(a) (i) Due to change in length and area of cross-section of a conductor, resistance of conductor changes while resistivity does not change. Why? (ii) Conductors of electric toasters and electric iron are made of an alloy rather than a pure metal. Why? (iii) Define the S.I. unit of electric current. OR (b) (i) How many bulbs of resistance $8\Omega$ each should be connected in parallel combination to draw a current of $2\mathrm{A}$ from a battery of $4\mathrm{V}$ ? (ii) Name the device used for measuring electric current. How is it connected in a circuit? (iii) State Joule's law of heating.

Electricity

Resistivity, Alloys, Current; Parallel Resistors, Ammeter, Joule's Law

Official Answer

(a) (i) $R=\rho l/A$ depends on length and area, while $\rho$ is a material constant; (ii) alloys have higher resistivity and resist oxidation at high temperature; (iii) 1 ampere is the current when 1 coulomb of charge flows per second.

(b) (i) 4 bulbs; (ii) Ammeter, connected in series; (iii) Joule's law: $H = I^2Rt$ .

resistivityalloyampereammeterseriesparallelJoule's lawheating

Marking Scheme

1Part (i)/(ii)/(iii) [or their OR alternatives] each carry 1 mark for correct concept/definition/calculation with brief justification.

Hint

For (b)(i): equivalent resistance needed is $V/I$ ; then use $1/R_p = n/R$ for n identical parallel resistors.

Quick Oral Answer

Resistivity is a fixed material property while resistance depends on geometry; alloys resist oxidation and have higher resistivity, making them ideal heating elements; the ampere is defined as one coulomb of charge flowing per second.

Analysis & Explanation

This question offers an internal choice between two 3-mark sets, both drawn directly from Sections 11.4–11.7 of the chapter.

Option (a) tests conceptual understanding of resistivity versus resistance (Section 11.5, Eq. 11.10), the practical use of alloys in heating devices (Section 11.5, Table 11.2 discussion), and the SI unit of current (Section 11.1, Eq. 11.1).

(i) Resistance $R=\rho l/A$ is an extrinsic property — it changes if you cut, stretch or thicken the wire because $l$ and $A$ change. Resistivity $\rho$ is an intrinsic property of the material itself (and temperature), so reshaping the same material does not alter $\rho$ .

(ii) The chapter states alloys generally have higher resistivity than their constituent metals and do not oxidise (burn) readily at high temperatures, which is why they are used in heating elements such as toasters and electric irons.

(iii) From Eq. (11.1), $I=Q/t$ ; the ampere is defined via this relation.

Option (b) tests application of the parallel resistance formula (Section 11.6.2, Eq. 11.18), the ammeter's role and connection (Section 11.1), and Joule's law (Section 11.7, Eq. 11.21).

(i) Using $R_p = V/I$ for the whole combination and $1/R_p = n/R$ for $n$ identical resistors in parallel, the number of 8 Ω bulbs needed is found.

(ii) The chapter states the ammeter measures current and is always connected in series.

(iii) Joule's law of heating, Eq. (11.21): $H=I^2Rt$ , i.e., heat produced is proportional to the square of the current, to resistance, and to time.

Common Mistakes

1Confusing resistance and resistivity as the same quantity that changes with wire dimensions.
2Thinking alloys are used because they have lower resistivity (it is actually higher, which combined with oxidation-resistance makes them suitable for heating elements).
3Connecting the ammeter in parallel instead of series in circuit diagrams.
4For (b)(i), using the series formula $R_s = R_1+R_2+\dots$ instead of the parallel formula, or forgetting to first find the required equivalent resistance from $V/I$ .

Interesting Facts

Nichrome, an alloy of nickel, chromium, manganese and iron, has a resistivity of $100\times10^{-6}\ \Omega\,\text{m}$ , about 60 times that of copper, which is exactly why it is chosen for the heating elements of toasters and irons.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2026?

This question carries 3 marks in the CBSE Class 10 Science 2026 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Electricity" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Resistivity, Alloys, Current; Parallel Resistors, Ammeter, Joule's Law" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2026 paper?

This is a Short Answer question from Section C in the CBSE Class 10 Science 2026 paper.