Electricity and magnetism

Why it matters

The ceiling fan, a torch, the metal detector at a station gate and the transformer on an electric pole all work on two linked ideas: electricity and magnetism.

Plain definition: Electricity is the effect of electric charges, either at rest (static electricity) or moving (electric current). Magnetism is the force shown by magnets and by moving charges. The two are connected: an electric current makes a magnetic field, and a changing magnetic field can make an electric current.

Where it appears in the Constable paper: General Science questions here are usually short facts — SI units, discoverers, energy changes in devices, how meters are connected — and one-step calculations such as power or electricity bills. These are fast marks if your basics are clear.

Core concept

Level 1 — Beginner: charge, current, voltage

Electric charge (Q) comes in two kinds, positive and negative. Like charges repel; unlike charges attract. SI unit: coulomb (C). The charge of one electron is 1.6 × 10⁻¹⁹ C (negative). So 1 coulomb is the charge of about 6.25 × 10¹⁸ electrons.

Electric current (I) is the rate of flow of charge: I = Q / t. SI unit: ampere (A); 1 A = 1 coulomb per second. It is measured with an ammeter, connected in series so the whole current passes through it; its very low resistance hardly reduces the current.

Direction of current: By convention, current flows from the positive to the negative terminal outside the cell; electrons actually move the opposite way. Exams mean the conventional direction unless they say "electrons".

Potential difference (V) is the work done to move a unit charge between two points: V = W / Q. SI unit: volt (V); 1 V = 1 joule per coulomb. It is measured with a voltmeter, connected in parallel across the part being tested; its very high resistance draws almost no current.

Why does current flow? Water flows from a higher tank to a lower one; in the same way, charge flows only when there is a potential difference. A cell creates it by changing chemical energy into electrical energy. A dry cell gives about 1.5 V. Alessandro Volta made the first electric cell.

Level 2 — Intermediate: resistance and Ohm's law

Ohm's law: At constant temperature, the current through a metal wire is directly proportional to the potential difference across it: V = I R. The constant R is the resistance, measured in ohm (Ω).

What decides resistance? For a wire, R = ρ L / A

• R increases with length (L) — a longer path means more collisions.
• R decreases with cross-sectional area (A) — a thicker wire is a wider path.
• ρ (resistivity) depends only on the material and its temperature, not on the shape. Unit: ohm metre (Ω m).
• For metals, resistance increases when temperature rises. For semiconductors (silicon, germanium) it decreases.

Material type	Examples	Use
Good conductors	silver (best), copper, aluminium	connecting wires (copper and aluminium are cheaper than silver)
High-resistivity alloys	nichrome	heater, iron and toaster elements
Very high melting point metal	tungsten	filament of incandescent bulb
Insulators	rubber, plastic, glass, dry wood	covering of wires, handles of tools

Nichrome suits heaters: high resistivity, and it does not oxidise easily when red hot.

Combining resistors

	Series	Parallel
Total resistance	R = R₁ + R₂ + R₃	1/R = 1/R₁ + 1/R₂ + 1/R₃
Same for every resistor	current	potential difference
Total compared with parts	larger than the largest	smaller than the smallest
n equal resistors, each R	nR	R/n
If one part breaks	whole circuit stops	others keep working

Household wiring is parallel, so every appliance gets the full voltage and has its own switch.

Level 3 — Advanced: heating, power and magnetism

Heating effect (Joule's law): H = I² R t. For the same current, doubling resistance doubles the heat; doubling the current makes it four times. Used in heaters, irons, bulbs and fuses.

Electric power (P) is the rate of using electrical energy: P = V I = I² R = V² / R. Unit: watt (W). 1 horsepower ≈ 746 W.

Commercial unit of energy: 1 kilowatt hour (kWh) = 1 "unit" on the electricity bill = 3.6 × 10⁶ J. Energy (kWh) = power (kW) × time (h).

Safety devices: A fuse is a short wire of low melting point (commonly a tin–lead alloy) placed in series with the live wire. Too much current heats it, it melts and breaks the circuit. An MCB (miniature circuit breaker) does the same job but can be reset. The earth wire connects the metal body of an appliance to the ground, so a leaking current goes to earth instead of through a person.

AC and DC: A cell or battery gives direct current (DC), which flows in one direction. Power stations supply alternating current (AC), which reverses direction regularly. Indian household supply is AC at about 220–230 V (textbooks use 220 V) and 50 Hz. At 50 Hz, the current changes direction 100 times every second (twice in each cycle).

Magnets: Every magnet has a north and a south pole; like poles repel, unlike attract. A freely hanging magnet rests in the north–south direction. A broken magnet gives two magnets, each with both poles. Iron, nickel and cobalt are strongly attracted (ferromagnetic).

Magnetic field lines go from N to S outside the magnet and S to N inside, forming closed loops. They never cross; where they are crowded, the field is strong. SI unit of magnetic field: tesla (T); 1 T = 10⁴ gauss. Magnetic flux is measured in weber (Wb).

The Earth is a magnet. A compass needle's north pole points to geographic north because the Earth's magnetic pole there behaves like a magnet's south pole.

Electricity makes magnetism: In 1820, Hans Christian Oersted saw a current-carrying wire deflect a compass needle. Right-hand thumb rule: thumb along the current, curled fingers show the field direction. A current-carrying solenoid (long coil) behaves like a bar magnet. With a soft iron core it becomes a strong electromagnet (electric bells, lifting cranes, relays). Soft iron gains and loses magnetism easily; steel keeps it, so steel is used for permanent magnets.

Motor principle: A current-carrying conductor placed in a magnetic field feels a force. Its direction is given by Fleming's left-hand rule. An electric motor converts electrical energy into mechanical energy.

Magnetism makes electricity: In 1831, Michael Faraday discovered electromagnetic induction — a changing magnetic field through a coil produces (induces) a current. Direction: Fleming's right-hand rule. A generator (dynamo) uses this to convert mechanical energy into electrical energy. A transformer also works on induction and works only with AC: a step-up transformer increases voltage, a step-down transformer decreases it.

Worked examples

Example 1 — Charge from current. A current of 0.5 A flows through a bulb for 10 minutes. How much charge passes?
Solution: Change time to seconds: 10 min = 600 s. Q = I × t = 0.5 × 600 = 300 C.
Why: Current is charge per second. Forgetting to convert minutes is the usual error.

Example 2 — Series and parallel. Resistors of 3 Ω and 6 Ω are connected to a 6 V battery. Find the total resistance and current (a) in series (b) in parallel.
(a) Series: R = 3 + 6 = 9 Ω; I = V/R = 6/9 = 2/3 A.
(b) Parallel: 1/R = 1/3 + 1/6 = 2/6 + 1/6 = 3/6, so R = 2 Ω; I = 6/2 = 3 A.
Check: 2 Ω is less than the smallest resistor (3 Ω), as it must be. Shortcut: product ÷ sum = 18 ÷ 9 = 2 Ω.

Example 3 — Electricity bill. A 1500 W geyser runs 2 hours a day for 30 days. At ₹5 per unit, what is the cost?
Solution: Power = 1.5 kW. Energy = 1.5 × 2 × 30 = 90 kWh = 90 units. Cost = 90 × 5 = ₹450.
Why: The bill counts kWh, so change watts to kilowatts first.

Example 4 — Stretching a wire. A wire of resistance R is stretched to twice its length. What is its new resistance?
Solution: The volume stays the same, so when length doubles, area becomes half. R = ρL/A becomes ρ(2L)/(A/2) = 4R.
Why: Longer and thinner both raise resistance, so 2 × 2 = 4.

Example 5 — Bulb rating. A bulb is marked 100 W, 220 V. Find its resistance and the current it draws.
Solution: R = V²/P = (220 × 220)/100 = 48400/100 = 484 Ω. I = P/V = 100/220 ≈ 0.45 A.

Example 6 — Fact to question. Fact: "A motor converts electrical energy into mechanical energy."
Question: Which of the following converts electrical energy into mechanical energy? (a) Generator (b) Electric motor (c) Transformer (d) Dry cell. Answer: (b). A generator does the reverse, a transformer changes voltage, a dry cell changes chemical to electrical energy.

Real-world connection

• Home wiring: sockets are in parallel; the MCB cuts power during a short circuit or overload. The longer, thicker pin of a 3-pin plug is the earth connection.
• Heaters and irons use nichrome coils; old-style bulbs use tungsten filaments in unreactive argon or nitrogen gas so the filament lasts longer.
• Power transmission: electricity travels long distances at very high voltage. For the same power, higher voltage means lower current, so less energy is lost as heat (I²R). Step-down transformers reduce the voltage before it reaches homes.
• Power plants use huge generators in which turbines rotate coils in a magnetic field; fans and water pumps run on motors.
• Metal detectors at airports and police checkpoints, and induction cooktops, work on electromagnetic induction.

How it is asked in the exam

You have about 54 seconds per question; most items here take 15–30 seconds, saving time for arithmetic.

Common formats:

1. Direct fact / unit: "The SI unit of magnetic field is —" (tesla).
2. Discoverer: "Electromagnetic induction was discovered by —" (Faraday).
3. Device and energy change: "A dynamo converts —" (mechanical into electrical energy).
4. Connection / rule: "A voltmeter is connected in —" (parallel).
5. One-step numerical: power, charge, resistor combinations, kWh bill.
6. Statement-based / match the following: check each statement or pair against the key facts.

Levels:

• Easy: "Which material is used for the filament of an electric bulb?" (tungsten)
• Medium: "Three resistors of 2 Ω, 3 Ω and 6 Ω are in parallel. Total resistance?" 1/2 + 1/3 + 1/6 = 3/6 + 2/6 + 1/6 = 1, so 1 Ω.
• Hard: "If the current through a resistor is doubled, the heat produced in the same time becomes —" (four times, as H ∝ I²), or the series-bulb trap.

తెలుగు సారాంశం (Telugu summary)

ఒక సెకనులో ప్రవహించే విద్యుత్ ఆవేశాన్ని విద్యుత్ ప్రవాహం అంటారు: I = Q/t. దీని ప్రమాణం ఆంపియర్, ఆవేశం ప్రమాణం కూలుంబ్.
పొటెన్షియల్ భేదం ప్రమాణం వోల్ట్, నిరోధం ప్రమాణం ఓమ్. ఉష్ణోగ్రత స్థిరంగా ఉంటే ఓమ్ నియమం V = IR వర్తిస్తుంది.
అమ్మీటర్‌ను వలయంలో శ్రేణిలో, వోల్ట్‌మీటర్‌ను సమాంతరంగా కలపాలి.
తీగ పొడవు పెరిగితే నిరోధం పెరుగుతుంది, మందం పెరిగితే నిరోధం తగ్గుతుంది. విశిష్ట నిరోధం పదార్థం, ఉష్ణోగ్రతపై మాత్రమే ఆధారపడుతుంది.
శ్రేణిలో మొత్తం నిరోధం పెరుగుతుంది; సమాంతరంలో అది అతి చిన్న నిరోధం కంటే కూడా తక్కువ. ఇళ్లలో వైరింగ్ సమాంతరంగా ఉంటుంది.
విద్యుత్ సామర్థ్యం P = VI, ప్రమాణం వాట్. కరెంటు బిల్లులో ఒక యూనిట్ అంటే ఒక కిలోవాట్ అవర్ (3.6 × 10⁶ జౌల్).
ఫ్యూజ్ తక్కువ ద్రవీభవన స్థానం గల తీగ; ఇది లైవ్ తీగతో శ్రేణిలో ఉంటుంది. ఎర్త్ తీగ విద్యుత్ షాక్ నుండి కాపాడుతుంది.
విద్యుత్ ప్రవాహం అయస్కాంత క్షేత్రాన్ని ఏర్పరుస్తుందని ఆయర్‌స్టెడ్ కనుగొన్నాడు. అయస్కాంత క్షేత్రం ప్రమాణం టెస్లా.
మోటార్ విద్యుత్ శక్తిని యాంత్రిక శక్తిగా మారుస్తుంది (ఫ్లెమింగ్ ఎడమచేతి నియమం). జనరేటర్ యాంత్రిక శక్తిని విద్యుత్ శక్తిగా మారుస్తుంది (కుడిచేతి నియమం, ఫారడే విద్యుదయస్కాంత ప్రేరణ).
ట్రాన్స్‌ఫార్మర్ AC తో మాత్రమే పనిచేస్తుంది. మన దేశంలో ఇళ్లకు 50 Hz AC సరఫరా అవుతుంది.

Key terms (English — తెలుగు):

• Electric charge — విద్యుత్ ఆవేశం
• Electric current — విద్యుత్ ప్రవాహం
• Potential difference — పొటెన్షియల్ భేదం
• Resistance — నిరోధం
• Resistivity — విశిష్ట నిరోధం
• Conductor — వాహకం
• Insulator — అవాహకం (బంధకం)
• Series connection — శ్రేణి సంధానం
• Parallel connection — సమాంతర సంధానం
• Electric power — విద్యుత్ సామర్థ్యం
• Magnet — అయస్కాంతం
• Magnetic field — అయస్కాంత క్షేత్రం
• Electromagnet — విద్యుదయస్కాంతం
• Electromagnetic induction — విద్యుదయస్కాంత ప్రేరణ
• Earthing — భూసంధానం (ఎర్తింగ్)