The magnetic strength of the solenoid is related to the following three factors: the number of turns of the coil; the size of the current in the coil; and whether there is an iron core. And the more turns, the greater the current and the stronger the magnetism; the magnetism is stronger when there is an iron core than when there is no iron core.
A. In this case, the magnitude of the current in the coil remains unchanged, and there is no iron core in both cases, but the number of turns of the coil increases, so the magnetism is enhanced, and the energized solenoid has an impact on the iron The attractive force of the block increases, so the indication of the force gauge increases.
B. The positions of switch S and slide P do not move. When the wooden rod is taken out of the solenoid, the number of turns of the coil, the current in the coil, and the condition of the iron core do not change. Changes, so the magnetic strength of the solenoid remains unchanged, so the reading of the dynamometer remains unchanged.
C. The positions of switch S and slide P do not move. Replace the wooden rod with an iron rod. The number of turns of the coil and the current in the coil, but there is no iron core in the coil. The iron core is inserted, so its magnetism is enhanced and the indication of the dynamometer becomes larger.
D. The position of switch S does not move. When sliding the slider P to end b, the resistance of the sliding rheostat connected to the circuit decreases and the circuit in the circuit increases, so the magnetism of the solenoid increases. , so the indication of the dynamometer increases.
Based on the above analysis, we choose B.