✔ 最佳答案
Where is the question ?
2013-04-02 16:25:31 補充：
Using the Right Hand Grip Rule,
Direction of magnetic field at P due to the upper wire is pointing into the paper;
direction of magnetic field at P due to the lower wire is also pointing into the paper.
Hence, these two fields are both in the same direction, and their strengths add up together.
Direction of magnetic field at P due to the left wire is pointing into the paper;
direction of magnetic field at P due to the right wire is pointing out of the paper.
Hence, these two fields are in opposite directions. Because the current carried by each of the wires and the distances of P from each wire are the same. The magnetic field strengths at P produced by the left and right hand wires are the same, but opposite in direction, thus the fields cancel each other out.
If the magentic field strength at P due to one wire is B, the the resultabt field due to these 4 wires is 2B, in direction pointing inot the paper.
