Example 1. As shown in figure 1, the balance can be used to measure the magnetic induction intensity. A rectangular coil with the width of L, ***N turns is suspended on the right arm of the balance, and the lower end of the coil is suspended in a uniform magnetic field with the magnetic field direction perpendicular to the paper surface. When the current I (the direction shown in the figure) passes through the coil, the balance is balanced by adding weights with mass to the left and right sides of the balance. When the current in the coil is reversed, add the weight M on the right side to rebalance the balance. This shows that ()
Figure 1
A the direction of magnetic induction intensity is perpendicular to the paper surface and inward, and the magnitude is;
B the direction of magnetic induction intensity is perpendicular to the paper surface and inward, and the magnitude is:
C the direction of magnetic induction intensity is perpendicular to the paper surface and the magnitude is:
D the direction of magnetic induction intensity is perpendicular to the paper, and the size is.
Analysis and solution: Because the right side needs to be weighed after the current is reversed, it can be seen that after the current is reversed, the energized coil is subjected to upward ampere force, and the left hand determines that the vertical line of the magnetic field is inward. Because the magnetic field acts on the coil:, before the current is reversed, the equilibrium condition is:, after the current is reversed, there is:, and the correct answer is B.
Second, measure the magnetic induction intensity b by using the induced electromotive force.
Example 2. In order to control the movement of water in the ocean, oceanographers sometimes measure the downward component b of the geomagnetic field according to the induced dynamic potential generated by water passing through the geomagnetic field and the velocity of water. An extracurricular activity interest group is composed of four members, A, B, C and D, and goes to a seaside place to measure the downward component B of geomagnetic field. Assuming that the water flow here is north-south and the velocity is V, which of the following determination methods is feasible? ()
A.a. Insert two electrodes into the water flow in the horizontal plane along the direction of the water flow, and measure the distance L between the two electrodes and the reading U of the sensitive instrument connected to the two electrodes for measuring the potential difference.
B, b, inserting two electrodes into the water flow in the horizontal plane along the direction perpendicular to the water flow, and measuring the distance L between the two electrodes and the reading U of a sensitive instrument connected with the two electrodes for measuring the potential difference;
C, c, inserting two electrodes into the water flow along the direction perpendicular to the sea level, and measuring the distance l between the two electrodes and the reading u of a sensitive instrument connected with the two electrodes for measuring the potential difference;
D.d. Insert two electrodes into the water flow in any direction on the horizontal plane, and measure the distance L between the two electrodes and the reading U of the sensitive instrument connected with the two electrodes, thus measuring the potential difference.
Analysis and solution: Because the downward component B of geomagnetic field needs to be measured, and the direction of water flow is north-south, which is equivalent to the conductor cutting the magnetic induction line in the east-west direction. At this time, the conductor should be perpendicular to the direction of water flow, that is, the electrode should be inserted into the water in the east-west direction, and the distance L and voltage U between the two poles can be measured. The correct answer is B.
3. Using the relationship between the electric quantity of the loop and the magnetic induction intensity when the induced electromotive force is generated, the magnetic induction intensity B is measured.
In physical experiments, an instrument called "pulse galvanometer" is often used to measure the amount of charge passing through the circuit. As shown in Figure 2, the detection coil can be used to measure the magnetic induction intensity of the magnetic field after being connected in series with the impact galvanometer. It is known that the number of turns of the coil is n, the area is s, and the loop resistance formed by the coil and the pulse galvanometer is r. When the coil is placed in the measured strong magnetic field, the plane of the coil is perpendicular to the magnetic field at first. Now turn the detection coil over180, and the amount of charge passing through the coil is measured by the pulse galvanometer as Q. From the above data, the magnetic induction intensity of the measured magnetic field can be measured as ().
Figure 2
A.B. C. D。
Analysis and solution: When the coil turns180, the magnetic flux in the coil changes = =2BS, so what's the difference? Apricot φ/font > E =, the average induced current in the coil and the amount of electricity passing through the coil are obtained from the above formula, so the correct answer is C.
Fourthly, the Hall effect is used to measure the magnetic induction B..
Magnetometer is an instrument for measuring magnetic induction intensity by using Hall effect. The principle is shown in Figure 3. Conductors with height L and thickness D are connected to four electrodes A, B, C and D respectively. As shown in the figure, the conductor is placed in a uniform magnetic field. When a current I passes between A and B, a voltmeter with extremely high sensitivity is connected to electrodes C and D, and the potential difference between the two electrodes is measured to be U. What is the magnetic induction intensity B of the uniform magnetic field?
Figure 3
Analysis and solution: When the potential difference between C and D poles is always U, let the electric field intensity between C and D poles be E, then U = El, so the electric field force of free charge in the conductor is balanced with Lorentz force, so V is the directional moving speed of free charge. Therefore ... let the number of free charges per unit volume in the conductor be n, then the current I = nqsv, therefore, therefore. As can be seen from the above formula, as long as the device is calibrated in a known magnetic field, the magnitude of magnetic induction intensity B can be determined by measuring U.