Physics
🔒 Log in to trackElectricity, magnetism and the EM spectrum
🔒 Log in to trackCurrent electricity
- Current I = Q/t (ampere); potential difference in volt; resistance in ohm.
- Ohm's law: V = IR (at constant temperature).
- Resistance R = ρL/A — doubles with length, falls with thicker wire; depends on material and temperature.
- Series: R = R₁ + R₂ + … (same current). Parallel: 1/R = 1/R₁ + 1/R₂ + … (same voltage). Household appliances are connected in parallel so each gets full voltage and works independently.
- Power P = VI = I²R = V²/R. Commercial energy unit = kWh (1 unit).
- Joule's heating H = I²Rt → heater, iron, filament bulb. Filament is tungsten (very high melting point); heating coils are nichrome (high resistivity). A fuse wire has a low melting point (tin–lead alloy) and is connected in series with the live wire.
- Indian domestic supply: AC, 220–230 V, 50 Hz. Wire colours: live — red/brown, neutral — black/blue, earth — green.
Magnetism and electromagnetism
- Oersted (1820): a current produces a magnetic field.
- Faraday: electromagnetic induction (1831) — basis of the generator/dynamo and transformer.
- Fleming's left-hand rule → motor; right-hand rule → generator.
- Transformer works only on AC; step-up for long-distance transmission (reduces current and I²R loss), step-down near homes.
- A galvanometer becomes an ammeter with a low resistance shunt in parallel, and a voltmeter with a high resistance in series.
- Rectifier (diode) converts AC → DC; inverter converts DC → AC. The transistor (1947, Bell Labs) amplifies and switches.
- Semiconductors: silicon, germanium. Doping with pentavalent (phosphorus, arsenic) → n-type; trivalent (boron, gallium) → p-type.
Electromagnetic spectrum (increasing wavelength)
Gamma rays → X-rays → Ultraviolet → Visible → Infrared → Microwaves → Radio waves.
| Wave | Use |
|---|---|
| Gamma | Cancer treatment, sterilising |
| X-ray | Bone imaging, airport scanners |
| UV | Sterilisation, vitamin D in skin |
| Infrared | TV remotes, night vision, heating |
| Microwave | Microwave ovens, radar, satellite links |
| Radio | Broadcasting, mobile communication |
Detailed notes
Current, voltage, resistance
Electric current is the flow of charge: I = Q/t, unit ampere (A). Charges move because a potential difference (voltage) pushes them — unit volt (V), measured by a voltmeter. The wire fights the flow with resistance — unit ohm (Ω), measured by connecting the meter readings: R = V/I. Ohm's law: at constant temperature, . Double the voltage and the current doubles. What changes resistance? R = ρL/A — more length, more resistance; thicker wire, less resistance; it depends on the material (its resistivity ρ) and on temperature. Metals conduct best; rubber, plastic and mica are insulators.
Series and parallel circuits
- Series: components in one loop; the same current flows through all; resistances add (R = R₁ + R₂ + …); one bulb fuses and the whole string goes dark.
- Parallel: branches across the same two points; the same voltage across each; 1/R = 1/R₁ + 1/R₂ + …, so total resistance is less than the smallest branch; each appliance works independently. Household appliances are connected in parallel so that each gets the full 220 V and switching off the TV does not kill the fan.
Power, energy and the heating effect
- Power P = VI = I²R = V²/R, unit watt. A 100 W bulb at 220 V draws about 0.45 A.
- Energy sold in kilowatt-hours: 1 unit = 1 kWh = 3.6 × 10⁶ J. Units = watts × hours ÷ 1000.
- Joule's law of heating: H = I²Rt — heat grows with current², resistance and time; this warms the iron, geyser and heater.
- Materials: the bulb filament is tungsten (extremely high melting point); heating coils are nichrome (high resistivity); a fuse wire (tin–lead alloy) has a low melting point, melts on excess current and breaks the circuit — it sits in series with the live wire.
- Indian domestic supply: AC, about 220 V at 50 Hz. Wiring colours: live — red/brown, neutral — black, earth — green (a safety path to the ground for metal bodies).
Magnetism and electromagnetic devices
Every magnet has a north and a south pole; like poles repel, unlike attract. Oersted (1820) showed that a current-carrying wire deflects a compass — electricity creates magnetism. A coil with a soft-iron core is an electromagnet (temporary magnet; cranes, electric bells); steel makes permanent magnets.
- Electromagnetic induction (Faraday, 1831): a changing magnetic field induces a current — the working of the generator/dynamo (motion → electricity).
- Electric motor: electricity + magnetic field → rotation. Remember the hands: Fleming's LEFT hand → motor, RIGHT hand → generator ('left = motor, both start with letters after F').
- Transformer: changes AC voltage by mutual induction — works only on AC. Step-up transformers raise voltage for long-distance transmission (current falls, so I²R line loss falls); step-down transformers bring it back near homes.
- Meters: a galvanometer becomes an ammeter with a low-resistance shunt in parallel (ammeter sits in series, so it must be nearly resistance-free); a voltmeter has high resistance in series inside and is connected in parallel with the component.
AC, DC and semiconductors
Current that reverses direction periodically is AC (mains supply); steady one-way current is DC (cells, batteries). A rectifier (diode) converts AC → DC; an inverter converts DC → AC. The transistor (1947, Bell Labs) amplifies and switches signals — the heart of electronics. Semiconductors (silicon, germanium) conduct halfway between metals and insulators. Doping with a pentavalent impurity (phosphorus, arsenic) adds free electrons → n-type; a trivalent one (boron, gallium) leaves holes → p-type. Joined p–n layers make diodes, chips and solar cells.
The electromagnetic spectrum
EM waves need no medium and travel at c = 3 × 10⁸ m/s. By increasing wavelength (decreasing frequency): Gamma rays → X-rays → Ultraviolet → Visible → Infrared → Microwaves → Radio waves.
| Wave | Typical use |
|---|---|
| Gamma | Cancer treatment, sterilising medical tools |
| X-rays | Bone imaging, airport security |
| UV | Sterilisation; makes vitamin D in skin |
| Visible | Seeing; lasers |
| Infrared | TV remotes, night vision, heaters |
| Microwave | Microwave ovens, radar, satellite links |
| Radio | Broadcasting, mobile phones |
Quick revision
- I = Q/t (A); V = IR (Ohm); R = ρL/A — longer more, thicker less.
- Series: same current, R adds. Parallel: same voltage, R shrinks; homes are parallel.
- P = VI; 1 unit = 1 kWh; H = I²Rt; filament tungsten, coil nichrome, fuse low-melting in the live wire.
- Oersted: current → magnetism; Faraday 1831: induction → generator. Left hand motor, right hand generator.
- Transformer: AC only; step-up for transmission. Rectifier AC→DC; transistor 1947.
- EM waves by rising wavelength: Gamma, X, UV, Visible, IR, Microwave, Radio.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Ohm's law and series/parallel circuitsvery common4 practice Q
Find total resistance of two or three resistors, or the current for a given voltage and resistance; 'what stays the same in series/parallel'.
- Series: just add the resistances; the current is common.
- Parallel: add the reciprocals, then invert; for two resistors use R1R2/(R1+R2). Three equal resistors R in parallel give R/3.
- Ohm's law: I = V/R. Why: one path shares current (series), branches share voltage (parallel).
Example: Resistors of 2 Ω, 3 Ω and 6 Ω are joined in parallel. Find the total resistance.
1/R = 1/2 + 1/3 + 1/6 = 1 → R = 1 Ω, less than the smallest resistor.
Type 2: Power, kWh and the heating effectvery common4 practice Q
Appliance watts into units/₹, H = I²Rt, 'filament is made of…', 'why is the fuse wire low-melting', transformer/transmission steps.
- Energy: kW × hours = units; multiply by the tariff for rupees.
- Heat: H = I²Rt with t in seconds.
- Materials: filament–tungsten (very high melting point), coil–nichrome (high resistivity), fuse–low-melting alloy in series with the live wire.
- Long transmission lines run at high voltage so the current (and I²R loss) stays small.
Example: A 2 kW heater runs for 3 hours. How many units does it use?
2 kW × 3 h = 6 kWh = 6 units.
Type 3: Electricity ↔ magnetism devicesvery common4 practice Q
'The electric motor works on…', 'a generator is based on…', 'a transformer works on…', which Fleming rule applies, who discovered electromagnetic induction.
- Oersted: current produces a magnetic field (compass deflects).
- Motor: magnetic force on a current-carrying conductor → rotation — Fleming's left-hand rule.
- Generator: electromagnetic induction (Faraday) — Fleming's right-hand rule.
- Transformer: mutual induction, AC only; step-up raises V, lowers I. Mnemonic: 'Left for Motor, Right for Generator'.
Example: The working of an electric motor is based on —
The magnetic force on a current-carrying conductor placed in a magnetic field — Fleming's left-hand rule gives the direction.
Type 4: Household wiring and safetycommon3 practice Q
'Appliances in a house are connected in…', colour of the earth wire, the job of the fuse, mains voltage and frequency in India.
- Homes: parallel — full voltage to each appliance, independent switches.
- Wires: live red/brown, neutral black, earth green — a safety path for leaking current.
- Fuse: low-melting wire in series with the live wire; melts first on overload.
- India: AC, about 220 V, 50 Hz.
Example: Which colour of insulation is used for the earth wire in India?
Green. Live is red/brown, neutral is black, earth is green.
Type 5: EM spectrum order and usesvery common3 practice Q
'Arrange gamma…radio by wavelength/frequency', 'TV remote uses…', 'which wave is used in radar/microwave oven', 'UV produces which vitamin'.
- Memorise rising wavelength: Gamma → X → UV → Visible → IR → Microwave → Radio (frequency rises the other way).
- Uses: gamma–cancer therapy, X–bones/scanner, UV–sterilising + vitamin D, IR–remotes/night vision, microwave–oven/radar, radio–broadcast.
- All EM waves travel at 3 × 10⁸ m/s in vacuum and need no medium.
Example: Which electromagnetic wave is used in TV remotes?
Infrared radiation — beyond the red end of the visible spectrum.
Type 6: Semiconductors, rectifiers and electronicsoccasional2 practice Q
'n-type is doped with…', 'which device converts AC to DC', facts about transistors and semiconductor materials.
- Semiconductors: silicon, germanium.
- Pentavalent doping (P, As) → n-type (extra electrons); trivalent (B, Ga) → p-type (holes).
- Diode = rectifier, AC → DC; inverter = DC → AC; transistor (1947) amplifies and switches.
Example: A pure silicon crystal doped with a pentavalent impurity such as phosphorus becomes —
n-type — each phosphorus atom donates one free electron (negative charge carriers).
Formulas
two resistors: R₁R₂/(R₁+R₂)
ρ = resistivity
Shortcut tricks
⚡ Fleming hands: 'Left for Motor, Right for Generator' (L-M, R-G)
Remember as the alphabet — L comes before M; G is 'generated' by the Right hand.
⚡ EM spectrum order: 'Good X-rays Use Visible Infra-Micro Radios'
Gamma → X-ray → UV → Visible → IR → Microwave → Radio: wavelength increases, frequency and energy decrease.
Example: Which has the longest wavelength: X-rays, UV, microwaves, gamma rays?
Microwaves.
Where students lose marks
Assuming a fuse should have a high melting point — it must melt easily.
Thinking a transformer works on DC — it needs changing current (AC).
Connecting an ammeter in parallel — it goes in series; a voltmeter goes in parallel.
Practice sets — 21 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 4 min · wrong answers go to your mistake notebook automatically.