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Physics · CDS

PC09 — Magnetism & Magnetic Effects of Current

📖 PC09 · CDS General Science — Physics 🎯 CDS Level : High Priority

Magnetism connects naturally to electricity — a flowing current creates a magnetic field, and a changing magnetic field creates electricity. This chapter covers basic magnetism, Earth's magnetic field, electromagnetism, and the principle behind motors and generators — all regularly tested in CDS.

📌 CDS focuses on: Properties of magnets and magnetic poles; Earth's magnetic field and its elements; Oersted's experiment (current produces magnetism); Fleming's Left-Hand Rule (motor action); Fleming's Right-Hand Rule (generator); electromagnetic induction (Faraday's law); practical devices — electric motor, generator, transformer.

Topics at a Glance

① Magnetism Basics

Properties, poles, magnetic field lines

② Earth's Magnetism

Geographic vs magnetic poles, declination

③ Magnetic Effect of Current

Oersted, solenoid, electromagnet

④ Force on a Conductor

Fleming's LHR; electric motor

⑤ Electromagnetic Induction

Faraday, Lenz, Fleming's RHR, generator

⑥ Transformer

Step-up, step-down; turns ratio

1. Magnetism — Basic Properties

1.1

Properties of Magnets & Magnetic Field Lines

Every magnet has two poles — and they follow strict rules

📍 Properties of Magnets

Every magnet has a North pole and a South pole — cannot be separated (monopoles don't exist)
Like poles repel; unlike poles attract
A freely suspended magnet always aligns N–S (used in compass)
Magnetic properties are concentrated near the poles
Magnets can be demagnetised by heating, hammering, or AC
Materials: Ferromagnetic (iron, nickel, cobalt) — strongly attracted

📍 Magnetic Field Lines

Closed loops from N to S outside the magnet; S to N inside
Never cross each other
Closer lines → stronger field
Tangent to field line = direction of field at that point
SI unit of magnetic field: Tesla (T)
Earth's field ~25–65 microtesla at surface

2. Earth's Magnetism

2.1

Geographic vs Magnetic Poles & Elements

Earth behaves like a giant bar magnet — with important offsets from geographic poles

⚡ Elements of Earth's Magnetism

Magnetic Declination: Angle between geographic north and magnetic north at any location. Compass does NOT point to exact geographic North — it points to magnetic North. Magnetic Dip (Inclination): Angle made by Earth's total magnetic field with the horizontal. At magnetic equator: dip = 0° (field is horizontal) At magnetic poles: dip = 90° (field is vertical) In India: dip is approximately 20°–30° (northern India) to 10° (southern India) Horizontal Component (H): The horizontal part of Earth's field — what a compass needle aligns with. H = B cos(dip angle)

The geographic North Pole is where Earth's rotation axis meets the surface. The magnetic North Pole is different — it is actually a south magnetic pole (since the north end of a compass needle is attracted to it). Earth's magnetic poles slowly drift over time.

3. Magnetic Effect of Electric Current

3.1

Oersted's Discovery & Electromagnets

A current-carrying conductor creates a magnetic field around it

Oersted's Experiment (1820): Hans Christian Oersted discovered that when a compass needle was placed near a current-carrying wire, it deflected. This proved that electric current produces a magnetic field — the first link between electricity and magnetism. Reversing the current reversed the deflection.

Fig. 1 — A solenoid (coil of wire) creates a magnetic field identical to a bar magnet when current flows. The right-hand thumb rule identifies the North pole.

📍 Electromagnet

A solenoid with a soft iron core — greatly amplifies the magnetic field
Soft iron: easy to magnetise and demagnetise (temporary magnet)
Hard steel: remains magnetised (permanent magnet)
Field strength increases with: more turns, more current, iron core
Used in: electric bells, doorbells, cranes (scrap metal lifting), MRI scanners

📍 Fleming's Left-Hand Rule (Motor)

Used to find the direction of force on a current-carrying conductor in a magnetic field
Forefinger → direction of magnetic Field (B)
Centre finger → direction of Current (I)
Thumb → direction of motion/Thrust (Force F)
Mnemonic: FBI — Force, B-field, current I
Applies to: electric motors, galvanometers

4. Electromagnetic Induction & Transformer

4.1

Faraday's Law, Lenz's Law & Fleming's Right-Hand Rule

A changing magnetic field induces an EMF — the principle behind generators

⚡ Laws of Electromagnetic Induction

Faraday's First Law: Whenever the magnetic flux through a circuit changes, an EMF is induced. (No change in flux → no induced EMF) Faraday's Second Law: Induced EMF (ε) is proportional to the rate of change of magnetic flux: ε = −dΦ/dt (Φ = magnetic flux = B × A × cos θ) Lenz's Law: The induced current opposes the change that caused it. (If you push a magnet in, induced current creates a field opposing entry) This is consistent with conservation of energy. Fleming's Right-Hand Rule (Generator): Forefinger → Magnetic Field (B) Centre finger → induced Current direction Thumb → Motion of conductor (Right hand for generator; Left hand for motor)

Motor vs Generator: Motor converts electrical energy to mechanical energy (uses LHR). Generator converts mechanical energy to electrical energy (uses RHR). Both use the interaction of current and magnetic fields.

📍 Transformer

Works on mutual induction — AC only (not DC)
Turns ratio: V₁/V₂ = N₁/N₂ = I₂/I₁
Step-up: N₂ > N₁ → V₂ > V₁ (increases voltage, reduces current)
Step-down: N₂ < N₁ → V₂ < V₁ (decreases voltage, increases current)
Ideal transformer: 100% efficient (P_in = P_out); real ones have core and copper losses
Power transmission: stepped up to high voltage (reduces I²R losses in lines)

📍 AC vs DC

DC (Direct Current): flows in one direction only; batteries, solar cells
AC (Alternating Current): reverses direction periodically; mains supply
India: 230V AC at 50 Hz (50 cycles per second)
Transformer only works with AC (DC doesn't change flux)
AC is used for power transmission — easier to step up/down
Rectifier converts AC to DC (used in chargers)

📝 CDS PYQ

Magnetism & Electromagnetism

Q1. Which rule is used to find the direction of force on a current-carrying conductor placed in a magnetic field?

(a) Right-hand thumb rule
(b) Fleming's Right-Hand Rule
(c) Fleming's Left-Hand Rule
(d) Lenz's Law

Answer: (c) Fleming's Left-Hand Rule
Fleming's Left-Hand Rule (FBI rule) gives the direction of force (thrust) on a current-carrying conductor in a magnetic field — the basis of the electric motor. Forefinger = Field, Centre = Current, Thumb = Force. Fleming's Right-Hand Rule is for generators (induced current). Right-hand thumb rule is for finding the magnetic field direction around a straight wire.

Q2. A transformer has 100 turns in the primary and 500 turns in the secondary coil. If the primary voltage is 220V, the secondary voltage is:

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

Answer: (b) 1100 V
V₂/V₁ = N₂/N₁ → V₂ = V₁ × N₂/N₁ = 220 × 500/100 = 220 × 5 = 1100 V. Since N₂ > N₁, this is a step-up transformer. Note: if the voltage increases, the current decreases proportionally (power is conserved in an ideal transformer).

Q3. Lenz's Law is a consequence of the law of conservation of:

(a) Charge
(b) Momentum
(c) Energy
(d) Mass

Answer: (c) Energy
Lenz's Law states that the induced current opposes the change causing it. This opposition requires work to be done — meaning mechanical energy is converted to electrical energy. If the induced current aided the change instead of opposing it, it would create energy from nothing, violating conservation of energy. Lenz's Law is fundamentally a consequence of energy conservation.

📚 Formula Sheet — PC09

⚡ Electromagnets

Right-hand thumb rule: thumb = N pole
Force on conductor: Fleming's LHR
Solenoid field ∝ n (turns/m) × I
Soft iron = temporary magnet (motors)
Hard steel = permanent magnet

📈 Induction

EMF = −dΦ/dt (Faraday)
Lenz: induced current opposes change
Generator: mechanical → electrical (RHR)
Motor: electrical → mechanical (LHR)
AC frequency in India: 50 Hz

⚡ Transformer

V₁/V₂ = N₁/N₂ (turns ratio)
I₁/I₂ = N₂/N₁ (current ratio)
Step-up: N₂>N₁; Step-down: N₂<N₁
Works only with AC
P₁ = P₂ (ideal transformer)

🌎 Earth's Magnetism

Declination: angle between geographic and magnetic north
Dip: 0° at equator; 90° at poles
Compass → magnetic north (not geographic)
Geographic N pole ≠ magnetic N pole
Magnetic poles drift over time

⚡ Quick Revision Booster — PC09

🤖 LHR vs RHR

LHR = Left Hand = Motor (F, B, I)
RHR = Right Hand = Generator
Motor: electrical → mechanical
Generator: mechanical → electrical
LHR: FBI — Force, B-field, current I

⚡ Transformer Rules

V₁/V₂ = N₁/N₂
Step-up: more turns → more voltage
Step-down: fewer turns → less voltage
AC only — DC won't work
Power transmission: step up (less I²R loss)

🚨 CDS Traps

LHR for motor; RHR for generator
Geographic N ≠ Magnetic N pole
Transformer needs AC not DC
Lenz → energy conservation
Dip = 0° at equator; 90° at poles

🔋 Practical Devices

Electric bell: electromagnet + AC
MRI: strong superconducting magnet
Induction cooker: eddy currents
AC generator: slip rings
DC motor: commutator (split rings)

⚖ Faraday's Laws

EMF induced when flux changes
More turns = more EMF
Faster motion = more EMF
Stronger magnet = more EMF
No change in flux → no EMF

📍 Magnets

Like poles repel; unlike attract
Monopoles don't exist
Field lines: N to S outside
Cannot be separated at poles
Demagnetise: heat or hammer

📝 Practice Exercise

Answer independently, then check.

E-01

A transformer steps 220V down to 11V. If the primary has 1000 turns, the secondary has:

(a) 50 turns
(b) 20000 turns
(c) 200 turns
(d) 500 turns

E-02

At the magnetic equator, the angle of dip is:

(a) 90°
(b) 45°
(c) 0°
(d) 180°

E-03

The device that converts mechanical energy into electrical energy is:

(a) Electric motor
(b) Transformer
(c) AC Generator
(d) Solenoid

E-04

Which material is best for making the core of an electromagnet?

(a) Hard steel
(b) Soft iron
(c) Copper
(d) Aluminium

Answers: E-01: (a) 50 turns [N₂ = N₁×V₂/V₁ = 1000×11/220 = 50] | E-02: (c) 0° [At equator field is horizontal — no vertical component — dip = 0°] | E-03: (c) AC Generator [converts mechanical → electrical; motor does the reverse] | E-04: (b) Soft iron [easy to magnetise and demagnetise — ideal for temporary electromagnets]

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