CDS Physics · Free notes
Electricity and Magnetism: CDS Physics
Electricity and Magnetism has 44 past-year questions in CDS General Knowledge, from 2016 (II) to 2026 (II): 13 EASY, 30 MODERATE and 1 HARD. Most ask a single fact or a one-line sum on power, resistance or units of energy, so the marks go to students who know the formulas cold and keep their units straight. Six questions turn on one idea: resistivity belongs to the material, not to the wire. Three more only ask you to name the right hand rule, so learn what each rule is for, and learn the colour of each house wire.
The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.
Subtopic notes
Charge and Current
7 PYQsStatic charge and what conducts it, the energy a charge gains across a potential difference, and how a cell drives a current that an ammeter measures.
Resistance and Circuits
12 PYQsOhm's law, how a wire's resistance depends on its length and thickness, resistivity as a property of the material, and resistors joined in series and in parallel.
Electric Power and Home Wiring
12 PYQsElectric power and the heating effect, the kilowatt-hour on the electricity bill, fuses, short circuits and the three wires of a house, and what a bulb does with energy.
Magnetism and Induction
13 PYQsMagnetic materials and field lines, the Earth's field, the field round a current and the hand rules, the force on a moving charge, and induced current with the transformer.
Formula & revision sheet
8 formulas · 6 reference tables · 32 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
8 formulas · 6 reference tables · 32 gotchas across all subtopics — the exam-eve cheat-sheet
Formulas (2)
Reference tables (1)
Static charge, conductors and insulators7 rows
| Material or device | What to remember |
|---|---|
| Silver, copper, aluminium | Good conductors: free electrons carry the charge |
| Human body | A conductor: the body's fluids carry dissolved ionsQ CDS 2021 (I): the human body is the conductor among a plastic sheet, distilled water and a thin wooden sheet. |
| Distilled water | A very poor conductor: almost no ions |
| Plastic, rubber, glass, dry wood | Insulators |
| Aircraft tyres | Made slightly conducting, so charge picked up by rubbing against the air flows to earth on landingQ CDS 2024 (II): conducting tyres remove the charge gathered by friction with the air. |
| Van de Graaff generator | Builds a very high voltage with a very small current |
| Lightning conductor | A pointed metal rod joined to a plate in the earth; carries the charge of a strike safely to the ground |
Watch out for (6)
- Pure water is a poor conductor→ Static charge, conductors and insulators
- An aircraft's charge comes from rubbing against the air→ Static charge, conductors and insulators
- Same voltage gives the same energy, not the same speed→ Potential difference and the energy a charge gains
- A volt is a joule per coulomb, not a joule→ Potential difference and the energy a charge gains
- Ammeter in series, voltmeter in parallel→ Current, cells and emf
- Primary cells cannot be recharged→ Current, cells and emf
Formulas (2)
Reference tables (1)
Resistivity: a property of the material7 rows
| Material | Resistivity at 20 °C (Ω m) | Where it is used |
|---|---|---|
| Silver | 1.60 × 10⁻⁸ | The best conductor; too costly for wiring |
| Copper | 1.62 × 10⁻⁸ | House wiring |
| Aluminium | 2.63 × 10⁻⁸ | Overhead power lines (light and cheap)Q CDS 2017 (II) and 2018 (I): an aluminium wire of the same size as a copper wire has more resistance, so it gives out more heat for the same current. |
| Tungsten | 5.20 × 10⁻⁸ | Bulb filaments |
| Nichrome (alloy) | About 1.1 × 10⁻⁶ | Heater coils |
| Glass | 10¹⁰ to 10¹⁴ | An insulatorQ CDS 2022 (I): silver, nichrome and glass in increasing order of resistivity. |
| Rubber | 10¹³ to 10¹⁶ | Covering on wires |
Watch out for (7)
- Thickness counts through the square of the diameter→ Ohm's law and the resistance of a wire
- Stretching changes both length and area→ Ohm's law and the resistance of a wire
- Resistivity is not resistance→ Resistivity: a property of the material
- The unit is ohm metre, not ohm per metre→ Resistivity: a property of the material
- Aluminium resists more than copper→ Resistivity: a property of the material
- The parallel total is smaller than the smallest resistor→ Resistors in series and in parallel
- Most heat from a battery comes from the smallest total resistance→ Resistors in series and in parallel
Formulas (2)
Reference tables (2)
Fuses, short circuits and house wiring6 rows
| Wire or part | Fact |
|---|---|
| Live and neutral | 220 V r.m.s. between themQ CDS 2023 (I): the r.m.s. voltage between live and neutral in Indian homes is 220 V. |
| Live wire | Red insulation (the code in NCERT); the fuse and switch go in it |
| Neutral wire | Black insulation (the code in NCERT) |
| Earth wire | Green insulation; joined to the metal body of an applianceQ CDS 2022 (I): a green wire is the earth wire, not the live wire. |
| Fuse | Low melting point wire in the live line; melts when the current is too bigQ CDS 2018 (II): a 5 A fuse at 220 V allows 1100 W, so eleven 100 W bulbs. |
| Short circuit | Live meets neutral; the current rises sharply and the fuse blows |
Bulbs and energy changes7 rows
| Device | Energy change |
|---|---|
| Filament bulb | Electrical to light, and much more heat |
| LED | Electrical to light, very little heat |
| Laser | Electrical to a narrow beam of light of one colour |
| Television set | Electrical to light and sound |
| Electric heater or iron | Electrical to heat |
| Electric motor or fan | Electrical to mechanical |
| Candle | Chemical to heat and lightQ CDS 2018 (II): the candle is the one that does not turn electrical energy into light. |
Watch out for (8)
- The resistance stays fixed, not the power→ Electric power and the heating effect
- More watts means less resistance→ Electric power and the heating effect
- Kilowatts times hours, not watts times hours→ Electrical energy and the unit on the bill
- A unit is energy, not power→ Electrical energy and the unit on the bill
- Green is the earth wire→ Fuses, short circuits and house wiring
- The fuse goes in the live wire→ Fuses, short circuits and house wiring
- 220 V is the r.m.s. value→ Fuses, short circuits and house wiring
- The gas in a bulb is for a longer life→ Bulbs and energy changes
Formulas (2)
Reference tables (2)
Magnets, magnetic materials and field lines3 rows
| Type | Behaviour near a magnet | Examples |
|---|---|---|
| Diamagnetic | Weakly pushed away | Bismuth, copper, silver, water |
| Paramagnetic | Weakly pulled in | Aluminium, sodium, oxygen |
| Ferromagnetic | Strongly pulled in; can become a magnet | Iron, cobalt, nickelQ CDS 2016 (II): liquids and gases never show ferromagnetism. |
Magnetic field of a current and the hand rules4 rows
| Rule | What it gives | How to hold the hand |
|---|---|---|
| Right-hand thumb rule | Direction of the field round a current | Thumb along the current; curled fingers show the fieldQ CDS 2025 (I): the right-hand thumb rule gives the field round a straight conductor. |
| Fleming's left-hand rule | Direction of the force on a current in a field (motor rule) | Forefinger field, middle finger current, thumb force |
| Fleming's right-hand rule | Direction of the induced current (generator rule) | Forefinger field, thumb motion, middle finger induced current |
| Hund's rule | Not a magnetism rule: it fills electrons into orbitals | Not a hand rule (chemistry) |
Watch out for (11)
- Ferromagnetism needs a solid→ Magnets, magnetic materials and field lines
- Field lines never cross, and need not be parallel→ Magnets, magnetic materials and field lines
- Declination alone does not give the direction→ Magnets, magnetic materials and field lines
- Thumb rule for the field, Fleming's rules for force and induced current→ Magnetic field of a current and the hand rules
- Between opposite currents the fields add→ Magnetic field of a current and the hand rules
- Inside a long solenoid the lines are straight→ Magnetic field of a current and the hand rules
- No force along the field→ Force on a moving charge and on a current
- Turn the answer round for an electron→ Force on a moving charge and on a current
- Step-up voltage means step-down current→ Electromagnetic induction and the transformer
- A magnet at rest induces nothing→ Electromagnetic induction and the transformer
- A transformer heats up from its own losses→ Electromagnetic induction and the transformer
PYQ weightage by concept
14 concepts · 44 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
14 concepts · 44 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Static charge, conductors and insulators | 3 | 7% |
| Potential difference and the energy a charge gains | 2 | 5% |
| Current, cells and emf | 2 | 5% |
| Concept | PYQs | Share |
|---|---|---|
| Resistivity: a property of the material | 6 | 14% |
| Resistors in series and in parallel | 4 | 9% |
| Ohm's law and the resistance of a wire | 2 | 5% |
| Concept | PYQs | Share |
|---|---|---|
| Fuses, short circuits and house wiring | 5 | 11% |
| Electric power and the heating effect | 3 | 7% |
| Electrical energy and the unit on the bill | 2 | 5% |
| Bulbs and energy changes | 2 | 5% |
| Concept | PYQs | Share |
|---|---|---|
| Magnets, magnetic materials and field lines | 4 | 9% |
| Magnetic field of a current and the hand rules | 4 | 9% |
| Electromagnetic induction and the transformer | 3 | 7% |
| Force on a moving charge and on a current | 2 | 5% |
Test yourself on Electricity and Magnetism
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.