The diagram below shows two parallel wires carrying current in the downward direction. The magnitude of the current in wire A is greater than in wire B. It is also observed that the wires exert force on each other.

(a) State the direction of the force experienced by wire A.

(b) Name the law/laws used to come to the conclusion in (a).

(c) When a compass is placed in between the two wires, it does not remain along the direction of the magnetic field of wire A or wire B at one of the points P, Q, or R. State with a reason at which point you will observe this.

(a) Since both wires carry current downward and are parallel, they attract each other. So, wire A pulls toward wire B, and vice versa.

(b) Right hand thumb rule and Fleming’s left-hand rule.

(c) At point R
Since, the magnitude of the current in wire A is greater than in wire B, neutral point will always be formed near the weaker current wire.

Explanation:

(a) To find the direction of the force experienced by wire A:

Using the Right-Hand Thumb Rule (to find magnetic field):
For wire B, on pointing the right thumb in the direction of current (downwards), the fingers curl around the wire in the direction of the magnetic field. Hence, at the position of wire A (to the left of B), the magnetic field due to B is into the page.

Using Fleming’s Left-Hand Rule (to find force):
Applying Fleming's Left-Hand Rule to wire A:

• Magnetic field due to wire B at the location of wire A is into the page (first finger).
• Current in wire A is downward (second finger).
• Using Fleming's Left-Hand Rule, the thumb will point to the right.

Therefore, wire A experiences a force to the right, i.e., towards wire B.

(b) Right hand thumb rule and Fleming’s left-hand rule as explained above.

(c) The compass will not remain along the direction of the magnetic field of wire A or wire B at point R.

Point P: At this point, the magnetic field due to wire A is stronger than that due to wire B. As a result, the net magnetic field is in the direction of wire A’s magnetic field. Therefore, the magnetic compass will align itself along the direction of the magnetic field produced by wire A.

Point Q: This point lies midway between the two wires, equidistant from each. However, the magnetic field produced by wire A is stronger than that of wire B because the current in wire A is greater. As a result, the magnetic compass at this location will align itself with the direction of the magnetic field created by wire A.

Point R: Since the current in wire A is greater than that in wire B, the neutral point,where the magnetic fields from both wires cancel each other—will be located closer to wire B, the weaker current source. Therefore, point R is the most probable location of the neutral point resulting from the magnetic fields of both wires.