Make sketches to illustrate your predictions Check your answ

Make sketches to illustrate your predictions. Check your answers. If the orientation of the compass needle is not what you predicted, resolve the discrepancy Sketch the magnetic field lines of a current-carrying wire. Include the direction of the current in the wire in your sketch. A wire is formed into a loop and the leads are twisted together. The sides of the loop are labeled A-D. The direction of the current is shown. (The diagram uses the convention that indicates current out of the page and indicates current into the page.) On the top two diagrams at right, sketch magnetic field lines for the loop. Base your answer on your knowledge of the magnetic field of a current-carrying wire. Explain why it is reasonable to ignore the effect of the magnetic field from the wire leads. Consider the magnetic field of a bar magnet. How are the magnetic field lines for the current loop similar to those for a short bar magnet? Can you identify a \"north\" and a \"south\" pole for a current loop? Devise a rule by which you can use your right hand to identify the magnetic poles of the loop from your knowledge of the direction of the current.

Solution

Part 1

For the magnetic fields, the hand right thumb rule is the shortest and effective technique. The field lines indicated in the image are pretty accurate.

Now regarding the wire leads. As it is mentioned that they are closely twisted together, it means that along the lenght of wire leads, pair of opposite current run almost together. So, magnetic field generated by one is cancelled by the other. Thats why we neglect magnetic field due to wire leads.

Part 2

We know that in a bar magnet, magnetic fields leave at north and enter at south poles. In a similar this current loop acts as a bar magnet of infitesimal length. Assume an imaginary plane at the centre of loop parallel to the above orientation. Then one face acts as a north pole and the other acts as south pole.

To be particular, the above shown face is north pole since we have magnetic lines coming out of this plane and the back face is the south pole.

The rule

Hold your thumb in the direction of current along the wire, then the face of loop in which the curl of fingers goes in the south pole and the other is north pole.

Ps: I have explained everything in words. If any further clarity is needed, you are welcome to comment.