Physics MCQs for NEET — Practice Questions with Answers

The NCERT text states, 'If $q_1q_2 > 0$, potential energy is positive. This is as expected, since for like charges ($q_1q_2 > 0$), electrostatic force is repulsive and a positive amount of work is needed to be done against this force to bring the charges from infinity to a finite distance apart.' Both positive-positive and negative-negative charges result in a positive product, hence positive potential energy.

The NCERT states, 'For unlike charges ($q_1q_2 < 0$), the electrostatic force is attractive. In that case, a positive amount of work is needed against this force to take the charges from the given location to infinity. In other words, a negative amount of work is needed for the reverse path (from infinity to the present locations), so the potential energy is negative.' This indicates an attractive force.

The NCERT text explains, 'To bring $q_1$ first from infinity to $r_1$, no work is required.' This is because there are no other charges present at that moment to exert a force against which work needs to be done.

Equation (2.26) in the NCERT text states, '$U = \frac{1}{4\pi\epsilon_0} \left( \frac{q_1q_2}{r_{12}} + \frac{q_1q_3}{r_{13}} + \frac{q_2q_3}{r_{23}} \right)$'. This formula accounts for the interaction energy between all pairs of charges in the system.

As per the NCERT text, 'Thus, work done in bringing a charge q from infinity to the point P in the external field is qV. This work is stored in the form of potential energy of q.' (Section 2.8.1)

The NCERT text defines, '1 eV = 1.6 \times 10^{-19}$ J.' This is the energy gained by an electron accelerated through a potential difference of 1 volt.

When bringing $q_2$ to its position, 'Work is done not only against the external field E but also against the field due to $q_1$.' (Section 2.8.2)

The NCERT states, 'The external field E is not produced by the given charge(s) whose potential energy we wish to calculate. E is produced by sources external to the given charge(s).' (Section 2.8)

The two charges are $7 \text{ mC}$ (positive) and $-2 \text{ mC}$ (negative). Since they are unlike charges, their product $q_1q_2 < 0$. According to the NCERT material, 'For unlike charges ($q_1q_2 < 0$), the electrostatic force is attractive... so the potential energy is negative.'

The NCERT states, 'Forces of this kind are called conservative forces. Spring force and gravitational force are examples of conservative forces.' It also mentions that 'Coulomb force between two (stationary) charges is also a conservative force.'