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SEC 2027 · G3 SEC 4 Combined Physics

Sec 4 Combined Physics
Visual Notes

Pictures first. Formulas boxed. Every exam keyword highlighted.

HOW TO STUDY WITH THIS

Read the 💡 box
the whole topic in 1 line

Study the diagram
cover it, redraw it

Learn the yellow
formulas & keywords

Redo the worked example
without looking

THE CHAPTERS

🌊

CH 1

General Properties of Waves

v = fλ

📡

CH 2

Electromagnetic Spectrum

order

💡

CH 3

Light: Reflection & Refraction

laws

🔍

CH 4

Lenses

converging lens

🔋

CH 5

Charge & Current Electricity

Q = It

🔀

CH 6

D.C. Circuits

series

🔌

CH 7

Practical Electricity

power

🧲

CH 8

Magnetism & Electromagnetism

magnetic fields

☢️

CH 9

Radioactivity

α β γ

📌 NOTESchools split Sec 3 / Sec 4 physics differently — check the other year if a chapter is missing.

🖨 PRINTCtrl+P → Save as PDF · Scale 100% · Margins None · Background graphics ON

Sec 4 Combined Physics · SEC K326

🌊

General Properties of Waves

v = fλ, transverse & longitudinal, sound

BIG IDEAWaves transfer energy without transferring matter. v = fλ · f = 1 ÷ T

crest-to-crest = wavelength; centre to crest = amplitude

Term	Meaning
amplitude A	max displacement from rest
wavelength λ	distance between successive crests
frequency f	waves per second (Hz)
period T	time for one wave (s)
wavefront	line joining points in phase (crests)

Transverse	Longitudinal
vibration ⟂ direction of travel	vibration ∥ direction of travel
crests & troughs	compressions & rarefactions
light, water, EM waves	sound

🔊 Sound

needs a medium (not in vacuum) · fastest in solids · loudness ↑ with amplitude · pitch ↑ with frequency · human range 20 Hz–20 kHz

Echo

distance = (speed × time) ÷ 2 — sound goes there and back

🔎 WHY IT WORKSWhy v = fλ: in one period T, the wave moves forward exactly one wavelength. So speed = λ ÷ T = fλ.

🧠 Worked example — follow the thinking

A boy claps 170 m from a wall and hears the echo 1.0 s later. Find the speed of sound.

Step	Why I do this
Total distance = 2 × 170 = 340 m	💭 Sound travels to the wall AND back.
v = 340 ÷ 1.0 = 340 m/s	💭 speed = distance ÷ time.

Sec 4 Combined Physics · SEC K326

📡

Electromagnetic Spectrum

order, uses, hazards

BIG IDEAAll EM waves are transverse, travel at 3.0 × 10⁸ m/s in vacuum and need no medium.

"Raging Martians Invaded Venus Using X-ray Guns"

Wave	Use	Hazard
radio	radio/TV broadcasting	—
microwave	cooking, mobile phones, satellites	internal heating of tissue
infrared	remote controls, thermal imaging, toasters	skin burns
visible	seeing, optical fibres, photography	eye damage (lasers)
ultraviolet	sterilising, detecting forged notes	sunburn, skin cancer
X-ray	medical images, airport security	ionising → cell damage, cancer
gamma	killing cancer cells, sterilising food	ionising → cancer, mutation

🔎 WHY IT WORKSWhy higher frequency is more dangerous: high-frequency EM waves carry more energy per "packet", enough to ionise atoms in cells and damage DNA. Low-frequency waves mainly just heat.

🧠 Worked example — follow the thinking

A radio station broadcasts at 100 MHz. Find the wavelength.

Step	Why I do this
v = 3.0 × 10⁸ m/s	💭 All EM waves have this speed.
λ = v ÷ f = 3.0 × 10⁸ ÷ 100 × 10⁶	💭 Rearrange v = fλ; M = 10⁶.
λ = 3.0 m	💭

Sec 4 Combined Physics · SEC K326

💡

Light: Reflection & Refraction

laws, refractive index

BIG IDEAReflection: i = r. Refraction: n = sin i ÷ sin r = c ÷ v. Light bends towards the normal entering a denser medium.

angles from the normal

Laws

reflection: i = r; incident ray, reflected ray and normal in the same plane
refraction: light changes speed → changes direction
plane-mirror image: same size, upright, laterally inverted, virtual, same distance behind

Refractive index n

n = speed in vacuum ÷ speed in medium
glass n ≈ 1.5 → light at 2 × 10⁸ m/s in glass
no bending if ray enters along the normal

Everyday

pool looks shallower · straw looks bent · mirage

🔎 WHY IT WORKSWhy a pool looks shallower: light from the bottom bends away from the normal as it leaves water. Your brain traces the ray back in a straight line, so the bottom seems higher than it is.

🧠 Worked example — follow the thinking

Light hits glass (n = 1.5) at i = 40°. Find r.

Step	Why I do this
sin r = sin 40° ÷ 1.5	💭 n = sin i ÷ sin r rearranged.
= 0.643 ÷ 1.5 = 0.4285	💭 Calculator in DEGREE mode.
r = sin⁻¹(0.4285) = 25.4°	💭 r < i: bends towards the normal (denser).

Sec 4 Combined Physics · SEC K326

🔍

Lenses

converging lens, focal length, images

BIG IDEAA converging (convex) lens bends parallel rays to meet at the focal point F. Focal length f = lens to F.

ray 1: parallel → through F · ray 2: through the optical centre → straight on

Object	Image	Image is	Use
beyond 2F	between F and 2F	real, inverted, diminished	camera, eye
at 2F	at 2F	real, inverted, same size	photocopier (1:1)
between F and 2F	beyond 2F	real, inverted, magnified	projector
at F	at infinity	—	spotlight
inside F	same side as object	virtual, upright, magnified	magnifying glass

Real vs virtual

real: rays actually meet → can be caught on a screen
virtual: rays only seem to come from it

Find f quickly

focus a distant object (window) on a screen → lens-to-screen distance ≈ f

🔎 WHY IT WORKSWhy the rays are chosen: we already know where these special rays go without measuring angles. Two rays from the top of the object are enough — where they meet is the top of the image.

🧠 Worked example — follow the thinking

An object is 30 cm from a lens of focal length 10 cm. Describe the image.

Step	Why I do this
2F = 20 cm, object at 30 cm → beyond 2F	💭 Compare the object distance with f and 2f.
Image: real, inverted, diminished, between F and 2F	💭 Use the table / two rays.

Sec 4 Combined Physics · SEC K326

🔋

Charge & Current Electricity

Q = It, e.m.f., p.d., resistance

BIG IDEAQ = It · V = W ÷ Q · R = V ÷ I. Current = rate of flow of charge.

Quantity	Definition	Unit
charge Q	—	coulomb C
current I	charge per unit time	ampere A
e.m.f. E	work done by source per unit charge to drive charge round the whole circuit	volt V
p.d. V	work done per unit charge across a component	volt V
resistance R	V ÷ I	ohm Ω

Direction

conventional current: + → −
electron flow: − → + (opposite)

Resistance of a wire

R ∝ length (longer → more collisions) · R ∝ 1 ÷ cross-sectional area (thicker → easier) · R = ρL ÷ A

🔎 WHY IT WORKSWhy a longer wire has more resistance: electrons collide with more metal ions along a longer path. A thicker wire gives more paths side by side, so the current flows more easily.

🧠 Worked example — follow the thinking

A charge of 120 C flows through a 6 V lamp in 1 minute. Find I, R and the energy transferred.

Step	Why I do this
I = Q ÷ t = 120 ÷ 60 = 2 A	💭 t must be in seconds.
R = V ÷ I = 6 ÷ 2 = 3 Ω	💭 Definition of resistance.
W = QV = 120 × 6 = 720 J	💭 p.d. = energy per unit charge.

Sec 4 Combined Physics · SEC K326

🔀

D.C. Circuits

series, parallel, effective resistance

BIG IDEASeries: same current, voltages add, R = R₁ + R₂. Parallel: same p.d., currents add, 1/R = 1/R₁ + 1/R₂.

Symbol	Component
—\|‖—	cell (long line +)
—⊗—	lamp
—▭—	fixed resistor
—▭↗—	variable resistor
—(A)—	ammeter (series)
—(V)—	voltmeter (parallel)
—▭~—	fuse
—▷\|—	diode / LED (arrows out)

Series

I same everywhere
V_total = V₁ + V₂
R_total = R₁ + R₂ (bigger than each)

Parallel

V same across each branch
I_total = I₁ + I₂
R_total smaller than the smallest branch

🔎 WHY IT WORKSWhy parallel R is smaller: each new branch is an extra path for charge, so more current flows for the same p.d. — and R = V ÷ I gets smaller.

🧠 Worked example — follow the thinking

6 Ω and 3 Ω in parallel, in series with 4 Ω, on a 12 V supply. Find the current from the supply.

Step	Why I do this
Parallel: 1/R = 1/6 + 1/3 = 1/2 → R = 2 Ω	💭 Combine the parallel pair first.
Total R = 2 + 4 = 6 Ω	💭 Then add in series.
I = V ÷ R = 12 ÷ 6 = 2 A	💭 Whole-circuit V = IR.

Sec 4 Combined Physics · SEC K326

🔌

Practical Electricity

power, energy cost, fuses, earthing, plug

BIG IDEAP = VI · E = VIt · cost = kWh × rate. Fuses and switches go on the live wire.

3-pin plug (UK/SG): live = brown, neutral = blue, earth = green/yellow

Safety devices

fuse: thin wire melts if I too large · rating just above normal current (3, 5, 13 A)
circuit breaker: trips, can be reset
earthing: metal case connected to earth — fault current flows to earth, blows fuse
double insulation (⧈): plastic casing, no earth wire needed

Hazards

damaged insulation → shock · overheating of cables (too much current) · damp / wet conditions → water conducts

Why on the live wire?

if the fuse/switch were on neutral, the appliance would still be at high voltage even when off/blown → shock risk

🔎 WHY IT WORKSWhy heating happens: charges collide with ions in the resistor, transferring energy to them — the resistor gets hot. Used in kettles, irons, heaters.

🧠 Worked example — follow the thinking

A 230 V kettle is rated 2.3 kW. Choose a fuse: 3 A, 5 A or 13 A.

Step	Why I do this
I = P ÷ V = 2300 ÷ 230 = 10 A	💭 Normal working current.
Fuse must be slightly higher than 10 A	💭 Too low → blows in normal use; too high → no protection.
Choose 13 A	💭

Sec 4 Combined Physics · SEC K326

🧲

Magnetism & Electromagnetism

magnetic fields, electromagnets, motor effect

BIG IDEAField lines go N → S. A current makes a magnetic field; a current in a field feels a force (Fleming's left-hand rule).

bar magnet field (closer lines = stronger)

Magnets

like poles repel, unlike attract · iron: soft — easily magnetised and loses it (temporary, electromagnets) · steel: hard — permanent magnets · induced magnetism: iron near a magnet becomes a magnet

Fields from currents

straight wire: circles round the wire — right-hand grip rule
solenoid: like a bar magnet; stronger with more turns, more current, iron core
reverse current → field reverses

✋ Fleming's left-hand rule

First finger = Field · seCond = Current · thuMb = Motion (force)
motor: coil in field → turning effect

🔎 WHY IT WORKSWhy a current-carrying wire moves in a field: the wire's own circular field adds to the magnet's field on one side and cancels it on the other. The wire is pushed from the strong-field side to the weak-field side.

🧠 Worked example — follow the thinking

A wire lies flat on the page. Field points to the right →, current flows up the page ↑. Which way is the force?

Step	Why I do this
First finger → field: point it right	💭 Fingers at right angles to each other.
Second finger → current: point it up the page	💭 Twist your LEFT hand to fit.
Thumb now points into the page → force into the page	💭 Thumb = motion. Reverse either I or B → force reverses.

Sec 4 Combined Physics · SEC K326

☢️

Radioactivity

α β γ, half-life, isotopes

BIG IDEAUnstable nuclei decay randomly and spontaneously, giving out α, β or γ. Half-life = time for half the nuclei to decay.

Nuclide notation

ᴬ_ZX
A = nucleon number (p + n) · Z = proton number
isotopes: same Z, different A (e.g. ¹²C, ¹⁴C)

each half-life halves the activity

	α alpha	β beta (β⁻)	γ gamma
Is	helium nucleus (2p 2n)	fast electron	EM wave
Ionising	strongest	medium	weakest
Stopped by	paper / few cm air	few mm aluminium	thick lead / concrete
Charge	+2	−1	0

Uses

γ: kill cancer cells, sterilise · β: paper thickness control · α: smoke detectors · tracers (short half-life)

Safety

background radiation from rocks, cosmic rays, food · minimise time, maximise distance, use shielding · tongs, lead aprons

🔎 WHY IT WORKSWhy we use half-life: we can never say WHEN one nucleus will decay — it is random. But with billions of nuclei, the fraction left after a set time is very predictable.

🧠 Worked example — follow the thinking

A sample has activity 800 Bq; half-life 3 days. Find activity after 12 days.

Step	Why I do this
12 ÷ 3 = 4 half-lives	💭 Count how many half-lives pass.
800 → 400 → 200 → 100 → 50	💭 Halve each time.
Activity = 50 Bq	💭

Sec 4 Combined Physics · SEC K326

Sec 4 Combined PhysicsVisual Notes

General Properties of Waves

Electromagnetic Spectrum

Light: Reflection & Refraction

Lenses

Charge & Current Electricity

D.C. Circuits

Practical Electricity

Magnetism & Electromagnetism

Radioactivity

Sec 4 Combined Physics
Visual Notes