Electricity — Class 10 Science

"Electricity: the energy that powers your phone, lights your home, runs every modern device."

1. About the Chapter

This is a HEAVY-MARK chapter on electrical circuits. Covers:

• Electric current and charge
• Potential difference (voltage)
• Ohm's Law
• Resistance and resistivity
• Series and parallel circuits
• Electric power and energy
• Heating effect

Why Important

• Foundation for all electrical engineering
• Used in every device (phone to factories)
• High-mark chapter (~10-12 marks)

2. Electric Current

Definition

Flow of ELECTRIC CHARGE (specifically, electrons) through a conductor.

Direction

• ELECTRONS flow from negative (-) to positive (+) terminal
• CONVENTIONAL CURRENT direction: positive (+) to negative (-) — OPPOSITE to electron flow
• (Historical convention from before electron was discovered.)

SI Unit

AMPERE (A) — named after André-Marie Ampère

1 Ampere = 1 Coulomb / 1 Second

Formula

I = Q / t

where:

• I = current (in amperes)
• Q = charge (in coulombs)
• t = time (in seconds)

Electron Charge

1 electron = 1.6 × 10⁻¹⁹ Coulomb 1 Coulomb ≈ 6.25 × 10¹⁸ electrons

3. Potential Difference (Voltage)

Definition

Work done per unit charge in moving a charge from one point to another.

SI Unit

VOLT (V) — named after Alessandro Volta

1 Volt = 1 Joule / 1 Coulomb

Formula

V = W / Q

where:

• V = potential difference (volts)
• W = work done (joules)
• Q = charge (coulombs)

Source

A BATTERY (or cell) maintains potential difference, driving current.

4. Ohm's Law

Statement

For a conductor at constant temperature, CURRENT is DIRECTLY PROPORTIONAL to VOLTAGE.

V ∝ I

Or: V = IR

where R = RESISTANCE (constant for a given conductor).

SI Unit of Resistance

OHM (Ω) — named after Georg Simon Ohm

1 Ohm = 1 Volt / 1 Ampere

Graphical Form

A V-I graph is a STRAIGHT LINE passing through origin.

• Slope = R (resistance)
• Steeper slope = higher resistance

What is Resistance?

The property of a conductor that OPPOSES the flow of electric current.

Factors Affecting Resistance

R = ρL/A

where:

• ρ (rho) = resistivity (depends on material)
• L = length of conductor
• A = cross-sectional area

Resistance:

• Increases with length (R ∝ L)
• Decreases with cross-section (R ∝ 1/A)
• Depends on material (ρ varies)
• Temperature affects (most: R increases with T)

Resistivity (ρ)

• Property of the material
• Unit: Ω·m
• Lower for good conductors (copper, silver)
• Higher for poor conductors (nichrome)
• Very high for insulators

Examples (Common Materials at 20°C)

• Silver: 1.6 × 10⁻⁸ Ω·m (best conductor)
• Copper: 1.7 × 10⁻⁸ Ω·m
• Aluminium: 2.7 × 10⁻⁸ Ω·m
• Nichrome: 1.1 × 10⁻⁶ Ω·m (high — used in heaters)
• Rubber: 10¹³-10¹⁶ Ω·m (insulator)

5. Series and Parallel Circuits

Series Connection

Components connected ONE AFTER ANOTHER. Single path for current.

Current: SAME through all components Voltage: DIVIDES across components

Total Resistance: R_total = R₁ + R₂ + R₃ + ...

Total ALWAYS GREATER than any individual resistance.

If one fails, ENTIRE circuit breaks.

Parallel Connection

Components on SEPARATE branches. Multiple paths for current.

Voltage: SAME across all components (= battery voltage) Current: DIVIDES through branches

Total Resistance: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + ...

Total ALWAYS LESS than smallest individual resistance.

If one fails, others continue (independent paths).

Examples

Two resistors in series:

• R₁ = 4Ω, R₂ = 6Ω
• R_total = 4 + 6 = 10Ω

Two resistors in parallel:

• R₁ = 4Ω, R₂ = 6Ω
• 1/R_total = 1/4 + 1/6 = 3/12 + 2/12 = 5/12
• R_total = 12/5 = 2.4Ω

Why Parallel for Homes?

• Each appliance gets full voltage (220V)
• If one fails, others work
• Used in all home wiring

Why Series Sometimes?

• Old-style Christmas lights
• Voltage dividers
• Fuses

6. Heating Effect of Current

Joule's Law

H = I²Rt

(Energy in JOULES when current I flows through resistance R for time t.)

Formula Forms

Energy = Power × Time E = Pt = VIt = I²Rt = V²t/R

Examples of Heating Effect

• Electric iron: nichrome wire heats clothes
• Electric heater/geyser: nichrome coil heats water
• Electric bulb (incandescent): tungsten filament heats and glows
• Fuse: thin wire melts when current too high (safety)
• Toaster

Why Tungsten in Bulbs?

• VERY HIGH MELTING POINT (~3400°C)
• High resistivity
• Glows white-hot

Filament Bulb vs Modern Alternatives

• Filament: ~95% energy as HEAT, only 5% as LIGHT
• LED: 80-90% energy as light, very efficient
• LED bulbs save energy in India significantly

7. Electric Power

Definition

Rate of doing electrical work, or rate of energy transfer.

SI Unit

WATT (W)

1 W = 1 Joule / 1 Second

Formulas

P = VI = I²R = V²/R

(All equivalent; use whichever fits given variables.)

Unit Conversion

• 1 W = 1 V × 1 A
• 1 kW (kilowatt) = 1000 W
• 1 MW (megawatt) = 10⁶ W

Electrical Energy (Billing)

Electricity consumed measured in kWh (kilowatt-hour) — also called UNIT.

1 kWh = 1 kW × 1 hour = 3.6 × 10⁶ Joules

This is the unit used in electricity bills.

Example

A 100 W bulb on for 10 hours:

• Energy = 100 × 10 = 1000 Wh = 1 kWh = 1 unit
• Cost (if ₹5/unit): ₹5

8. Worked Examples

Example 1: Current

A current of 0.5 A flows through a circuit for 2 minutes. Find charge.

• I = Q/t
• Q = It = 0.5 × 120 = 60 Coulombs

Example 2: Voltage

A 100 J of work is done in moving 5 C of charge between two points. Find potential difference.

• V = W/Q = 100/5 = 20 Volts

Example 3: Ohm's Law

A bulb has resistance 100Ω and 220V supply. Find current.

• I = V/R = 220/100 = 2.2 A

Example 4: Series

Two resistors 5Ω and 10Ω in series, connected to 30V battery. Find current.

• R_total = 5 + 10 = 15Ω
• I = V/R = 30/15 = 2 A
• (Same current through both)

Example 5: Parallel

Two resistors 6Ω and 12Ω in parallel, connected to 6V battery. Find total current.

• 1/R_total = 1/6 + 1/12 = 2/12 + 1/12 = 3/12 = 1/4
• R_total = 4Ω
• I = V/R = 6/4 = 1.5 A

Example 6: Power

A heater has resistance 50Ω, voltage 220V. Find power.

• P = V²/R = (220)²/50 = 48400/50 = 968 W

Example 7: Energy Cost

A 1000 W heater used for 5 hours daily. Monthly energy and cost (₹5/unit)?

• Daily energy = 1000 × 5 = 5000 Wh = 5 kWh
• Monthly (30 days): 5 × 30 = 150 kWh = 150 units
• Cost: 150 × 5 = ₹750

9. Common Mistakes

1. Confusing series and parallel formulas

   • Series: R = R₁ + R₂ + ... (ADD)
   • Parallel: 1/R = 1/R₁ + 1/R₂ + ... (RECIPROCALS)

2. Wrong power formula

   • P = VI = I²R = V²/R — three forms, use what fits.

3. Units confusion

   • Watt = Joule/second. kWh ≠ kW.

4. Current direction

   • Conventional current: + to -. Electrons: - to +.

5. Resistance vs resistivity

   • RESISTANCE depends on shape (L, A) AND material
   • RESISTIVITY (ρ) is just material property

10. Real-World Applications

Indian Electricity Sector

• 95% of India electrified (2026)
• Major sources: coal (50%), solar (15%), hydro (12%), wind, nuclear
• Target: 500 GW renewable by 2030

Indian Power Companies

• NTPC (National Thermal Power)
• Tata Power
• Adani Power
• State utilities

Modern Uses

• Smartphones (battery + charging circuit)
• Electric vehicles (Tata, Mahindra EVs)
• Solar panels (DC to AC inverters)
• LED lights (saved India many GW)

11. Conclusion

Electricity is the FOUNDATION of modern life:

• Current: flow of charge
• Voltage: drives current
• Ohm's Law: V = IR
• Series and parallel: different rules
• Power: P = VI
• Energy: measured in kWh (units)

Master:

• Ohm's law
• Series/parallel formulas
• Power and energy
• Heating effect (Joule's law)
• Unit calculations (for bills!)

Practice 20+ problems. This is HIGH-MARK chapter (~10-12 marks).

Electricity: invisible but indispensable.