Class 10 – Science Extra Questions – Ch. 11 – Electricity

Ans: A

Explanation: Potential difference between two points is the amount of work done per unit charge in moving a charge between those two points. This quantity is commonly referred to as voltage. Work is the energy transferred, electric power is the rate at which energy is transferred or work is done, and rating typically refers to the power capacity of a device. Therefore, voltage is the correct term for potential difference.
Correct Option: A

Ans: C

Explanation: The fundamental formula for electrical power is P = VI (Power equals Voltage times Current). Ohm’s Law states V = IR (Voltage equals Current times Resistance). To express power in terms of voltage and resistance only, we can substitute for current (I) from Ohm’s Law into the power formula. From V = IR, we get I = V/R. Substituting this into P = VI gives P = V(V/R) = V²/R. Therefore, option C correctly expresses electrical power in terms of voltage and resistance only. Option B is correct if expressed in terms of current and resistance only (P = I²R), and option A is correct if expressed in terms of voltage and current only. Option D is not a standard physics formula for power.
Correct Option: C

Ans: D

Explanation: The question asks for a property that directly relates to how strongly a material opposes the flow of electric current, independent of the object’s shape or size. Resistance is the opposition to current flow for a specific object and depends on shape and size. Conductivity is the inverse of resistivity and measures how well a material conducts electricity. Permittivity relates to how a material affects electric fields. Resistivity is an intrinsic property of a material that quantifies its opposition to electrical current flow. It is independent of the object’s shape or size. Therefore, resistivity directly relates to how strongly a material opposes the flow of electric current, independent of the object’s shape or size.

However, upon re-reading the question, it asks for a property that *directly relates* to how strongly a material opposes the flow of electric current. While resistivity is the intrinsic property, resistance is what we *measure* for a specific object which reflects this opposition. The wording “how strongly a material opposes the flow of electric current” can be interpreted as the inherent quality of the material itself, which is resistivity. However, the phrase “a characteristic independent of the object’s shape or size” strongly points to an intrinsic material property.

Let’s analyze the options again:
A. Conductivity: This is the ability to conduct electricity, the *opposite* of opposing it.
B. Permittivity: This relates to electric fields, not current opposition.
C. Resistance: This is the opposition to current flow for a *specific object*. It *does* depend on shape and size. For example, a thin wire has more resistance than a thick wire of the same length and material.
D. Resistivity: This is the *intrinsic property* of a material that quantifies its opposition to current flow. It is independent of the object’s shape or size. For a given material, resistivity multiplied by the length and divided by the cross-sectional area gives the resistance of an object made from that material.

The question asks for a property that relates to how strongly a *material* opposes current flow, and explicitly states it is independent of shape or size. This is the definition of resistivity.

Let’s reconsider the phrasing. “How strongly a material opposes the flow of electric current” is directly measured by its resistivity. Resistivity is a fundamental material property. Resistance, on the other hand, is the opposition for a specific object, and it *does* depend on shape and size. Therefore, resistivity is the correct answer as it is independent of shape and size.

However, the question’s phrasing “how strongly a material opposes the flow of electric current” is also a conceptual description of what resistance *represents*, but then it contradicts itself by saying “independent of the object’s shape or size,” which eliminates resistance.

Let’s assume the question is asking for the intrinsic property of the material. In that case, it would be resistivity.

Let’s consider a common way this concept is taught. Resistivity is the inherent property of a material that determines its resistance. If we take a standard sample of a material (e.g., a cube of 1 meter by 1 meter by 1 meter), its resistance would be numerically equal to its resistivity. So, resistivity is the fundamental measure of opposition.

Revisiting the options and the question’s phrasing:
“how strongly a material opposes the flow of electric current” – this describes the inherent characteristic of the material.
“a characteristic independent of the object’s shape or size” – this further emphasizes it’s a material property.

Resistance is an extrinsic property (depends on geometry), while resistivity is an intrinsic property (material-dependent). Conductivity is the inverse of resistivity. Permittivity is unrelated to electrical resistance.

Therefore, resistivity is the property that directly relates to how strongly a material opposes the flow of electric current, and it is independent of the object’s shape or size.

Correct Option: D

Ans: D

Explanation: The wire is designed to melt and break the circuit when the current exceeds a safe limit. This melting is due to the heat generated by the current flowing through the wire (Joule heating). When the current is too high, the wire heats up beyond its melting point and breaks, interrupting the flow of electricity and protecting other components in the circuit from damage. This action is analogous to a switch that automatically opens due to an overload. Option A is incorrect because the wire’s primary function isn’t to increase resistance, although its resistance does contribute to heating. Option B is incorrect; a melting wire at high currents indicates a failure to conduct efficiently. Option C is incorrect; while heat is generated, its purpose is not for warming but for circuit protection. Option D accurately describes the function of the wire as an automatic switch that opens under overload conditions.
Correct Option: D

Ans: B

Explanation: A single movable pulley has the load supported by two segments of the rope. In a perfect pulley system (where there is no friction and the pulley is weightless), the tension in the rope is the same throughout. If the effort applied to pull the rope is F, then each segment of the rope supporting the load exerts a force of F. Therefore, the total upward force on the load is 2F. The mechanical advantage (MA) is defined as the ratio of the load (L) to the effort (E), or MA = L/E. In this case, L = 2F, and E = F, so MA = 2F/F = 2.
Correct Option: B

Ans: A

Explanation: To achieve the minimum resistance using multiple resistors, they should be connected in parallel. When resistors are connected in parallel, the reciprocal of the equivalent resistance is equal to the sum of the reciprocals of individual resistances. In this case, we have three resistors each with a resistance of 2 Ω.
Let R1 = 2 Ω, R2 = 2 Ω, and R3 = 2 Ω.
The formula for equivalent resistance in parallel is:
1/R_eq = 1/R1 + 1/R2 + 1/R3
1/R_eq = 1/2 + 1/2 + 1/2
1/R_eq = 3/2
R_eq = 2/3 Ω
Therefore, the minimum resistance that can be made is 2/3 Ω.
Correct Option: A

Ans: B

Explanation: Electric energy is the capacity to do work by means of electric charge. Power is the rate at which energy is transferred or converted. Power is measured in watts (W), which is equivalent to joules per second (J/s). Energy is power multiplied by time. Therefore, energy can be measured in watts multiplied by seconds (W·s). Since 1 watt is 1 joule per second, 1 watt-second (W·s) is equal to (1 J/s) * 1 s = 1 Joule. Therefore, watt-second is a unit equivalent to the unit of electric energy. Volt-ampere is the unit of apparent power. Kilowatt is a unit of power, not energy. Joule-volt is not a standard unit of energy.
Correct Option: B

Ans: A

Explanation:Let the resistance of each identical resistor be R.
When three identical resistors are connected in series, the total resistance R_s is the sum of individual resistances:
R_s = R + R + R = 3R

When three identical resistors are connected in parallel, the reciprocal of the total resistance R_p is the sum of the reciprocals of individual resistances:
1/R_p = 1/R + 1/R + 1/R
1/R_p = 3/R
Therefore, R_p = R/3

Now we need to find the relationship between R_s and R_p.
We have R_s = 3R and R_p = R/3.
From R_p = R/3, we can express R in terms of R_p: R = 3R_p.
Substitute this expression for R into the equation for R_s:
R_s = 3 * (3R_p)
R_s = 9R_p

Therefore, the relationship between R_s and R_p is R_s = 9 R_p.
Correct Option: A

Ans: C

Explanation: Electrical conductivity in metals is primarily determined by the ease with which free electrons can move through the material. A higher number of free electrons (A) means more charge carriers, increasing conductivity. The drift velocity (B) of these electrons under an electric field directly relates to the current flow and thus conductivity. The frequency of collisions (D) between electrons and lattice ions impedes electron flow, reducing conductivity. The length of the metallic sample (C) affects the resistance (R = ρL/A), but not the intrinsic conductivity (ρ) of the material itself. Conductivity is a material property, independent of the sample’s dimensions.
Correct Option: C

Ans: D

Explanation: When resistors are connected in series, their resistances add up, resulting in a larger equivalent resistance. When resistors are connected in parallel, the reciprocal of the equivalent resistance is equal to the sum of the reciprocals of individual resistances. This means that connecting resistors in parallel always results in an equivalent resistance smaller than the smallest individual resistance. To obtain the smallest possible equivalent resistance, we want to maximize the effect of parallel connections. Connecting all three resistors in parallel will result in the smallest equivalent resistance among all the given options.
Correct Option: D