Current Electricity MCQs for NEET — Physics Questions with Answers

The passage states, 'The formulae we have derived earlier for series and parallel combinations of resistors are not always sufficient to determine all the currents and potential differences in the circuit. Two rules, called Kirchhoff’s rules, are very useful for analysis of electric circuits.'

While not explicitly stated as a formula, the example 3.6 demonstrates that applying the loop rule to remaining closed loops 'does not provide any additional independent equation' once a sufficient number of independent equations has been formed to solve for the unknowns. The number of independent equations from the loop rule is generally equal to the number of unknown currents that cannot be determined by the junction rule.

The junction rule states: 'At any junction, the sum of the currents entering the junction is equal to the sum of currents leaving the junction.' Therefore, if $I_1$ and $I_2$ enter and $I_3$ leaves, then $I_1 + I_2 = I_3$.

The context mentions, 'If, while labelling the current I through the cell one goes from P to N, then of course V = e + I r (3.60)'. This implies that when current flows from P to N through the cell, the terminal voltage is $e+Ir$.

The text states: 'Given a circuit, we start by labelling currents in each resistor by a symbol, say I, and a directed arrow to indicate that a current I flows along the resistor in the direction indicated.'

In the absence of an electric field, electrons move randomly. If we consider all the electrons, their average velocity will be zero since their directions are random. (NCERT, Section 3.5, page 85)

From the NCERT text, the average velocity $v_d$ is given by $v_d = -\frac{eE\tau}{m}$. This is derived from averaging the acceleration experienced by electrons between collisions. (NCERT, Eq. 3.17, page 86)

The amount of charge crossing area A in time $\Delta t$ is $I\Delta t$. This is also equal to $neA|v_d|\Delta t$. Therefore, $I = neA|v_d|$. (NCERT, Eq. 3.18, page 86)

The relaxation time ($\tau$) is defined as the average time between successive collisions of an electron with the positive ions in the conductor. The NCERT text states, 'The average value of $t_i$ then is $\tau$ (known as relaxation time).' (NCERT, page 86)

Each 'free' electron does accelerate, increasing its drift speed until it collides with a positive ion of the metal. It loses its drift speed after collision but starts to accelerate and increases its drift speed again only to suffer a collision again and so on. On the average, therefore, electrons acquire only a drift speed. (NCERT, Example 3.2 (b), page 88)