Q4. Assertion: when a Wheatstone bridge is balanced, the galvanometer and the cell can be interchanged and the bridge stays balanced. Reason: the balance condition can be written R1R4 = R2R3, the products of opposite arms. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion. With the cell across BD, the routes from B to D are B–A–D (R2, then R1) and B–C–D (R4, then R3). A and C are at the same potential when R2/R1 = R4/R3, which is again R1R4 = R2R3. Opposite arms stay opposite, so the condition is unchanged (reasoned from NCERT Eq. 3.64(a), page 101).
Q5. In a bridge, R1 = 4 Ω, R2 = 8 Ω and R4 = 12 Ω. What R3 balances it?
R3 = R4R1/R2 = 12 × 4/8 = 6 Ω (NCERT Eq. 3.64(a), page 101).
Q6. In NCERT Example 3.7, find the current I2 through the 60 Ω arm.
I2 = 31.5Ig = 31.5 × 4.87 mA ≈ 0.153 A (NCERT page 102).