'Electricity: Magnetic and Heating Effects' — Practice

Question 1

Which physicist discovered the magnetic effect of current?

Accepted Answer

✦ Answer: Hans Christian Ørsted (Danish physicist, in 1820). He noticed a compass needle deflecting near a current-carrying wire.

Question 2

Name the safety device that melts to break a circuit when current is too high.

Accepted Answer

✦ Answer: Electric FUSE. It contains a thin wire with low melting point. When current exceeds the limit, wire melts, breaks circuit, prevents fire. Modern alternative: MCB (Miniature Circuit Breaker).

Question 3

Explain how an electromagnet works and how to make a strong one.

Accepted Answer

Step 1 — Principle.
  When electric current flows through a coil wrapped around an iron core, the iron becomes a powerful magnet (magnetic effect of current).

Step 2 — Construction.
  • Take a SOFT IRON core (e.g., a nail or rod)
  • Wrap INSULATED COPPER WIRE in tight coils around the core
  • Connect ends to a battery with a switch

Step 3 — Operation.
  • Close switch → current flows → iron becomes magnet → attracts iron objects
  • Open switch → current stops → magnetism disappears

Step 4 — Make stronger.
  Three ways to increase strength:
  • Increase NUMBER OF TURNS in coil
  • Increase CURRENT through coil
  • Use SOFT IRON core (not steel — steel retains magnetism)

Step 5 — Why soft iron?
  Soft iron magnetises EASILY when current flows AND demagnetises EASILY when current stops. Steel retains magnetism. For an ON/OFF magnet, soft iron is ideal.

✦ Answer: An electromagnet is a coil of insulated wire around a soft iron core. When current flows, the iron becomes a magnet; when current stops, magnetism disappears. To make stronger: increase turns, current, and use soft iron core. Used in motors, MRI, junkyard cranes, doorbells.

Question 4

A home is wired so all appliances are in parallel. Explain why parallel is preferred over series for home wiring, and discuss safety devices used.

Accepted Answer

Step 1 — Why parallel for homes.
  Parallel has THREE major advantages:
  (a) EACH APPLIANCE WORKS INDEPENDENTLY. If TV fails, fridge still runs.
  (b) SAME VOLTAGE (220-240V) reaches every appliance.
  (c) APPLIANCES CAN BE SWITCHED ON/OFF separately.

Step 2 — Why series is bad for homes.
  In series:
  (a) ALL appliances must be on or none.
  (b) Voltage DIVIDES — each device gets only a fraction.
  (c) If ONE device fails, ALL stop working.

Step 3 — Safety devices in modern Indian homes.
  (a) MAIN SWITCH — at meter, controls entire house
  (b) MCB (Miniature Circuit Breaker) — automatically trips when current too high (overload). Reusable (unlike fuses).
  (c) ELCB / RCCB (Residual Current Circuit Breaker) — detects current leakage to earth (saves lives from electric shock).
  (d) EARTHING — third pin in 3-pin sockets connects appliance body to ground. If current leaks (e.g., short circuit), it flows to earth not through user.
  (e) FUSE — older safety device; thin wire that melts on overload. Replaced by MCBs in modern homes.

Step 4 — Why earthing is crucial.
  Appliances like fridge, geyser, washing machine have metal bodies. If insulation fails, current could flow through body. Without earthing, anyone touching it gets electrocuted. Earthing diverts the leaked current safely to ground.

Step 5 — Safe practices.
  • Use ISI-marked appliances
  • Don't overload sockets
  • Disconnect before repair
  • Keep electrical items away from water
  • Install MCB and ELCB at main panel

Step 6 — Indian context.
  India has 220-240V AC at 50 Hz. Modern wiring uses 3-pin sockets with earth. Approximately 3,000 deaths/year from electrocution — preventable with proper safety.

✦ Answer: Parallel is preferred because (a) each appliance works independently, (b) gets full voltage, (c) failures don't cascade. Safety devices: MCB (overload protection), ELCB (leak detection), Earthing (diverts leaks), Fuse (older alternative). India: 220V/50Hz with 3-pin earthed sockets.

Practice — 'Electricity: Magnetic and Heating Effects'

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