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Science & TechnologyNCERT Class 10 · Science (Class 10)

Electricity

This chapter explains how electric charge flows through a circuit and what governs that flow — current, potential difference, resistance, and the V = IR relationship known as Ohm's law.

⏱ 7 min readGS-III6 sections5 memory tricks
Why this matters for UPSC

Electricity is foundational Science & Technology that UPSC tests for conceptual clarity, not derivations — typically by matching a physical quantity with its SI unit and the scientist it honours (current–ampere–Ampère, PD–volt–Volta, resistance–ohm–Ohm), or how the two meters are connected. For GS-III it underpins energy and infrastructure themes such as power transmission, conductor choice (copper/aluminium), and transmission/heating losses captured by R = ρl/A. A clear grasp here also de-mystifies grid efficiency and electrical-appliance technology questions.

Understand the chapter

Electric Current and the Circuit

Electric current is the rate of flow of electric charge through a conductor; if a net charge Q crosses a cross-section in time t, then I = Q/t. A continuous, closed conducting path is an electric circuit, and breaking it anywhere (opening a switch) stops the current so the bulb stops glowing. Because electrons were not known when electricity was first studied, conventional current is defined as the flow of positive charge — taken opposite to the actual electron drift.

  • I = Q/t; SI unit of current = ampere (A), with 1 A = 1 coulomb/second.
  • SI unit of charge = coulomb (C); 1 C ≈ 6 × 10^18 electrons; electron charge = 1.6 × 10^-19 C.
  • Conventional current direction is opposite to the direction of electron flow.
  • Sub-units: 1 mA = 10^-3 A, 1 μA = 10^-6 A.

Potential Difference — the Driving Force

Charges flow only when there is a difference of 'electric pressure' — the potential difference — across the conductor, just as water flows when there is a pressure or height difference between the ends of a tube. A cell or battery maintains this potential difference using chemical energy stored in it, and must keep expending that energy to sustain the current. Potential difference is defined as the work done to move a unit charge between two points: V = W/Q.

  • SI unit = volt (V); 1 V = 1 joule/coulomb (1 J C^-1).
  • A cell's chemical action creates PD across its terminals even when no current is drawn.
  • A battery is one or more cells combined to provide the required potential difference.

Ohm's Law and Resistance

Georg Simon Ohm (1827) established that, at constant temperature, the potential difference across a metallic conductor is directly proportional to the current through it: V ∝ I, hence V = IR. The constant R is the resistance — the property of a conductor to oppose the flow of charge — with SI unit ohm (Ω). For a fixed voltage the current is inversely proportional to resistance, so doubling R halves the current.

  • V = IR; R = V/I; 1 Ω = 1 volt/ampere.
  • Resistance arises because moving electrons are retarded by the attraction of the conductor's atoms.
  • Rheostat = a variable resistance used to regulate current without changing the source voltage.
  • Low resistance = good conductor; appreciable resistance = resistor; very high = insulator.

Factors on Which Resistance Depends

Experiment shows resistance is directly proportional to length (l) and inversely proportional to area of cross-section (A), and also depends on the nature of the material: R = ρl/A. Here ρ (rho) is the electrical resistivity, a characteristic property of the material with SI unit ohm-metre (Ω m). Both resistance and resistivity vary with temperature.

  • R ∝ l (longer wire → more resistance); R ∝ 1/A (thicker wire → less resistance).
  • Resistivity ρ is independent of the conductor's dimensions — it is a material property.
  • Doubling the length doubles R (halves current); a thicker wire lowers R (raises current).

Conductors, Alloys and Their Uses

Metals and alloys have very low resistivity (10^-8 to 10^-6 Ω m) and are good conductors, whereas insulators like glass and rubber have resistivity of the order of 10^12 to 10^17 Ω m. Crucially, an alloy generally has higher resistivity than its constituent metals and does not oxidise (burn) readily at high temperatures, which is exactly why alloys are used in heating appliances. Different materials are therefore chosen for different jobs based on these properties.

  • Nichrome (alloy of Ni, Cr, Mn, Fe) and similar alloys → electric iron, toaster, heaters.
  • Tungsten → filaments of electric bulbs (almost exclusively, for its high melting point).
  • Copper and aluminium → electrical transmission lines.
  • Silver has the lowest resistivity (1.60 × 10^-8 Ω m); copper is next (1.62 × 10^-8 Ω m).

Measuring Instruments and Circuit Symbols

Current and potential difference are measured by two different instruments connected in two different ways. An ammeter measures current and is always connected in series in the circuit, while a voltmeter measures potential difference and is always connected in parallel across the two points. Circuit diagrams use standard conventional symbols for cells, batteries, open/closed plug keys, resistors, rheostats and meters.

  • Ammeter → connected in series (ideally very low resistance).
  • Voltmeter → connected in parallel (ideally very high resistance).
  • Standard symbols exist for cell, battery, plug key (open/closed), resistor, rheostat and bulb.

Key terms

Electric current
Rate of flow of electric charge through a conductor; I = Q/t, measured in ampere.
Electric circuit
A continuous, closed conducting path through which an electric current flows.
Coulomb (C)
SI unit of electric charge; equal to the charge of about 6 × 10^18 electrons.
Ampere (A)
SI unit of electric current; 1 A = one coulomb of charge flowing per second.
Potential difference
Work done to move a unit charge between two points; V = W/Q, measured in volt.
Volt (V)
SI unit of potential difference; 1 V = 1 joule per coulomb (1 J C^-1).
Ohm's law
At constant temperature the PD across a conductor is directly proportional to the current: V = IR.
Resistance (R)
Property of a conductor opposing the flow of charge; SI unit ohm (Ω), R = V/I.
Resistivity (ρ)
Characteristic material property linking resistance to dimensions: R = ρl/A; SI unit ohm-metre (Ω m).
Rheostat
A variable resistance used to change the current in a circuit without altering the source voltage.

Must-know facts exam-ready

  • Electric current I = Q/t — it is the rate of flow of electric charge.
  • SI unit of charge is the coulomb (C); 1 C ≈ 6 × 10^18 electrons; one electron carries 1.6 × 10^-19 C.
  • Ampere (A), the unit of current, is named after French scientist André-Marie Ampère (1775–1836); 1 A = 1 C/s, and it is an SI base unit.
  • Volt (V), the unit of potential difference, honours Italian physicist Alessandro Volta (1745–1827); 1 V = 1 J/C.
  • Ohm's law (V = IR) was given by German physicist Georg Simon Ohm in 1827 and holds at constant temperature.
  • Ohm (Ω) is the SI unit of resistance; 1 Ω = 1 V/A, and R = V/I.
  • Conventional current flows opposite to the direction of electron (negative charge) flow.
  • Ammeter is connected in series; voltmeter is connected in parallel.
  • R = ρl/A: resistance ∝ length and ∝ 1/area; resistivity ρ (unit Ω m) is a material property.
  • Conductors (metals/alloys) have resistivity 10^-8–10^-6 Ω m; insulators 10^12–10^17 Ω m.
  • Silver has the lowest resistivity; copper/aluminium are used for transmission lines, tungsten for bulb filaments, and nichrome (Ni-Cr-Mn-Fe) plus other alloys for heating devices because they resist oxidation at high temperature.

Memory tricks remember it for good

VIR
V = I × R, i.e. Volt = Ampere × Ohm.
💡 Recalls Ohm's law and instantly pairs each quantity with its SI unit.
A-V-O = Ampère, Volta, Ohm
Ampère → current (ampere); Volta → potential difference (volt); Ohm → resistance (ohm).
💡 Matches each scientist to the quantity and the SI unit named after them.
'A.S.V.P.' — Ammeter Series, Voltmeter Parallel
Ammeter is connected in Series; Voltmeter is connected in Parallel.
💡 Fixes how the two meters are wired into a circuit.
RAIL
R goes up with Area-Inverse and Length — R ∝ l and R ∝ 1/A (with ρ = material).
💡 Recalls R = ρl/A and what raises or lowers resistance.
'CAT lines, TUNG lights, NICHrome heats'
Copper/Aluminium → Transmission lines; Tungsten → bulb filaments; Nichrome/alloys → heaters.
💡 Recalls which material is chosen for which application.

Traps to avoid

  • Conventional current is OPPOSITE to electron flow, not in the same direction.
  • Ammeter goes in series and voltmeter in parallel — aspirants routinely swap the two.
  • Ohm's law holds only at constant temperature; not every conductor is ohmic.
  • Resistance vs resistivity: resistance depends on dimensions (l, A), while resistivity (ρ) is a dimension-independent material property.
  • High resistivity is not 'bad' — nichrome's high resistivity plus resistance to oxidation is precisely why it is used in heaters; tungsten is chosen for filaments for its high melting point, not the lowest resistivity.
  • Silver has the lowest resistivity, yet copper/aluminium are used for transmission lines (cost/abundance) — the best conductor is not always the one deployed.

Exam focus

🧠 Prelims angles

  • Match quantity–unit–scientist: current/ampere/Ampère, PD/volt/Volta, resistance/ohm/Ohm.
  • Unit definitions and relations: 1 A = 1 C/s, 1 V = 1 J/C, 1 Ω = 1 V/A.
  • Instrument connection: ammeter (series) versus voltmeter (parallel).
  • Material applications: tungsten (filament), copper/aluminium (transmission), nichrome/alloys (heating).
  • Conductors vs insulators by order-of-magnitude resistivity (10^-8–10^-6 vs 10^12–10^17 Ω m).
  • Factors affecting resistance (length, area, material, temperature) and the formula R = ρl/A.

✍️ Mains angles GS-III

  • How do basic electrical properties of conductors and alloys translate into India's power and appliance technologies?Link low-resistivity copper/aluminium to transmission, high-resistivity non-oxidising alloys to heating devices, and tungsten to lighting.
  • Why are transmission losses a core efficiency concern for India's grid, and how does conductor science address them?Use R = ρl/A and the heating effect of current to justify high-voltage transmission and careful conductor selection.
  • Role of material science in energy infrastructure and efficiency.Connect resistivity, alloys and conductor choice to reliable, low-loss power delivery and appliance design.
Practice Science & Technology questions from this syllabus →

Last-minute revision tick as you recall

  • I = Q/t; current = rate of flow of charge; SI charge = coulomb (~6 × 10^18 electrons).
  • Ampere (Ampère, French): 1 A = 1 C/s; ammeter in series.
  • Volt (Volta, Italian): V = W/Q, 1 V = 1 J/C; voltmeter in parallel.
  • Ohm's law (Georg Ohm, 1827): V = IR at constant temperature; 1 Ω = 1 V/A.
  • R = ρl/A; resistivity unit Ω m; R rises with length, falls with thickness.
  • Conventional current flows opposite to electron flow.
  • Metals/alloys: 10^-8–10^-6 Ω m; insulators: 10^12–10^17 Ω m.
  • Copper/aluminium → transmission; tungsten → bulb filament; nichrome/alloys → heaters.
  • Silver = lowest resistivity; alloys have higher resistivity than constituent metals and resist oxidation.

Distilled from NCERT Class 10 · Science (Class 10) for UPSC. Always cross-check facts with the original NCERT.