10th Class Physics Chapter 9 Notes PDF 2026 – Electromagnetic Induction and Electromagnetic Waves

Definition

Electromagnetic induction is the production of an e.m.f. (and so current, in a closed circuit) by a changing magnetic field linking a coil — either the field itself changes, or the coil/magnet moves relative to each other.

$ε = N \frac{ΔΦ}{Δ t}$ ( $N$ = number of turns, $ΔΦ$ = change in magnetic flux, $Δ t$ = time taken)

Rule

The induced e.m.f. increases if the rate of change of magnetic flux increases — this is exactly why pushing a magnet into a coil faster induces a larger e.m.f.: the same total flux change happens in less time, giving a bigger $ΔΦ/Δ t$ .

Definition

Lenz's law: the direction of an induced current is always such that it opposes the change that produced it. This supports the law of conservation of energy — if the induced current instead helped the change along, the effect would build on itself and create energy from nothing, which isn't possible; a real-life example is the resistance felt pushing a magnet into a coil, which is exactly the induced current's opposing magnetic field pushing back.

Example 1 — induced e.m.f. from changing flux

A $50$ -turn coil experiences a flux change from $0.3$ Wb to $0.8$ Wb over $0.4$ s. Find the average induced e.m.f.

$ΔΦ = 0.8 - 0.3 = 0.5$ Wb.
$ε = N \frac{ΔΦ}{Δ t} = 50 \times \frac{0.5}{0.4}$ .

Answer: $ε = 62.5$ V

Example 2 — induced e.m.f. and current in a loop

A circular loop, radius $0.2$ m, resistance $5 Ω$ , sits perpendicular to a field of $0.5$ T. The field drops to zero in $0.1$ s. Find (a) the induced e.m.f. (b) the induced current and its direction.

Area $A = π r^{2} = π (0.2)^{2} \approx 0.126$ m $^{2}$ .
Initial flux $Φ_{1} = B A = 0.5 \times 0.126 \approx 0.0628$ Wb; final flux $Φ_{2} = 0$ .
$ε = N \frac{ΔΦ}{Δ t} = 1 \times \frac{0.0628}{0.1} \approx 0.628$ V.
$I = \frac{ε}{R} = \frac{0.628}{5}$ .
By Lenz's law, the induced current opposes the decreasing into-the-page field, so it flows to try to maintain it — anticlockwise (as seen from the side the field points towards).

Answer: $ε \approx 0.628$ V, $I \approx 0.126$ A, anticlockwise

Definition

An A.C. generator converts mechanical energy into electrical energy: a coil is rotated in a magnetic field, continuously changing the flux through it and inducing an alternating e.m.f.

Slip rings (continuous, unsplit rings, unlike a D.C. motor's split commutator) connect the rotating coil to the external circuit while allowing the current's natural direction reversal each half-turn to pass through unchanged — this is exactly why they, rather than a commutator, are used to produce genuinely alternating output.

Induced e.m.f. is zero when the coil is momentarily parallel to the field (moving along the field lines rather than across them, so no flux is being cut at that instant) and maximum when the coil is perpendicular to the field.

Definition

A transformer transfers electrical energy from a primary coil to a secondary coil purely through a changing magnetic field (mutual induction) linking the two coils, wound on a shared iron core, with no direct electrical connection between them.

An iron core is used (rather than wood or plastic) because iron is easily magnetized and concentrates the magnetic field, efficiently guiding nearly all the flux from the primary coil to the secondary coil — wood or plastic wouldn't support or channel a magnetic field at all.

A transformer only works on A.C., not D.C., because it relies on a continuously changing flux to induce an e.m.f. — a steady D.C. current produces a constant, unchanging flux, so after the initial switch-on moment there's nothing left to induce anything in the secondary coil.

$\frac{V _{s}}{V _{p}} = \frac{N _{s}}{N _{p}}$ ( $V_{p}, V_{s}$ = primary/secondary voltage, $N_{p}, N_{s}$ = primary/secondary turns)

Definition

Step-up transformer: $N_{s} > N_{p}$ , so $V_{s} > V_{p}$ — used to raise voltage for efficient long-distance power transmission (lower current, less resistive loss). Step-down transformer: $N_{s} < N_{p}$ , so $V_{s} < V_{p}$ — used to lower voltage to safe levels for household and industrial use.

Example 3 — secondary voltage of a transformer

A transformer has $500$ primary turns and $100$ secondary turns; primary voltage is $220$ V. Find (a) the secondary voltage (b) whether it's step-up or step-down.

$\frac{V _{s}}{V _{p}} = \frac{N _{s}}{N _{p}} \Rightarrow V_{s} = V_{p} \times \frac{N _{s}}{N _{p}} = 220 \times \frac{100}{500}$ .
$N_{s} < N_{p}$ , so voltage decreases.

Answer: $V_{s} = 44$ V — a step-down transformer

Example 4 — step-up transformer output

A step-up transformer has turns ratio $2 : 100$ . An A.C. voltage of $25$ V is applied to the primary. Find the secondary voltage.

$V_{s} = V_{p} \times \frac{N _{s}}{N _{p}} = 25 \times \frac{100}{2}$ .

Answer: $V_{s} = 1250$ V

Example 5 — step-down transformer output

A step-down transformer has turns ratio $100 : 5$ . An A.C. voltage of $190$ V is applied to the primary. Find the secondary voltage.

$V_{s} = V_{p} \times \frac{N _{s}}{N _{p}} = 190 \times \frac{5}{100}$ .

Answer: $V_{s} = 9.5$ V

Definition

An oscilloscope displays how a voltage signal varies with time, tracing the waveform on a screen — letting you read off a signal's amplitude, frequency, and shape directly. Its main components are the electron gun (produces a beam of electrons), deflection plates (steer the beam vertically according to the input voltage, and horizontally at a steady sweep rate to represent time), and a fluorescent screen (glows where the beam strikes it, tracing the waveform).

Definition

An electron moving at right angles to a uniform magnetic field follows a circular path — the magnetic force on it is always perpendicular to its velocity (never speeding it up or slowing it down, only changing direction), which is exactly the condition for uniform circular motion, with the magnetic force acting as the centripetal force.

Definition

Light can be modelled as travelling in discrete packets of energy called photons — massless particles that carry momentum, which is why sunlight can exert a tiny but real push on an object (radiation pressure).

Solar sails use this photon momentum: sunlight striking a large, reflective sail transfers momentum to it with every reflected photon, providing continuous (if very gentle) propulsion through space without any fuel.

Definition

The sky looks blue during the day because sunlight scatters off gas molecules in the atmosphere, and shorter wavelengths (blue/violet) scatter far more strongly than longer wavelengths (red) — so blue light is scattered in all directions across the sky, reaching your eyes from every part of it, while most red light passes straight through.

At sunset, sunlight travels through a much longer path of atmosphere to reach your eyes; nearly all the strongly-scattered blue light is scattered away out of the direct line of sight along that long path, leaving mostly the weakly-scattered red and orange light to reach you directly — hence a red sky at sunset.

Quick Recap

● $ε = N \frac{ΔΦ}{Δ t}$ ; faster change $\to$ larger induced e.m.f.
●Lenz's law: induced current opposes the change that caused it (conserves energy)
●A.C. generator: rotating coil, slip rings give alternating output; e.m.f. zero when coil parallel to field, max when perpendicular
●Transformer: $\frac{V _{s}}{V _{p}} = \frac{N _{s}}{N _{p}}$ ; iron core concentrates flux; needs changing flux, so A.C. only
●Oscilloscope: electron gun $+$ deflection plates $+$ fluorescent screen, traces voltage vs time
●Charged particle $⊥$ field $\to$ circular path (magnetic force $=$ centripetal force)
●Light $=$ photons (massless, carry momentum $\to$ solar sails); sky blue by day (blue scatters most), red at sunset (long path scatters blue away)

10th Class Physics Chapter 9 Notes

Electromagnetic Induction

The A.C. Generator

Transformers

The Oscilloscope

A Charged Particle in a Magnetic Field

Light as Photons and the Colour of the Sky

More 10th Class Physics notes

Thermal Physics

Thermal Physics

Electronics

Electronics

Atomic and Nuclear Physics

Atomic and Nuclear Physics