Science and Technology Class 9 Notes Chapter 3 Maharashtra Board

Current Electricity

Introduction to Electricity

Importance of Electricity:

• We use electricity daily for appliances like fridges, fans, washing machines, etc.
• Industries use it for motors and furnaces.
• Hospitals, banks, and offices use generators to avoid power cuts.
• Even some animals, like eels, use electricity for hunting or defense.
• Lightning is a natural form of electricity.

Flow of Electricity:

• Electricity flows like water-from a higher level to a lower level.
• Example: Water flows from a dam at a higher level to a lower level due to gravity.

Potential and Potential Difference

Electric Potential:

• It’s like the “level” of electric charge at a point, similar to the height of water.
• Positive charges flow from higher potential to lower potential.
• Electrons (negative charges) flow from lower potential to higher potential.

Potential Difference:

• The difference in potential between two points (A and B).
• It causes the flow of electrons.
• Example: In a circuit, electrons flow from the negative terminal (lower potential) to the positive terminal (higher potential) of a cell until the potential difference becomes zero.

Activity (Figure 3.1):

• Two bottles connected by a tube with a clamp:
  • When the clamp is removed, water flows from the higher bottle to the lower one.
  • Water stops when levels are equal.
  • To keep it flowing longer, maintain a height difference (like potential difference in electricity).

Potential Difference of a Cell:

• It’s the difference between the positive and negative terminals of a cell.
• Caused by chemical reactions inside the cell.
• Formula: Potential Difference (V) = Work Done (W) / Charge (Q).
• Unit: Volt (V). 1 V = 1 J / 1 C (1 Joule of work to move 1 Coulomb of charge).

Free Electrons and Current

Free Electrons:

• In metals, outermost electrons are weakly attached to atoms and can move freely (called free electrons).
• These electrons carry negative charge in a conductor.

Flow of Electrons:

• Without a cell, electrons move randomly in a wire (no current).
• When connected to a cell, electrons move from the negative terminal (lower potential) to the positive terminal (higher potential), creating a current.
• Conventional current direction: Positive to negative (opposite to electron flow).

Electric Current (I):

• The flow of electrons through a conductor.
• Formula: I = Q / t (Charge / Time).
• Unit: Ampere (A). 1 A = 1 C / 1 s (1 Coulomb per second).
• Smaller units:
  • 1 mA (milliampere) = 10⁻³ A.
  • 1 μA (microampere) = 10⁻⁶ A.
• Example: If 0.4 A flows for 5 minutes (300 s), charge (Q) = I × t = 0.4 × 300 = 120 C.

Resistance and Ohm’s Law

Resistance (R):

• It’s the opposition to the flow of electrons in a conductor.
• Electrons collide with atoms/ions, causing resistance.
• Unit: Ohm (Ω).

Ohm’s Law:

• At a constant temperature, the current (I) through a conductor is directly proportional to the potential difference (V) across it.
• Formula: V = I × R (or R = V / I).
• 1 Ω = 1 V / 1 A (resistance when 1 A flows with 1 V applied).
• Example: A bulb with R = 1000 Ω and V = 230 V has I = V / R = 230 / 1000 = 0.23 A.

Resistivity (ρ):

• Resistance depends on:
  • Length (L): R ∝ L (longer wire, more resistance).
  • Area of cross-section (A): R ∝ 1/A (thicker wire, less resistance).
  • Material of the wire.
• Formula: R = ρ × (L / A).
• ρ (resistivity) is a property of the material.
• Unit of ρ: Ohm-meter (Ωm).
• Example: Copper (ρ = 1.7 × 10⁻⁸ Ωm), Nichrome (ρ = 1.1 × 10⁻⁶ Ωm), Diamond (very high).
• Example: A wire (L = 50 cm, r = 0.5 mm, R = 30 Ω) has ρ = (R × A) / L = 4.71 × 10⁻⁵ Ωm.

Activity (Ohm’s Law Verification, Figure 3.7):

• Use a nichrome wire, cells, ammeter, and voltmeter.
• Increase the number of cells (1 to 4) and measure V and I.
• Calculate R = V / I for each case.
• Plot a V vs. I graph-it’s a straight line, proving Ohm’s Law (V ∝ I).

Conductors and Insulators

Conductors:

• Materials with low resistance, allowing easy current flow.
• Examples: Copper, aluminum (used in wires).

Insulators:

• Materials with very high resistance, blocking current flow.
• Examples: Rubber, glass (used for safety).

Activity (Figure 3.6):

• Test materials (copper, aluminum, glass, rubber) in a circuit.
• Conductors (copper, aluminum) show high current; insulators (glass, rubber) show no current.

Why Do Some Conduct?

• Conductors have many free electrons; insulators have few or none.
• Our body conducts because it contains water with ions (charged particles).

Electric Circuit

What is a Circuit?

• A continuous path for current flow, including a cell, wires, and resistors.
• Circuit Diagram: A drawing using symbols to show connections.

Components (Figure 3.5):

• Ammeter: Measures current (connected in series).
• Voltmeter: Measures potential difference (connected in parallel).
• Resistance: Opposes current.
• Cell: Provides potential difference.
• Key/Switch: Opens or closes the circuit.

Resistors in Series

Series Connection (Figure 3.8):

• Resistors are connected end-to-end (one after another).
• Same current (I) flows through each resistor.
• Total potential difference (V) = V₁ + V₂ + V₃.
• Effective resistance (Rₛ): Rₛ = R₁ + R₂ + R₃ + … + Rₙ.
• Features:
  • Total resistance increases (sum of all resistances).
  • If one resistor fails, the circuit breaks (e.g., bulbs in series dim or stop).
• Example: R₁ = 15 Ω, R₂ = 3 Ω, R₃ = 4 Ω → Rₛ = 15 + 3 + 4 = 22 Ω.
• Example: R₁ = 16 Ω, R₂ = 14 Ω, V = 18 V → Rₛ = 30 Ω, I = 18 / 30 = 0.6 A, V₁ = 9.6 V, V₂ = 8.4 V.

Resistors in Parallel

Parallel Connection (Figure 3.9):

• Resistors are connected side by side (same ends connected).
• Total current (I) = I₁ + I₂ + I₃.
• Potential difference (V) is the same across all resistors.
• Effective resistance (Rₚ): 1/Rₚ = 1/R₁ + 1/R₂ + 1/R₃ + … + 1/Rₙ.

Features:

• • Total resistance decreases (less than the smallest resistance).
  • If one resistor fails, others still work (current finds another path).
  • Bulbs in parallel shine equally bright.
• Example: R₁ = 15 Ω, R₂ = 20 Ω, R₃ = 10 Ω → 1/Rₚ = 1/15 + 1/20 + 1/10 = 13/60 → Rₚ = 60/13 ≈ 4.615 Ω.
• Example: R₁ = 5 Ω, R₂ = 10 Ω, R₃ = 30 Ω, V = 12 V → I₁ = 2.4 A, I₂ = 1.2 A, I₃ = 0.4 A, I = 4 A, Rₚ = 3 Ω.

Domestic Electrical Connections

Wiring at Home:

• Three wires:
  • Live wire (red/brown): Brings current (220 V in India).
  • Neutral wire (blue/black): Returns current.
  • Earth wire (green/yellow): For safety, connected to the ground.
• Appliances are connected in parallel:
  • Each gets the same voltage (220 V).
  • If one fails, others still work.

Fuse Wire:

• Protects appliances by melting if current is too high.
• Made of a material with a low melting point.
• Common ratings: 1 A, 2 A, 5 A, etc.

Safety Precautions:

• Keep switches/sockets out of children’s reach.
• Don’t touch appliances with wet hands; use rubber soles.
• If someone gets a shock, don’t touch them-turn off the main switch or use a wooden stick to move them.