Electricity and magnetism: Ohm's law, household…

Core facts

Electric charge: unit coulomb (C). Current = charge / time; unit ampere (A), measured with an ammeter (connected in series).
Potential difference: unit volt (V), measured with a voltmeter (in parallel).
Ohm's law: V = IR at constant temperature. Resistance unit: ohm.
Resistance of a wire rises with length and falls with thicker cross-section. Resistivity depends on material and temperature.
Series: same current; resistances add. Parallel: same voltage; reciprocal sum. Household wiring is parallel.
Electric power P = VI (watt). Heating effect (Joule's law): H = I²Rt.
Fuse: thin wire of low melting point in the live wire; it melts if current exceeds the safe limit. MCB is a resettable switch.
Earthing: the metal body of an appliance connects to the ground through the earth wire, so leakage current flows away and prevents shock.
Indian supply is AC, about 230 V at 50 Hz. Traditional colour code: red (live), black (neutral), green (earth).
Oersted (1820) showed a current produces a magnetic field. Faraday (1831) discovered electromagnetic induction.
Motor: electrical to mechanical energy. Generator (dynamo): mechanical to electrical energy.
Transformer: changes AC voltage; works on AC only.

Explained

Circuits and appliances

A bulb filament is of tungsten (high melting point, about 3,400 °C), in an inert gas. Heater coils use nichrome, a high-resistance alloy. Fuse wire is tin-lead.
Appliances of high power (geyser, air conditioner) need thick wires and a separate circuit.
Electrical energy is billed in kilowatt-hour: 1 kWh = 1 "unit" = 3.6 x 10⁶ J.

Magnetism basics

Like poles repel, unlike attract. Field lines emerge from the north pole and enter the south pole outside the magnet, forming closed loops; they never cross.
Earth acts like a bar magnet; a freely suspended magnet settles in the north-south direction (compass).
Right-hand thumb rule gives field direction around a current-carrying wire. A solenoid with soft iron core forms an electromagnet (cranes, electric bells).

Rules for direction

Fleming's left-hand rule: force on a current-carrying conductor in a magnetic field (motor).
Fleming's right-hand rule: direction of induced current (generator).

Electromagnetic induction. Moving a magnet near a coil, or changing the field through it, induces a current in the coil. AC generators produce alternating current; DC generators use a split-ring commutator.

Transformer

Based on mutual induction: Vs/Vp = Ns/Np.
Step-up: more turns in the secondary. Step-down: fewer turns.
Power is transmitted at high voltage so that current, and thus I²R heat loss, is small; it is stepped down near homes.

Common traps

✗ "Ammeter is connected in parallel." ✓ Series; voltmeter is in parallel.
✗ "House circuits are in series." ✓ They are parallel, so each appliance gets full voltage.
✗ "Transformer works on DC." ✓ AC only; DC gives no changing flux.
✗ "Fuse is placed in the neutral wire." ✓ It goes in the live wire.
✗ "Motor and generator use the same Fleming rule." ✓ Motor: left-hand; generator: right-hand.
✗ "Faraday discovered that current makes magnetism." ✓ That was Oersted; Faraday found induction.

Quick revision

V = IR; unit of resistance ohm.
Ammeter in series; voltmeter in parallel.
Household wiring parallel; supply about 230 V, 50 Hz.
Fuse in live wire; earthing prevents shock.
1 kWh = 1 unit = 3.6 x 10⁶ J.
Motor: left-hand rule; generator: right-hand rule.
Oersted 1820; Faraday 1831.
Transformer: AC only; high-voltage transmission cuts loss.

Go deeper: Deeper, Mains writing (beyond what APPSC Paper I usually asks)

Deeper Advanced

Lenz's law: induced current opposes the change that causes it (conservation of energy).
AC vs DC: AC can be transformed easily, so it dominates transmission. DC is stored in batteries; solar cells give DC and need inverters. Direct current is also used in high-voltage long-distance links (HVDC).
Cells: primary cells (dry cell) cannot be recharged; secondary cells (lead-acid, lithium-ion) can. A lead-acid cell gives about 2 V.
Semiconductors: conductivity between metals and insulators; silicon and germanium. A diode conducts one way (rectifier); LED emits light; a transistor amplifies or switches signals.
Superconductivity: zero resistance below a critical temperature; discovered by Kamerlingh Onnes in 1911 (mercury at about 4 K). Used in MRI magnets and maglev.
National grid: India's regional grids were synchronised into one national grid in December 2013 (the southern grid joined last).
Short circuit (live touches neutral) and overloading (too many appliances) are the two causes of excess current in a household circuit.

Mains writing Mains writing

Frame: electricity supply is the base of development; its safety and efficiency depend on simple physical principles.
Analysis:
- Transmission and distribution losses reduce supplied power; high-voltage lines cut I²R loss.
- Renewable power needs inverters and storage, bringing DC/AC conversion and battery technology to the fore.
- Electrical accidents are largely due to poor earthing and faulty wiring.
Way forward: smart meters, ISI-marked appliances, residual-current devices and a stronger grid for renewables.
Example: electric vehicles are motors driven by batteries; regenerative braking turns the motor into a generator to recover energy.

Medha AI writes its own notes and checks them against the sources listed. Medha AI is an independent preparation service. It is not affiliated with, endorsed by or connected to APPSC, TSPSC, MPSC, UPSC or any other commission or government body.

2.1.7 Electricity and magnetism: Ohm's law, household circuits, motors and transformers

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