⚡ Static Electricity
- Static electricity is the accumulation of electric charges on the surface of an object.
- Objects become electrically charged by friction — when two objects are rubbed together, electrons transfer from one to the other.
- There are two types of charges: positive (+) and negative (−).
- Like charges repel each other; unlike charges attract each other.
- Conductors allow charges to flow freely (e.g. metals). Insulators do not allow charges to flow (e.g. rubber, plastic).
Examples of static electricity: Lightning is a massive static discharge between clouds. Clothes clinging together when taken out of a dryer. Hair standing on end when combed with a plastic comb.
🔋 Electric Current and Circuits
- Electric current (I) is the rate of flow of electric charge. SI unit: Ampere (A).
- Conventional current flows from the positive terminal to the negative terminal of a cell (outside the cell).
- Electron flow is actually in the opposite direction (from negative to positive).
- A cell converts chemical energy to electrical energy. A battery is a combination of two or more cells.
- A circuit is a complete, closed path through which electric current can flow.
- Potential difference (V) drives the current in a circuit. SI unit: Volt (V).
Common Circuit Symbols (ICSE)
- Cell: a short and a long parallel line. Battery: multiple such pairs.
- Switch (open/closed), resistor (rectangle), bulb (circle with cross), ammeter (A), voltmeter (V), wire (straight line).
📐 Ohm's Law and Resistance
Ohm's Law: At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends.
V = I × R (Voltage = Current × Resistance)
- Resistance (R) is the opposition to the flow of current. SI unit: Ohm (Ω).
- R = V ÷ I | I = V ÷ R
- Resistance depends on: length of conductor, cross-sectional area, material, and temperature.
- Longer wire → more resistance. Thicker wire → less resistance.
- Measured using an ohmmeter; or calculated from V and I using a voltmeter and ammeter.
🔌 Series and Parallel Circuits
| Property | Series Circuit | Parallel Circuit |
|---|---|---|
| Connection | Components connected end-to-end in one loop | Components connected across the same two points (branches) |
| Current | Same through all components | Divides at junctions; total I = I₁ + I₂ + … |
| Voltage | Divides across components; total V = V₁ + V₂ + … | Same across each branch |
| Total resistance | R = R₁ + R₂ + … (increases) | 1/R = 1/R₁ + 1/R₂ + … (decreases) |
| If one component fails | Entire circuit breaks | Other branches still work |
| Use | Fairy lights (old type), some sensors | Home electrical wiring, domestic appliances |
💡 Effects of Electric Current
Heating Effect
- When current flows through a resistor, electrical energy is converted to heat energy.
- Used in: electric iron, heater, electric bulb (filament), electric kettle, fuse wire.
- Fuse: A safety device made of a low-melting-point alloy. It melts and breaks the circuit if excess current flows, preventing damage to appliances.
Chemical Effect
- Electric current through a solution (electrolyte) causes chemical changes — called electrolysis.
- Used in: electroplating (coating metals with another metal, e.g. silver-plating cutlery), purification of metals.
Magnetic Effect
- A current-carrying conductor produces a magnetic field around it.
- Electromagnet: A coil of wire wound around an iron core — behaves as a magnet when current passes through it. Used in electric bells, cranes, MRI machines.
🏷️ Key Terms
Static electricity
Charge
Conductor
Insulator
Electric current
Ampere
Voltage
Volt
Resistance
Ohm
Ohm's Law
Series circuit
Parallel circuit
Fuse
Electrolysis
Electroplating
Electromagnet
Ammeter
Voltmeter