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I have tried to learn it myself but I was unable to understand. Can you please explain it in detaails with simple example.


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2 Answers 2

By lines of force, do you mean the magnetic field? If so, you could determine the direction of the B-field by right-hand rule.

The reason is that a current element flowing up will create a B-field circulating out of the screen on the wire's left, and into the screen on the right's right. This direction is chosen by convention.

Wrap the wire into a loop (making it flows counterclockwise), we find that the B-field will point into the screen outside of the loop,

and out of the screen inside the loop

Summing up all elements (Biot-Savart law) we will find that the B-field should look like

(Picture source: http://frank.mtsu.edu/~phys2020/Lectures/L12-L18/L17/Current_Loops/current_loops.html; Wikipedia)

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A circular current, running counter-clockwise when viewed from above, induces a magnetic field whose field lines point mostly up through the center of the loop and then curl around to point down outside the loop. A clockwise current's magnetic field lines point the opposite direction.

Here is a picture from this source magnetic field lines

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