Q4. Assertion: a bar magnet resting inside a coil induces no current in it. Reason: an emf is induced in a coil only when the magnetic flux through it changes with time. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: with nothing moving or changing, the flux through the coil stays the same, so there is no induced emf (NCERT pages 155 and 157).
Q5. A magnet is pulled away from a 100-turn coil, and the flux through each turn falls from 4.0 × 10−4 Wb to 1.0 × 10−4 Wb in 0.10 s. Neglect self-inductance and find the magnitudes of the average emf and average current if the circuit’s resistance is 15 Ω.
ε = NΔΦB/Δt = 100 × 3.0 × 10−4/0.10 = 0.30 V, and I = 0.30/15 = 0.020 A (NCERT Eq. (6.4), page 158).
Q6. Why can sensitive electrical instruments near an electromagnet be damaged when it is switched on or off?
Switching it on or off changes its magnetic field quickly, so the flux through the instruments’ circuits changes and induces emfs and currents in them (NCERT footnote, page 157).
Q7. In Experiment 6.2, coils C1 and C2 are moved together at the same velocity, keeping their separation fixed. Is a current induced in C1?
No. There is no relative motion, so the flux from C2 through C1 does not change (reasoned from NCERT page 156).
Q8. A flat coil moves at constant velocity, without turning, through a region of uniform magnetic field, staying wholly inside it. Is a current induced?
No. B, A and θ all stay the same, so the flux through the coil does not change. Relative motion induces a current only when it changes the flux (reasoned from NCERT Eq. (6.1) and page 158).