LET Elementary Physics — Electricity, Magnetism, Light and Modern PhysicsRevision Notes

Exam Tips

• Remember the phrase 'GAINS electrons = NEGATIVE' to quickly answer charge-type questions.
• If a question asks what type of charging a balloon-on-hair scenario uses, the answer is FRICTION (triboelectric charging).
• LET items on lightning will ask about the DISCHARGE of static electricity — lightning is NOT current electricity in the household sense.
• Grounding questions often appear as 'Why does the ground pin prevent electric shock?' — answer: it provides a safe path for excess charge to flow to Earth.

Key Points

• Matter contains two types of electric charge: positive (protons) and negative (electrons).
• The fundamental rule: LIKE charges REPEL each other; UNLIKE (opposite) charges ATTRACT each other.
• Objects become charged mainly by FRICTION (rubbing), which TRANSFERS electrons from one object to another — it does NOT create new charge.
• The object that GAINS electrons becomes NEGATIVELY charged; the object that LOSES electrons becomes POSITIVELY charged.
• Classic example: Rubbing a balloon on hair transfers electrons to the balloon (balloon = negative, hair = positive), so the balloon sticks to walls and lifts hair strands.
• Charging by INDUCTION rearranges charges in a neutral object without direct contact, allowing a charged object to attract neutral ones.
• The Law of Conservation of Charge: total charge is always conserved — one object gains exactly what the other loses.
• LIGHTNING is a massive natural discharge of static electricity built up in storm clouds between regions of opposite charge.
• GROUNDING (earthing) safely drains excess charge into the Earth, which is why lightning rods and the third pin (ground pin) on appliances exist.
• Static electricity is a practical safety topic for elementary Science — pupils can observe it by rubbing a plastic ruler on cloth and attracting small paper bits.

Definitions

Section Title

Static Electricity

Common Mistakes

• Thinking friction CREATES charge — friction only TRANSFERS electrons; total charge is always conserved.
• Confusing which object becomes positive or negative: the one that LOSES electrons is POSITIVE; the one that GAINS electrons is NEGATIVE.
• Assuming induction requires contact — induction works WITHOUT touching.
• Forgetting that static electricity involves STATIONARY (non-flowing) charges, unlike current electricity.

Formulas

Exam Tips

• Memorize the OHM'S LAW TRIANGLE: Place V on top, I and R at the bottom. Cover the unknown to see the formula.
• For quick calculation: if V = 220 V and R = 110 Ω, then I = 220/110 = 2 A — LET problems use clean numbers.
• Philippine classroom context: the standard household voltage is 220 V; exam problems will use 220 V or 110 V as typical values.
• When asked which is a better conductor — choose METALS (copper, silver, aluminum); when asked which is an insulator — choose RUBBER, PLASTIC, or GLASS.
• Energy billing: 1 kWh = 1,000 watt-hours; if a 500 W appliance runs for 4 hours, that is 0.5 kW × 4 h = 2 kWh.

Key Points

• Current electricity is the STEADY, CONTINUOUS FLOW of electrons (charge) through a conductor such as a copper wire.
• Three fundamental quantities: VOLTAGE (V) — the electrical 'push' or pressure driving the current; CURRENT (I) — the rate of charge flow; RESISTANCE (R) — the opposition to flow.
• Units: Voltage in VOLTS (V), Current in AMPERES or AMPS (A), Resistance in OHMS (Ω).
• OHM'S LAW: V = I × R (Voltage equals Current times Resistance).
• Rearranged forms: I = V ÷ R (to find current) and R = V ÷ I (to find resistance).
• CONDUCTORS — materials like copper, aluminum, silver — allow electrons to flow easily because they have free electrons.
• INSULATORS — materials like rubber, plastic, glass, dry wood — block electron flow and are used as wire coatings for safety.
• SEMICONDUCTORS (silicon, germanium) have conductivity between conductors and insulators — basis of electronic devices.
• Electrical POWER (P) = V × I, measured in WATTS (W).
• Electrical ENERGY is billed in KILOWATT-HOURS (kWh): Energy = Power (kW) × Time (hours).
• Alternative power formulas: P = I²R and P = V²÷R (derived by combining P = VI with Ohm's Law).

Definitions

Section Title

Current Electricity and Ohm's Law

Common Mistakes

• Confusing VOLTAGE (the push) with CURRENT (the flow) — voltage is like water pressure; current is like the water actually flowing.
• Forgetting to rearrange Ohm's Law correctly: I = V/R (NOT I = R/V).
• Using watts instead of kilowatts when computing kWh — always convert watts to kilowatts by dividing by 1,000.
• Mixing up the units: volts for voltage, amperes for current, ohms for resistance — the LET often tests unit identification.
• Thinking more resistance always means more power — actually P = V²/R, so MORE resistance means LESS power at constant voltage.

Formulas

Exam Tips

• KEY CONTRAST TABLE: Series = same I, shared V, resistances add, one break stops all. Parallel = same V, shared I, lower total R, one break leaves others working.
• LET shortcut for two equal resistors in parallel: R_total = R/2 (e.g., two 6 Ω resistors → R_total = 3 Ω).
• When asked 'Why is home wiring parallel?' — answer: so each appliance gets FULL voltage and one can be switched off independently.
• Series vs. parallel identification: if you can trace only ONE path from battery to back, it is SERIES; if there are BRANCHES, it is PARALLEL.
• A fuse protects against OVERCURRENT (too much current), not overvoltage.

Key Points

• A CIRCUIT is a complete, unbroken loop through which current can flow from a power source (battery), through components, and back.
• An OPEN CIRCUIT is a broken loop — current cannot flow. A CLOSED CIRCUIT is complete — current flows.
• Components can be connected in SERIES (single path) or PARALLEL (multiple branches).
• SERIES CIRCUITS: all components share ONE path; current is the SAME through every component; voltage DIVIDES among components; total resistance = R1 + R2 + R3 + ...; if one component fails, the ENTIRE circuit stops.
• PARALLEL CIRCUITS: components are in SEPARATE branches; voltage is the SAME across every branch; current DIVIDES among branches; total resistance is LESS than the smallest individual resistance; if one branch fails, the OTHERS keep working.
• Formula for total resistance in parallel: 1/R_total = 1/R1 + 1/R2 + 1/R3 + ...
• For TWO resistors in parallel: R_total = (R1 × R2) ÷ (R1 + R2).
• HOUSEHOLD (home) wiring in the Philippines uses PARALLEL circuits so every appliance gets the same 220 V and one appliance can be switched off without affecting others.
• Old-style series Christmas lights (parol-style): one bulb burns out, the whole string goes dark — classic series circuit illustration.
• FUSE: a thin wire that MELTS and breaks the circuit if current becomes dangerously high, preventing overheating and fire.
• CIRCUIT BREAKER: a resettable switch that TRIPS (opens) the circuit automatically when current exceeds a safe limit.

Definitions

Section Title

Simple Circuits: Series and Parallel

Common Mistakes

• Saying total parallel resistance is found by simply adding — WRONG; use the reciprocal formula 1/R_total = 1/R1 + 1/R2.
• Forgetting that in a SERIES circuit, the CURRENT is the same everywhere but VOLTAGE divides.
• Forgetting that in a PARALLEL circuit, the VOLTAGE is the same everywhere but CURRENT divides.
• Assuming that more branches in parallel means more total resistance — it is actually LESS total resistance.
• Thinking a fuse is reusable — a fuse MELTS and must be replaced; a circuit breaker can be RESET.

Exam Tips

• Memorize: LIKE poles REPEL, UNLIKE poles ATTRACT — same rule as electric charges.
• For compass questions: the needle's NORTH tip points toward geographic north because Earth's geographic north is a magnetic SOUTH pole.
• LET question type: 'What happens when you cut a bar magnet in half?' → Each piece becomes a COMPLETE magnet with N and S poles.
• Strongest magnetic attraction is between N and S poles of different magnets; strongest repulsion is between two N poles or two S poles.

Key Points

• A MAGNET has two poles: NORTH (N) and SOUTH (S).
• Magnetic rule (mirrors the electric charge rule): LIKE poles REPEL; UNLIKE poles ATTRACT.
• Magnetic poles CANNOT be isolated — if you cut a magnet in half, each piece becomes a NEW complete magnet with its own north and south pole.
• A MAGNETIC FIELD surrounds every magnet — visualized as field LINES running from the NORTH pole to the SOUTH pole outside the magnet.
• FERROMAGNETIC materials (iron, nickel, cobalt, and their alloys like steel) can be strongly magnetized. Most other materials (wood, copper, plastic) are not.
• EARTH is a giant magnet. The geographic NORTH POLE is actually a magnetic SOUTH pole — that is why the compass needle's NORTH tip is attracted toward it.
• A COMPASS aligns along Earth's magnetic field lines, pointing roughly toward geographic north.
• Magnets are used in everyday devices: compasses, speakers, hard disk drives, MRI machines, and electric motors.
• PERMANENT magnets (like bar magnets and ceramic magnets) keep their magnetism. TEMPORARY magnets (like soft iron) lose magnetism when the magnetizing force is removed.

Definitions

Section Title

Magnetism

Common Mistakes

• Thinking you can isolate a single magnetic pole by cutting a magnet — each piece always has BOTH poles.
• Confusing geographic north with magnetic north — Earth's geographic north is a MAGNETIC SOUTH, attracting compass needles' north tips.
• Saying ALL metals are magnetic — only IRON, NICKEL, COBALT (ferromagnetic) are strongly magnetic; copper, aluminum, and gold are not.
• Drawing magnetic field lines going from south to north outside the magnet — they go from NORTH to SOUTH outside.

Exam Tips

• KEY MEMORY AID: Motor = M for Motion (output); Generator = G for Generate electricity (output).
• LET question: 'What principle underlies an electric generator?' → Electromagnetic INDUCTION (Faraday's Law).
• LET question: 'What principle underlies an electric motor?' → Force on a current-carrying conductor in a magnetic field (Oersted/Ampere).
• To increase electromagnet strength: (1) increase current, (2) add more wire turns, (3) use soft iron core — LET often lists these as options.
• Transformers only work with ALTERNATING CURRENT (AC), not direct current (DC) — because they need a CHANGING magnetic field.

Key Points

• Electricity and magnetism are two aspects of a SINGLE force called ELECTROMAGNETISM.
• OERSTED'S DISCOVERY (1820): an electric current flowing through a wire produces a MAGNETIC FIELD around that wire — this linked electricity to magnetism for the first time.
• An ELECTROMAGNET is made by coiling a wire around an iron core and passing current through it. Its strength increases with MORE TURNS of wire or MORE CURRENT, and it can be SWITCHED ON AND OFF.
• FARADAY'S LAW — ELECTROMAGNETIC INDUCTION: moving a magnet near a coil (or moving a coil in a magnetic field) INDUCES a voltage and current in the coil. This is how generators work.
• KEY PRINCIPLE: a changing magnetic field produces an electric current; an electric current produces a magnetic field.
• ELECTRIC MOTOR: converts ELECTRICAL energy into MECHANICAL (motion) energy — uses the force on a current-carrying wire in a magnetic field.
• ELECTRIC GENERATOR: converts MECHANICAL energy into ELECTRICAL energy — uses electromagnetic induction (moving coil in a magnetic field).
• TRANSFORMER: changes the voltage of alternating current (AC) using electromagnetic induction between two coils; step-up transformers increase voltage, step-down transformers decrease it.
• APPLICATIONS: electric motors (electric fans, washing machines), generators (power plants, portable gensets), transformers (power distribution), doorbells, speakers, electromagnets in cranes and MRI machines.
• MOTOR vs. GENERATOR are physical OPPOSITES: motor = electricity → motion; generator = motion → electricity.

Definitions

Section Title

Electromagnetism

Common Mistakes

• Mixing up motor and generator: MOTOR uses electricity to produce MOTION; GENERATOR uses MOTION to produce electricity.
• Thinking a static (non-moving) magnet induces current — induction requires RELATIVE MOTION between magnet and coil, or a CHANGING magnetic field.
• Saying Oersted discovered induction — OERSTED discovered that current makes a magnetic field; FARADAY discovered induction (moving magnet makes current).
• Forgetting that an electromagnet is TEMPORARY — it loses its magnetism when the current is switched off.

Formulas

Exam Tips

• REFLECTION MEMORY AID: 'The angle in equals the angle out' — both from the NORMAL.
• REFRACTION MEMORY AID: 'Dense medium = bends toward normal; less dense = bends away from normal.'
• For mirror types: Concave mirrors can form REAL images (projected on a screen) at certain distances; convex mirrors ALWAYS form virtual, smaller, upright images.
• For lenses: CONvex = CONverging = Corrects farsightEDness; CONcave = DIVerging = corrects nearsightEDness.
• Rainbow question: red refracts the LEAST (outer arc), violet refracts the MOST (inner arc) — think R(red) at the Rim.
• LET tip: if a question says 'a side mirror on a jeepney' or 'a store security mirror,' the answer is always CONVEX mirror.

Key Points

• Light travels in STRAIGHT LINES (rectilinear propagation) at 300,000 km/s (3 × 10⁸ m/s) in a vacuum.
• When light hits a surface, it can be REFLECTED (bounced back), REFRACTED (bent as it passes through), or ABSORBED.
• LAW OF REFLECTION: the ANGLE OF INCIDENCE equals the ANGLE OF REFLECTION (both measured from the NORMAL — an imaginary line perpendicular to the surface at the point of contact). Angle in = Angle out.
• SMOOTH SURFACES (mirrors, calm water) produce SPECULAR (clear) reflection — a distinct image forms.
• ROUGH SURFACES produce DIFFUSE reflection — light scatters in many directions; no clear image but the surface is visible.
• PLANE (FLAT) MIRROR produces an image that is: (1) VIRTUAL — appears behind the mirror, (2) UPRIGHT — same orientation as object, (3) SAME SIZE as object, (4) LATERALLY REVERSED — left and right are swapped, (5) as far BEHIND the mirror as the object is in FRONT.
• CONCAVE MIRROR: curves INWARD like a bowl; CONVERGES light to a focal point; used in flashlights, car headlights, shaving/makeup mirrors, and reflecting telescopes.
• CONVEX MIRROR: curves OUTWARD; DIVERGES light; produces a SMALLER, UPRIGHT, WIDER field-of-view image; used as vehicle side mirrors and store security/CCTV mirrors.
• REFRACTION: the BENDING of light as it passes from one MEDIUM to another because its SPEED CHANGES.
• Light SLOWS DOWN and bends TOWARD the NORMAL when entering a DENSER medium (e.g., air to water/glass).
• Light SPEEDS UP and bends AWAY FROM the NORMAL when entering a LESS DENSE medium (e.g., water to air).
• REAL-WORLD REFRACTION EXAMPLES: bent straw in a glass of water, a swimming pool appearing shallower than it is, a fish in water appearing closer to the surface than it actually is.
• CONVEX (CONVERGING) LENS: thicker in the middle; bends light toward a focal point; used in magnifying glasses, cameras, microscopes, and to correct FARSIGHTEDNESS (hyperopia).
• CONCAVE (DIVERGING) LENS: thinner in the middle; spreads light outward; used to correct NEARSIGHTEDNESS (myopia).
• DISPERSION: a PRISM separates white light into the VISIBLE SPECTRUM (ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet) because each color refracts at a slightly different angle. This explains the formation of a RAINBOW.

Definitions

Section Title

Reflection and Refraction of Light

Common Mistakes

• Measuring reflection/refraction angles from the SURFACE instead of from the NORMAL — always use the NORMAL.
• Confusing concave and convex: CONcave = like a CAVE curving inward; CONvex = curving outward.
• Saying the bent straw is caused by REFLECTION — it is caused by REFRACTION (bending at the air-water boundary).
• Confusing which lens corrects which vision problem: CONVEX lens → farsightedness; CONCAVE lens → nearsightedness.
• Thinking light bends toward the normal when going from dense to less dense — it bends AWAY from the normal when moving to a LESS dense medium.
• Listing rainbow colors in the wrong order — remember ROYGBIV (Red on the outside of the arc, Violet on the inside).

Exam Tips

• PENETRATION ORDER (least to most): Alpha (paper) → Beta (aluminum) → Gamma (lead/concrete). Memory aid: ABG — A=paper, B=aluminum, G=lead.
• ATOMIC MODEL SEQUENCE: Dalton (solid sphere) → Thomson (plum pudding) → Rutherford (nucleus) → Bohr (energy shells) → Modern quantum (electron cloud).
• HALF-LIFE question: if a 100 g sample has a half-life of 10 years, after 10 years = 50 g remains; after 20 years = 25 g remains; after 30 years = 12.5 g remains.
• CARBON DATING uses the half-life of carbon-14 (approximately 5,730 years) — a common LET application question.
• Gamma rays are electromagnetic waves (like X-rays and visible light), so they travel at the speed of light and have no mass — the LET may ask this distinction.

Key Points

• MODERN PHYSICS explores phenomena that classical (Newtonian) physics could not explain — primarily the behavior of atoms and subatomic particles.
• ATOMIC STRUCTURE: the atom has a tiny, dense central NUCLEUS containing PROTONS (positive charge) and NEUTRONS (no charge), surrounded by ELECTRONS (negative charge) in shells or energy levels.
• The ATOMIC NUMBER = number of PROTONS in the nucleus; it uniquely identifies each element.
• In a NEUTRAL atom, the number of PROTONS equals the number of ELECTRONS.
• ATOMIC MODELS evolved in historical order: Dalton → Thomson → Rutherford → Bohr → Modern Quantum Model.
• RADIOACTIVITY is the SPONTANEOUS breakdown of unstable atomic nuclei, releasing energy and particles. It is a property of the NUCLEUS, not the electrons.
• THREE TYPES OF RADIATION: ALPHA (α), BETA (β), and GAMMA (γ) — they differ in composition, charge, and penetrating power.
• ALPHA PARTICLES: helium nuclei (2 protons + 2 neutrons), positively charged, LEAST penetrating — stopped by a sheet of paper or human skin.
• BETA PARTICLES: high-speed electrons emitted from the nucleus, negatively charged, MODERATE penetrating power — stopped by a few millimeters of aluminum or plastic.
• GAMMA RAYS: high-energy electromagnetic radiation (like very energetic X-rays), NO charge, MOST penetrating — needs THICK LEAD or CONCRETE to block.
• HALF-LIFE: the time required for HALF of a radioactive sample to decay into a different element or isotope. Used in CARBON DATING (determining the age of fossils and artifacts).
• BENEFICIAL USES of radioactivity: cancer treatment (radiotherapy), medical imaging (PET scans, tracers), sterilization of medical equipment, nuclear power generation.
• HARMFUL EFFECTS: high doses of radiation damage and kill living cells, causing radiation sickness and cancer — proper SHIELDING and safety protocols are essential.

Definitions

Section Title

Introduction to Modern Physics: Atomic Models and Radioactivity

Common Mistakes

• Mixing up the atomic models — Rutherford discovered the NUCLEUS; Bohr added ENERGY LEVELS (shells); Thomson proposed the 'plum pudding' model BEFORE Rutherford.
• Saying alpha is most penetrating — GAMMA is the most penetrating; ALPHA is the LEAST penetrating.
• Thinking alpha particles are electrons — alpha particles are HELIUM NUCLEI (2 protons + 2 neutrons), which are positively charged.
• Confusing ATOMIC NUMBER with MASS NUMBER — atomic number = protons only; mass number = protons + neutrons.
• Saying neutral atoms have more protons than electrons — in a NEUTRAL atom, protons = electrons.