CBSE Class 10 Science: Chapter 12 Electricity Assertion-Reason Questions
Latest Pattern Assertion-Reason (A/R) Questions
Welcome to Part 5 of our 8-part series on Chapter 12, Electricity. This post focuses on Assertion-Reason (A/R) Questions. These questions test your in-depth conceptual understanding and your ability to link two related statements.
Part 5: Assertion-Reason (A/R) Questions
Question 1
CBSE 2023
Assertion (A): A voltmeter is always connected in parallel across the component whose voltage is to be measured.
Reason (R): A voltmeter has a very high resistance so that it draws negligible current from the circuit.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
A voltmeter measures potential difference between two points. It is connected in parallel. To ensure it doesn't change the circuit's current, it must have a very high resistance, forcing almost all the current to flow through the main component. Thus, the reason correctly explains the assertion.
Question 2
CBSE 2022
Assertion (A): Alloys like Nichrome are commonly used in electrical heating devices.
Reason (R): Alloys have a high resistivity and a high melting point.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
Heating devices require a large amount of heat (H = I²Rt). Alloys like Nichrome are used because their high resistivity (ρ) produces more heat, and their high melting point prevents them from melting or oxidizing at high temperatures.
Question 3
Assertion (A): An ammeter is connected in series in a circuit.
Reason (R): An ammeter has a very low resistance.
(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
Both statements are true. An ammeter is connected in series to measure the current flowing *through* the circuit. It has a low resistance so that it doesn't significantly change the total resistance of the circuit. However, the reason it's in series isn't *because* its resistance is low. The low resistance is a *requirement* for it to function correctly *in series*.
Question 4
CBSE 2023
Assertion (A): When resistors are connected in parallel, the equivalent resistance is less than the smallest individual resistance.
Reason (R): Connecting resistors in parallel provides more paths for the current to flow, thereby decreasing the total opposition.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
The assertion is a known fact. The reason correctly explains why. By adding parallel paths, the total effective area for current flow increases, which (like a thicker wire) reduces the overall resistance.
Question 5
Assertion (A): The SI unit of electric power is Watt.
Reason (R): Power is defined as the rate of energy consumption (P = E/t).
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
Power is the rate of doing work or consuming energy (Reason). P = E/t. The unit of Energy (E) is Joule (J) and time (t) is second (s). Therefore, the unit of power is J/s, which is defined as a Watt (W). The reason correctly defines the assertion.
Question 6
CBSE 2021
Assertion (A): A fuse wire has a low melting point and is made of a material with high resistivity.
Reason (R): A fuse wire is used to protect the circuit from overloading and short-circuiting.
(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
(A) is true: A fuse needs high resistivity to heat up (H = I²Rt) and a low melting point to break the circuit. (R) is also true: That is the function of a fuse. However, (R) explains the *purpose* of a fuse, not *why* it is made of high resistivity/low melting point material.
Question 7
Assertion (A): When two bulbs (60W, 220V) and (40W, 220V) are connected in series, the 40W bulb glows brighter.
Reason (R): In a series circuit, power is directly proportional to resistance, and the 40W bulb has higher resistance.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
First, R = V²/P. The 40W bulb has higher resistance than the 60W bulb. In series, current (I) is constant. Brightness (Power) is P = I²R. Since I is constant, P ∝ R. The 40W bulb (higher R) glows brighter. The reason is the correct explanation.
Question 8
Assertion (A): When the length of a wire is doubled, its resistance also gets doubled.
Reason (R): Resistance is directly proportional to the length of the wire (R ∝ L).
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
The formula for resistance is R = ρ(L/A). If all other factors (ρ, A) are constant, then R ∝ L. Therefore, doubling the length (L) will double the resistance (R).
Question 9
Assertion (A): A wire is stretched to double its length. Its new resistance will become four times the original.
Reason (R): When a wire is stretched, its length increases, and its area of cross-section decreases.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
When stretched, L' = 2L. Since Volume (L×A) is constant, the new area A' = A/2.
New Resistance R' = ρ(L'/A') = ρ(2L / (A/2)) = ρ(4L/A) = 4 * [ρ(L/A)] = 4R.
The reason (that both length *and* area change) is the correct explanation for why the resistance becomes four times, not just double.
Question 10
Assertion (A): In a parallel circuit, the total resistance is 1/Req = 1/R1 + 1/R2 + ...
Reason (R): In a parallel circuit, the voltage across each resistor is constant.
(c) Assertion (A) is true, but Reason (R) is false.
The formula for parallel resistance (Assertion) is correct. However, the reason is false. In a parallel circuit, the *voltage* across each resistor is constant, not the current.
Question 11
Assertion (A): The commercial unit of electrical energy is the kilowatt-hour (kWh).
Reason (R): The SI unit of electrical energy is the Joule (J).
(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
Both statements are independently true. The commercial unit *is* kWh, and the SI unit *is* Joule. However, the fact that the SI unit is Joule is not the *reason* why the commercial unit is kWh. The commercial unit is kWh because it is a larger, more convenient unit for billing purposes.
Question 12
Assertion (A): Wires carrying electricity are coated with an insulating material like PVC.
Reason (R): Insulators have very low resistivity and prevent electric shocks.
(c) Assertion (A) is true, but Reason (R) is false.
The assertion is true; wires are coated with insulators (like PVC). The reason is false. Insulators have very *high* resistivity (or low conductivity), not low resistivity. This high resistivity is what stops the current and prevents shocks.
Question 13
Assertion (A): If a graph is plotted between potential difference (V) and current (I), the slope of the V-I graph gives the resistance (R).
Reason (R): According to Ohm's Law, V = IR.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
Reason (R) states V = IR. Comparing this to the equation of a line, y = mx + c, if V is on the y-axis (y=V) and I is on the x-axis (x=I), then V = R*I. The slope (m) is indeed the resistance (R). The reason (Ohm's Law) is the basis for the assertion.
Question 14
Assertion (A): In a series circuit, if one appliance fails, all other appliances stop working.
Reason (R): In a series circuit, there is only one continuous path for the current to flow.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
The reason is the definition of a series circuit. If one appliance (like a bulb filament) fails, it breaks this single path. This stops the flow of current to all other appliances in the same path, causing them to stop working.
Question 15
CBSE 2023
Assertion (A): Resistivity of a material does not change if the length or area of the wire is changed.
Reason (R): Resistivity is a characteristic property of the material and only depends on temperature.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
Resistance (R) depends on length and area (R = ρL/A). However, Resistivity (ρ) is an intrinsic property of the material itself (like density or melting point). It does not depend on the wire's dimensions. It only changes with temperature.
Question 16
Assertion (A): Domestic circuits in a house are connected in parallel.
Reason (R): A parallel connection ensures that all appliances receive the same voltage (e.g., 220V) and can be operated independently.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
Houses are wired in parallel precisely for the reasons stated: (1) Every appliance gets the full mains voltage, and (2) if one appliance fails or is switched off, it doesn't break the circuit for the others.
Question 17
Assertion (A): When the radius of a wire is doubled, its resistance becomes one-fourth.
Reason (R): Resistance is inversely proportional to the area of cross-section (A = πr²).
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
R ∝ 1/A, and A = πr². So, R ∝ 1/r².
If the radius (r) is doubled (r' = 2r), the new area A' = π(2r)² = 4πr² = 4A.
The new resistance R' ∝ 1/A' = 1/(4A).
Therefore, R' = R/4. The resistance becomes one-fourth.
Question 18
Assertion (A): Copper and aluminum wires are used for electrical transmission.
Reason (R): Copper and aluminum have very high resistivity.
(c) Assertion (A) is true, but Reason (R) is false.
Assertion is true; these metals are used for transmission. The reason is false. Copper and aluminum are used precisely because they are good conductors, meaning they have very *low* resistivity, which minimizes power loss (as heat) during transmission.
Question 19
Assertion (A): 1 kWh is equal to 3.6 × 10⁶ Joules.
Reason (R): 1 kWh = 1 kilowatt × 1 hour = 1000 Watt × 3600 seconds.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
The reason shows the exact calculation.
1 kWh = 1000 W × 3600 s = 3,600,000 W·s.
Since 1 Watt = 1 Joule/second, 1 W·s = 1 Joule.
Therefore, 1 kWh = 3,600,000 J = 3.6 × 10⁶ J.
Question 20
Assertion (A): If a 2A current flows through a 5Ω resistor for 10 seconds, the heat produced is 200 J.
Reason (R): Heat produced is given by Joule's Law of Heating, H = I²Rt.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
The reason (R) gives the formula H = I²Rt.
Using this formula to check the assertion (A):
H = (2A)² × 5Ω × 10s = 4 × 5 × 10 = 200 J.
The assertion is a correct application of the reason.
Question 21
Assertion (A): The unit of current is Ampere (A).
Reason (R): 1 Ampere is defined as the flow of 1 Coulomb of charge per second (I = Q/t).
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
The assertion is true. The reason provides the fundamental definition of the Ampere (1 A = 1 C/s), which correctly explains what the unit represents.
Related Posts
Question 22
Assertion (A): Ohm's law is a universal law.
Reason (R): All conductors obey Ohm's law.
(d) Assertion (A) is false, but Reason (R) is true.
This is tricky. Assertion (A) is false; Ohm's law is not a universal law (like the law of gravitation). It is an empirical law that only applies to certain materials (ohmic conductors) under specific conditions (like constant temperature).
Reason (R) is also false. Not *all* conductors obey Ohm's law. Devices like semiconductor diodes, transistors, and electrolytes are conductors, but they are *non-ohmic* (their V-I graph is not a straight line).
Therefore, both (A) and (R) are false. *Correction*: The provided options do not have "Both are false". Let's re-evaluate.
(A) Ohm's law is a universal law. (False)
(R) All conductors obey Ohm's law. (False)
*If both are false, the correct CBSE option is (d)*. In some contexts, (d) is "Both A and R are false". Assuming the provided (a,b,c,d) options are standard:
(A) is false. (R) is false. The closest fit is (d) "Assertion (A) is false, but Reason (R) is true" is also incorrect.
*Let's assume there's a typo in the question and re-interpret Reason (R) as "Ohmic conductors obey Ohm's Law".*
*Let's stick to the text:*
(A) Ohm's law is a universal law. (False)
(R) All conductors obey Ohm's law. (False)
*Since there is no (e) Both are false, let's re-read the options.*
(a) A true, R true, R explains A
(b) A true, R true, R not explains A
(c) A true, R false
(d) A false, R true
*There must be a mistake in my analysis or the question's premise. Let's reconsider.*
*Re-analysis:*
Is (R) true? "All conductors obey Ohm's law." No, semiconductors are non-ohmic. So (R) is definitively False.
Is (A) true? "Ohm's law is a universal law." No, it's not.
This question is flawed as both statements are false. Let's find a replacement.
*Replacement Question 22:*
Assertion (A): When the temperature of a pure metal conductor increases, its resistance increases.
Reason (R): As temperature increases, the thermal vibrations of metal ions increase, which increases the number of collisions for the free electrons.
Question 22
Assertion (A): When the temperature of a pure metal conductor increases, its resistance increases.
Reason (R): As temperature increases, the thermal vibrations of metal ions increase, which increases the number of collisions for the free electrons.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
Increased temperature causes the fixed positive ions (kernels) in the metal lattice to vibrate more vigorously. This increased vibration obstructs the path of the free electrons (current) more frequently, leading to more collisions and thus higher resistance.
Question 23
Assertion (A): In a simple battery circuit, the point of lower potential is the positive terminal.
Reason (R): The conventional direction of current is from higher potential to lower potential.
(d) Assertion (A) is false, but Reason (R) is true.
The assertion is false. The positive terminal of a battery is the point of *higher* potential, and the negative terminal is the point of *lower* potential. The reason is true; this is the definition of conventional current flow.
Question 24
Assertion (A): When a 4 Ω resistor and a 6 Ω resistor are connected in parallel, the total resistance is 2.4 Ω.
Reason (R): The equivalent resistance in parallel (Rp) is given by 1/Rp = 1/R1 + 1/R2.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
The reason gives the correct formula. Let's check the assertion using the formula:
1/Rp = 1/4 + 1/6
Common denominator is 12.
1/Rp = 3/12 + 2/12 = 5/12
Rp = 12/5 = 2.4 Ω.
The assertion is a correct calculation based on the reason.
Question 25
Assertion (A): When the resistance of a circuit is halved, keeping the voltage constant, the current is doubled.
Reason (R): According to Ohm's Law, current is inversely proportional to resistance (I ∝ 1/R).
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
From Ohm's Law, V = IR. If V is constant, then I = V/R, which means I ∝ 1/R. If R is halved (R' = R/2), the new current I' = V/(R/2) = 2(V/R) = 2I. The current is doubled. The reason perfectly explains the assertion.
