Electricity
1. Static Electricity
Electric charges at REST.
Electrostatic charges are produced by FRICTION.
| Charging by Friction | Example |
|---|---|
| Rubbing a plastic comb with dry hair | Comb attracts paper bits |
| Rubbing a balloon on woollen cloth | Balloon sticks to wall |
Like charges REPEL. Unlike charges ATTRACT.
'Benjamin Franklin's experiment with a kite in a thunderstorm proved that LIGHTNING is static electricity on a HUGE scale.'
2. Current Electricity
The FLOW of electric charges (electrons) through a conductor.
Electric current (I): The rate of flow of charge through a conductor. Unit: Ampere (A)
3. Conductors and Insulators
| Conductors | Insulators |
|---|---|
| Allow current to FLOW | DO NOT allow current to flow |
| Have FREE electrons | Have NO free electrons |
| Examples: Copper, aluminium, silver, iron | Examples: Plastic, wood, rubber, glass, ebonite |
Semiconductors: Materials whose conductivity is BETWEEN conductors and insulators. Examples: Silicon, germanium.
4. Simple Electric Circuit
A closed path through which electric current flows.
Components
| Component | Symbol | Function |
|---|---|---|
| Cell/Battery | — | +— |
| Switch | —( )—— | Opens or CLOSES the circuit |
| Bulb | —⊕— | Converts electrical energy to LIGHT |
| Resistor | —∧∧∧— | OPPOSES current flow |
| Ammeter | —(A)— | Measures CURRENT (connected in SERIES) |
| Voltmeter | —(V)— | Measures VOLTAGE (connected in PARALLEL) |
| Fuse | —°— | SAFETY device — melts if current EXCEEDS limit |
5. Ohm's Law
V = I × R
Where: V = Potential DIFFERENCE (voltage) in volts (V) I = Current in AMPERES (A) R = Resistance in OHMS (Ω)
'Ohm's law states that the current flowing through a conductor is DIRECTLY proportional to the potential difference across it, PROVIDED temperature and other physical conditions remain constant.'
6. Series and Parallel Circuits
Series Circuit
| Aspect | Series |
|---|---|
| Current | SAME through ALL components |
| Voltage | SHARED across components |
| Total resistance | SUM of individual resistances (R = R₁ + R₂) |
| If one component FAILS | ENTIRE circuit breaks |
Parallel Circuit
| Aspect | Parallel |
|---|---|
| Current | DIVIDES among branches |
| Voltage | SAME across ALL branches |
| Total resistance | LESS than the smallest resistance |
| If one component FAILS | Other branches STILL work |
Worked Example: Two resistors of 4 Ω and 6 Ω are connected in series. Find the total resistance.
R = 4 + 6 = 10 Ω
Worked Example: Two resistors of 4 Ω and 6 Ω are connected in parallel. Find the total resistance.
1/R = 1/4 + 1/6 = 3/12 + 2/12 = 5/12 R = 12/5 = 2.4 Ω
7. Household Wiring and Safety
Earthing
- Connects the METAL body of an appliance to the EARTH
- If a 'LIVE' wire touches the metal body, current FLOWS to the ground instead of through the USER
- Prevents ELECTRIC shock
Fuse
- A THIN wire of low melting point
- Connected in SERIES with the live wire
- If current EXCEEDS the safe limit, the fuse MELTS and BREAKS the circuit
- 'Sacrificial' device — PROTECTS the appliance and wiring
Miniature Circuit Breaker (MCB)
- MODERN alternative to a fuse
- Automatically TRIPS (switches off) when current exceeds the limit
- Can be REUSED (just flip the switch back)
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Ammeter is connected in parallel' | Ammeter is ALWAYS connected in SERIES (it measures current flowing THROUGH it) |
| 'Voltmeter is connected in series' | Voltmeter is ALWAYS connected in PARALLEL (it measures voltage ACROSS a component) |
| 'Fuse is connected in the neutral wire' | Fuse is ALWAYS in the LIVE wire — so it breaks the circuit when needed |
| 'Short circuit means the circuit is too short' | A short circuit is a LOW-RESISTANCE path that allows VERY HIGH current to flow |
ICSE Exam Focus (6–8 marks)
- 2-mark questions: Define current, potential difference, resistance with SI units
- 3-mark questions: Distinguish series and parallel circuits
- 4-mark questions: Ohm's law numericals (V = IR)
- 6-mark questions: Household wiring — fuse, earthing, safety
- 8-mark questions: Combined numericals with series/parallel resistors
Self-Test
Q1. A current of 2 A flows through a resistor of 6 Ω. Find the voltage across it. A1. V = IR = 2 × 6 = 12 V.
Q2. Why are homes wired in parallel instead of series? A2. In parallel, each appliance gets the FULL voltage (240 V) and can work INDEPENDENTLY. In series, if one appliance fails, ALL stop working.
Q3. What is the function of a fuse in a circuit? A3. A fuse is a SAFETY device. If current exceeds the safe limit, the fuse wire MELTS and breaks the circuit, preventing FIRE or damage.
Q4. Calculate the total resistance when 3 Ω and 6 Ω resistors are connected in parallel. A4. 1/R = 1/3 + 1/6 = 2/6 + 1/6 = 3/6. R = 6/3 = 2 Ω.
Q5. Why is earthing necessary in electrical appliances? A5. Earthing provides a SAFE path for leakage current to flow into the ground. Without earthing, a person touching a faulty appliance could receive a FATAL shock.
Q6. State Ohm's law. A toaster draws 5 A from a 240 V supply. Find its resistance. A6. Ohm's law: V = IR. R = V/I = 240/5 = 48 Ω.
