Chapter 11: Electricity

1. Introduction

Electricity plays a very important role in our daily lives. It runs our lights, fans, TVs, computers, and many other appliances. In this chapter, we’ll understand what electricity is, how current flows, and how it is measured. We’ll also learn about potential difference, resistance, Ohm’s law, and electrical energy.

2. Electric Current

Definition:
 The flow of electric charge through a conductor is called electric current.

  • Symbol: I
  • Formula:
    I = Q / t
     where,
     I = current (in ampere, A)
     Q = charge (in coulomb, C)
     t = time (in seconds, s)

1 Ampere:
 If 1 coulomb of charge flows through a conductor in 1 second, the current is said to be 1 ampere.

Unit: Ampere (A)
 Measuring instrument: Ammeter

3. Electric Circuit

An electric circuit is a closed conducting path in which current flows.
 It usually consists of a battery, wires, a switch, and a resistor or bulb.

  • Closed Circuit: Current flows when the switch is closed.
  • Open Circuit: Current does not flow when the switch is open.

Direction of Current:
 Electric current flows from the positive terminal to the negative terminal of the battery (conventional direction).

4. Electric Potential and Potential Difference

Electric Potential:
 It is the capacity of a charged body to do work due to its charge.

Potential Difference (V):
 The work done to move a unit positive charge from one point to another.

Formula:
 V = W / Q
 where,
 V = potential difference (volt, V)
 W = work done (joule, J)
 Q = charge (coulomb, C)

1 Volt:
 Potential difference is 1 volt when 1 joule of work is done to move 1 coulomb of charge.

Instrument used: Voltmeter (always connected in parallel to the circuit component)

5. Circuit Symbols

Component

Symbol

Cell

⚫➖⚫

Battery

⚫➖⚫⚫➖⚫

Switch (open)

⎯/⎯

Switch (closed)

———

Resistor

Zigzag line

Voltmeter

Ⓥ

Ammeter

Ⓐ

Connecting wire

Straight line

6. Ohm’s Law

Statement:
 At constant temperature, the potential difference (V) across a conductor is directly proportional to the current (I) flowing through it.

Formula:
 V ∝ I
 or
 V = IR

where,
 V = potential difference (volt)
 I = current (ampere)
 R = resistance (ohm)

Unit of Resistance: Ohm (Ω)

1 Ohm:
 A conductor has resistance of 1 ohm if a current of 1 ampere flows through it when 1 volt is applied across its ends.

7. Resistance

Definition:
 Resistance is the property of a conductor that opposes the flow of electric current.

Formula:
 R = V / I

Factors affecting resistance:

  1. Length (L): R ∝ L
     (Longer wire → more resistance)
  2. Area of cross-section (A): R ∝ 1/A
     (Thicker wire → less resistance)
  3. Material: Different materials have different resistances.
  4. Temperature: Resistance increases with increase in temperature for metals.

Resistivity (ρ):
 R = ρ × (L / A)
 where ρ (rho) is the resistivity of the material.

Unit: ohm-meter (Ω·m)

8. Resistance in Series and Parallel

(a) Series Connection
  • Current is same in all resistors.
  • Total resistance (R) = R₁ + R₂ + R₃
  • Equivalent resistance is greater than individual resistors.
(b) Parallel Connection
  • Voltage is same across all resistors.
  • Total resistance:
    1/R = 1/R₁ + 1/R₂ + 1/R₃
  • Equivalent resistance is less than the smallest resistor.

9. Electric Power and Energy

Electric Power (P):
 Rate of doing work or rate of consumption of electrical energy.

Formulae:

  1. P = VI
  2. P = I²R
  3. P = V² / R

Unit of Power: Watt (W)
 1 kilowatt (kW) = 1000 watt

Electric Energy:
 E = P × t
 or
 E = V × I × t

Commercial Unit of Energy:

  • 1 kilowatt hour (kWh) = 1 unit of electricity
  • 1 kWh = 3.6 × 10⁶ joules

10. Heating Effect of Electric Current

When current flows through a conductor, some electrical energy is converted into heat.

Heat produced (H):
 H = I²Rt

where,
 I = current
 R = resistance
 t = time

This is known as Joule’s Law of Heating.

Applications:

  • Electric heater
  • Electric iron
  • Electric bulb

Explanation for Bulb:
 The filament of a bulb is made of tungsten because it has high resistance and a high melting point (~3380°C).

11. Electric Fuse

A fuse is a safety device used to protect electric circuits from excessive current.

Working:
 If current exceeds the safety limit, the fuse wire melts and breaks the circuit, preventing damage.

Material:
 Fuse wire is made of alloy of tin and lead (low melting point).

Rating:
 Given in amperes (A), indicates maximum current it can carry safely.

12. Practical Uses of Series and Parallel Circuits

  • Series: Used in fairy lights; if one bulb fuses, circuit breaks.
  • Parallel: Used in homes; all appliances get same voltage and work independently.

13. Electric Power Calculation Example

If V = 220 V, I = 5 A
 Then,
 P = VI = 220 × 5 = 1100 W

So, the device consumes 1.1 kW power.

14. Summary

Quantity

Symbol

Formula

Unit

Current

I

Q / t

Ampere (A)

Potential Difference

V

W / Q

Volt (V)

Resistance

R

V / I

Ohm (Ω)

Power

P

VI, I²R, V²/R

Watt (W)

Energy

E

P × t

Joule (J)

15. Key Points to Remember

  • Current flows from positive to negative terminal (conventional current).
  • Voltmeter is connected in parallel; ammeter in series.
  • Resistance increases with length, decreases with thickness.
  • Fuse protects appliances from overcurrent.
  • Unit of energy in electricity bill = kilowatt-hour (kWh).
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