Current Electricity MCQs for NEET — Physics Questions with Answers

The text states: 'Electric circuits generally consist of a number of resistors and cells interconnected sometimes in a complicated way. 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.'

The context states: 'Bending or reorienting the wire does not change the validity of Kirchhoff’s junction rule.'

The example provided states: 'It is easily verified that Kirchhoff’s second rule applied to the remaining closed loops does not provide any additional independent equation, that is, the above values of currents satisfy the second rule for every closed loop of the network.'

The context mentions: '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. Similarly, for each source (i.e., cell or some other source of electrical power) the positive and negative electrodes are labelled.' There is no mention of assuming all resistors are identical.

The context explains: 'This rule is also obvious, since electric potential is dependent on the location of the point. Thus starting with any point if we come back to the same point, the total change must be zero. In a closed loop, we do come back to the starting point and hence the rule.'

According to the NCERT text, 'The equivalent internal resistance of a series combination of n cells is just the sum of their internal resistances.' (Eq. 3.46 for two cells extends to n cells).

The NCERT text states, 'Consider first two cells in series (Fig. 3.13), where one terminal of the two cells is joined together leaving the other terminal in either cell free. ...eeq = $\epsilon_1$ + $\epsilon_2$' (Eq. 3.45). This refers to the arrangement where the negative of one is connected to the positive of the other, resulting in additive emfs.

The NCERT text mentions: 'If instead we connect the two negatives, Eq. (3.42) would change to $V_{BC} = -\epsilon_2 - Ir_2$ and we will get $e_{eq} = \epsilon_1 - \epsilon_2$ (${\epsilon_1} > {\epsilon_2}$)' (Eq. 3.47). This indicates that when cells are connected in opposition (e.g., negative to negative or positive to positive), their emfs subtract.

The NCERT text states, 'The equivalent internal resistance of a series combination of n cells is just the sum of their internal resistances.' For n identical cells, $r_{eq} = r + r + ... + r$ (n times) $= n r$.

For cells in parallel (Figure 3.14), the NCERT text states, 'At the point B1, I1 and I2 flow in whereas the current I flows out. Since as much charge flows in as out, we have $I = I_1 + I_2$' (Eq. 3.48). This follows Kirchhoff's current rule (junction rule).