Short Answer Questions — Chapter 4: Electricity
Ohm's Law: At constant temperature, the electric current flowing through a conductor is directly proportional to the potential difference applied across its ends.
Mathematical form:
V ∝ I (at constant temperature)
V = IR
Where:
- V = Potential difference (Volts, V)
- I = Current (Amperes, A)
- R = Resistance (Ohms, Ω)
Graph: A graph of V vs I for an ohmic conductor is a straight line through the origin. The slope = R (resistance).
Conditions: Ohm's Law holds when:
- Temperature is constant
- Physical conditions remain unchanged
| Property | Series Circuit | Parallel Circuit |
|---|---|---|
| Current | Same through all components | Divides among branches |
| Voltage | Divides across components | Same across all components |
| Total Resistance | R = R₁ + R₂ + ... (larger) | 1/R = 1/R₁ + 1/R₂ + ... (smaller) |
| If one component fails | Entire circuit breaks (all go off) | Only that branch fails, others work |
| Use | String lights (old) | Household wiring, parallel appliances |
| Example | Simple series bulbs | Home electrical circuit |
Why homes use parallel: If one appliance is turned off or fails, others continue working. Also, each appliance gets full supply voltage.
Electric resistance is the opposition offered by a conductor to the flow of electric current through it.
SI unit: Ohm (Ω)
Factors affecting resistance:
| Factor | Effect on Resistance |
|---|---|
| Length (L) | R ∝ L — longer wire has more resistance |
| Cross-sectional area (A) | R ∝ 1/A — thicker wire has less resistance |
| Material (ρ) | Resistivity ρ depends on material; metals have low ρ |
| Temperature | For metals: R increases with temperature; For semiconductors: R decreases with temperature |
Joule's Law of Heating: The heat produced in a conductor is directly proportional to: 1. The square of the current (I²) 2. The resistance (R) of the conductor 3. The time (t) for which current flows
Where:
- H = Heat produced (Joules)
- I = Current (Amperes)
- R = Resistance (Ohms)
- t = Time (seconds)
Using V = IR: H = VIt = V²t/R
Applications of Joule's heating:
- Electric bulb (tungsten filament)
- Electric iron, heater, toaster
- Electric fuse (protection)
- Electric welding
Electric power is the rate at which electrical energy is consumed or the rate of doing work.
P = W/t = Energy/time
Using V = IR and Q = It:
P = VI = I²R = V²/R
Where:
- P = Power (Watts, W)
- V = Voltage (Volts, V)
- I = Current (Amperes, A)
- R = Resistance (Ohms, Ω)
SI unit: Watt (W). 1 W = 1 J/s.
Larger units:
- Kilowatt (kW) = 1000 W
- Megawatt (MW) = 10⁶ W
Commercial unit of energy: Kilowatt-hour (kWh) 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J
Electric fuse: A safety device that protects electrical circuits and appliances from damage due to excessive current (overloading or short circuit).
Structure: A short piece of thin wire with low melting point and high resistance, enclosed in a ceramic or glass holder.
- When excess current flows, the fuse wire heats up rapidly (Joule's heating: H = I²Rt)
- Since fuse wire has low melting point, it melts and breaks the circuit
- This stops the current and protects the connected appliances
Why in series: The fuse must carry the full current of the circuit. In series, all current passes through the fuse. If placed in parallel, most current would bypass it and the fuse would not melt when there is overload.
Fuse wire material: Low melting point alloy (usually tin-lead alloy, 37% lead + 63% tin)
Modern alternatives: Miniature circuit breakers (MCBs) — can be reset after tripping.
In domestic wiring (India):
Three wires come from the electricity board:
| Wire | Colour | Function |
|---|---|---|
| Live wire | Red (or Brown) | Carries current from power station (at high potential ~230 V) |
| Neutral wire | Black (or Blue) | Returns current; at ground potential (0 V) |
| Earth wire | Green | Safety wire connected to earth; protects from electric shock |
How it works:
- All appliances are connected between the live wire and neutral wire (parallel connection)
- The earth wire is connected to the metal body of appliances
- If a live wire touches the metal body, current flows to earth → fuse blows → circuit breaks
Safety devices:
- Fuse / MCB: Breaks circuit on overload
- Earth wire: Prevents electric shock
- ELCB (Earth Leakage Circuit Breaker): Detects current leaking to earth and trips
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