course of meridians: isogons emerge from one area and converge in another, almost opposite one. The difference from the meridians, which converge near the poles, is that in each hemisphere there are two areas of convergence of the isogons: one is the magnetic pole, the other is the geographic pole. There, the D values ​​vary within ±180 0.

Lines equal values I – isoclines. Isoclinic maps are a family of latitudinal curves. The zero isocline (magnetic equator) goes around globe near the equator, moving away from it by 15 0 in the region of South America. In the region of the south magnetic pole (Northern Hemisphere) I = +90 0, in the region of the North magnetic pole (Southern Hemisphere) I = -90 0.

Lines of equal values ​​of H and Z are isodines. Isodyne (Z) maps repeat isocline maps: at the magnetic equator Z = 0; at the poles Z = N t = 48-55 A/m. The values ​​of the horizontal component Нт – Н vary from Н = 0 at the poles to Н = 32 A/m at the magnetic equator, where Н = Нт.

Isopore maps show the displacement rate of any EEM. The period of complete circulation of the MPZ is approximately 2 thousand years.

The Earth as a whole is a spherical magnet, the poles of which lie near the geographic poles: near the north geographic pole there is a south magnetic pole S (~ 11.5º to the Earth’s rotation axis), and near the south geographic pole there is a north magnetic pole N. The magnetic poles drift, presumably the south magnetic pole to the northwest.

The angle between the geographical and magnetic meridian is called magnetic declination β (Fig. 1).

The vector of total intensity (magnetic induction B=μ 0 H) is directed tangentially to the lines of force of the Earth's magnetic field. A magnetic needle suspended on a thread is set in the direction of the vector of the total strength of the Earth's magnetic field, which can be decomposed into two components: horizontal H g and vertical H b (Fig. 4).

The ratio between the horizontal and vertical components depends on geographical location. The closer to the north, the steeper the arrow is set down. Therefore, to characterize the Earth’s magnetic field, an angle is introduced α – inclination angle.

A magnetic needle, which can only rotate about a vertical axis, will deviate only under the influence of the vector H r, settling in the plane of the magnetic meridian. This property of the magnetic needle is used in compasses.

So, to characterize the Earth’s magnetic field, the following are used:

1. Magnetic declination β

2. Inclination angle α

3. Horizontal component of the Earth’s magnetic field H g:

N g = Нcosα or B g = Bcosα

Methodology for measuring horizontal (H g) and vertical H in the components of the Earth's magnetic field.

The quantities characterizing the Earth's magnetic field can be measured by two methods.

1)The tangent compass method allows you to determine the horizontal component of the magnetic field H g .

A compass is placed inside the coil. The plane of the coil is set in the plane of the magnetic meridian, i.e. along the magnetic needle of the compass. When current passes through a coil, a magnetic field is created in it perpendicular to the plane coils and the compass needle are set in the direction of the resulting magnetic field.

Figure 5 shows the cross section of the coil.

Magnetic field strength at the center of a circular current , and in the center of a circular coil with current, taking into account the number of turns:

From Fig. 5 it follows that , Then:

.

After logarithmic differentiation of this formula, we obtain a formula for calculating the error

(2)

it follows that the error will be minimal if sin 2α =1 i.e. α =45°. This means that you need to choose such a current strength in the circuit that the deviation of the magnetic needle is close to 45° and then

Where N– number of coil turns, N=400 turns; R – average radius coils, R=35 mm.

2) A method using the phenomenon of electromagnetic induction allows us to determine the horizontal H g and vertical H components of the induction of the Earth's magnetic field.

The installation consists of an inductor (Fig. 1) and a measuring device that calculates the average flow value induced emf that occurs in the coil when it rotates.

Magnetic induction B g and B b is determined by the formula.

where S is the area of ​​the coil.

If the frame on which the coil is mounted is installed horizontally, then (the axis of rotation of the coil is horizontal) the measuring device measures the flow<E i Δt> created by the vertical component B in.

If the frame is installed vertically, the measuring device measures the flow<E i Δt> created by the horizontal component B g.

Because in the absence of a medium, magnetic induction and magnetic field strength are related by the relation:

where - magnetic constant = 4 10 -7 H/m.

§ 15. Terrestrial magnetism and its elements. Magnetic cards

The position of the magnetic poles does not remain unchanged; their coordinates slowly change. The approximate coordinates of the magnetic poles in 1950 were as follows:

Northern - φ ~ 76°N; L ~ 96°W;

South - φ ~ 75°S; L ~ 150° O st .

The Earth's magnetic axis is a straight line connecting magnetic poles, passes outside the center of the Earth, and makes approximately an angle of about 1G.5 with its axis of rotation.

The strength of the Earth's magnetic field is characterized by the intensity vector T, which at any point of the Earth's magnetic field is directed tangent to the lines of force. In Fig. 18 the force of earth's magnetism at point A is depicted by the magnitude and direction of the vector AF. The vertical plane NmAZF, in which the vector AF is located, and therefore the axis of the freely suspended magnetic needle, is called plane of the magnetic meridian. This plane makes an angle RAS with the plane of the true meridian NuAZM, which is called magnetic declination and denoted by the letter d.

Rice. 18.

The angle NmAF formed by the vector AF with the plane of the true horizon NuAH is called magnetic inclination and is designated by the letter v.

Magnetic inclination is measured from the horizontal plane downwards from 0 to 90° and is considered positive if the northern end of the magnetic needle is lowered, and negative if the southern end is lowered.

Points on earth's surface, in which the vector T is directed horizontally, form a closed line that crosses twice geographic equator and called magnetic equator. Full force terrestrial magnetism - vector T - can be decomposed into horizontal H and vertical Z components in the plane of the magnetic meridian. From Fig. 18 we have:

H = TcosO, Z=TsinO or Z = HtgO.

The quantities d, H, Z and O that determine the Earth’s magnetic field at a given point are called elements of earth magnetism.

The distribution of the elements of terrestrial magnetism over the surface of the globe is usually depicted on special maps in the form of curved lines connecting points with the same value of a particular element. Such lines are called isolines. Equal magnetic declination curves - isogons put isogons on maps (Fig. 19); curves connecting points with equal magnetic voltage, called isodynes, or isodynamics. Curves connecting points of equal magnetic inclination - isoclines, plot isoclines on maps.

Rice. 19.

All elements of earth's magnetism at any point on the earth's surface are subject to changes called variations. Changes in the elements of earth's magnetism are divided into periodic and non-periodic (or disturbances).

Periodic changes include secular, annual (seasonal) and daily changes. Of these, daily and annual variations are small and are not taken into account for navigation. Secular variations are a complex phenomenon with a period of several centuries. The magnitude of the secular change in magnetic declination varies at different points on the earth's surface in the range from 0 to 0.2-0.3° per year. Therefore, on nautical charts, the magnetic declination of the compass is reduced to a specific year, indicating the amount of annual increase or decrease.

To adjust the declination to the year of navigation, you need to calculate its change over the elapsed time and use the resulting correction to increase or decrease the declination indicated on the map in the navigation area.

Example 18. The voyage takes place in 1968. The compass declination, taken from the map, d = 11°, 5 O st is given to 1960. The annual increase in declination is 5". Reduce the declination to 1968.

Solution. The time period from 1968 to 1960 is eight years; change Ad = 8 x 5 = 40" ~0°.7. Compass declination in 1968 d = 11°.5 + 0°.7 = - 12°, 2 O st

Sudden short-term changes in the elements of earth's magnetism (disturbances) are called magnetic storms, the occurrence of which is determined by the northern lights and the number of sunspots. At the same time, changes in declination are observed in temperate latitudes up to 7°, and in polar regions- up to 50°.

In some areas of the earth's surface, the declination differs sharply in magnitude and sign from its values ​​at adjacent points. This phenomenon is called a magnetic anomaly. Marine maps indicate the boundaries of magnetic anomaly areas. When sailing in these areas, you must pay close attention to the operation magnetic compass, since the accuracy of the work is impaired.

Elements of terrestrial magnetism

The Earth as a whole is a huge spherical magnet. At any point in the space surrounding the Earth and its surface, the action of magnetic lines of force is detected. In other words, a magnetic field is created in the space surrounding the Earth, the lines of force of which are shown in Figure 19.1. The north magnetic pole is located at the southern geographic pole, and the south magnetic pole is located at the north. The Earth's magnetic field is directed horizontally at the equator, and vertically at the magnetic poles. At other points on the earth's surface, the earth's magnetic field is directed at a certain angle.

The existence of a magnetic field at any point on the Earth can be established using a magnetic needle. If you hang a magnetic needle N.S. on a thread L(Fig. 19.2) so that the suspension point coincides with the center of gravity of the arrow, then the arrow will be installed in the direction of the tangent to the line of force of the Earth’s magnetic field. In the northern hemisphere, the southern end will be inclined towards the Earth and the arrow axis will make an angle of inclination with the horizon q(at the magnetic equator the inclination is 0). The vertical plane in which the arrow axis is located is called the plane of the magnetic meridian. All planes of magnetic meridians intersect in a straight line N.S., and traces of magnetic meridians on the Earth’s surface converge at the magnetic poles N And S. Since the magnetic poles do not coincide with geographic poles, then the arrow axis will deviate from the geographic meridian.

Magnetic field of circular current

According to the theory, the magnetic field strength at the center ABOUT, created by the length element dl circular turn radius R, through which current flows I, can be determined by the Biot-Savart-Laplace law

And vector recording this law looks like

In this expression: r– module of the radius vector drawn from the conductor element dl to the field point in question; 1/4 p- proportionality coefficient for writing the formula in the SI system of units.

In the example under consideration, the radius vector is perpendicular to the current element, and in modulus equal to the radius turn, so

The magnetic field strength vector is directed perpendicular to the drawing plane, in which the vectors and lie, and is oriented according to the gimlet rule.