Physics · Moving Charges And Magnetism · NEET
Same direction attracts, opposite direction repels. This is the reverse of what most students expect (magnets: like poles repel). The reason: wire A's field at wire B, combined with F = I L x B, points toward A when currents are parallel. Memory hook: parallel currents are 'friends who come together', anti-parallel currents 'push apart'.
F/L = mu_0 * I1 * I2 / (2*pi*d), where d is the separation and mu_0 = 4*pi*10^-7 T m/A. This comes from wire 1 making a field B = mu_0*I1/(2*pi*d) at wire 2, and wire 2 feeling F = I2 * L * B. Note the force is per METRE of length, not a single total force, unless a length is given.
Yes. By Newton's third law the two wires feel equal and opposite forces, even when I1 does not equal I2. The formula is symmetric in I1 and I2, so F/L is identical on each wire. A wire with double the current does NOT feel double the force.
For a wire to hang in equilibrium, the magnetic force per unit length must exactly cancel its weight per unit length: mu_0*I1*I2/(2*pi*d) = lambda*g, where lambda is mass per unit length. If the upper wire attracts the lower one upward, the lower wire lifts off when magnetic force exceeds lambda*g.
Compute the force per unit length from EACH outer wire separately using mu_0*I^2/(2*pi*d), then add them as VECTORS (not simple numbers). If the two forces are perpendicular, the resultant is sqrt(2) times one force. If they are opposite (equal currents same side), they cancel.
An arrangement of three parallel straight wires placed perpendicular to the plane of paper carrying the same current I along the same direction is shown, with the middle wire B at distance d from each of the other two, and lines BA and BC at 90 degrees. The magnitude of the force per unit length on the middle wire B is:
Two infinitely long parallel wires A and B carry currents I and 2I in the same direction. Wire A (mass per unit length lambda) lies on an insulated floor; wire B is fixed at height h above it. The minimum h so that wire A does NOT rise from the floor is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Currents in the same direction attract; currents in opposite directions repel. This is the opposite of the 'like poles repel' rule for magnets, which is why it is a common trap.
Force per unit length F/L = mu_0 * I1 * I2 / (2 * pi * d), where d is the distance between the wires and mu_0 = 4*pi*10^-7 T m/A.
One ampere is the steady current in each of two long parallel wires 1 metre apart that produces a force of 2*10^-7 N per metre of length on each wire. Putting I1 = I2 = 1 A and d = 1 m into the formula gives exactly 2*10^-7 N/m.
Yes. By Newton's third law and the symmetric formula, both wires feel the same magnitude of force per unit length, even if one carries more current.
When the magnetic force per unit length balances gravity: mu_0*I1*I2/(2*pi*d) = lambda*g, where lambda is the wire's mass per unit length.
