Electricity: Class 10 short notes and formulas
Electricity is formula-heavy but small. Almost every board question uses one of six relations, and the series-versus-parallel comparison comes up every year. Here they are on one page.
Short notes
- Electric current is the rate of flow of charge: I = Q/t. Its unit is the ampere (A); 1 A = 1 coulomb per second.
- One coulomb is the charge of about 6 × 10¹⁸ electrons.
- Potential difference is the work done to move a unit charge: V = W/Q. Its unit is the volt; 1 V = 1 J/C.
- An ammeter is connected in series; a voltmeter in parallel across the part being measured.
- Ohm's law: at constant temperature, V = IR. A graph of V against I is a straight line.
- Resistance depends on the conductor: R = ρl/A. It increases with length, decreases with cross-sectional area, and depends on the material through its resistivity ρ (unit: ohm metre).
- Alloys such as nichrome have high resistivity and don't oxidise easily when hot, so they're used in heaters and irons. Copper and aluminium are used for wires.
- In series, the same current flows through each resistor: R = R₁ + R₂ + R₃.
- In parallel, each resistor has the same voltage: 1/R = 1/R₁ + 1/R₂ + 1/R₃. Household appliances are connected in parallel so each gets full voltage and can be switched on its own.
- Heating effect (Joule's law): H = I²Rt. It's used in heaters, irons, bulbs and fuses.
- A bulb's filament is made of tungsten because of its very high melting point (3380 °C).
- Power P = VI = I²R = V²/R; unit watt (W). Electrical energy sold at home is measured in kilowatt-hours: 1 kWh = 3.6 × 10⁶ J, one "unit" on your bill.
At a glance
| Series | Parallel | |
|---|---|---|
| Current | Same through each | Divides between branches |
| Voltage | Divides between resistors | Same across each |
| Total resistance | R = R₁ + R₂ + … | 1/R = 1/R₁ + 1/R₂ + … |
| Total vs. smallest | More than the largest | Less than the smallest |
| One part fails | Whole circuit stops | Others keep working |
Memory tricks
VIR triangle
Write V on top, I and R below. Cover the one you want: V = IR, I = V/R, R = V/I.
PIV and its cousins
P = VI. Put V = IR in it to get I²R; put I = V/R to get V²/R. You only need to remember P = VI.
Parallel is always less
Total resistance in parallel is always less than the smallest resistor. Use it to check your answer.
Practice questions
Two 6 Ω resistors are connected in parallel. The total resistance is:
Show answer
3 Ω. 1/R = 1/6 + 1/6 = 1/3, so R = 3 Ω.
If the length of a wire is doubled (same material and thickness), its resistance:
Show answer
Doubles. R = ρl/A, so R is directly proportional to length.
1 kWh is equal to:
Show answer
3.6 × 10⁶ J. 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J.
A 100 W bulb runs on 220 V. The current through it is about:
Show answer
0.45 A. I = P/V = 100/220 ≈ 0.45 A.
Questions
Which numericals come most often from Electricity?
Combinations of resistors in series and parallel, R = ρl/A, heat produced (H = I²Rt), and power and energy cost in kWh.
Why are household circuits connected in parallel?
Each appliance gets the full mains voltage, draws only the current it needs, and can be switched on or off without affecting the others.
Can TellBrief solve doubts from my own physics notes?
Yes. Add your chapter PDF or notes, choose CBSE once, and ask any doubt in Hindi or English. Answers come from your material, with the page.