Electricity — 50 Q&A

Electric current is rate of flow of charge through a conductor. Unit: ampere (A). 1 A = 1 coulomb / second.

Potential difference between two points is work done in moving a unit charge from one point to the other. Unit: volt (V). 1 V = 1 joule / coulomb.

Ohm's law: the potential difference across a conductor is directly proportional to the current through it, provided temperature remains constant. V = I R.

Resistance depends on material (resistivity ρ), length l (∝ l) and cross-sectional area A (∝ 1/A). Formula: R = ρ l / A. Temperature also affects resistance for most materials.

Resistivity (ρ) is a material's intrinsic property indicating how strongly it opposes current. Unit: ohm·metre (Ω·m). Low ρ → good conductor.

In series: same current through each resistor. Total resistance R_eq = R1 + R2 + R3 + .... Voltage divides across resistors.

In parallel: same potential difference across each resistor. Total: 1/R_eq = 1/R1 + 1/R2 + .... For two resistors: R_eq = (R1·R2)/(R1+R2).

• Series: same current, voltages add, R increases.
• Parallel: same voltage, currents add, R decreases.
• Practical: household circuits use parallel so appliances get full voltage independently.

Power = rate of energy transfer. P = V·I. Using Ohm's law: P = I^2 R = V^2 / R. Unit: watt (W). Energy over time: E = P·t (joules or kWh).

Energy used = power × time. Utility bills use kilowatt-hour (kWh): 1 kWh = 1000 W × 1 h. Bill = energy (kWh) × rate (₹/kWh).

Joule's law: heat produced H = I^2 R t. Used in electric heaters, kettles, bulbs, fuses. Efficient in appliances designed to convert electrical energy to heat.

A fuse is a safety device with a thin wire that melts when excessive current flows, breaking the circuit to protect wiring and appliances. Fuses have rated current values; choose slightly above normal operating current.

Live (hot): carries current to appliance (often red/brown). Neutral: return path (black/blue). Earth (green/yellow): safety; provides low-resistance path to ground to prevent shock and operate protective devices.

Earthing prevents electric shock by providing a low resistance path for fault current to flow to ground, ensuring metal parts do not remain at dangerous potential.

Ammeter measures current — connected in series with low internal resistance. Voltmeter measures potential difference — connected in parallel across component and has high internal resistance.

1. Junction rule: total current entering a junction = total current leaving (conservation of charge).
2. Loop rule: sum of potential differences around any closed loop = 0 (energy conservation).

Domestic meters (electromechanical induction types) have a rotating aluminium disc whose speed is proportional to power consumed. Digital meters compute energy from measured voltage & current. Energy read in kWh.

Parallel wiring ensures each appliance gets full supply voltage and can operate independently; switching off or fault in one doesn't affect others.

Short circuit: low-resistance fault between live and neutral causing very high current which can heat and damage wiring or start fires. Protection: fuses, MCBs, proper insulation.

MCB (Miniature Circuit Breaker) trips automatically on overcurrent and can be reset. Fuses melt and must be replaced. MCBs offer quicker, reusable protection and better selectivity.

Identify series or parallel groups, replace them by their equivalent, repeat until a single equivalent resistance remains. Use series/parallel formulas.

Two resistors in series form a potential divider. Voltage across R2: V2 = ε · R2 / (R1 + R2). Used to obtain required lower voltages.

Conductors: metals (copper, aluminium) — have free electrons. Insulators: rubber, glass, plastic — no free charges; used for coating wires and as supports.

Metals conduct because of free electrons. Under electric field electrons drift opposite to field with drift velocity v_d. Current density J = n e v_d (n = electrons per m³, e = electron charge).

LDR's resistance decreases with increasing light intensity. Used in night lights, street light controllers and light sensors.

Nichrome has relatively high resistivity and high melting point, produces heat when current passes and survives high temperatures — ideal for heaters, toasters and electric irons.

• Use energy-efficient appliances (LEDs, efficient motors).
• Insulate to reduce heating/cooling load.
• Turn off unused devices, use timers and smart controls.

Earthing electrode (rod/plate) driven into ground with low-resistance connection to earth wire. Regular maintenance ensures low contact resistance (moistening soil, corrosion checks).

Distribution systems use neutral earthing schemes (TN-S, TN-C, TT etc.) to maintain safety and fault clearing. (Details are technical; follow local electrical codes for installations.)

Power loss in lines ∝ I^2 R. For fixed power, raising voltage reduces current and hence losses. Transformers step-up voltage for transmission and step-down for distribution.

Transformer uses mutual induction between windings on a core to step voltage up/down in AC circuits. Power (neglecting losses) ~ conserved: V1/V2 = N1/N2.

Lightning conductor (air terminal) provides a preferred path for lightning strikes to ground via heavy conductors and earth electrodes. It protects structures from direct lightning damage; earthing in electrical systems protects against electrical faults.

For metals, resistance increases with temperature (positive temperature coefficient). For semiconductors, resistance typically decreases with temperature (negative coefficient).

Superconductors have (nearly) zero electrical resistance below a critical temperature — allowing lossless currents. Applications: MRI magnets, maglev trains, sensitive detectors (research & specialized tech).

Total R = 3 + 6 = 9 Ω. Current I = V/R = 12/9 = 4/3 A ≈ 1.333 A. Voltage across 3 Ω: V1 = I·3 = 4 V. Across 6 Ω: V2 = I·6 = 8 V.

1/R_eq = 1/4 + 1/12 = 3/12 + 1/12 = 4/12 = 1/3 ⇒ R_eq = 3 Ω. Total current: I = 12/3 = 4 A. Current through 4 Ω: I1 = 12/4 = 3 A. Through 12 Ω: I2 = 12/12 = 1 A. (I = I1 + I2).

Fuse rating slightly above normal operating current. E.g., an appliance uses 800 W at 230 V → I = P/V ≈ 3.48 A → choose common fuse 5 A (nearest standard) to allow normal operation but blow on fault.

RCCB detects imbalance between live and neutral currents indicating leakage (e.g., to earth) and trips quickly to prevent electric shock. Unlike MCB (overcurrent), RCCB senses leakage current.

In parallel each bulb receives full voltage and brightness is independent. In series bulbs share voltage — one failing would break the circuit and others dim.

If live touches metal body, the body may become live causing shock if touched. Earthing gives a low-resistance path for fault current to flow to ground and operate protective devices (fuse/MCCB/RCCB) — reducing danger.

Solar cell converts light to electricity by photovoltaic effect in semiconductor junctions. Arrays form panels used for off-grid or grid-connected power generation.

Primary cells (non-rechargeable): e.g., alkaline cells. Secondary (rechargeable): e.g., lead-acid (batteries), NiMH, lithium-ion used in phones & vehicles.

• Keep electrical appliances away from water; never touch with wet hands.
• Switch off before repairing; use insulated tools and trained electrician.
• Fit proper earthing and protective devices; don't overload sockets.

Heaters, immersion rods, electric irons, toasters use resistive elements (nichrome) that heat when current passes (I^2R). Control thermostats regulate temperature.

Current through coil produces magnetic field that attracts iron hammer (plunger) to strike the bell. Contact breaks; spring returns hammer and cycle repeats while switch closed.

Lenz's law: induced emf/current opposes the change in magnetic flux that produced it (consequence of conservation of energy). It gives direction of induced current in generators and coils.

• Ohm's law: V = I R.
• Power: P = V·I = I^2 R = V^2/R.
• Energy: E = P·t (kWh for billing).
• Resistors: series R_sum, parallel 1/R_eq = Σ 1/R.
• Battery: V_terminal = ε − I r.

• Understand V = I R, and how resistance depends on ρ, l, A.
• Series vs parallel behaviour for current/voltage and how to compute equivalents.
• Power & energy formulas for billing and appliance sizing; heating effect → applications & safety.
• Domestic wiring: live/neutral/earth, earthing, fuses/MCBs/RCCBs — practical safety essentials.