Electricity and Magnetism
Introduction
This chapter covers the fundamental concepts of electric current, resistance, circuits, household electricity, magnetism, and electromagnetic effects. It combines the study of electricity with magnetic effects of current.
Ohm's Law
The current flowing through a conductor is directly proportional to the potential difference across its ends, provided physical conditions (temperature, etc.) remain constant.
V ∝ I or V = IR
Where V = potential difference (V), I = current (A), R = resistance (Ω).
Factors Affecting Resistance
R = ρ × l / A
Where ρ = resistivity of the material, l = length, A = cross-sectional area.
- Longer wire → higher resistance.
- Thicker wire → lower resistance.
- Higher temperature → higher resistance (for metals).
Electromotive Force (EMF) vs Terminal Voltage
| Term | Definition |
|---|---|
| EMF (ε) | Total energy supplied by the cell per unit charge |
| Terminal voltage (V) | Voltage across the terminals when current is flowing |
| Internal resistance (r) | Resistance within the cell itself |
V = ε − Ir
Series and Parallel Circuits
Series Circuit
Rₛ = R₁ + R₂ + R₃ + ...
- Same current flows through all components.
- Voltage divides across components.
- If one component fails, the circuit is broken.
Parallel Circuit
1/Rₚ = 1/R₁ + 1/R₂ + 1/R₃ + ...
- Same voltage across all branches.
- Current divides among branches.
- If one branch fails, others still work.
Household Circuits
Wiring System
- Live wire (Red/Brown) — Carries current to the appliance.
- Neutral wire (Black/Blue) — Returns current from the appliance.
- Earth wire (Green/Yellow) — Provides a safety path for leakage current.
Safety Devices
| Device | Function |
|---|---|
| Fuse | Melts when current exceeds safe limit |
| MCB (Miniature Circuit Breaker) | Trips automatically on overload |
| Earthing | Provides an escape route for fault current to prevent electric shock |
Magnetic Effect of Current
Magnetic Field Lines
- Direction: From N to S outside the magnet, S to N inside.
- Closer field lines → stronger magnetic field.
- Field lines never intersect.
Electromagnet
A coil of wire wound around a soft iron core that behaves as a magnet when current flows through it.
Factors affecting strength:
- Number of turns in the coil.
- Current flowing through the coil.
- Nature of the core material.
Fleming's Left Hand Rule
If we arrange the thumb, forefinger, and middle finger of the left hand mutually perpendicular, with:
- Forefinger pointing in the direction of the magnetic field (N to S)
- Middle finger pointing in the direction of current
- Then the thumb points in the direction of the force (motion)
DC Motor
A device that converts electrical energy into mechanical energy.
Working Principle: A current-carrying conductor placed in a magnetic field experiences a force (Fleming's Left Hand Rule).
Key Components: Armature coil, split-ring commutator, brushes, permanent magnet or electromagnet.
Worked Numericals
Example 1: Ohm's Law
A potential difference of 12 V is applied across a resistor of 4 Ω. Find the current.
Solution: I = V/R = 12/4 = 3 A
Example 2: Series Circuit
Three resistors of 2 Ω, 3 Ω, and 5 Ω are connected in series. Find the total resistance.
Solution: Rₛ = 2 + 3 + 5 = 10 Ω
Example 3: Parallel Circuit
Two resistors of 6 Ω and 3 Ω are connected in parallel. Find the total resistance.
Solution: 1/Rₚ = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2 Rₚ = 2 Ω
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Adding parallel resistors like series | Use 1/Rₚ = 1/R₁ + 1/R₂ |
| Forgetting units | Voltage in V, current in A, resistance in Ω |
| Confusing live and neutral wires | Live carries current to the appliance; neutral completes the circuit |
| Wrong hand rule application | Use LEFT hand for motor effect (current × field → force) |
ICSE Exam Focus
This chapter carries 10–12 marks. Key topics: Ohm's law numericals, series/parallel circuits, magnetic field patterns, Fleming's left hand rule, DC motor, household wiring.
Marks Blueprint: Ohm's law/Resistance — 3 marks, Circuit problems — 4 marks, Magnetism/Electromagnet — 2 marks, DC motor — 2 marks.
Self-Test Questions
-
Find the resistance of a wire of length 2 m, cross-sectional area 0.5 mm², and resistivity 1.6 × 10⁻⁸ Ωm.
-
Three resistors of 4 Ω, 6 Ω, and 12 Ω are connected in parallel. Find the total resistance.
-
State Ohm's law. Draw the V-I graph for a conductor obeying Ohm's law.
-
Explain the working of a DC motor with a labelled diagram.
-
What is an electromagnet? List two factors that affect its strength.
-
Distinguish between series and parallel circuits.
