Electricity: CBSE Class 10 Physics practice questions

A. $6\times10^{18}$

$N = \dfrac{1\ \text{C}}{1.6\times10^{-19}\ \text{C}} \approx 6.25\times10^{18}$, so nearly $6\times10^{18}$ electrons. $1.6\times10^{-19}$ is the charge of one electron, not their number, and the powers of ten with the wrong sign are the typical slips from dividing by a small number.

C. $300\ \text{C}$

From $I = Q/t$ we get $Q = It$. Time must be in seconds: $10\ \text{min} = 600\ \text{s}$, so $Q = 0.5\ \text{A} \times 600\ \text{s} = 300\ \text{C}$. Using $10$ directly gives the trap $5\ \text{C}$; dividing $t$ by $I$ gives $1200\ \text{C}$; and $10/0.5 = 20$ comes from the same slip with minutes.

B. $4800\ \Omega$

$I = 2.5\ \text{mA} = 2.5\times10^{-3}\ \text{A}$, so $R = V/I = 12/(2.5\times10^{-3}) = 4800\ \Omega$. Using $2.5$ directly gives the trap $4.8\ \Omega$; $12\times2.5 = 30$ multiplies instead of dividing; and $I/V$ inverts the ratio.