Simply CBSE

Electricity — Class 10 Science

Quick revision notes for exam preparation.

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📌 Key Points

  • Electric current is the rate of flow of charge: I = Q/t, SI unit is Ampere (A)
  • Potential difference is work done per unit charge: V = W/Q, SI unit is Volt (V)
  • Ammeter is connected in SERIES; Voltmeter is connected in PARALLEL
  • Ohm’s law: V = IR (at constant temperature). V-I graph is a straight line through origin
  • Resistance R = V/I, SI unit is Ohm (Ω). R = ρl/A
  • Resistivity (ρ) depends only on material and temperature, SI unit is Ωm
  • Series: Rₛ = R₁ + R₂ + R₃. Current same, voltage divides
  • Parallel: 1/Rₚ = 1/R₁ + 1/R₂ + 1/R₃. Voltage same, current divides
  • Series total R > largest R. Parallel total R < smallest R
  • Household wiring is in parallel (same voltage, independent operation)
  • Heating effect: H = I²Rt (Joule’s law of heating)
  • Nichrome is used in heating elements (high resistivity, high melting point)
  • Tungsten is used for bulb filaments (melting point 3380°C)
  • Fuse wire has LOW melting point, connected in series with live wire
  • Electric power P = VI = I²R = V²/R. Commercial unit: kWh (1 kWh = 3.6 × 10⁶ J)

📘 Important Definitions

Electric Current
The rate of flow of electric charge through a conductor. I = Q/t. SI unit: Ampere (A).
Potential Difference
Work done to move a unit charge from one point to another. V = W/Q. SI unit: Volt (V).
Resistance
The property of a conductor that opposes the flow of electric current. R = V/I. SI unit: Ohm (Ω).
Resistivity
The resistance of a conductor of unit length and unit area of cross-section. ρ = RA/l. SI unit: Ωm.
Ohm’s Law
The potential difference across a conductor is directly proportional to the current, at constant temperature. V = IR.
Electric Power
The rate at which electrical energy is consumed or dissipated. P = VI. SI unit: Watt (W).
Kilowatt-hour (kWh)
The commercial unit of electrical energy. 1 kWh = energy consumed when 1 kW power is used for 1 hour = 3.6 × 10⁶ J.
Electric Fuse
A safety device made of a wire with low melting point that melts and breaks the circuit when excessive current flows.

🔢 Formulas & Laws

Electric Current

I = Q / t

Current = Charge / Time

Potential Difference

V = W / Q

Voltage = Work done / Charge

Ohm’s Law

V = IR

Also I = V/R and R = V/I

Resistance Formula

R = ρl / A

ρ = resistivity, l = length, A = area

Series Resistance

Rₛ = R₁ + R₂ + R₃

Total resistance = sum of all resistances

Parallel Resistance

1/Rₚ = 1/R₁ + 1/R₂ + 1/R₃

For two: Rₚ = R₁R₂/(R₁+R₂)

Joule’s Law of Heating

H = I²Rt

Also H = VIt = V²t/R

⚠️ Common Mistakes

✗ Wrong: Connecting ammeter in parallel or voltmeter in series

✓ Correct: Ammeter is ALWAYS in series (low resistance). Voltmeter is ALWAYS in parallel (high resistance).

✗ Wrong: Confusing conventional current direction with electron flow

✓ Correct: Conventional current: + to −. Electron flow: − to + (opposite direction).

✗ Wrong: Forgetting to take reciprocal in parallel resistance calculation

✓ Correct: If 1/Rₚ = 5/6, then Rₚ = 6/5 = 1.2 Ω (NOT 5/6 Ω).

✗ Wrong: Confusing kW (power) with kWh (energy)

✓ Correct: kW is a unit of power (rate of consumption). kWh is a unit of energy (total consumption). E = P × t.

✗ Wrong: Mixing up fuse wire and bulb filament properties

✓ Correct: Fuse wire = LOW melting point (melts easily). Bulb filament (tungsten) = HIGH melting point.

📝 Exam Focus

These questions are frequently asked in CBSE exams:

State and explain Ohm’s law with V-I graph
3m★★★
Derive Rₛ = R₁+R₂+R₃ for series combination
3m★★★
Derive 1/Rₚ = 1/R₁+1/R₂+1/R₃ for parallel combination
3m★★★
Factors affecting resistance of a conductor
3m★★★
Why is household wiring done in parallel?
2m★★★
Numericals on series-parallel combinations
5m★★★
Joule’s law of heating and its applications
3m★★
Electricity bill calculation (kWh)
3m★★

Diagram to practice: Circuit diagrams with ammeter/voltmeter placement, series circuit, parallel circuit, V-I graph for Ohm’s law, circuit symbols

🎯 Last-Minute Recall

Close your eyes and try to recall: Key definitions, formulas, and 3 common mistakes. If you can recall 80% without looking, you're exam-ready!