PA08 — Magnetism & Magnetic Effects of Current

✈ Physics – PA08 · AFCAT General Awareness AFCAT Level

Magnetism connects electricity with motion — a flowing current creates a magnetic field, and a moving conductor in a field generates electricity. For AFCAT, focus on practical outcomes: which hand rule for which device, the transformer turns ratio, and what an electromagnet is used for.

📌 AFCAT Focus: Fleming's Left-Hand Rule (electric motor) vs Fleming's Right-Hand Rule (generator); transformer formula V₁/V₂ = N₁/N₂; properties of magnets (like poles repel, unlike attract); Earth's magnetic elements (declination, dip); Oersted's experiment (current produces magnetism).

1. Basic Magnetism

📍 Properties of Magnets

Every magnet has two poles: North (N) and South (S)
Like poles repel; Unlike poles attract
Poles cannot be isolated — cut a magnet, each piece has both poles (no monopoles)
A freely suspended magnet aligns along N-S direction (compass)
Magnetic field lines: from N to S outside; S to N inside
Field lines never cross each other

📍 Earth's Magnetism

Earth acts like a giant bar magnet
Geographic North ≠ Magnetic North (compass shows magnetic north)
Declination: Angle between geographic and magnetic north at a location
Dip (Inclination): Angle the Earth's field makes with horizontal. At equator: 0°; At poles: 90°
Magnetic poles drift slowly over years

2. Magnetic Effect of Current — Oersted's Experiment

Fig. 1 — Oersted's Discovery: Current Creates Magnetic Field Around a Wire

3. Fleming's Rules & Devices

Fig. 2 — Fleming's Left-Hand Rule (Motor) vs Right-Hand Rule (Generator)

4. Electromagnetic Induction & Transformer

Faraday's Law & Transformer:

● Faraday's Law: EMF is induced whenever magnetic flux through a circuit changes
● Lenz's Law: The induced current always opposes the change causing it (conservation of energy)

● Transformer Ratio: V₁/V₂ = N₁/N₂ = I₂/I₁
Step-up: N₂ > N₁ → V₂ > V₁ (increases voltage, decreases current)
Step-down: N₂ < N₁ → V₂ < V₁ (decreases voltage, increases current)
● Transformers work on AC only (NOT DC)
● Used in power transmission: step UP voltage (reduces I²R power loss in lines)

📝 AFCAT PYQs — Magnetism

Q1. Fleming's Left-Hand Rule is used to find the direction of force on: AFCAT PYQ

(a) A magnetic pole in a magnetic field(b) A current-carrying conductor in a magnetic field(c) An induced current(d) A moving charge in electric field

✔ Answer: (b) A current-carrying conductor in a magnetic field

Fleming's Left-Hand Rule (FBI rule) gives the direction of force (thrust) on a current-carrying conductor placed in a magnetic field — the principle behind the electric motor. Forefinger = Field direction; Middle finger = Current direction; Thumb = Force/motion direction. Left Hand = Motor; Right Hand = Generator.

Q2. A transformer has primary turns N₁ = 200 and secondary turns N₂ = 1000. Primary voltage = 220V. Secondary voltage is: AFCAT PYQ

(a) 44 V(b) 1100 V(c) 220 V(d) 2200 V

✔ Answer: (b) 1100 V

V₁/V₂ = N₁/N₂ → V₂ = V₁ × N₂/N₁ = 220 × 1000/200 = 220 × 5 = 1100 V. Since N₂ > N₁, this is a step-up transformer — voltage increases, current decreases proportionally. Power input = Power output (ideal transformer). Used to transmit electricity at high voltage (reduces line losses).

Q3. Lenz's Law is a consequence of which conservation law? AFCAT PYQ

(a) Conservation of charge(b) Conservation of momentum(c) Conservation of energy(d) Conservation of mass

✔ Answer: (c) Conservation of energy

Lenz's Law states the induced current opposes the change (motion or field change) that causes it. This opposition means you must do work against it — that work is converted to electrical energy. If the induced current aided the change, it would create energy from nothing — violating conservation of energy. Lenz's Law is fundamentally an energy conservation principle.

🧠 Quick Memory Chart — PA08 Magnetism

🤕 Hand Rules

Left Hand = Motor (electrical→mechanical)
Right Hand = Generator (mechanical→electrical)
FBI: Field, current (I), Force
Lenz: opposes the change
Lenz = energy conservation

⚡ Transformer

V₁/V₂ = N₁/N₂
Step-up: N₂>N₁ (more V)
Step-down: N₂<N₁ (less V)
AC only; NOT DC
Power in = Power out (ideal)

📍 Magnets & Earth

Like poles repel; unlike attract
No monopoles — cut = 2 magnets
Declination: geographic vs magnetic N
Dip: 0° equator; 90° poles
Oersted: current → magnetic field

📝 Practice Exercise

E1. Which device converts mechanical energy into electrical energy?

(a) Motor(b) Transformer(c) Generator(d) Electromagnet

E2. A transformer steps voltage from 11,000V to 220V. If primary has 500 turns, secondary turns are:

(a) 25,000(b) 100(c) 10(d) 1000

E3. The angle of dip at the Earth's equator is:

(a) 90°(b) 45°(c) 0°(d) 30°

Answers: E1 → (c) Generator [converts mechanical → electrical; motor does reverse] | E2 → (c) 10 [N₂ = N₁ × V₂/V₁ = 500 × 220/11000 = 10] | E3 → (c) 0° [Earth's field is horizontal at equator → no vertical component → dip = 0°]

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