Q4. Assertion: a solid metal plate swinging into a magnetic field comes to rest much sooner than a slotted one. Reason: the slots break the large current loops, so the eddy currents and their opposing force are much smaller. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: the large eddy currents in the solid plate oppose its motion strongly (reasoned from Faraday’s and Lenz’s laws).
Q5. In the ring model of Example 2, the field changes four times as fast. By what factor does the heating power change?
The emf is proportional to dB/dt, so it becomes four times as large, and P = ε2/R becomes 16 times as large (reasoned from NCERT Eq. (6.3)).
Q6. Two identical magnets are dropped from the same height, one down a vertical copper pipe and one down an identical pipe with a slit along its whole length. Which reaches the bottom first?
The magnet in the slit pipe. The slit breaks the circular current paths round the pipe, so the eddy currents and their braking force are much smaller (reasoned from Lenz’s law).
Q7. Why does an eddy current brake become weaker as a train slows down, so that ordinary brakes are still needed to stop it completely?
The induced emf depends on the rate of change of flux, which falls as the wheels slow. The eddy currents and their braking force fall too, becoming very small near rest (reasoned from Faraday’s law, NCERT Eq. (6.3)).