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Class 10 Electricity: cheat sheet
CBSE Science. Current, potential difference and resistance; Ohm's law; resistors in series and parallel; electric power and the heating effect.
Key ideas
- Electric current is the rate of flow of charge, measured in amperes with an ammeter connected in series.
- Potential difference is the work done per unit charge, measured in volts with a voltmeter connected in parallel.
- Ohm's law: at a constant temperature, the current through a conductor is proportional to the potential difference across it.
- Resistance increases with length, decreases with thickness, and depends on the material (resistivity).
- In series the current is the same through each resistor; in parallel the potential difference is the same across each.
- Resistors in parallel give a total resistance smaller than the smallest one.
- Current produces heat in a resistor (used in heaters, irons and fuses).
Remember
| Current | I = Q ÷ t |
| Ohm's law | V = IR |
| Resistance of a wire | R = ρL ÷ A |
| Series / parallel | R = R₁ + R₂ + R₃; 1/R = 1/R₁ + 1/R₂ + 1/R₃ |
| Power | P = VI = I²R = V² ÷ R |
| Heating | H = I²Rt; 1 kWh = 3.6 × 10⁶ J |
Watch out
- Connecting the voltmeter in series (it goes in parallel).
- Adding resistances in parallel as if they were in series.
- Confusing kW (power) with kWh (energy).
Worked example
Two bulbs are rated 40 W and 200 W, both at 220 V. Find their resistances. Which has the higher resistance? (Give the answer to 2 decimal places where needed.)
Working: R = V²/P: 220² ÷ 40 and 220² ÷ 200; the lower-power bulb has more resistance
Answer: 1,210 Ω and 242 Ω; the 40 W bulb
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