Question

Two parallel wires carry currents of 20 A and 40 A in opposite directions. Another wire carying a current anti parallel to 20 A is placed midway between the two wires. T he magnetic force on it will be

Options

• towards 20 A

• towards 40 A

• zero

• perpendicular to the plane of the currents 

Answer 1

 towards 40 A
According to Fleming's left-hand rule, if the forefinger and middle finger of our left hand point towards the magnetic field acting on a wire and the current flowing in the wire, respectively, then the thumb will point towards the direction in which the force will act (keeping all three perpendicular). Direction of force can be determined using Fleming's left-hand rule. 

`vecF` is the sum is forces acting on the middle wire due to the wires on the both sides of it . 

h = 20 A, i2 = 40A 

`vec(B) =vec(B)_1 + vec(B)_2`

In the figure, dotted circle shows the magnetic filed lines due to both the wires.
Magnetic field at any point on the middle wire will be acting along the tangent to the masgnetic field lines at that point.
Therefore, the wire will experience a magnetic field pointing ​towards the 40 A wire.
Due to AB, the force will be towards right and due to CD, the force on the wire will be towards right. So, both the forces will add to give a resultant force, which will be towards right, that is, towards the 40 A current-carrying wire.