NEET Practice Questions (MCQs) with Answers & Solutions

Using the formula for potential energy of two charges $U = \frac{1}{4\pi\epsilon_0} \frac{q_1q_2}{r_{12}}$. Here $q_1 = 7 \times 10^{-6} C$, $q_2 = -2 \times 10^{-6} C$, and $r_{12} = 9 cm - (-9 cm) = 18 cm = 0.18 m$. $U = (9 \times 10^9) \frac{(7 \times 10^{-6})(-2 \times 10^{-6})}{0.18} = 9 \times 10^9 \times \frac{-14 \times 10^{-12}}{0.18} = \frac{-126 \times 10^{-3}}{0.18} = -0.7 J$. This matches the Example 2.5(a) in the NCERT.

The NCERT states for a system of charges in an external field, the total potential energy includes the mutual interaction energy of the charges plus the energy of interaction of each charge with the external electric field. For two charges, this additional part is $q_1V(r_1) + q_2V(r_2)$. (Example 2.5(c) and equation 2.29).

By definition, 'V at a point P is the work done in bringing a unit positive charge from infinity to the point P.' (Section 2.8.1). Also confirmed earlier in Section 2.3, 'Potential at P due to the charge Q is $V(r) = \frac{Q}{4\pi\epsilon_0 r}$'.

The NCERT states, 'The path-independence of work done by an electrostatic field can be proved using the Coulomb’s law... The concept of the potential energy would not be meaningful if the work depended on the path.' (Section 2.2(i)). This highlights the conservative nature of electrostatic forces.

The provided text explains: 'The amount of work done by the external torque will be given by $pE (\cos\theta_0 - \cos\theta_1)$. This work is stored as the potential energy of the system.' (Section 2.8.3).

In the process of assembling a system of charges, when the first charge ($q_1$) is brought from infinity, there are no other charges yet present in the field to exert any electrostatic force on it. Hence, no work is done against an existing electrostatic field. The NCERT implies this when discussing 'Potential energy of a system of charges' (Section 2.7): 'To bring $q_1$ first from infinity to $r_1$, no work is required.'

According to the provided text, "The specific heat capacity of a substance is defined by $s = \frac{1}{m} \frac{\Delta Q}{\Delta T}$ where m is the mass of the substance and $\Delta Q$ is the heat required to change its temperature by $\Delta T$." This directly corresponds to option 1.

The text states: "The molar specific heat capacity of a substance is defined by $C = \frac{1}{\mu} \frac{\Delta Q}{\Delta T}$ where $\mu$ is the number of moles of the substance." This matches option 1.

The definition provided is: "The latent heat of fusion ($L_f$) is the heat per unit mass required to change a substance from solid into liquid at the same temperature and pressure." This aligns with option 2.

As per the text: "The latent heat of vaporisation ($L_v$) is the heat per unit mass required to change a substance from liquid to the vapour state without change in the temperature and pressure." This corresponds to option 2.