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Class 10 Electricity notes: NCERT Science Chapter 11 revision

CMDLearn · 3 November 2026

These are short revision notes for Chapter 11 of the NCERT Class 10 Science textbook (Reprint 2026-27), pages 171 to

1. Electricity is the chapter with the most formulas in Class 10 Science, so these notes put every formula next to

its unit and its meaning, then give the key words, the circuit diagram to practise, the mistakes students make most often and 8 quick questions of our own with worked answers.

Keep the textbook open next to these notes. The wording here is our own; the textbook is the authority.

How much it counts

CBSE's Class 10 Science curriculum for 2026-27 puts Electricity in Unit IV, Effects of Current, worth 13 of the 80 theory marks, shared with Magnetic Effects of Electric Current (curriculum, PDF page 4). For this chapter the unit lists current and potential difference, Ohm's law, resistance and resistivity and what they depend on, series and parallel resistors and their uses, the heating effect and its uses, and electric power with the links between P, V, I and R (PDF page 6). More on the units: CBSE Class 10 Science syllabus 2026-27.

The formulas on one page

1. Electric current and circuits (section 11.1, pages 171-172)

• An electric current is a flow of charge through a conductor; in metal wires the moving charges are electrons.
• A circuit is a continuous, closed path for the current. A switch completes or breaks the path.
• Current is the rate of flow of charge: I = Q/t.
• The coulomb (C) is the unit of charge; it is the charge of nearly 6 × 10¹⁸ electrons. One electron carries 1.6 × 10⁻¹⁹ C of negative charge.
• 1 ampere = 1 coulomb flowing per second. Smaller units: 1 mA = 10⁻³ A, 1 µA = 10⁻⁶ A.
• By convention, current flows from the positive terminal to the negative, which is opposite to the direction in which electrons move. The rule was fixed before electrons were known.
• An ammeter measures current and is always joined in series.

2. Potential difference (section 11.2, pages 173-174)

• Charges move along a wire only when there is a potential difference across it, as water flows in a pipe only when there is a pressure difference.
• A cell or battery makes the potential difference by chemical action, and uses up its stored chemical energy to keep a current going.
• Potential difference = work done ÷ charge moved: V = W/Q.
• 1 volt = 1 joule of work to move 1 coulomb between two points.
• A voltmeter measures potential difference and is always joined in parallel across the two points.

3. Circuit symbols (section 11.3, pages 174-175)

Table 11.1 gives the symbols you must draw: an electric cell, a battery (several cells), a plug key or switch (open and closed), a wire joint, wires crossing without joining, an electric bulb, a resistor, a variable resistance (rheostat), an ammeter and a voltmeter. In the cell symbol, the longer line is the positive terminal.

4. Ohm's law and resistance (section 11.4, pages 175-177)

• Ohm's law: for a metal wire, the potential difference and the current rise together in direct proportion, provided its temperature stays the same. V ∝ I, so V = IR.
• A V-I graph for such a wire is a straight line through the origin (Fig. 11.3).
• Resistance (R) is a conductor's property of opposing the flow of charge. 1 ohm: 1 A flows when 1 V is applied.
• For a fixed voltage, current is inversely proportional to resistance: double R and the current halves.
• A rheostat (variable resistance) changes the current without changing the source.
• Why resistance exists: electrons are held back by the attraction of the atoms they move among.
• Same size, different behaviour: a good conductor has low resistance, a resistor has appreciable resistance, and an insulator has much higher resistance still.

5. What resistance depends on (section 11.5, pages 177-180)

• Resistance depends on the wire's length, its area of cross-section and its material.
• R ∝ l and R ∝ 1/A, so R = ρl/A, where ρ (rho) is the resistivity, a property of the material (unit Ω m).
• Doubling the length doubles R; a thicker wire of the same length has a lower R.
• Both resistance and resistivity change with temperature.
• Typical resistivity (Table 11.2): metals and alloys about 10⁻⁸ to 10⁻⁶ Ω m (good conductors); insulators such as rubber and glass about 10¹² to 10¹⁷ Ω m. You need not learn the table's values; it is there for sums.
• Alloys have a higher resistivity than the metals they are made of and do not oxidise easily when hot, so they are used in heaters, irons and toasters.
• Tungsten is used for bulb filaments; copper and aluminium for transmission lines.

6. Resistors in series and in parallel (section 11.6, pages 181-187)

Why homes are not wired in series (page 187):

• devices such as a bulb and a heater need very different currents, but a series circuit forces one current on all;
• when one part fails, everything stops (the book's example is a string of festival lights, where each bulb has to be tested to find the dead one).

A parallel circuit lowers the total resistance and lets each device draw its own current.

7. Heating effect of current (section 11.7, pages 188-190)

• Part of the energy a source supplies does useful work (turning a fan); the rest warms the device. In a circuit of resistors only, all of it becomes heat.
• Heat produced: H = VIt, and with V = IR, H = I²Rt (Joule's law of heating). The heat is proportional to: - the square of the current (for a given resistance); - the resistance (for a given current); - the time the current flows.
• Useful applications: electric iron, toaster, oven, kettle and heater.
• Electric bulb: the filament must get very hot without melting, so it is made of tungsten (melting point 3380°C), held on insulating supports, and the bulb is filled with nitrogen and argon to make the filament last. Most of the power still comes out as heat; only a small part is light.
• Fuse: a wire of suitable melting point joined in series with the device. Too large a current heats it until it melts and breaks the circuit. Fuses for homes come in ratings such as 1, 2, 3, 5 and 10 ampere.
• Unwanted heating wastes energy and can change how circuit parts behave.

8. Electric power and energy (section 11.8, pages 191-192)

• Power is the rate at which electrical energy is used: P = VI = I²R = V²/R.
• 1 watt = the power of a device carrying 1 A at 1 V. 1 kilowatt = 1000 W.
• Energy = power × time. The commercial unit is the kilowatt hour (kW h), the "unit" on an electricity bill: 1 kW h = 3.6 × 10⁶ J.
• The bill pays for energy, not for electrons: electrons are not used up in a circuit (the "More to Know" box, page 191).

Key words at a glance

• Electric current: rate of flow of charge (A).
• Potential difference: work done per unit charge between two points (V).
• Resistance: opposition to the flow of charge (Ω).
• Resistivity: resistance property of a material, independent of the wire's size (Ω m).
• Ohmic conductor: one whose V-I graph is a straight line at constant temperature.
• Rheostat: a variable resistance.
• Joule's law of heating: H = I²Rt.
• Fuse: a safety wire in series that melts if the current is too large.
• Kilowatt hour: the "unit" of electrical energy, 3.6 × 10⁶ J.

The diagram to know: a circuit for Ohm's law

Board papers often ask you to draw a circuit with meters in the right places. Practise this one (it is the set-up of Fig. 11.2):

1. Draw a battery (two or more cell symbols joined), a plug key, a resistor and an ammeter in one closed loop.
2. Put the ammeter in series, in the loop itself.
3. Draw the voltmeter in parallel, on a separate pair of wires joined across the two ends of the resistor only.
4. Mark the ammeter and voltmeter terminals + and −, with the + side towards the battery's positive terminal.
5. Add an arrow for the conventional current, from the battery's + terminal round to its − terminal.

Mistakes that cost marks

• Putting the ammeter in parallel or the voltmeter in series. Ammeter: series. Voltmeter: parallel.
• Leaving out "at constant temperature" from Ohm's law. It is part of the law.
• Thinking resistivity changes with length. Resistance does; resistivity is a property of the material.
• Forgetting the last step in parallel sums. 1/Rp is not Rp: turn the fraction over.
• Using I²R when the voltage is fixed. For mains appliances, V²/R or VI is usually quicker and safer.
• Mixing up kW h and kW. kW is power; kW h is energy. Multiply power by time in hours.
• Saying electrons get used up. Energy is used; the electrons keep flowing round.

Quick check: 8 questions of our own

These are our own questions, written for these notes and checked against the textbook pages named; no practising teacher has reviewed them yet. Every number was re-computed by a script.

Question 1. Arjun sets up a cell, a resistor, an ammeter and a voltmeter. He joins the voltmeter in the main loop with the resistor, and the ammeter across the two ends of the resistor. (a) What two mistakes has he made? (b) Which way does the conventional current flow through the resistor compared with the electrons?

Answer: (a) the ammeter should be in series in the loop, and the voltmeter in parallel across the resistor; (b) opposite to the electrons. An ammeter is always connected in series with the part whose current it measures (page 172), and a voltmeter always goes in parallel, across the two points it compares (page 173). Conventional current runs from the positive terminal to the negative, which is opposite to the direction of electron flow (page 171).

Question 2. A phone charger cable carries a steady current of 1.5 A for 20 minutes. (a) How much charge passes through it? (b) About how many electrons is that?

Answer: (a) 1,800 C; (b) about 1.1 × 10²² electrons. (a) Q = It = 1.5 A × 1,200 s = 1,800 C (page 172). (b) One coulomb is the charge of nearly 6 × 10¹⁸ electrons, so 1,800 C is about 1,800 × 6 × 10¹⁸ = 1.08 × 10²² electrons. Using one electron's charge, 1.6 × 10⁻¹⁹ C, gives 1,800 ÷ (1.6 × 10⁻¹⁹) = 1.125 × 10²², which rounds to the same 1.1 × 10²² (page 172).

Question 3. Neha measures a small coil of wire: at 1 V the current is 0.1 A, at 2 V it is 0.2 A, at 4 V it is 0.4 A, but at 8 V it is only 0.6 A, and the coil is now noticeably warm. (a) Find V/I for each reading. (b) Does the coil follow Ohm's law over the whole range? (c) Give the most likely reason for the last reading.

Answer: (a) 10, 10, 10 and about 13.3 Ω; (b) only for the first three readings; (c) the coil heated up and its resistance rose. (a) 1/0.1 = 2/0.2 = 4/0.4 = 10 Ω, but 8/0.6 ≈ 13.3 Ω. (b) Ohm's law says V/I stays constant only while the temperature stays the same (page 176). (c) Resistance changes with temperature (page 179), and the heating effect (page 188) warmed the coil at the larger current.

Question 4. Two wires are made of the same metal. Wire P is 3 m long with a diameter of 0.4 mm. Wire Q is 1.5 m long with a diameter of 0.8 mm. How many times the resistance of Q is the resistance of P?

Answer: 8 times. R = ρl/A (page 178), and the cross-section area A depends on the square of the diameter. P is twice as long (3 ÷ 1.5 = 2), which doubles its resistance. Q's diameter is twice P's, so Q's area is 2² = 4 times P's, which makes P's resistance 4 times larger again. Together: 2 × 4 = 8.

Question 5. A kitchen has an electric kettle rated 1,500 W and a mixer rated 500 W, both for a 220 V supply. Home fuses come in ratings of 1, 2, 3, 5 and 10 ampere. (a) What current does each draw? (b) Which fuse suits each one? (c) How is a fuse joined to the device, and what does it do?

Answer: (a) kettle about 6.8 A, mixer about 2.3 A; (b) 10 A for the kettle, 3 A for the mixer; (c) in series; it melts and breaks the circuit if the current is too high. (a) I = P/V (page 188): 1,500 ÷ 220 ≈ 6.82 A and 500 ÷ 220 ≈ 2.27 A. (b) Pick the smallest listed rating above the working current: 10 A (5 A would melt in normal use) and 3 A. (c) The fuse sits in series with the appliance it protects; a current above its rating heats the fuse wire until it melts (page 190).

Question 6. A heater coil makes 6,000 J of heat in a certain time. Using Joule's law, how much heat would it make if (a) the current through it were doubled, the time staying the same; (b) it ran for three times as long at the same current; (c) a coil of twice the resistance carried the same current for the same time?

Answer: (a) 24,000 J; (b) 18,000 J; (c) 12,000 J. H = I²Rt (page 189): heat goes with the square of the current, so doubling I multiplies H by 4; it is directly proportional to time, so ×3; and directly proportional to resistance at a given current, so ×2.

Question 7. Riya says: "The electricity bill charges us for the electrons our lamps use up." Her family's 5 W LED night lamp runs for 10 hours a night. (a) What is wrong with her statement? (b) How many units of energy does the lamp use in 30 nights? (c) How many joules is that?

Answer: (a) electrons are not used up; the bill pays for energy; (b) 1.5 units (kW h); (c) 5.4 × 10⁶ J. (a) The book's "More to Know" box says electrons are not consumed in a circuit; we pay for the energy that moves them (page 191). (b) 5 W = 0.005 kW, and 0.005 kW × 10 h × 30 = 1.5 kW h. (c) 1 kW h = 3.6 × 10⁶ J (page 191), so 1.5 × 3.6 × 10⁶ = 5.4 × 10⁶ J.

Question 8. A car's 12 V battery feeds two lamps joined in parallel: a headlamp of 3 Ω and a dashboard lamp of 24 Ω. (a) Find the current through each lamp. (b) Find the total current taken from the battery. (c) Find the equivalent resistance, and check it with Ohm's law. (d) If the dashboard lamp fails, does the headlamp go out?

Answer: (a) 4 A and 0.5 A; (b) 4.5 A; (c) about 2.67 Ω; (d) no. (a) In parallel each lamp has the full 12 V across it (page 185): 12 ÷ 3 = 4 A and 12 ÷ 24 = 0.5 A. (b) The branch currents add (page 186): 4 + 0.5 = 4.5 A. (c) 1/Rp = 1/3 + 1/24 = 8/24 + 1/24 = 9/24, so Rp = 24/9 ≈ 2.67 Ω (page 186); check: 12 V ÷ 4.5 A ≈ 2.67 Ω. It is less than the smaller resistance, as a parallel combination always is. (d) No: a parallel circuit keeps a separate path for each branch, while in series a single failure breaks the whole circuit (page 187).

Revise with questions

Test yourself next: Class 10 Electricity, 8 practice questions with answers. For worked examples of every section, see NCERT Class 10 Science Chapter 11, Electricity, explained. The other notes in this set: Life Processes, Metals and Non-metals and Heredity. For timed practice, the free Class 10 revision papers include a Science paper laid out in 1, 2, 3 and 5-mark sections.

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Sources

All checked on 3 November 2026. NCERT page numbers are the printed page numbers of the textbook.

• NCERT, Science, Textbook for Class X, Chapter 11 "Electricity" (Reprint 2026-27), pages 171-194: sections 11.1 and 11.2 (pages 171-174); Table 11.1 (pages 174-175); 11.4 and Fig. 11.3 (pages 175-177); 11.5 and Table 11.2 (pages 177-180); 11.6 (pages 181-187); 11.7 and 11.7.1 (pages 188-190); 11.8, "More to Know" and "What you have learnt" (pages 191-192). ncert.nic.in/textbook/pdf/jesc111.pdf
• CBSE, Science curriculum, Class X, 2026-27: unit marks (PDF page 4) and Unit IV contents (PDF page 6): cbseacademic.nic.in/web_material/CurriculumMain27/SecPart1/Science_SecP1_2026-27.pdf

The facts, formulas, section numbers and figure numbers are from the documents above. The wording of these notes, the diagram steps, the list of mistakes and the 8 questions are our own.

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