NDA Physics · Electricity and Magnetism

Electrical Devices and Safety

The recall layer of the chapter: which material does what (nichrome heats, tungsten lights), how fuses and earthing keep us safe, how generators, motors and transformers work, and what ammeters, voltmeters and galvanometers measure.

All Electricity and Magnetism notes NDA Physics strategy

Why this matters

Fifteen PYQs — almost all EASY single-fact recall, the most reliable marks in the chapter. The bank reuses the same facts every year: nichrome heating elements, tungsten filaments, low-melting-point fuses, the red/green/black wiring code, generator-vs-motor, transformers needing AC, and the ammeter-in-series / voltmeter-in-parallel rule. Drill the tables and bank the marks.

Concept 1 of 5

Heating elements and bulb filaments

Intuition

Two jobs, two metals. A heater needs a wire that resists current (to get hot) without melting — that's nichrome. A bulb filament needs to glow white-hot without melting at all — that's tungsten, the metal with the highest melting point in common use.

Definition

Match the device to the material the bank tests:

Device / part	Material	Why
Heating element (iron, heater, toaster)	Nichrome	High resistivity (heats well) + high melting point + doesn't oxidiseQ
Incandescent bulb filament	Tungsten	Highest melting point (~3400°C) — glows white-hot without meltingQ
Photoelectric cell	Rubidium / caesium	Alkali metals have a low work function — emit electrons easily under lightQ NDA 2018 Apr — photo-cell metal is rubidium (an alkali metal), NOT tungsten or copper.

Nichrome HEATS, tungsten LIGHTS, alkali metals (rubidium/caesium) EMIT electrons in photocells.

Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 1Electricity and MagnetismEASY

The heating element in an electric iron is usually made of

[Q54 · Sep · 2023]

Nichrome heats, tungsten lights — don't swap them

Tungsten's selling point is its melting point (for a glowing filament); nichrome's is high resistivity with durability (for a heater). Swapping the two metals is the standard distractor.

Drill 3 more on heating elements and bulb filaments

Concept 2 of 5

Fuses, earthing and household wiring

Intuition

Safety devices all exploit one idea: a dangerous fault means a big current. A fuse is a thin wire with a LOW melting point placed in series — too much current and it melts, breaking the circuit. The three household wires are colour-coded so the earth wire can safely carry away fault current.

Definition

Household electrical safety facts the bank tests:

Item	Key fact
Fuse wire	Conducting, low melting point; in SERIES — melts and breaks the circuit on excess currentQ
Short circuit	Resistance drops near zero ⟹ current increases substantially (which is what blows the fuse)Q
Three-wire colour code	Red = live, Green = earth (ground), Black = neutralQ NDA 2018 Sep — the OLD Indian code: red live, green earth, black neutral (don't confuse with newer brown/green-yellow/blue).

A fault ⟹ large current ⟹ the low-melting-point fuse melts first, protecting the rest of the circuit.

Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 2Electricity and MagnetismEASY

A fuse wire must be

[Q120 · Apr · 2019]

Fuse = LOW melting point (and conducting)

A fuse must conduct normally but melt easily on overload, so it needs a LOW melting point. 'High melting point' and 'insulator' are both wrong — an insulator wouldn't carry the normal current at all.

Drill 2 more on fuses, earthing and household wiring

Concept 3 of 5

Generators, motors and the AC/DC distinction

Intuition

A generator turns motion into current (electromagnetic induction); a motor does the reverse, turning current into motion. The ONLY hardware difference between an AC and a DC generator is how the coil connects to the outside world: slip rings give AC, a split-ring commutator gives DC.

Definition

Rotating-machine facts:

Device / question	Answer
Generator / dynamo works on…	Faraday's law of electromagnetic inductionQ
Device used to produce electric current	Generator (a motor consumes current; a galvanometer detects it)Q
Convert an AC generator to DC	Replace slip rings with a split-ring commutatorQ NDA 2023 Sep — slip rings ⟹ AC output; a split-ring commutator ⟹ DC output. That ring is the only change.

Generator = motion → current (induction). Motor = current → motion. Commutator (split-ring) = the AC→DC converter.

Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 3Electricity and MagnetismEASY

A DC generator works on the principle of

[Q100 · Sep · 2022]

Generator produces current; motor consumes it

A motor runs ON current (it doesn't produce it); a galvanometer DETECTS current. The device that PRODUCES current is the generator. And the AC↔DC switch is purely the slip-ring vs split-ring choice.

Drill 2 more on generators, motors and the ac/dc distinction

Concept 4 of 5

Transformers — changing AC voltage

Intuition

A transformer trades voltage for current (and vice versa) using two coils linked by a changing magnetic field. More turns on the output coil means higher voltage (step-up); fewer means lower (step-down). Crucially, it only works on AC — a steady DC makes no changing flux, so nothing is induced.

Definition

A transformer changes an AC voltage using the ratio of turns: ** $\dfrac{V_s}{V_p} = \dfrac{N_s}{N_p}$ — more secondary turns ⟹ step-up (higher voltage); fewer ⟹ step-down**. It transfers power (ideally $V_pI_p = V_sI_s$ ), so stepping voltage UP steps current DOWN. It cannot work on DC (no changing flux).

Transformer turns ratio

\dfrac{V_s}{V_p} = \dfrac{N_s}{N_p}

V_p, V_sprimary / secondary voltage
N_p, N_sprimary / secondary turns

Worked example

A transformer has 100 turns on the primary and 500 on the secondary, with 220 V applied to the primary. Is it step-up or step-down, and what is the secondary voltage?

More secondary turns (500 > 100) ⟹ step-UP.
$V_s = V_p \times N_s/N_p = 220 \times 500/100$ .
$V_s = 220 \times 5 = 1100$ V.

Answer:Step-up; secondary voltage = 1100 V.

Practice this conceptself-check · 3 quick reps

From the bank · past-year question

Example 4Electricity and MagnetismEASY

Step-up transformers are used for

[Q113 · Sep · 2017]

Transformers change VOLTAGE, not power — and need AC

A step-up transformer raises voltage (and lowers current); it does NOT 'increase electrical power'. And it works only on AC. The distractors 'increases power' and any DC use are wrong.

Drill 1 more on transformers — changing ac voltage

Concept 5 of 5

Meters, conductors and insulators

Intuition

Each meter has a job and a placement. An ammeter measures current, so it goes IN the path (series) and must have LOW resistance. A voltmeter measures voltage ACROSS something (parallel) and must have HIGH resistance so it barely draws current. A galvanometer just detects whether current flows.

Definition

Measuring instruments and conductor facts:

Instrument / term	Connection	Key property
Ammeter	In series	Low resistance (so it doesn't reduce the current)
Voltmeter	In parallel	High resistance (so it draws almost no current)Q NDA 2025 Apr — the WRONG statement is 'voltmeter low resistance, ammeter high resistance' — it's the reverse.
Galvanometer	—	Detects the presence of current in a circuitQ
Insulator	—	Electrons do NOT flow through it easily (few free electrons)Q

Ammeter: series + low R. Voltmeter: parallel + high R. Galvanometer: detects current. Insulator: electrons can't flow easily.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 5Electricity and MagnetismEASY

Which one of the following statements is NOT correct?

[Q146 · Apr · 2025]

Voltmeter HIGH resistance, ammeter LOW — the common swap

The false statement to catch: 'a voltmeter has low resistance and an ammeter has high resistance.' It's reversed. Voltmeter = high R (parallel); ammeter = low R (series).

Drill 2 more on meters, conductors and insulators

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Transformers — changing AC voltage
Transformer turns ratio
$\dfrac{V_s}{V_p} = \dfrac{N_s}{N_p}$

Reference tables (4)

Heating elements and bulb filaments3 rows

Device / part	Material	Why
Heating element (iron, heater, toaster)	Nichrome	High resistivity (heats well) + high melting point + doesn't oxidiseQ
Incandescent bulb filament	Tungsten	Highest melting point (~3400°C) — glows white-hot without meltingQ
Photoelectric cell	Rubidium / caesium	Alkali metals have a low work function — emit electrons easily under lightQ NDA 2018 Apr — photo-cell metal is rubidium (an alkali metal), NOT tungsten or copper.

Nichrome HEATS, tungsten LIGHTS, alkali metals (rubidium/caesium) EMIT electrons in photocells.

Fuses, earthing and household wiring3 rows

Item	Key fact
Fuse wire	Conducting, low melting point; in SERIES — melts and breaks the circuit on excess currentQ
Short circuit	Resistance drops near zero ⟹ current increases substantially (which is what blows the fuse)Q
Three-wire colour code	Red = live, Green = earth (ground), Black = neutralQ NDA 2018 Sep — the OLD Indian code: red live, green earth, black neutral (don't confuse with newer brown/green-yellow/blue).

A fault ⟹ large current ⟹ the low-melting-point fuse melts first, protecting the rest of the circuit.

Generators, motors and the AC/DC distinction3 rows

Device / question	Answer
Generator / dynamo works on…	Faraday's law of electromagnetic inductionQ
Device used to produce electric current	Generator (a motor consumes current; a galvanometer detects it)Q
Convert an AC generator to DC	Replace slip rings with a split-ring commutatorQ NDA 2023 Sep — slip rings ⟹ AC output; a split-ring commutator ⟹ DC output. That ring is the only change.

Generator = motion → current (induction). Motor = current → motion. Commutator (split-ring) = the AC→DC converter.

Meters, conductors and insulators4 rows

Instrument / term	Connection	Key property
Ammeter	In series	Low resistance (so it doesn't reduce the current)
Voltmeter	In parallel	High resistance (so it draws almost no current)Q NDA 2025 Apr — the WRONG statement is 'voltmeter low resistance, ammeter high resistance' — it's the reverse.
Galvanometer	—	Detects the presence of current in a circuitQ
Insulator	—	Electrons do NOT flow through it easily (few free electrons)Q

Ammeter: series + low R. Voltmeter: parallel + high R. Galvanometer: detects current. Insulator: electrons can't flow easily.

Watch out for (5)

Nichrome heats, tungsten lights — don't swap them
→ Heating elements and bulb filaments
Fuse = LOW melting point (and conducting)
→ Fuses, earthing and household wiring
Generator produces current; motor consumes it
→ Generators, motors and the AC/DC distinction
Transformers change VOLTAGE, not power — and need AC
→ Transformers — changing AC voltage
Voltmeter HIGH resistance, ammeter LOW — the common swap
→ Meters, conductors and insulators

Drill every past-year question on this subtopic

15 questions from the bank — paginated, with cart and Word-export support.

Drill the 15 questions