Every Book Back multiple-choice question from Current Electricity (12th Standard Physics, Samacheer Kalvi) — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
The following graph shows current versus voltage values of some unknown conductor. What is the resistance of this conductor?
- A. 2 ohmCorrect
- B. 4 ohm
- C. 8 ohm
- D. 1 ohm
Explanation. Resistance is determined by calculating the reciprocal of the slope of the current-voltage graph, where the value is obtained by dividing the potential difference by the corresponding current at any point on the linear graph.
Q2
A wire of resistance 2 ohms per meter is bent to form a circle of radius 1m. The equivalent resistance between its two diametrically opposite points, A and B as shown in the figure is
- A. \(\pi \Omega\)Correct
- B. \(\frac{\pi}{2} \Omega\)
- C. \(2\pi \Omega\)
- D. \(\frac{\pi}{4} \Omega\)
Explanation. The circular wire acts as two semi-circular resistors connected in parallel between the diametrically opposite points. The total resistance is calculated by applying the parallel combination formula to the resistances of these two equal semi-circular arcs.
Q3
A toaster operating at 240 V has a resistance of 120 \(\Omega\). Its power is
- A. 400 W
- B. 2 W
- C. 480 WCorrect
- D. 240 W
Explanation. The electric power dissipated is calculated using the formula \(P = \frac{V^2}{R}\). Substituting 240 volts for voltage and 120 ohms for resistance gives a result of 480 watts.
Q4
A carbon resistor of (47 \(\pm\) 4.7 ) k \(\Omega\) to be marked with rings of different colours for its identification. The colour code sequence will be
- A. Yellow – Green – Violet – Gold
- B. Yellow – Violet – Orange – SilverCorrect
- C. Violet – Yellow – Orange – Silver
- D. Green – Orange – Violet - Gold
Explanation. According to the carbon resistor color code, the digits 4 and 7 correspond to yellow and violet. The kilo-ohm multiplier represents three zeros (orange), and the ten percent tolerance is indicated by a silver ring.
Q5
What is the value of resistance of the following resistor?
- A. 100 k \(\Omega\)Correct
- B. 10 k \(\Omega\)
- C. 1 k \(\Omega\)
- D. 1000 k \(\Omega\)
Explanation. The resistance value is determined using the standard color code table for carbon resistors, where the sequence of colored bands represents significant digits, the decimal multiplier, and the tolerance level.
Q6
Two wires of A and B with circular cross section are made up of the same material with equal lengths. Suppose \(R_A = 3 R_B\), then what is the ratio of radius of wire A to that of B?
- A. 3
- B. \(\sqrt{3}\)
- C. \(\frac{1}{\sqrt{3}}\)Correct
- D. \(\frac{1}{3}\)
Explanation. Resistance is inversely proportional to the square of the radius for wires of identical material and length. Therefore, the ratio of radii is the square root of the inverse ratio of their resistances.
Q7
A wire connected to a power supply of 230 V has power dissipation \(P_1\). Suppose the wire is cut into two equal pieces and connected parallel to the same power supply. In this case power dissipation is \(P_2\). The ratio \(P_2/P_1\) is
- A. 1
- B. 2
- C. 3
- D. 4Correct
Explanation. Halving the length of a wire halves its resistance. Connecting two such halves in parallel creates an equivalent resistance that is one-fourth of the original, leading to four times the power dissipation at constant voltage.
Q8
In India electricity is supplied for domestic use at 220 V. It is supplied at 110 V in USA. If the resistance of a 60W bulb for use in India is R, the resistance of a 60W bulb for use in USA will be
- A. R
- B. 2R
- C. \(\frac{R}{4}\)Correct
- D. \(\frac{R}{2}\)
Explanation. For a constant power rating, resistance is directly proportional to the square of the voltage. Since the voltage in the USA is half that of India, the resistance must be one-fourth to maintain sixty watts.
Q9
In a large building, there are 15 bulbs of 40 W, 5 bulbs of 100 W, 5 fans of 80 W and 1 heater of 1k W are connected. The voltage of electric mains is 220 V. The maximum capacity of the main fuse of the building will be
- A. 14 A
- B. 8 A
- C. 10 A
- D. 12 ACorrect
Explanation. The total power is the sum of all individual appliance ratings. The total current is found by dividing this total power by the supply voltage, and the fuse must exceed this calculated current value.
Q10
There is a current of 1.0 A in the circuit shown below. What is the resistance of P?
- A. 1.5 \(\Omega\)
- B. 2.5 \(\Omega\)
- C. 3.5 \(\Omega\)Correct
- D. 4.5 \(\Omega\)
Explanation. The value of the unknown resistance is found using the series circuit formula where total voltage equals the product of current and the sum of all resistances, allowing for the calculation of the missing resistor value.
Q11
What is the current drawn out from the battery?
- A. 1 ACorrect
- B. 2 A
- C. 3 A
- D. 4 A
Explanation. The network's equivalent resistance is calculated by identifying series and parallel branches. The total current drawn is then determined by applying Ohm's law, dividing the battery voltage by this equivalent resistance.
Q12
The temperature coefficient of resistance of a wire is 0.00125 per °C. At 20°C, its resistance is 1 \(\Omega\). The resistance of the wire will be 2 \(\Omega\) at
- A. 800 °C
- B. 700 °C
- C. 850 °C
- D. 820 °CCorrect
Explanation. The final temperature is calculated using the temperature dependence formula for resistance, \(R = R_0[1 + \alpha(T - T_0)]\), by substituting the known resistance values, temperature coefficient, and initial temperature.
Q13
The internal resistance of a 2.1 V cell which gives a current of 0.2 A through a resistance of 10 \(\Omega\) is
- A. 0.2 \(\Omega\)
- B. 0.5 \(\Omega\)Correct
- C. 0.8 \(\Omega\)
- D. 1.0 \(\Omega\)
Explanation. The internal resistance is derived from the formula for total current in a circuit, \(I = \frac{\varepsilon}{R + r}\), where electromotive force, current, and external resistance are given.
Q14
A piece of copper and another of germanium are cooled from room temperature to 80 K. The resistance of
- A. each of them increases
- B. each of them decreases
- C. copper increases and germanium decreases
- D. copper decreases and germanium increasesCorrect
Explanation. Copper, being a conductor, has a positive temperature coefficient causing resistance to decrease upon cooling. Germanium, a semiconductor, has a negative coefficient, meaning its resistance increases as the temperature decreases.
Q15
In Joule’s heating law, when R and t are constant, if the H is taken along the y axis and \(I^2\) along the x axis, the graph is
- A. straight lineCorrect
- B. parabola
- C. circle
- D. ellipse
Explanation. Based on the formula \(H = I^2Rt\), if resistance and time are kept constant, heat produced is directly proportional to the square of the current, which mathematically defines a linear relationship and a straight-line graph.
