Q4. Assertion: a diode is a non-ohmic device. Reason: reversing the potential difference across a diode, keeping its size fixed, does not give a current of the same size in the opposite direction. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: this is NCERT’s type (b) departure from Ohm’s law (NCERT page 89).
Q5. A device gives 0.50 A at 2.0 V and 1.5 A at 4.0 V. Find V/I at each reading. Is the device ohmic?
V/I = 2.0/0.50 = 4.0 Ω and 4.0/1.5 ≈ 2.7 Ω. The ratio changes, so V is not proportional to I and the device is non-ohmic (NCERT Eq. (3.3), page 83, and page 89).
Q6. Why does NCERT draw the two halves of the diode’s characteristic to different scales?
Because the current for one sign of V is in milliamperes and for the other only in microamperes; each voltage mark on the negative side stands for ten times as many volts (−2 V against 0.2 V). On one scale the small current would not show (NCERT Figure 3.6, page 89).