Electricity and Magnetism
1. Static Electricity
STATIC ELECTRICITY is the accumulation of electric charges on an object.
How Charges Are Produced
When two objects are RUBBED together, electrons TRANSFER from one to the other.
- Object that LOSES electrons becomes POSITIVELY charged.
- Object that GAINS electrons becomes NEGATIVELY charged.
Examples
- Rubbing a plastic comb with dry hair — comb attracts small pieces of paper.
- Rubbing a glass rod with silk — glass rod becomes positively charged.
- Rubbing an ebonite rod with fur — ebonite rod becomes negatively charged.
Charging by Induction
A charged object can induce opposite charge on a NEARBY object without direct contact.
Law of Charges
- LIKE charges REPEL each other.
- UNLIKE charges ATTRACT each other.
2. Electric Current
Electric current is the FLOW of electric charges (electrons) through a conductor.
SI unit: ampere (A).
Direction of Current
- Conventional current: Flows from POSITIVE to NEGATIVE terminal.
- Electron flow: Flows from NEGATIVE to POSITIVE terminal.
Electric Cell
An electric cell provides ENERGY to push electrons around a circuit.
Parts of a cell:
- Positive terminal (marked +)
- Negative terminal (marked -)
- Electrolyte (chemical paste)
- Electrodes
3. Simple Electric Circuit
A closed, continuous PATH through which electrons can flow.
Components of a Simple Circuit
| Component | Symbol | Function |
|---|---|---|
| Cell | — | |
| Bulb | ⭘ | Converts electrical energy to light and heat |
| Switch | —◠— | Opens or closes the circuit |
| Wire | ——— | Conducts electricity between components |
Circuit Diagram
A circuit diagram uses STANDARD SYMBOLS to represent components.
Open and Closed Circuit
- Closed circuit: Complete path → current flows → bulb GLOWS.
- Open circuit: Gap in path → current does NOT flow → bulb does NOT glow.
4. Conductors and Insulators
Conductors
Materials that ALLOW electric current to flow through them.
Examples: Copper, aluminium, silver, iron, graphite, tap water, human body.
Why: Have FREE ELECTRONS that can move easily.
Insulators
Materials that do NOT allow electric current to flow through them.
Examples: Rubber, plastic, wood, glass, dry air, pure water, paper, cloth.
Why: NO free electrons. Electrons are tightly bound.
Important
- DRY wood is an insulator. WET wood can conduct electricity.
- PURE water is an insulator. TAP water (with dissolved salts) is a conductor.
5. Magnetic Materials
Magnets
A MAGNET is a material that ATTRACTS iron, nickel, cobalt, and some other materials.
Types of Magnets
| Type | Description |
|---|---|
| Natural magnets | Found in nature (lodestone / magnetite — Fe₃O₄) |
| Artificial magnets | Made by humans (bar magnet, horseshoe magnet, U-shaped magnet, electromagnet) |
| Temporary magnets | Lose magnetism easily (electromagnets, soft iron) |
| Permanent magnets | Retain magnetism for long time (steel, alnico) |
Magnetic Poles
- The ends of a magnet where the magnetic force is STRONGEST.
- Every magnet has TWO poles: NORTH (N) and SOUTH (S).
- A MAGNETIC NEEDLE (compass) is a small magnet that points North-South.
Law of Magnetic Poles
- LIKE poles REPEL (N-N, S-S).
- UNLIKE poles ATTRACT (N-S).
6. Magnetic Field and Earth's Magnetism
- A MAGNETIC FIELD is the region around a magnet where its force can be detected.
- Represented by MAGNETIC FIELD LINES that go from N to S (outside the magnet).
- The Earth itself behaves like a HUGE MAGNET.
- The Earth's magnetic SOUTH pole is near the geographic North pole, and vice versa.
- A freely suspended magnet aligns N-S due to Earth's magnetic field.
7. Electromagnet
An ELECTROMAGNET is a magnet made by passing current through a coil wound around a soft iron core.
Properties:
- Can be made VERY STRONG.
- Polarity can be REVERSED by reversing current.
- Strength can be VARIED by changing current or number of turns.
- It is a TEMPORARY magnet — loses magnetism when current is switched off.
Uses of Electromagnets
- Electric bells and buzzers.
- Cranes to lift heavy iron/steel objects.
- Generators and motors.
- Magnetic resonance imaging (MRI) in hospitals.
8. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Static electricity (law of charges) | 2 marks | High |
| Simple circuit and diagram | 2-3 marks | Very High |
| Conductors and insulators | 2 marks | High |
| Magnetic poles and law | 2 marks | Very High |
| Electromagnet uses | 2 marks | Medium |
Common Mistakes
- Thinking LIKE charges attract — they REPEL.
- Confusing magnetic poles and electric charges (similar rules but different phenomena).
- Drawing circuit diagrams without using STANDARD symbols.
- Saying the Earth's magnetic North pole is at the geographic North pole (it is actually near the geographic South pole).
Self-Test (5 Questions)
Q1. Like charges ___ and unlike charges ___. (1 mark)
- A) attract, repel
- B) repel, attract
- C) attract, attract
- D) repel, repel
Q2. What is the SI unit of electric current? (1 mark)
Q3. Give two examples of magnetic materials. (2 marks)
Q4. 'Draw a simple circuit with a cell, bulb, and switch in the closed position.' (3 marks)
Q5. 'Why is an electromagnet called a temporary magnet?' (2 marks)
Answers
A1. B) repel, attract. A2. Ampere (A). A3. Iron, nickel, cobalt (any two). A4. Diagram showing cell (two parallel lines), bulb (circle with cross), switch (connected), and connecting wires forming a closed loop. A5. An electromagnet loses its magnetism when the electric current is switched OFF. Unlike a permanent magnet, it does NOT retain magnetism.
