Q1. A bulb draws a current of 0.5 A when connected to a 220 V supply. Find its resistance and power.
Given: I = 0.5 A, V = 220 V
Resistance: R = V ÷ I = 220 ÷ 0.5 = 440 Ω
Power: P = V × I = 220 × 0.5 = 110 W
Electricity — Class 10
Stop memorising and start seeing it. Drag the sliders below to watch Ohm's law come alive, build your own series and parallel circuits, and solve every type of board numerical step by step.
Electric current (I)Rate of flow of charge, I = Q/t. Unit: ampere (A). Measured with an ammeter (in series).
Potential difference (V)Work done per unit charge, V = W/Q. Unit: volt (V). Measured with a voltmeter (in parallel).
Resistance (R)Opposition to current, R = V/I. Unit: ohm (Ω). Depends on length, area, material & temperature.
Ohm's lawAt constant temperature, V = I × R. A straight V–I graph through the origin means the conductor is ohmic.
1. Ohm's Law Lab Interactive
Change the battery voltage and the resistance. The current, power and bulb brightness update instantly — exactly as V = I R predicts.
Current0.30 AI = V ÷ R
Power1.8 WP = V × I
Medium voltage and resistance — the bulb glows steadily.
Try this: keep the voltage fixed and slide the resistance up — the current (and brightness) fall. That inverse relationship is the heart of Ohm's law.
2. Series vs Parallel Calculator Interactive
Enter three resistor values and switch between series and parallel. See the equivalent resistance, the working, and the total current from a 6 V battery.
Equivalent resistance60 Ω
Total current (at 6 V)0.10 A
Remember: series resistance is always bigger than the largest resistor; parallel resistance is always smaller than the smallest one. Home wiring is parallel so each appliance gets the full 220 V.
3. Board Numericals — Solved Step by Step
These are the exact patterns CBSE repeats. Read the question, try it yourself, then reveal the full solution.
Q2. Three resistors of 4 Ω, 6 Ω and 12 Ω are joined in parallel across a 6 V battery. Find the equivalent resistance and the current drawn from the battery.
Parallel: 1/R = 1/4 + 1/6 + 1/12 = 3/12 + 2/12 + 1/12 = 6/12
So R = 12 ÷ 6 = 2 Ω
Current: I = V ÷ R = 6 ÷ 2 = 3 A
Q3. An electric heater of 1000 W runs for 2 hours daily. Find the energy used in a 30-day month and the cost at ₹6 per unit.
Energy per day: 1000 W × 2 h = 2000 Wh = 2 kWh (units)
Per month: 2 × 30 = 60 units
Cost: 60 × ₹6 = ₹360
4. Every formula you need (one place)
| Quantity | Formula | SI unit |
|---|---|---|
| Electric current | I = Q / t | ampere (A) |
| Potential difference | V = W / Q | volt (V) |
| Ohm's law | V = I R | — |
| Resistance of a wire | R = ρ L / A | ohm (Ω) |
| Resistors in series | R = R₁ + R₂ + R₃ | ohm (Ω) |
| Resistors in parallel | 1/R = 1/R₁ + 1/R₂ + 1/R₃ | ohm (Ω) |
| Electric power | P = VI = I²R = V²/R | watt (W) |
| Heat produced (Joule) | H = I²Rt | joule (J) |
| Electrical energy | E = P × t | kWh / joule |
5. Common mistakes & quick exam wins
- Ammeter in series, voltmeter in parallel. Swapping them is the most common 1-mark slip. (An ideal ammeter has zero resistance, an ideal voltmeter infinite resistance.)
- Parallel resistance is smaller, not larger. If you add resistors in parallel and get a bigger number, you've made an error.
- 1 unit of electricity = 1 kWh = 3.6 × 10⁶ J. Numericals on cost almost always use this.
- Heating ∝ I²R. That's why the thin filament/heating element gets hot, not the thick connecting wires.
- Resistance of a wire doubles if you double its length and halves if you double its area of cross-section.
Frequently asked questions
What is Ohm's law (Class 10)?
At constant temperature, the current through a conductor is directly proportional to the potential difference across it: V = I R. On a V–I graph this gives a straight line through the origin.
Why are home appliances connected in parallel?
So every appliance gets the same full voltage (220 V), each can be switched on/off independently, and if one fails the others keep working. A series connection would dim everything and one failure would break the whole circuit.
What is the difference between resistance and resistivity?
Resistance (R, in Ω) depends on the size and shape of the specific wire. Resistivity (ρ, in Ω·m) is a property of the material only and does not change with length or area.
How is 1 unit of electricity defined?
One unit = one kilowatt-hour (kWh) = the energy used by a 1000 W appliance running for 1 hour = 3.6 × 10⁶ joules.