EasyPhysicsHigh School (11–12)Electricity & Magnetism › Magnetic Effects of CurrentNumericalBoard ExamAPA-LevelIB
A loop in the plane of the page surrounds two long wires…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A loop in the plane of the page surrounds two long wires. One carries A out of the page and the other carries A into the page. Two more wires, with A out of the page and A into it, are outside the loop. Find the net current through the loop, and round it with out of the page as positive. A second, larger loop surrounds all four wires. Find the circulation round it.
Need a reminder? The key ideaShow
Ampere's law: , where only the net current through the loop counts. A round path of radius round a long wire gives . In a wire of radius with an even current, for .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The first loop. out of the page. The two outside wires do not count.
Step 2. Its circulation. .
Step 3. The larger loop. A. So .
Answer: A, , and .
Watch out. The field at the first loop includes the fields of the outside wires. They change at each point, but not the circulation.
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