13. At location of wire W , find the B due to X, Y and Z. The resultant B can be found . Use LHR to find the force on wire W .
At location of wire Y, find the B due to X, W and Z. The resulant of B can be found. It may be tricky here if you do not know that B due to wire W is weak as it is along a diagonal. Finally Use LHR to find force on wire Y.
14. Use F = NBIL along the current which is perpendicular to I. The direction of this force is equal and opposite acting at a distance 0.12 m apart. Hence torque = F x 0.12 = 0.34 Nm.
15. Current balance. Both forces are downwards on opposite side of PQ. BIL and mg are the two forces. When the frame is balanced, mg ( 8 cm) = BIL ( 15 cm). Calculate B .
To produce a force downwards in the plan view is into the page. B due to solenoid is to the right. Use LHR the force into page , hence the current is C to B. Hence QCBP.
16. The wire is coiled round the south pole hence the length is N(circumference) = 18.8 m.
b) The force F= BIL as the current is perpendicular to the B direction.
Using LHR the force is either left or right with reference to the diagram on the right.
c) If ac is supplied the current will alternate and direction of force will alternate left to right.
d) The signals that passes the wire cause the diaphragm of the speaker to vibrate producing sound waves.