Class 10 Physics • Chapter 09 (Deep Detail)
Power generated at power stations (11 kV) is stepped up (132 kV) for transmission to minimize heat loss ($I^2R$), then stepped down at city substations.
Ring System: Appliances are connected in Parallel. Advantages:
REASONING Why are appliances connected in parallel and not sequence?
Ans: In series, resistance increases reducing current. Also, if one appliance fails, the circuit breaks. Parallel allows independent operation at full voltage.
ANALYSIS Identify the error in the circuit diagram above. What is the danger?
Ans: The Fuse is connected in the Neutral wire. It should be in the Live wire.
Danger: If the fuse blows, the current stops, but the appliance remains connected to the high potential Live wire. Anyone touching it can get a shock.
THINKING What is the advantage of connecting appliances in parallel (Ring System)?
Ans: Independent operation, Constant Voltage (220V), and Reduced wire cost (Ring).
A safety device connected in Series with the Live Wire.
Principle: Heating effect of current.
Material: Alloy of Lead and Tin (Low melting point).
Function: Melts and breaks the circuit if current exceeds safe limit (short circuit or overload).
Q: Which wire is the fuse connected to?
Ans: Live Wire. Always in series.
Connected in Series with the Live Wire.
Reason: If switch is on Neutral, the appliance remains connected to Live (High Potential) even when off. User may get shock.
Dual Control Switch: Used in staircases to operate one light from two places.
Connecting the metallic body of an appliance to the earth.
Reason: If live wire touches the metal body, current flows to earth instantly (low resistance path). Fuse blows, protecting the user from fatal shock.
Q: Why is local earthing required?
Ans: To provide a safe low-resistance path for leakage current to ground, preventing shock.
REASONING How does earthing prevent electric shock?
Ans: As earth wire has negligible resistance, the leakage current flows through it to the ground instead of the human body (high resistance). This high current also instantly blows the fuse, cutting off supply.
NUMERICAL An electric oven of power 2.5 kW is used on a 250 V mains supply. Calculate the current rating of the fuse to be used.
Solution: $I = P/V = 2500/250 = 10 \text{ A}$. Use fuse slightly higher, e.g., 10 A or 13 A.
REASONING Why should a switch not be connected to the neutral wire?
Ans: The appliance remains connected to Live wire even when switched off. Risk of shock to user touching it.
CONCEPTUAL A 5A fuse is protecting a refrigerator. If the fuse is moved from Live to Neutral wire, will it still blow during a fault?
Ans: Yes, it will still blow if current exceeds 5A (as current is same in series). HOWEVER, it will not protect the user from shock because the appliance stays connected to Live potential.
3-Pin Plug:
CONCEPTUAL Why are the pins of a plug split at the ends?
Ans: To provide a spring action. It ensures a tight fit in the socket, preventing loose connection and sparking.
CONCEPTUAL Why is the Earth pin thicker and longer?
Ans: Longer: To connect Earth first (safety). Thicker: To prevent insertion into Live/Neutral holes.
| Wire | Old Convention | New Convention |
|---|---|---|
| Live (L) | Red | Brown |
| Neutral (N) | Black | Blue |
| Earth (E) | Green | Yellow/Green |