Electricity & Magnetism

🎯 Learning Objective

Apply Ohm's law, tell series from parallel, know conductors, insulators and the fuse, and recall key magnet facts.

💡 Concept

• Ohm's Law: V = I × R (Voltage = Current × Resistance) at constant temperature; unit of resistance = ohm (Ω)
• Current (I) is measured in amperes by an ammeter (connected in series); voltage (V) is measured in volts by a voltmeter (connected in parallel)
• In a SERIES circuit there is one path, the same current flows, resistances add up, and if one part fails the whole circuit stops
• In a PARALLEL circuit there are multiple paths and the voltage is the same across each branch — used in household wiring
• Conductors allow current to flow: metals like copper, aluminium and silver (silver is the best conductor)
• Insulators block current: rubber, glass, plastic, wood and dry air
• A fuse is a safety device — a thin, low-melting-point wire that melts during an overload or short circuit and breaks the circuit, preventing fire
• Household supply in India is about 220–230 volts AC at 50 hertz
• A magnet has two poles (North and South); like poles repel and unlike poles attract
• Magnetic poles always exist in pairs — breaking a magnet gives two smaller magnets; a freely suspended magnet points North–South (the basis of the compass)

🧮 Key Formulas

V = I × R (Ohm's Law)

>

Series: R = R₁ + R₂ + R₃ …

>

Household supply: 220–230 V, 50 Hz AC

✏️ Easy Example

Q. According to Ohm's law, if voltage is 10 V and resistance is 5 Ω, the current is: (a) 2 A (b) 50 A (c) 0.5 A (d) 15 A

1. Ohm's law: V = I × R, so I = V / R
2. I = 10 / 5 = 2 amperes

Answer: (a) 2 A

🇮🇳 Real-Life Example

Your home's wiring is parallel — that's why switching off the fan doesn't turn off the lights. And the fuse (or MCB) in your meter box is a tiny sacrificial wire that melts to save the whole house from an electrical fire.

📝 Exam-Level Example

Q. Which of the following is the best conductor of electricity? (a) Copper (b) Silver (c) Iron (d) Aluminium

1. All metals conduct, but one leads
2. Silver is the best conductor of electricity

Answer: (b) Silver

📝 Exam-Level Example

Q. A fuse wire is made of a material having a: (a) high melting point (b) low melting point (c) high density (d) zero resistance

1. A fuse must melt quickly when current gets too high
2. So it uses a wire with a low melting point

Answer: (b) low melting point

🪄 Memory Trick

Ohm's triangle: cover the one you want — V on top, I and R below. Cover V → I×R; cover I → V/R; cover R → V/I.

⚠️ Common Mistakes

• ❌ Connecting an ammeter in parallel — it must be in series (voltmeter goes in parallel)
• ❌ Thinking a broken magnet gives a single pole — you always get two smaller magnets
• ❌ Assuming copper is the best conductor — silver is best; copper is just cheaper

🏆 Exam Tips

• ✅ V = I × R — rearrange for I = V/R and R = V/I as needed
• ✅ Household electricity is 220–230 V, 50 Hz AC — a frequent factual question

📌 Summary

• Ohm's Law: V = I × R; resistance in ohms (Ω)
• Series = one path, current same, resistances add; parallel = many paths, voltage same
• Silver = best conductor; rubber/plastic = insulators; fuse = low-melting safety wire
• Magnets have two poles; like repel, unlike attract; no single pole exists