As shown in figure a, the smooth parallel guide rails Mn and PQ are placed horizontally without resistance. The distance between the two guide rails is d = 10cm. The conductor rods AB and CD are placed on the guide rails and perpendicular to the guide rails. The resistance of each rod between the guide rails is r = 1.0 Ω. The two rods are tied with insulated wire with length of L = 20cm. When the whole device is in a uniform magnetic field. T = 0, the magnetic field direction is vertical and downward, and the wire is just in the ungrounded position After that, the change of magnetic induction intensity B with time t is shown in Fig. B. the influence of the induced current magnetic field is not taken into account. The whole process of the wire is not broken

As shown in figure a, the smooth parallel guide rails Mn and PQ are placed horizontally without resistance. The distance between the two guide rails is d = 10cm. The conductor rods AB and CD are placed on the guide rails and perpendicular to the guide rails. The resistance of each rod between the guide rails is r = 1.0 Ω. The two rods are tied with insulated wire with length of L = 20cm. When the whole device is in a uniform magnetic field. T = 0, the magnetic field direction is vertical and downward, and the wire is just in the ungrounded position After that, the change of magnetic induction intensity B with time t is shown in Fig. B. the influence of the induced current magnetic field is not taken into account. The whole process of the wire is not broken

(1) According to the picture, △ B △ t = 0.1t/se = n △ Φ △ t = △ BS △ t = 0.1 × 0.1 × 0.2V = 0.002v, then I = Er = 0.0022a = 0.001a, so the induced current in the circuit is 0.001a. (2) the horizontal tension and Ampere force balance of conductor bar t = FA = bil = 0.1 × 0.001 × 0.1N = 1 ×

The distance between two sufficiently long parallel smooth metal guides Mn and PQ is l, and the angle between the guide plane and the horizontal plane is a = 30 degrees

The whole device is in a uniform magnetic field with upward value. The metal rod CD with length L is placed on the guide rail perpendicular to Mn and PQ, and keeps good electrical contact with the guide rail. The mass of the metal rod is m, the resistance is r, and the acceleration of gravity is g. now the metal rod is released from static state

When the metal bar AB is placed on the horizontal smooth frame cdef, the contact between the bar and the frame is good, and the uniform magnetic field is inclined downward perpendicular to the bar ab. from a certain moment, the magnetic induction intensity decreases uniformly, and a horizontal external force F is applied to keep the metal bar ab still, then f ()
A. Direction right, and constant force B. direction right, and variable force C. direction left, and variable force D. direction left, and constant force

(1) According to Lenz's law, AB tends to move to the right in order to prevent the decrease of Φ. In order to keep the Abba bar stationary, the applied force F is directed to the left. (2) when the magnetic induction B decreases uniformly, the change rate of magnetic induction △ B △ t remains unchanged