Current Electricity Questions and Answers

Answer: C

The key's answer is (A) is false, but (R) is true.

Electric current has a magnitude and is assigned a direction (the direction of flow of positive charge), which is what the Reason says. But it is a scalar, because currents do not combine by the laws of vector addition: the currents meeting at a junction simply add arithmetically (Kirchhoff's junction rule), whatever the angles between the wires.

So the Assertion that current is a vector is false. Having a direction is necessary for a vector quantity but not sufficient, which is why options treating both statements as true are wrong.

Answer: D

(A) is true and (R) is false.

Nichrome, an alloy of nickel and chromium, is used for heating elements in toasters, irons and geysers.

The reason is that it has a high resistivity, so a coil produces plenty of heat, together with a high melting point and resistance to oxidation when hot. (R) wrongly says low resistivity, so it is false.

Copper, with its low resistivity, is used for the connecting wires precisely because it does not heat up much.

Answer: B

A fuse wire should have high resistance, be thin and have a low melting point.

A fuse works by melting when too much current flows. Heat produced in a wire is I²R, so a high resistance makes it heat up quickly with excess current. A thin wire has higher resistance and heats faster, and a low melting point makes sure it melts and breaks the circuit before the appliance or wiring is damaged. Fuse wire is usually an alloy of tin and lead.

A thick wire (option A) would take too long to melt. Low resistance and high melting point (C, D) are the properties of connecting wires, which must not melt.

Answer: C

She is likely to find that distilled water is a poor conductor of electricity.

Water conducts electricity because of the salts dissolved in it, which provide ions. Distilled water is almost free of dissolved salts, so very few ions are present and very little current flows; with a simple bulb tester the bulb does not glow. Adding a pinch of salt makes it conduct well.

It is therefore not a good conductor. Calling it an insulator overstates the case, since even pure water has a very small number of ions, so the key treats it as a poor conductor.

Answer: A

The statement that is not correct is 'There is a minimum limit of current which can safely flow in the electric circuit.'

Every circuit has a maximum safe current. Above it the wires overheat and can cause a fire, which is why fuses are fitted in the electric circuits of all buildings: a fuse wire melts and breaks the circuit when the current exceeds the safe limit.

There is no minimum safe current: a smaller current is always safe, it simply does less work. So statements B, C and D are correct and A is the incorrect one.

Answer: D

One can use a blade.

A circuit needs a conducting path. A blade is made of metal (steel), which conducts electricity, so it can bridge the gap left by a missing wire, as in the simple tester activities of the NCERT science books.

Paper, rubber and wood are insulators: they do not allow current to pass, so a paper strip, a rubber belt or a wooden stick would leave the circuit open.

Answer: B

The bulbs glow brightest in Arrangement B, with all bulbs in parallel.

In parallel, each bulb is connected directly across the source, so each gets the full voltage and draws its full current. Each bulb's power, and so its brightness, is the greatest possible.

In series the source voltage is shared among the three bulbs, so each is dim (one third of the voltage, one ninth of the power). The mixed arrangements put at least some bulbs on a reduced share of the voltage, so they cannot match the all-parallel circuit.

Answer: A

The least heat is produced by wire C.

With the same cell, the voltage V is fixed, so the heat produced in a given time is \(H = \frac{V^2 t}{R}\): the higher the resistance, the less the heat. Resistance is \(R = \rho \frac{l}{A}\), so for wires of the same material it is proportional to length divided by area.

Taking l/A in units of cm per \(10^{-8}\ \text{m}^2\):

\[ \begin{aligned} A &: 10/10 = 1 \\ B &: 10/8 = 1.25 \\ C &: 12/6 = 2 \\ D &: 5/10 = 0.5 \end{aligned} \]

C has the highest resistance and so draws the least current and produces the least heat. D, the shortest and thickest, produces the most. Choosing D comes from using \(H = I^2Rt\) as if the current were the same in every wire, which is not so when each wire is connected to the same cell.

Answer: C

The set of poor conductors is shampoo, vegetable oil, distilled water.

A liquid conducts electricity when it contains enough dissolved ions. Distilled water is almost free of dissolved salts, vegetable oil has no ions, and shampoo conducts only weakly; in the school tester activity none of them makes the bulb glow, so they are classed as poor conductors.

Each other set contains good conductors. Tap water carries dissolved salts. Salt solution, lemon juice and vinegar contain ions from salts and acids. Sugar solution does not conduct, but vinegar and lemon juice in the same set do.

Answer: B

The difference is \(\frac{8}{9}\ \Omega\).

The maximum resistance comes from connecting all three in series and the minimum from connecting all three in parallel:

\[ \begin{aligned} R_{max} &= 3 \times \tfrac{1}{3} = 1\ \Omega \\ \frac{1}{R_{min}} &= 3 + 3 + 3 = 9 \Rightarrow R_{min} = \tfrac{1}{9}\ \Omega \\ R_{max} - R_{min} &= 1 - \tfrac{1}{9} = \tfrac{8}{9}\ \Omega \end{aligned} \]

1 Ω and 1/9 Ω are the maximum and minimum themselves, not their difference, and 2/3 Ω does not follow from either arrangement.