Physics — Class 10 (CBSE)

Board: CBSE | Class: 10 | Subject: Science (Physics Chapters) Class 10 Physics is part of the integrated Science curriculum, covering Light, Electricity, and Magnetic Effects of Current.

Overview

Class 10 Physics covers three major areas: optics (light and its behaviour), electricity (circuits and current), and electromagnetism (magnetic effects of electric current). These are high-weightage topics in the Class 10 Board Exam.

Light — Reflection and Refraction

Reflection of Light

Laws of Reflection 1. Angle of incidence = Angle of reflection (∠i = ∠r) 2. Incident ray, reflected ray, and normal are all in the same plane

Spherical Mirrors

Feature	Concave Mirror	Convex Mirror
Reflecting surface	Inner (cave)	Outer (bulging)
Nature of image (distant object)	Real, inverted	Virtual, erect, diminished
Uses	Torch, shaving mirror, solar concentrators	Rear-view mirrors, security mirrors

Mirror Formula

1/f = 1/v + 1/u

Magnification m = -v/u = h'/h

Sign Convention (New Cartesian): All distances measured from pole; distances in direction of incident light are positive.

Object Position	Image Position	Nature
At infinity	At F	Real, inverted, point-sized
Beyond C	Between F and C	Real, inverted, diminished
At C	At C	Real, inverted, same size
Between C and F	Beyond C	Real, inverted, enlarged
At F	At infinity	Real, highly enlarged
Between F and P	Behind mirror	Virtual, erect, enlarged

Refraction of Light

Laws of Refraction (Snell's Law)

n₁ sin θ₁ = n₂ sin θ₂

Refractive index (n) = Speed of light in vacuum / Speed of light in medium
                     = c / v

Key phenomena: - Light bends towards normal when entering denser medium - Total Internal Reflection — when light travels from denser to rarer medium beyond critical angle - Apparent depth — objects in water appear closer; n = real depth / apparent depth

Lenses

Feature	Convex Lens	Concave Lens
Nature	Converging	Diverging
Image of distant object	Real, inverted at F	Virtual, erect at F
Uses	Magnifying glass, camera, eye correction (hypermetropia)	Spectacles for myopia

Lens Formula

1/f = 1/v - 1/u

Power of a lens

P = 1/f (f in metres)

SI unit: Dioptre (D); Convex lens → +P; Concave lens → −P

Human Eye defects: | Defect | Cause | Correction | |---|---|---| | Myopia (short-sightedness) | Image forms before retina | Concave lens | | Hypermetropia (far-sightedness) | Image forms behind retina | Convex lens | | Presbyopia | Loss of accommodation with age | Bifocal lens |

Electricity

Electric Charge and Current

Electric current (I) = Charge (Q) / Time (t)
SI unit: Ampere (A); 1 A = 1 C/s
Conventional current flows from + to −; electrons flow from − to +
Potential difference (V) = Work done / Charge; SI unit: Volt (V)

Ohm's Law

V = IR

Resistance (R) measured in Ohm (Ω)
Factors affecting resistance: length ↑ → R ↑; area ↑ → R ↓; temperature ↑ → R ↑ (for metals)
R = ρL/A where ρ = resistivity (intrinsic property of material)

Resistors in Circuits

Series combination

Rₛ = R₁ + R₂ + R₃

Same current through all; voltage divides

Parallel combination

1/Rₚ = 1/R₁ + 1/R₂ + 1/R₃

Same voltage across all; current divides; total resistance < smallest R

Electric Power

P = VI = I²R = V²/R

SI unit: Watt (W)
1 kWh = 1 unit of electricity = 1000 W × 3600 s = 3.6 × 10⁶ J
Heating effect of current (Joule's Law): H = I²Rt

Domestic Electric Circuits

Live wire — red/brown (230 V AC)
Neutral wire — black/blue (0 V)
Earth wire — green/yellow (safety)
Fuse wire — low melting point; melts when excess current flows; protects devices
MCB (Miniature Circuit Breaker) — modern replacement for fuse; can be reset

Magnetic Effects of Electric Current

Magnetic Field

Magnetic field lines emerge from N-pole and enter S-pole (outside magnet)
Field lines never intersect; closer lines = stronger field

Oersted's Discovery: A current-carrying conductor produces a magnetic field around it.

Right-Hand Thumb Rule

If right thumb points in direction of current, curled fingers show direction of magnetic field.

Magnetic Force on Current-Carrying Conductor

A current-carrying conductor in a magnetic field experiences a force. Fleming's Left-Hand Rule: Hold left hand such that index finger points in direction of B, middle finger in direction of I → thumb points in direction of force (motion).

Electric motor: Converts electrical energy → mechanical energy (uses this principle)

Electromagnetic Induction

Faraday's Law: A changing magnetic flux induces an EMF (and hence current) in a conductor. Fleming's Right-Hand Rule: For generators (opposite of left-hand rule).

Electric generator: Converts mechanical energy → electrical energy

Direct Current (DC) vs Alternating Current (AC)

Feature	DC	AC
Direction	Constant	Changes periodically
Source	Battery, cell	Generator, power plants
Frequency	0 Hz	50 Hz (India)
Transmission loss	High	Low (transformers possible)
India supply	—	230 V, 50 Hz

Sources of Energy

Conventional Sources

Fossil fuels — coal, petroleum, natural gas; non-renewable; cause pollution
Thermal power plants — burn coal to produce steam → turbine → generator
Hydroelectric power — potential energy of water → kinetic energy → electrical energy

Non-Conventional (Renewable) Sources

Source	Working Principle	Limitation
Solar energy	Photovoltaic cells convert sunlight to electricity	Depends on sunshine; expensive
Wind energy	Wind turbines	Requires consistent wind speed
Biogas	Decomposition of organic waste; mainly CH₄	Requires waste management
Tidal energy	Tidal rise and fall	Limited locations
Geothermal	Earth's internal heat	Limited to volcanic regions

Nuclear Energy

Fission — heavy nucleus splits; releases enormous energy
Fusion — lighter nuclei combine; more energy; not yet commercially viable
Nuclear power plants: controlled fission of U-235

Waves

Waves transfer energy from one place to another without transferring matter. Class 10 wave topics cover the general behaviour of waves, light as a wave, the full electromagnetic spectrum, and sound.

General Properties of Waves

A wave is a disturbance that transfers energy from one place to another without transferring matter. When a wave passes, the particles of the medium only vibrate about a fixed position — they do not travel along with the wave.

Example: When you drop a stone in a pond, ripples spread outwards carrying energy, but a floating leaf just bobs up and down — it does not move outward with the ripple. This proves energy moves, matter does not.

Types of Wave Motion

Waves are grouped in two ways.

1. By what they travel through: - Mechanical waves need a material medium to travel (sound, water waves, waves on a spring). They cannot travel through a vacuum. - Electromagnetic waves can travel through a vacuum (light, radio waves, X-rays).

2. By the direction the particles vibrate: - Transverse waves — particles vibrate at right angles to the direction of travel. - Longitudinal waves — particles vibrate along (parallel to) the direction of travel.

Transverse Waves

In a transverse wave the particles move up and down (perpendicular) while the wave moves forward (horizontal).

The high points are crests; the low points are troughs.
Examples: light and all electromagnetic waves, water surface ripples, a wave sent along a rope, seismic S-waves.

Demonstration: Fix one end of a rope and shake the other end up and down. A "hump" travels along the rope while each part of the rope only moves up and down — not forward with the wave.

Longitudinal Waves

In a longitudinal wave the particles vibrate backwards and forwards in the same direction the wave travels.

Regions where particles bunch together are compressions (high pressure).
Regions where particles spread apart are rarefactions (low pressure).
Examples: sound waves, a "push–pull" wave on a slinky spring, seismic P-waves.

Demonstration: Push and pull one end of a slinky spring along its length. You see moving compressed regions and stretched regions travelling along the spring.

Feature	Transverse	Longitudinal
Particle vibration	Perpendicular to travel	Parallel to travel
Features	Crests and troughs	Compressions and rarefactions
Travels through vacuum?	Only if EM wave	No (needs a medium)
Examples	Light, water, rope waves	Sound, slinky push waves

Describing Waves

To describe any wave we use five quantities: wavelength, frequency, wave speed, amplitude, and phase. A displacement–distance graph or displacement–time graph is used to picture them.

Wavelength (λ)

The wavelength is the distance between two consecutive points that are in step — for example, from one crest to the next crest, or one compression to the next compression.

Symbol: λ (Greek "lambda"); SI unit: metre (m).
On a displacement–distance graph, wavelength is the length of one complete cycle.

Frequency (f)

The frequency is the number of complete waves (cycles) produced or passing a point per second.

Symbol: f; SI unit: hertz (Hz). 1 Hz = 1 wave per second.
Related to the time period T (time for one wave) by:

f = 1 / T        and        T = 1 / f

Example: If 5 complete waves pass a point in 1 second, f = 5 Hz. If one wave takes 0.2 s, then f = 1 / 0.2 = 5 Hz.

Wave Speed (v)

The wave speed is the distance the wave travels per second, measured in metres per second (m/s). It depends on the medium — e.g. sound travels faster in water than in air.

Amplitude (a)

The amplitude is the maximum displacement of a particle from its rest (undisturbed) position.

Measured in metres (m) for the displacement.
Larger amplitude → more energy carried. For sound this means louder; for light this means brighter.

Phase

Two points on a wave are in phase if they are moving in the same direction with the same displacement (for example, two neighbouring crests). They are out of phase (specifically antiphase) if one is at a crest while the other is at a trough.

Points exactly one wavelength apart are always in phase.
Points half a wavelength apart are in antiphase (completely out of step).

The Wave Equation

The speed, frequency and wavelength of a wave are linked by the wave equation:

v = f λ

where v = speed (m/s), f = frequency (Hz), λ = wavelength (m).

This can be rearranged:

f = v / λ            λ = v / f

Worked Examples

Example 1 — Finding wave speed A wave has a frequency of 50 Hz and a wavelength of 0.4 m. Find its speed.

v = f λ = 50 × 0.4 = 20 m/s

Example 2 — Finding frequency Sound travels at 340 m/s. A sound wave has a wavelength of 1.7 m. Find its frequency.

f = v / λ = 340 / 1.7 = 200 Hz

Example 3 — Using the time period Water waves pass a post every 0.5 s and are 2 m apart. Find their speed.

f = 1 / T = 1 / 0.5 = 2 Hz
v = f λ = 2 × 2 = 4 m/s

Example 4 — Finding wavelength A radio station broadcasts at 100 MHz (100 × 10⁶ Hz). Radio waves travel at 3 × 10⁸ m/s. Find the wavelength.

λ = v / f = (3 × 10⁸) / (100 × 10⁶) = 3 m

Wavefronts and Rays

A wavefront is a line (or surface) joining all points on a wave that are in phase — for example, the line along the top of a crest. Consecutive wavefronts are one wavelength apart.
A ray is a line drawn at right angles to the wavefront showing the direction the wave travels.
Plane waves have straight, parallel wavefronts; circular waves (from a point source) have curved wavefronts spreading outward.

Reflection at a Plane Surface

When a wave hits a straight barrier it reflects. - The angle of incidence = the angle of reflection (measured from the normal). - The wavelength, frequency and speed do not change on reflection. - In a ripple tank, straight water wavefronts hitting a straight barrier bounce off at an equal angle.

Refraction

Refraction is the change in direction of a wave when it moves from one medium into another and changes speed. - When a wave slows down (e.g. water waves entering shallower water), the wavelength decreases and the wave bends towards the normal. - The frequency stays the same; only speed and wavelength change.

Example: In a ripple tank, placing a glass plate makes the water shallower. Waves slow down over the plate, their wavelength shortens, and their direction bends.

Diffraction

Diffraction is the spreading out of waves as they pass through a gap or around the edge of an obstacle. - Diffraction is greatest when the gap width is about equal to the wavelength. - If the gap is much wider than the wavelength, the wave passes almost straight through with only slight spreading at the edges. - If the gap is about the same size as the wavelength, the wave spreads out in strong semicircular wavefronts.

Diffraction due to an Edge or Wide Gap

At a sharp edge, waves bend slightly around the corner into the "shadow" region.
Through a wide gap (much larger than λ), the middle of the beam continues straight while only the edges show curved, diffracted wavefronts — so most of the wave passes undeviated.

This is why we can hear sound (long wavelength) around a doorway easily, but light (very short wavelength) casts sharp shadows and barely diffracts through the same doorway.

Wave Theory

Wave theory treats light and other radiation as waves and uses wavefronts to explain how they behave. It successfully explains reflection, refraction, diffraction, interference and polarisation — behaviours that confirm the wave nature of light.

Reflection and Wave Theory

Using wavefronts: as a straight wavefront meets a barrier, each part of it reflects in turn. Because all parts travel at the same speed, the reflected wavefront makes the same angle with the surface as the incoming one — explaining why angle of incidence = angle of reflection.

Refraction and Wave Theory

When a wavefront enters a slower medium at an angle, the part that enters first slows down first. This makes the wavefront pivot (change direction) and the wavelength shorten, bending the wave towards the normal — exactly what wave theory predicts and what we observe.

Diffraction and Wave Theory

Every point on a wavefront can be treated as a source of new "wavelets" (Huygens' idea). At a gap, these wavelets spread out and overlap, curving the wavefront into the region beyond the edges — explaining why waves diffract, and why narrow gaps diffract most.

Interference

Interference happens when two waves of the same type overlap. - Constructive interference — crest meets crest (waves in phase) → larger amplitude (louder/brighter). - Destructive interference — crest meets trough (waves in antiphase) → smaller or zero amplitude (quieter/darker).

Example: Two loudspeakers playing the same note create alternating loud and soft regions as you walk past — evidence of sound interference.

Polarisation

Polarisation restricts the vibrations of a transverse wave to a single plane. - Only transverse waves can be polarised; longitudinal waves (like sound) cannot — this is a key test of whether a wave is transverse. - A polarising filter (e.g. Polaroid sunglasses) allows through only light vibrating in one direction, cutting glare.

Exam-Style Questions

Q1. A wave travels at 12 m/s and has a wavelength of 3 m. Calculate its frequency. (2 marks)

Answer: f = v / λ = 12 / 3 = 4 Hz

Q2. State one difference between a transverse and a longitudinal wave, and give one example of each. (3 marks)

Answer: In a transverse wave particles vibrate perpendicular to the direction of travel (e.g. light); in a longitudinal wave they vibrate parallel to it (e.g. sound).

Q3. Water waves slow down as they move into shallower water. State what happens to (a) their frequency and (b) their wavelength. (2 marks)

Answer: (a) Frequency stays the same. (b) Wavelength decreases.

Q4. Explain why sound can be heard around a corner but a clear shadow is formed in light. (3 marks)

Answer: Sound has a long wavelength similar to the width of a doorway, so it diffracts (spreads) strongly around corners. Light has an extremely short wavelength compared with the gap, so it diffracts very little and travels almost straight, forming a sharp shadow.

Q5. Explain why sound cannot be polarised but light can. (2 marks)

Answer: Polarisation only affects transverse waves. Light is transverse, so it can be polarised; sound is longitudinal, so it cannot.

Light

Light is a form of energy that travels as a transverse electromagnetic wave and lets us see. It travels extremely fast — about 3 × 10⁸ m/s in a vacuum — and, unlike sound, needs no medium to travel.

Sources of Light

Luminous objects make their own light — e.g. the Sun, a lamp, a candle flame, a glowing LED.
Non-luminous objects do not make light; we see them only because they reflect light from a luminous source — e.g. the Moon, a book, a wall, a mirror.

Example: We see the Moon because it reflects sunlight; it is non-luminous. The Sun is luminous because it produces its own light.

Rays and Beams

A ray is the straight-line path along which light travels; it is drawn as a line with an arrow showing direction.
A beam is a stream (group) of light rays. Beams can be:
Parallel — rays travel side by side (e.g. a laser, light from a very distant source like the Sun).
Diverging — rays spread out from a point (e.g. a bare lamp bulb).
Converging — rays come together to a point (e.g. light passing through a magnifying/convex lens).

Light travels in straight lines — this is called rectilinear propagation and is shown by the fact that light cannot bend around corners like sound does.

Shadows

Because light travels in straight lines, an opaque object blocks it and forms a shadow. - A small (point) source gives a sharp shadow with just one dark region — the umbra. - A large (extended) source gives a shadow with two parts: a fully dark centre (umbra) and a partly lit fuzzy edge (penumbra), where only part of the source is blocked.

Example: During a solar eclipse, the Moon casts its shadow on Earth — people in the umbra see a total eclipse, and those in the penumbra see a partial eclipse.

Speed of Light

In a vacuum (and approximately in air) light travels at c = 3 × 10⁸ m/s — the fastest speed possible in nature.
Light slows down in denser media: it is slower in water and slower still in glass.
Because light is so much faster than sound, we see lightning before we hear the thunder, even though both happen together.

Example: Light from the Sun (about 1.5 × 10¹¹ m away) takes roughly 8 minutes to reach Earth: time = distance ÷ speed = (1.5 × 10¹¹) ÷ (3 × 10⁸) ≈ 500 s ≈ 8 min.

Reflection of Light

Reflection is the bouncing back of light when it hits a surface. - Regular (specular) reflection — from a smooth, shiny surface (like a mirror); parallel rays stay parallel and form a clear image. - Diffuse reflection — from a rough surface (like paper or a wall); rays scatter in all directions, so no image forms but we can see the object from any angle.

Some useful terms: - Incident ray — the ray arriving at the surface. - Reflected ray — the ray leaving the surface. - Normal — an imaginary line drawn at 90° to the surface at the point where the ray hits. - Angle of incidence (i) — between incident ray and normal. - Angle of reflection (r) — between reflected ray and normal.

Laws of Reflection

The angle of incidence equals the angle of reflection (i = r).
The incident ray, the reflected ray, and the normal all lie in the same plane.

Image in a plane mirror is: - the same size as the object, - as far behind the mirror as the object is in front, - virtual (cannot be formed on a screen), - upright but laterally inverted (left–right swapped — which is why "AMBULANCE" is written mirror-reversed on the front of the vehicle).

Worked example: If a ray strikes a mirror at an angle of incidence of 30°, the angle of reflection is also 30°, and the angle between the incident and reflected rays is 30° + 30° = 60°.

Periscope

A periscope lets you see over the top of an obstacle (used in submarines and to look over crowds). - It uses two plane mirrors fixed inside a tube, each set at 45° and parallel to each other. - Light from the object reflects off the top mirror, travels down the tube, reflects off the bottom mirror, and enters the eye. - Each mirror turns the light through 90°, so the total turn is 180° and the image seen is upright and the correct size.

Electromagnetic Spectrum

The electromagnetic (EM) spectrum is the full range of EM waves. All EM waves travel at the speed of light (3 × 10⁸ m/s) in a vacuum and are transverse.

Order of increasing frequency (decreasing wavelength):

Wave	Typical Use	Danger
Radio waves	Broadcasting, communication	Low
Microwaves	Cooking, mobile/satellite signals	Internal heating
Infrared (IR)	Remote controls, thermal imaging, heaters	Skin burns
Visible light	Vision, optical fibres, photography	Generally safe
Ultraviolet (UV)	Sterilisation, security marking, tanning	Skin cancer, eye damage
X-rays	Medical imaging, security scanners	Cell damage, cancer
Gamma rays	Cancer treatment, sterilising equipment	Severe cell damage

Memory aid (long → short wavelength): Radio Microwaves Infrared Visible Ultraviolet X-rays Gamma → "Rich Men In Vegas Use eXpensive Gadgets".

Sound

Sound is a longitudinal wave produced by vibrating objects. It needs a material medium (solid, liquid, or gas) to travel — it cannot travel through a vacuum.

Compressions — regions where particles are close together (high pressure)
Rarefactions — regions where particles are spread apart (low pressure)
Speed of sound in air ≈ 340 m/s (much slower than light); fastest in solids, slowest in gases

Properties we hear | Property | Depends on | Effect | |---|---|---| | Loudness | Amplitude | Bigger amplitude → louder | | Pitch | Frequency | Higher frequency → higher pitch |

Range of hearing - Human audible range: 20 Hz – 20,000 Hz - Infrasound: below 20 Hz | Ultrasound: above 20,000 Hz

Echo — reflected sound heard distinctly after the original; used in SONAR and ultrasound scanning to measure distances: distance = ½ × speed × time.

Electricity and Magnetism

This topic explains magnetism, the quantities used to describe electricity, how circuits are built, how to stay safe with electricity, and the effects produced when electricity and magnetism interact.

Simple Phenomena of Magnetism

Magnetic materials: iron, steel, cobalt, nickel are attracted to magnets; most other materials are non-magnetic
Poles: every magnet has a North and a South pole; like poles repel, unlike poles attract
Magnetic field: the region around a magnet where a magnetic force acts; represented by field lines running N → S outside the magnet
Field lines are closer together where the field is stronger; they never cross

Induced magnetism — a magnetic material becomes a magnet when placed in a magnetic field.

Magnetisation methods: stroking with a magnet, using a DC electric current (electromagnet). Demagnetisation: heating, hammering, or using alternating current.

Property	Soft iron	Steel
Magnetises	Easily	Harder
Keeps magnetism	Loses quickly (temporary)	Retains (permanent)
Use	Electromagnets	Permanent magnets

Electrical Quantities

Quantity	Symbol	Definition	Unit
Charge	Q	Quantity of electricity	Coulomb (C)
Current	I	Rate of flow of charge `I = Q/t`	Ampere (A)
Potential difference (voltage)	V	Energy per unit charge `V = W/Q`	Volt (V)
Resistance	R	Opposition to current `R = V/I`	Ohm (Ω)
Power	P	Rate of energy transfer `P = VI`	Watt (W)

Ohm's Law

V = I R

Current is directly proportional to voltage (at constant temperature).

Conductors (metals) allow current to flow easily; insulators (plastic, rubber) do not
Charge measured by Q = I t; energy by E = P t = V I t

Electric Circuits

Circuit symbols — cell, battery, switch, lamp, resistor, variable resistor (rheostat), ammeter, voltmeter.

Ammeter — measures current; connected in series
Voltmeter — measures voltage; connected in parallel across a component

Series circuit

R_total = R₁ + R₂ + R₃

Same current everywhere; voltage shared between components
If one component fails, the whole circuit breaks

Parallel circuit

1/R_total = 1/R₁ + 1/R₂ + 1/R₃

Same voltage across each branch; current is shared
Components work independently (used in house wiring)

Electrical Safety

Live wire (brown) carries the high voltage; Neutral (blue) completes the circuit; Earth (green/yellow) is a safety wire
Fuse — a thin wire that melts and breaks the circuit if the current gets too high; placed in the live wire
Circuit breaker (MCB) — switches off automatically on excess current; can be reset
Earthing — connects the metal case of an appliance to the ground so it cannot become live and give a shock
Double insulation — appliances with plastic casing that need no earth wire

Common hazards: damaged insulation, overloaded sockets/plugs, wet conditions near electricity, long or coiled cables overheating.

Electromagnetic Effects

Magnetic effect of a current A current-carrying wire creates a magnetic field around it (Oersted's discovery). A coil (solenoid) carrying current behaves like a bar magnet — this is an electromagnet, made stronger by more turns, more current, or an iron core.

Right-Hand Grip Rule: thumb points along the current, curled fingers show the field direction.

Force on a current-carrying conductor A wire carrying current in a magnetic field experiences a force. - Fleming's Left-Hand Rule: thuMb = Motion, First finger = Field, seCond finger = Current - This is the principle of the electric motor (electrical energy → mechanical energy)

Electromagnetic induction Moving a conductor through a magnetic field (or changing the field through a coil) induces a voltage/current — Faraday's Law. - Larger EMF from: faster movement, stronger field, more coil turns - Fleming's Right-Hand Rule gives the induced current direction - Principle of the generator/dynamo (mechanical energy → electrical energy)

Transformer — changes the size of an AC voltage using two coils on an iron core:

V_p / V_s = N_p / N_s

Step-up transformers increase voltage; step-down transformers decrease it. Used to transmit electricity efficiently over long distances.

Key Formulas — Quick Reference

Concept	Formula	Unit
Mirror formula	1/f = 1/v + 1/u	cm or m
Magnification (mirror)	m = -v/u	—
Snell's Law	n₁sinθ₁ = n₂sinθ₂	—
Refractive index	n = c/v	—
Lens formula	1/f = 1/v - 1/u	m
Power of lens	P = 1/f	Dioptre (D)
Ohm's Law	V = IR	V, A, Ω
Resistance	R = ρL/A	Ω
Power	P = VI = I²R	Watt (W)
Joule's heating	H = I²Rt	Joule (J)
Electrical energy	E = Pt	kWh or J
Wave speed	v = f λ	m/s
Time period	T = 1/f	s
Charge	Q = I t	Coulomb (C)
Transformer	Vₚ/Vₛ = Nₚ/Nₛ	—

Exam Tips

Mirror and lens formula sign convention: distances measured from optical centre/pole; incident light direction is positive
Resistors in parallel always give less resistance than the smallest individual resistor
Fleming's Left-Hand Rule → Motor; Right-Hand Rule → Generator
AC is used for transmission because voltage can be stepped up/down with transformers
Always show all steps and units in numerical problems for full marks