Unit 2 High-Yield Topics | Flowxiom<\/title>\n<meta name=\"description\" content=\"Unit 2 High-Yield Topics \u2014 Edexcel A-level Physics WPH. Question types: Explain why wave theory fails; calculate threshold frequency or maximum kinetic energ...\">\n<link rel=\"canonical\" href=\"https:\/\/flowxiom.com\/edexcel-physics-unit-2-high-yield-topics\/\">\n<script type=\"application\/ld+json\">\n{\n \"@context\": \"https:\/\/schema.org\",\n \"@type\": \"FAQPage\",\n \"mainEntity\": [\n {\n \"@type\": \"Question\",\n \"name\": \"Q1. Work function \\\\(\\\\phi = 3.0 \\\\times 10^{-19}\\\\) J, light frequency \\\\(f = 8.0 \\\\times 10^{14}\\\\) Hz. Find the maximum kinetic energy of the photoelectron. (\\\\(h = 6.63 \\\\times 10^{-34}\\\\) J s)\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"\\\\(E_k = hf - \\\\phi = 6.63\\\\times10^{-34} \\\\times 8.0\\\\times10^{14} - 3.0\\\\times10^{-19} = \\\\mathbf{2.3\\\\times10^{-19}\\\\ \\\\text{J}}\\\\)\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"Q2. Diffraction grating: 400 lines per mm, wavelength 600 nm. What is the maximum order of bright fringe?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"\\\\(d = 1\/400\\\\ \\\\text{mm} = 2.5\\\\times10^{-6}\\\\ \\\\text{m}\\\\) \\n \\\\(n_{max} = d\/\\\\lambda = 2.5\\\\times10^{-6} \/ 600\\\\times10^{-9} = 4.17\\\\) \\n Maximum order \\\\(= \\\\mathbf{4}\\\\) (round down)\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"Q3. \\\\(6\\\\ \\\\Omega\\\\) and \\\\(3\\\\ \\\\Omega\\\\) in parallel, then in series with \\\\(2\\\\ \\\\Omega\\\\). Find total resistance.\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"Parallel: \\\\(R = \\\\dfrac{6\\\\times3}{6+3} = 2\\\\ \\\\Omega\\\\) \\n Series: \\\\(R_{total} = 2 + 2 = \\\\mathbf{4\\\\ \\\\Omega}\\\\)\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"Q4. Glass with refractive index \\\\(n = 1.5\\\\). Find the critical angle.\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"\\\\(\\\\sin c = \\\\dfrac{1}{1.5} = 0.667\\\\) \\n \\\\(c = \\\\sin^{-1}(0.667) = \\\\mathbf{41.8\u00b0}\\\\)\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"Q5. Two energy levels in hydrogen differ by \\\\(\\\\Delta E = 2.55\\\\ \\\\text{eV}\\\\). Find the wavelength of the emitted photon. (\\\\(h = 6.63\\\\times10^{-34}\\\\) J s, \\\\(c = 3.00\\\\times10^8\\\\) m\/s, \\\\(1\\\\ \\\\text{eV} = 1.6\\\\times10^{-19}\\\\ \\\\text{J}\\\\))\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"\\\\(E = 2.55 \\\\times 1.6\\\\times10^{-19} = 4.08\\\\times10^{-19}\\\\ \\\\text{J}\\\\) \\n \\\\(\\\\lambda = \\\\dfrac{hc}{E} = \\\\dfrac{6.63\\\\times10^{-34} \\\\times 3.00\\\\times10^8}{4.08\\\\times10^{-19}} = \\\\mathbf{4.88\\\\times10^{-7}\\\\ \\\\text{m}}\\\\) (488 nm \u2014 blue-green)\"\n }\n }\n ]\n}\n<\/script>\n\n<script>\nMathJax = {\n tex: { inlineMath: [['\\\\(','\\\\)']], displayMath: [['\\\\[','\\\\]']] },\n svg: { fontCache: 'global' }\n};\n<\/script>\n<script async src=\"https:\/\/cdn.jsdelivr.net\/npm\/mathjax@3\/es5\/tex-svg.js\"><\/script>\n\n<style>\n:root{--accent:#2563eb;--warn-bg:#fef9c3;--warn-border:#ca8a04;--formula-bg:#eff6ff;--formula-border:#2563eb;}\nbody{font-family:system-ui,sans-serif;max-width:860px;margin:0 auto;padding:1.5rem;line-height:1.7;color:#1e293b;}\nh1{font-size:2rem;border-bottom:3px solid var(--accent);padding-bottom:.4rem;}\nh2{font-size:1.4rem;color:var(--accent);margin-top:2rem;}\nh3{font-size:1.1rem;margin-top:1.4rem;}\ntable{border-collapse:collapse;width:100%;margin:1rem 0;}\nth,td{border:1px solid #cbd5e1;padding:.5rem .75rem;text-align:left;}\nth{background:#e2e8f0;}\npre,code{background:#f1f5f9;border-radius:4px;}\npre{padding:1rem;overflow-x:auto;}\ncode{padding:.1rem .3rem;font-size:.9em;}\n.seo-warning-box{background:var(--warn-bg);border-left:4px solid var(--warn-border);padding:.75rem 1rem;margin:1rem 0;border-radius:0 6px 6px 0;}\n.seo-note-box{background:#f0fdf4;border-left:4px solid #16a34a;padding:.75rem 1rem;margin:1rem 0;border-radius:0 6px 6px 0;}\nsection.formula-card{background:var(--formula-bg);border:1px solid var(--formula-border);border-radius:8px;padding:1rem 1.25rem;margin:1.5rem 0;}\ndetails{border:1px solid #e2e8f0;border-radius:6px;padding:.5rem 1rem;margin:.75rem 0;}\nsummary{cursor:pointer;font-weight:600;}\n.answer-block{margin-top:.5rem;padding-top:.5rem;border-top:1px solid #e2e8f0;overflow-x:auto;}\n.answer-block p{margin:.25rem 0;}\n.numbered-step{padding-left:1.5rem;position:relative;}\nhr{border:none;border-top:1px solid #e2e8f0;margin:2rem 0;}\n.toc{background:#f8fafc;border:1px solid #e2e8f0;border-radius:8px;padding:1rem 1.5rem;margin-bottom:2rem;}\n.toc h2{margin-top:0;font-size:1.1rem;}\n.toc ul{margin:0;padding-left:1.2rem;}\n.toc a{color:var(--accent);text-decoration:none;}\n.site-footer{margin-top:3rem;padding-top:1rem;border-top:2px solid var(--accent);font-size:.9rem;color:#64748b;}\n<\/style>\n<\/head>\n<body>\n<nav class=\"toc\"><h2>Contents<\/h2><ul>\n <li><a href=\"#topic-1-the-photoelectric-effect-frequent-6mark-question\">Topic 1: The Photoelectric Effect (frequent 6-mark question)<\/a><\/li>\n <li><a href=\"#topic-2-doubleslit-interference-and-diffraction-grating\">Topic 2: Double-Slit Interference and Diffraction Grating<\/a><\/li>\n <li><a href=\"#topic-3-stationary-standing-waves\">Topic 3: Stationary (Standing) Waves<\/a><\/li>\n <li><a href=\"#topic-4-dc-circuits-kirchhoffs-laws\">Topic 4: DC Circuits (Kirchhoff’s Laws)<\/a><\/li>\n <li><a href=\"#topic-5-power-formulae\">Topic 5: Power Formulae<\/a><\/li>\n <li><a href=\"#topic-6-basic-wave-properties\">Topic 6: Basic Wave Properties<\/a><\/li>\n <li><a href=\"#topic-7-refraction-and-total-internal-reflection\">Topic 7: Refraction and Total Internal Reflection<\/a><\/li>\n <li><a href=\"#topic-8-atomic-energy-levels-and-spectra\">Topic 8: Atomic Energy Levels and Spectra<\/a><\/li>\n <li><a href=\"#topic-9-resistivity-and-drift-velocity\">Topic 9: Resistivity and Drift Velocity<\/a><\/li>\n <li><a href=\"#practice-questions\">Practice Questions<\/a><\/li>\n<\/ul><\/nav>\n<h1>Unit 2 High-Yield Topics<\/h1>\n<p><strong>Free resource by <a href=\"https:\/\/flowxiom.com\">Flowxiom<\/a> \u2014 Edexcel A-level Physics<\/strong><\/p>\n<p><em>Not everything. Just what’s on the paper. High-frequency topics only \u2014 covering ~80% of exam marks.<\/em><\/p>\n<p>Edexcel A-level Physics | Waves, Optics & DC Circuits | WPH11 & WPH12<\/p>\n<hr>\n<h2 id=\"topic-1-the-photoelectric-effect-frequent-6mark-question\">Topic 1: The Photoelectric Effect (frequent 6-mark question)<\/h2>\n<p><strong>Question types:<\/strong> Explain why wave theory fails; calculate threshold frequency or maximum kinetic energy.<\/p>\n<h3 id=\"einsteins-photoelectric-equation\">Einstein’s Photoelectric Equation<\/h3>\n<p>\\[hf = \\phi + E_{k(max)}\\]<\/p>\n<table>\n<thead><tr><th>Symbol<\/th><th>Meaning<\/th><\/tr><\/thead>\n<tr><td>\\(hf\\)<\/td><td>Incident photon energy (\\(h = 6.63 \\times 10^{-34}\\) J s)<\/td><\/tr>\n<tr><td>\\(\\phi\\)<\/td><td>Work function \u2014 minimum energy to escape the metal<\/td><\/tr>\n<tr><td>\\(E_{k(max)}\\)<\/td><td>Maximum kinetic energy of photoelectron<\/td><\/tr>\n<\/table>\n\n<p>Threshold frequency: \\(f_0 = \\dfrac{\\phi}{h}\\) (at this point \\(E_k = 0\\))<\/p>\n<p>Stopping voltage: \\(E_{k(max)} = eV_s\\)<\/p>\n<h3 id=\"why-wave-theory-fails-threepoint-answer-chain\">Why Wave Theory Fails \u2014 Three-Point Answer Chain<\/h3>\n<table>\n<thead><tr><th>Observation<\/th><th>Wave theory predicts (wrong)<\/th><th>Photon theory explains (correct)<\/th><\/tr><\/thead>\n<tr><td>Threshold frequency exists<\/td><td>Sufficient intensity should eject electrons at any frequency<\/td><td>Single photon energy \\(hf\\) insufficient; more photons don’t help<\/td><\/tr>\n<tr><td>No time delay<\/td><td>Weak light needs time to build up energy<\/td><td>One photon transfers energy to one electron instantly<\/td><\/tr>\n<tr><td>Kinetic energy independent of intensity<\/td><td>Higher intensity \u2192 higher kinetic energy<\/td><td>Intensity increases photon count, not individual photon energy<\/td><\/tr>\n<\/table>\n\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c Must state: single photon energy \\(hf < \\phi\\), so electron cannot escape<\/li>\n<li>\u274c Gradient of stopping voltage vs frequency graph = \\(h\/e\\), not \\(h\\)<\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"topic-2-doubleslit-interference-and-diffraction-grating\">Topic 2: Double-Slit Interference and Diffraction Grating<\/h2>\n<p><strong>Question types:<\/strong> Fringe spacing calculation, maximum order for diffraction grating, effect of changing parameters.<\/p>\n<h3 id=\"doubleslit-fringe-spacing\">Double-Slit Fringe Spacing<\/h3>\n<p>\\[\\Delta y = \\frac{\\lambda L}{d}\\]<\/p>\n<ul>\n<li>\\(\\lambda\\): wavelength, \\(L\\): slit-to-screen distance, \\(d\\): slit separation<\/li>\n<li>Wider fringes: increase \\(\\lambda\\) \/ increase \\(L\\) \/ decrease \\(d\\)<\/li>\n<\/ul>\n<h3 id=\"diffraction-grating\">Diffraction Grating<\/h3>\n<p>\\[d\\sin\\theta = n\\lambda\\]<\/p>\n<ul>\n<li>\\(d\\): grating spacing (if question gives N lines per mm, then \\(d = 1\/N\\) mm)<\/li>\n<li>Maximum order: set \\(\\sin\\theta = 1\\), \\(n_{max} = d\/\\lambda\\) (round down to integer)<\/li>\n<\/ul>\n<h3 id=\"conditions-for-interference\">Conditions for Interference<\/h3>\n<p class=\"numbered-step\"><strong>Coherent<\/strong>: same frequency + constant phase difference<\/p>\n<p class=\"numbered-step\">Same type of wave<\/p>\n<h3 id=\"path-difference-rule\">Path Difference Rule<\/h3>\n<table>\n<thead><tr><th>Path difference<\/th><th>Result<\/th><\/tr><\/thead>\n<tr><td>\\(n\\lambda\\) (whole number of wavelengths)<\/td><td>Constructive interference \u2014 bright fringe<\/td><\/tr>\n<tr><td>\\((n+0.5)\\lambda\\)<\/td><td>Destructive interference \u2014 dark fringe<\/td><\/tr>\n<\/table>\n\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c \\(d\\) = slit separation (centre to centre), not slit width<\/li>\n<li>\u274c For the grating: \\(d\\) is the separation between adjacent slits, not the slit width<\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"topic-3-stationary-standing-waves\">Topic 3: Stationary (Standing) Waves<\/h2>\n<p><strong>Question types:<\/strong> Resonance conditions, counting nodes and antinodes, frequency calculation.<\/p>\n<h3 id=\"resonance-condition\">Resonance Condition<\/h3>\n<p>String with both ends fixed: \\(L = \\dfrac{n\\lambda}{2}\\), \\(n = 1, 2, 3…\\)<\/p>\n<ul>\n<li>Fundamental (\\(n=1\\)): \\(f_1 = \\dfrac{v}{2L}\\)<\/li>\n<li>\\(n\\)th harmonic: \\(f_n = nf_1\\)<\/li>\n<\/ul>\n<h3 id=\"node-vs-antinode\">Node vs Antinode<\/h3>\n<ul>\n<li><strong>Node<\/strong>: point of zero displacement (fixed end)<\/li>\n<li><strong>Antinode<\/strong>: point of maximum displacement<\/li>\n<\/ul>\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c Recount nodes\/antinodes by drawing the wave pattern<\/li>\n<li>\u274c Distance between adjacent nodes = \\(\\lambda\/2\\), not \\(\\lambda\\)<\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"topic-4-dc-circuits-kirchhoffs-laws\">Topic 4: DC Circuits (Kirchhoff’s Laws)<\/h2>\n<p><strong>Question types:<\/strong> Series\/parallel equivalent resistance, complex circuit analysis, internal resistance and terminal voltage.<\/p>\n<h3 id=\"series-and-parallel-rules\">Series and Parallel Rules<\/h3>\n<table>\n<thead><tr><th><\/th><th>Series<\/th><th>Parallel<\/th><\/tr><\/thead>\n<tr><td>Current<\/td><td>Same throughout<\/td><td>Branch currents sum to total<\/td><\/tr>\n<tr><td>Voltage<\/td><td>Sum of p.d.s = total voltage<\/td><td>Same across all branches<\/td><\/tr>\n<tr><td>Resistance<\/td><td>\\(R_{total} = R_1 + R_2 + …\\)<\/td><td>\\(\\dfrac{1}{R_{total}} = \\dfrac{1}{R_1} + \\dfrac{1}{R_2} + …\\)<\/td><\/tr>\n<\/table>\n\n<p>Two resistors in parallel: \\(R_{total} = \\dfrac{R_1 R_2}{R_1 + R_2}\\)<\/p>\n<h3 id=\"internal-resistance-and-terminal-voltage\">Internal Resistance and Terminal Voltage<\/h3>\n<p>\\[\\varepsilon = I(R + r) \\qquad V_{terminal} = \\varepsilon – Ir\\]<\/p>\n<p>Higher current \u2192 lower terminal voltage (more voltage lost across internal resistance).<\/p>\n<h3 id=\"kirchhoffs-laws\">Kirchhoff’s Laws<\/h3>\n<ul>\n<li><strong>KCL<\/strong>: Sum of currents into a junction = sum of currents out<\/li>\n<li><strong>KVL<\/strong>: Sum of e.m.f.s = sum of p.d.s around any closed loop<\/li>\n<\/ul>\n<h3 id=\"potential-divider\">Potential Divider<\/h3>\n<p>\\[V_{out} = \\frac{R_2}{R_1 + R_2} \\times V_{in}\\]<\/p>\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c Parallel resistance is always <strong>less<\/strong> than the smallest individual resistance<\/li>\n<li>\u274c Current through internal resistance = total circuit current<\/li>\n<li>\u274c Terminal voltage < e.m.f. whenever current flows<\/li>\n<\/ul>\n<\/div>\n<hr>\n<section class=\"formula-card\" id=\"topic-5-power-formulae\">\n<h2>Topic 5: Power Formulae<\/h2>\n<p>\\[P = IV = I^2R = \\frac{V^2}{R}\\]<\/p>\n<ul>\n<li>Know \\(I\\) and \\(R\\): use \\(P = I^2R\\)<\/li>\n<li>Know \\(V\\) and \\(R\\): use \\(P = V^2\/R\\)<\/li>\n<li>Series circuit: same current \u2192 larger resistance dissipates more power<\/li>\n<li>Parallel circuit: same voltage \u2192 smaller resistance dissipates more power<\/li>\n<\/ul>\n<hr>\n<h2 id=\"topic-6-basic-wave-properties\">Topic 6: Basic Wave Properties<\/h2>\n<p><strong>Question types:<\/strong> Classify transverse\/longitudinal waves, wave speed calculation, phase difference, polarisation.<\/p>\n<h3 id=\"basic-equations\">Basic Equations<\/h3>\n<p>\\[v = f\\lambda \\qquad T = \\frac{1}{f}\\]<\/p>\n<h3 id=\"transverse-vs-longitudinal\">Transverse vs Longitudinal<\/h3>\n<table>\n<thead><tr><th><\/th><th>Transverse<\/th><th>Longitudinal<\/th><\/tr><\/thead>\n<tr><td>Oscillation direction<\/td><td>Perpendicular to propagation<\/td><td>Parallel to propagation<\/td><\/tr>\n<tr><td>Can be polarised?<\/td><td><strong>Yes<\/strong><\/td><td>No<\/td><\/tr>\n<tr><td>Examples<\/td><td>Light, EM waves, rope<\/td><td>Sound, ultrasound<\/td><\/tr>\n<\/table>\n\n<div class=\"seo-note-box\">\n<p>Polarisation only occurs in transverse waves \u2014 this is evidence that light is a transverse wave.<\/p>\n<\/div>\n<h3 id=\"phase-difference\">Phase Difference<\/h3>\n<p>\\[\\Delta\\phi = \\frac{2\\pi\\,\\Delta x}{\\lambda}\\]<\/p>\n<ul>\n<li>In phase: \\(\\Delta\\phi = 2n\\pi\\) (whole number of wavelengths)<\/li>\n<li>Antiphase: \\(\\Delta\\phi = (2n+1)\\pi\\) (half-integer wavelengths)<\/li>\n<\/ul>\n<h3 id=\"intensity-relationships\">Intensity Relationships<\/h3>\n<p>\\[I \\propto A^2 \\qquad I \\propto \\frac{1}{r^2} \\text{ (point source, spherical spread)}\\]<\/p>\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c Sound is longitudinal \u2014 cannot be polarised<\/li>\n<li>\u274c Wave entering new medium: frequency unchanged; speed and wavelength change<\/li>\n<li>\u274c Path difference \u2192 phase difference: \\(\\Delta\\phi = 2\\pi\\Delta x\/\\lambda\\)<\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"topic-7-refraction-and-total-internal-reflection\">Topic 7: Refraction and Total Internal Reflection<\/h2>\n<p><strong>Question types:<\/strong> Calculate refractive index, critical angle, conditions for TIR, optical fibre principle.<\/p>\n<h3 id=\"snells-law\">Snell’s Law<\/h3>\n<p>\\[n_1\\sin\\theta_1 = n_2\\sin\\theta_2\\]<\/p>\n<p>Air (\\(n\\approx1\\)) \u2192 medium: \\(n = \\dfrac{\\sin i}{\\sin r}\\)<\/p>\n<p>Physical meaning: \\(n = \\dfrac{c}{v}\\) (speed in vacuum \/ speed in medium, \\(n > 1\\))<\/p>\n<h3 id=\"two-conditions-for-tir\">Two Conditions for TIR<\/h3>\n<p class=\"numbered-step\">Light travels from optically <strong>denser<\/strong> to optically <strong>less dense<\/strong> medium<\/p>\n<p class=\"numbered-step\">Angle of incidence \\(\\geq\\) critical angle \\(c\\)<\/p>\n<p>\\[\\sin c = \\frac{1}{n} \\quad \\text{(from medium of index }n\\text{ into air)}\\]<\/p>\n<h3 id=\"optical-fibre-principle\">Optical Fibre Principle<\/h3>\n<p>Light in the glass core hits the core-cladding boundary at an angle > critical angle \u2192 TIR \u2192 light propagates along the fibre without loss.<\/p>\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c TIR occurs when going from <strong>denser to less dense<\/strong> medium \u2014 not the other way<\/li>\n<li>\u274c Formula \\(\\sin c = 1\/n\\) assumes the less dense medium is air (\\(n=1\\))<\/li>\n<li>\u274c Entering denser medium: speed decreases, wavelength decreases, <strong>frequency unchanged<\/strong><\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"topic-8-atomic-energy-levels-and-spectra\">Topic 8: Atomic Energy Levels and Spectra<\/h2>\n<p><strong>Question types:<\/strong> Explain line spectra, calculate photon frequency from energy level diagram, emission vs absorption spectra.<\/p>\n<h3 id=\"photon-energy\">Photon Energy<\/h3>\n<p>\\[E_{photon} = hf = \\frac{hc}{\\lambda} = E_{high} – E_{low}\\]<\/p>\n<h3 id=\"transition-rules\">Transition Rules<\/h3>\n<table>\n<thead><tr><th>Direction<\/th><th>Result<\/th><\/tr><\/thead>\n<tr><td>Electron drops to lower level<\/td><td><strong>Emits<\/strong> a photon<\/td><\/tr>\n<tr><td>Electron rises to higher level<\/td><td><strong>Absorbs<\/strong> a photon of specific frequency<\/td><\/tr>\n<\/table>\n\n<p>Electrons can only occupy discrete energy levels \u2192 only specific frequencies emitted\/absorbed \u2192 <strong>line spectrum<\/strong>.<\/p>\n<h3 id=\"emission-vs-absorption-spectra\">Emission vs Absorption Spectra<\/h3>\n<table>\n<thead><tr><th>Type<\/th><th>Appearance<\/th><th>Cause<\/th><\/tr><\/thead>\n<tr><td>Emission spectrum<\/td><td>Bright lines on dark background<\/td><td>Excited electrons fall to lower level<\/td><\/tr>\n<tr><td>Absorption spectrum<\/td><td>Dark lines on continuous spectrum<\/td><td>Electrons absorb specific frequencies<\/td><\/tr>\n<\/table>\n\n<p>Same frequencies for the same element.<\/p>\n<h3 id=\"ionisation\">Ionisation<\/h3>\n<p>Energy levels are negative values; the ionisation energy = energy needed to move an electron from ground state to \\(E = 0\\).<\/p>\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c Energy levels are <strong>negative<\/strong>; \\(\\Delta E = E_{high} – E_{low}\\) (positive result)<\/li>\n<li>\u274c One photon is absorbed by one electron only \u2014 photons cannot combine<\/li>\n<li>\u274c Each line represents a specific wavelength\/frequency, not a position in space<\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"topic-9-resistivity-and-drift-velocity\">Topic 9: Resistivity and Drift Velocity<\/h2>\n<p><strong>Question types:<\/strong> Calculate wire resistance, derive drift velocity, explain resistance-temperature behaviour.<\/p>\n<h3 id=\"resistivity-formula\">Resistivity Formula<\/h3>\n<p>\\[R = \\frac{\\rho L}{A}\\]<\/p>\n<ul>\n<li>\\(\\rho\\): resistivity (unit: \\(\\Omega\\cdot\\text{m}\\)) \u2014 material property, independent of shape<\/li>\n<li>\\(R \\propto L\\) (longer wire \u2192 greater resistance)<\/li>\n<li>\\(R \\propto 1\/A\\) (larger cross-section \u2192 smaller resistance)<\/li>\n<\/ul>\n<h3 id=\"drift-velocity-formula\">Drift Velocity Formula<\/h3>\n<p>\\[I = nAve\\]<\/p>\n<table>\n<thead><tr><th>Symbol<\/th><th>Meaning<\/th><th>Unit<\/th><\/tr><\/thead>\n<tr><td>\\(n\\)<\/td><td>Number density of charge carriers<\/td><td>m\u207b\u00b3<\/td><\/tr>\n<tr><td>\\(A\\)<\/td><td>Cross-sectional area<\/td><td>m\u00b2<\/td><\/tr>\n<tr><td>\\(v\\)<\/td><td>Drift velocity<\/td><td>m\/s<\/td><\/tr>\n<tr><td>\\(e\\)<\/td><td>Elementary charge \\(1.6\\times10^{-19}\\)<\/td><td>C<\/td><\/tr>\n<\/table>\n\n<div class=\"seo-note-box\">\n<p>Drift velocity is very slow (~\\(10^{-4}\\) m\/s). Light turns on instantly because the electric field propagates at near light speed \u2014 not because electrons move fast.<\/p>\n<\/div>\n<h3 id=\"temperature-effect\">Temperature Effect<\/h3>\n<table>\n<thead><tr><th>Material<\/th><th>As temperature rises<\/th><th>Resistance change<\/th><th>Reason<\/th><\/tr><\/thead>\n<tr><td>Metal conductor<\/td><td>Ions vibrate more, more collisions<\/td><td><strong>Increases<\/strong><\/td><td>\\(n\\) unchanged, but collision frequency rises<\/td><\/tr>\n<tr><td>NTC thermistor (semiconductor)<\/td><td>More electrons gain sufficient energy<\/td><td><strong>Decreases<\/strong><\/td><td>\\(n\\) increases exponentially<\/td><\/tr>\n<\/table>\n\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c Unit of resistivity: \\(\\Omega\\cdot\\text{m}\\), not \\(\\Omega\/\\text{m}\\)<\/li>\n<li>\u274c Resistivity \\(\\rho\\) does not change with dimensions \u2014 it is a material property<\/li>\n<li>\u274c Same current, larger cross-section \u2192 <strong>smaller<\/strong> drift velocity<\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"practice-questions\">Practice Questions<\/h2>\n<p><strong>Q1.<\/strong> Work function \\(\\phi = 3.0 \\times 10^{-19}\\) J, light frequency \\(f = 8.0 \\times 10^{14}\\) Hz. Find the maximum kinetic energy of the photoelectron. (\\(h = 6.63 \\times 10^{-34}\\) J s)<\/p>\n<details><summary>Answer<\/summary><div class=\"answer-block\"><p>\\(E_k = hf – \\phi = 6.63\\times10^{-34} \\times 8.0\\times10^{14} – 3.0\\times10^{-19} = \\mathbf{2.3\\times10^{-19}\\ \\text{J}}\\)<\/p><\/div><\/details>\n<hr>\n<p><strong>Q2.<\/strong> Diffraction grating: 400 lines per mm, wavelength 600 nm. What is the maximum order of bright fringe?<\/p>\n<details><summary>Answer<\/summary><div class=\"answer-block\"><p>\\(d = 1\/400\\ \\text{mm} = 2.5\\times10^{-6}\\ \\text{m}\\)<\/p>\n<p>\\(n_{max} = d\/\\lambda = 2.5\\times10^{-6} \/ 600\\times10^{-9} = 4.17\\)<\/p>\n<p>Maximum order \\(= \\mathbf{4}\\) (round down)<\/p><\/div><\/details>\n<hr>\n<p><strong>Q3.<\/strong> \\(6\\ \\Omega\\) and \\(3\\ \\Omega\\) in parallel, then in series with \\(2\\ \\Omega\\). Find total resistance.<\/p>\n<details><summary>Answer<\/summary><div class=\"answer-block\"><p>Parallel: \\(R = \\dfrac{6\\times3}{6+3} = 2\\ \\Omega\\)<\/p>\n<p>Series: \\(R_{total} = 2 + 2 = \\mathbf{4\\ \\Omega}\\)<\/p><\/div><\/details>\n<hr>\n<p><strong>Q4.<\/strong> Glass with refractive index \\(n = 1.5\\). Find the critical angle.<\/p>\n<details><summary>Answer<\/summary><div class=\"answer-block\"><p>\\(\\sin c = \\dfrac{1}{1.5} = 0.667\\)<\/p>\n<p>\\(c = \\sin^{-1}(0.667) = \\mathbf{41.8\u00b0}\\)<\/p><\/div><\/details>\n<hr>\n<p><strong>Q5.<\/strong> Two energy levels in hydrogen differ by \\(\\Delta E = 2.55\\ \\text{eV}\\). Find the wavelength of the emitted photon. (\\(h = 6.63\\times10^{-34}\\) J s, \\(c = 3.00\\times10^8\\) m\/s, \\(1\\ \\text{eV} = 1.6\\times10^{-19}\\ \\text{J}\\))<\/p>\n<details><summary>Answer<\/summary><div class=\"answer-block\"><p>\\(E = 2.55 \\times 1.6\\times10^{-19} = 4.08\\times10^{-19}\\ \\text{J}\\)<\/p>\n<p>\\(\\lambda = \\dfrac{hc}{E} = \\dfrac{6.63\\times10^{-34} \\times 3.00\\times10^8}{4.08\\times10^{-19}} = \\mathbf{4.88\\times10^{-7}\\ \\text{m}}\\) (488 nm \u2014 blue-green)<\/p><\/div><\/details>\n<hr>\n<p><em>Want more? Visit <a href=\"https:\/\/flowxiom.com\">flowxiom.com<\/a><\/em><\/p>\n<footer class=\"site-footer\">\n <p>Free resource by <a href=\"https:\/\/flowxiom.com\">Flowxiom<\/a> \u2014 Edexcel A-level Physics<br>\n High-frequency topics only, covering ~80% of exam marks.<\/p>\n<\/footer>\n<\/body>\n<\/html>\n\n\n\n\n<p><\/p>\n","protected":false},"excerpt":{"rendered":"<p>This guide is a complete “cheat sheet” for Unit 2 Physics. It focuses on the topics that appear most often in exams, such as electricity and wave behavior. Key sections include:<\/p>\n<p>Quantum Physics: Explaining the photoelectric effect and energy levels.<\/p>\n<p>Waves & Optics: Simple rules for double-slit interference, diffraction, and fiber optics.<\/p>\n<p>DC Circuits: Easy-to-follow steps for series\/parallel resistance and Kirchhoff\u2019s Laws.<\/p>\n<p>Materials & Resistance: Understanding resistivity and how temperature affects different components.<\/p>\n<p>Exam Support: A “Common Mistakes” section for every topic to help you keep your marks.<\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[],"class_list":["post-38","post","type-post","status-publish","format-standard","hentry","category-exam-sprint-pack-physics-exam-sprint-pack"],"_links":{"self":[{"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/posts\/38","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/comments?post=38"}],"version-history":[{"count":2,"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/posts\/38\/revisions"}],"predecessor-version":[{"id":40,"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/posts\/38\/revisions\/40"}],"wp:attachment":[{"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/media?parent=38"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/categories?post=38"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/tags?post=38"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}

{"id":38,"date":"2026-04-17T16:14:32","date_gmt":"2026-04-17T16:14:32","guid":{"rendered":"https:\/\/flowxiom.com\/?p=38"},"modified":"2026-04-17T16:16:31","modified_gmt":"2026-04-17T16:16:31","slug":"unit-2-high-yield-topics","status":"publish","type":"post","link":"https:\/\/flowxiom.com\/index.php\/2026\/04\/17\/unit-2-high-yield-topics\/","title":{"rendered":"Unit 2 High-Yield Topics"},"content":{"rendered":"\n\n\n\n\n\nUnit 2 High-Yield Topics | Flowxiom<\/title>\n<meta name=\"description\" content=\"Unit 2 High-Yield Topics \u2014 Edexcel A-level Physics WPH. Question types: Explain why wave theory fails; calculate threshold frequency or maximum kinetic energ...\">\n<link rel=\"canonical\" href=\"https:\/\/flowxiom.com\/edexcel-physics-unit-2-high-yield-topics\/\">\n<script type=\"application\/ld+json\">\n{\n \"@context\": \"https:\/\/schema.org\",\n \"@type\": \"FAQPage\",\n \"mainEntity\": [\n {\n \"@type\": \"Question\",\n \"name\": \"Q1. Work function \\\\(\\\\phi = 3.0 \\\\times 10^{-19}\\\\) J, light frequency \\\\(f = 8.0 \\\\times 10^{14}\\\\) Hz. Find the maximum kinetic energy of the photoelectron. (\\\\(h = 6.63 \\\\times 10^{-34}\\\\) J s)\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"\\\\(E_k = hf - \\\\phi = 6.63\\\\times10^{-34} \\\\times 8.0\\\\times10^{14} - 3.0\\\\times10^{-19} = \\\\mathbf{2.3\\\\times10^{-19}\\\\ \\\\text{J}}\\\\)\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"Q2. Diffraction grating: 400 lines per mm, wavelength 600 nm. What is the maximum order of bright fringe?\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"\\\\(d = 1\/400\\\\ \\\\text{mm} = 2.5\\\\times10^{-6}\\\\ \\\\text{m}\\\\) \\n \\\\(n_{max} = d\/\\\\lambda = 2.5\\\\times10^{-6} \/ 600\\\\times10^{-9} = 4.17\\\\) \\n Maximum order \\\\(= \\\\mathbf{4}\\\\) (round down)\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"Q3. \\\\(6\\\\ \\\\Omega\\\\) and \\\\(3\\\\ \\\\Omega\\\\) in parallel, then in series with \\\\(2\\\\ \\\\Omega\\\\). Find total resistance.\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"Parallel: \\\\(R = \\\\dfrac{6\\\\times3}{6+3} = 2\\\\ \\\\Omega\\\\) \\n Series: \\\\(R_{total} = 2 + 2 = \\\\mathbf{4\\\\ \\\\Omega}\\\\)\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"Q4. Glass with refractive index \\\\(n = 1.5\\\\). Find the critical angle.\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"\\\\(\\\\sin c = \\\\dfrac{1}{1.5} = 0.667\\\\) \\n \\\\(c = \\\\sin^{-1}(0.667) = \\\\mathbf{41.8\u00b0}\\\\)\"\n }\n },\n {\n \"@type\": \"Question\",\n \"name\": \"Q5. Two energy levels in hydrogen differ by \\\\(\\\\Delta E = 2.55\\\\ \\\\text{eV}\\\\). Find the wavelength of the emitted photon. (\\\\(h = 6.63\\\\times10^{-34}\\\\) J s, \\\\(c = 3.00\\\\times10^8\\\\) m\/s, \\\\(1\\\\ \\\\text{eV} = 1.6\\\\times10^{-19}\\\\ \\\\text{J}\\\\))\",\n \"acceptedAnswer\": {\n \"@type\": \"Answer\",\n \"text\": \"\\\\(E = 2.55 \\\\times 1.6\\\\times10^{-19} = 4.08\\\\times10^{-19}\\\\ \\\\text{J}\\\\) \\n \\\\(\\\\lambda = \\\\dfrac{hc}{E} = \\\\dfrac{6.63\\\\times10^{-34} \\\\times 3.00\\\\times10^8}{4.08\\\\times10^{-19}} = \\\\mathbf{4.88\\\\times10^{-7}\\\\ \\\\text{m}}\\\\) (488 nm \u2014 blue-green)\"\n }\n }\n ]\n}\n<\/script>\n\n<script>\nMathJax = {\n tex: { inlineMath: [['\\\\(','\\\\)']], displayMath: [['\\\\[','\\\\]']] },\n svg: { fontCache: 'global' }\n};\n<\/script>\n<script async src=\"https:\/\/cdn.jsdelivr.net\/npm\/mathjax@3\/es5\/tex-svg.js\"><\/script>\n\n<style>\n:root{--accent:#2563eb;--warn-bg:#fef9c3;--warn-border:#ca8a04;--formula-bg:#eff6ff;--formula-border:#2563eb;}\nbody{font-family:system-ui,sans-serif;max-width:860px;margin:0 auto;padding:1.5rem;line-height:1.7;color:#1e293b;}\nh1{font-size:2rem;border-bottom:3px solid var(--accent);padding-bottom:.4rem;}\nh2{font-size:1.4rem;color:var(--accent);margin-top:2rem;}\nh3{font-size:1.1rem;margin-top:1.4rem;}\ntable{border-collapse:collapse;width:100%;margin:1rem 0;}\nth,td{border:1px solid #cbd5e1;padding:.5rem .75rem;text-align:left;}\nth{background:#e2e8f0;}\npre,code{background:#f1f5f9;border-radius:4px;}\npre{padding:1rem;overflow-x:auto;}\ncode{padding:.1rem .3rem;font-size:.9em;}\n.seo-warning-box{background:var(--warn-bg);border-left:4px solid var(--warn-border);padding:.75rem 1rem;margin:1rem 0;border-radius:0 6px 6px 0;}\n.seo-note-box{background:#f0fdf4;border-left:4px solid #16a34a;padding:.75rem 1rem;margin:1rem 0;border-radius:0 6px 6px 0;}\nsection.formula-card{background:var(--formula-bg);border:1px solid var(--formula-border);border-radius:8px;padding:1rem 1.25rem;margin:1.5rem 0;}\ndetails{border:1px solid #e2e8f0;border-radius:6px;padding:.5rem 1rem;margin:.75rem 0;}\nsummary{cursor:pointer;font-weight:600;}\n.answer-block{margin-top:.5rem;padding-top:.5rem;border-top:1px solid #e2e8f0;overflow-x:auto;}\n.answer-block p{margin:.25rem 0;}\n.numbered-step{padding-left:1.5rem;position:relative;}\nhr{border:none;border-top:1px solid #e2e8f0;margin:2rem 0;}\n.toc{background:#f8fafc;border:1px solid #e2e8f0;border-radius:8px;padding:1rem 1.5rem;margin-bottom:2rem;}\n.toc h2{margin-top:0;font-size:1.1rem;}\n.toc ul{margin:0;padding-left:1.2rem;}\n.toc a{color:var(--accent);text-decoration:none;}\n.site-footer{margin-top:3rem;padding-top:1rem;border-top:2px solid var(--accent);font-size:.9rem;color:#64748b;}\n<\/style>\n<\/head>\n<body>\n<nav class=\"toc\"><h2>Contents<\/h2><ul>\n <li><a href=\"#topic-1-the-photoelectric-effect-frequent-6mark-question\">Topic 1: The Photoelectric Effect (frequent 6-mark question)<\/a><\/li>\n <li><a href=\"#topic-2-doubleslit-interference-and-diffraction-grating\">Topic 2: Double-Slit Interference and Diffraction Grating<\/a><\/li>\n <li><a href=\"#topic-3-stationary-standing-waves\">Topic 3: Stationary (Standing) Waves<\/a><\/li>\n <li><a href=\"#topic-4-dc-circuits-kirchhoffs-laws\">Topic 4: DC Circuits (Kirchhoff’s Laws)<\/a><\/li>\n <li><a href=\"#topic-5-power-formulae\">Topic 5: Power Formulae<\/a><\/li>\n <li><a href=\"#topic-6-basic-wave-properties\">Topic 6: Basic Wave Properties<\/a><\/li>\n <li><a href=\"#topic-7-refraction-and-total-internal-reflection\">Topic 7: Refraction and Total Internal Reflection<\/a><\/li>\n <li><a href=\"#topic-8-atomic-energy-levels-and-spectra\">Topic 8: Atomic Energy Levels and Spectra<\/a><\/li>\n <li><a href=\"#topic-9-resistivity-and-drift-velocity\">Topic 9: Resistivity and Drift Velocity<\/a><\/li>\n <li><a href=\"#practice-questions\">Practice Questions<\/a><\/li>\n<\/ul><\/nav>\n<h1>Unit 2 High-Yield Topics<\/h1>\n<p><strong>Free resource by <a href=\"https:\/\/flowxiom.com\">Flowxiom<\/a> \u2014 Edexcel A-level Physics<\/strong><\/p>\n<p><em>Not everything. Just what’s on the paper. High-frequency topics only \u2014 covering ~80% of exam marks.<\/em><\/p>\n<p>Edexcel A-level Physics | Waves, Optics & DC Circuits | WPH11 & WPH12<\/p>\n<hr>\n<h2 id=\"topic-1-the-photoelectric-effect-frequent-6mark-question\">Topic 1: The Photoelectric Effect (frequent 6-mark question)<\/h2>\n<p><strong>Question types:<\/strong> Explain why wave theory fails; calculate threshold frequency or maximum kinetic energy.<\/p>\n<h3 id=\"einsteins-photoelectric-equation\">Einstein’s Photoelectric Equation<\/h3>\n<p>\\[hf = \\phi + E_{k(max)}\\]<\/p>\n<table>\n<thead><tr><th>Symbol<\/th><th>Meaning<\/th><\/tr><\/thead>\n<tr><td>\\(hf\\)<\/td><td>Incident photon energy (\\(h = 6.63 \\times 10^{-34}\\) J s)<\/td><\/tr>\n<tr><td>\\(\\phi\\)<\/td><td>Work function \u2014 minimum energy to escape the metal<\/td><\/tr>\n<tr><td>\\(E_{k(max)}\\)<\/td><td>Maximum kinetic energy of photoelectron<\/td><\/tr>\n<\/table>\n\n<p>Threshold frequency: \\(f_0 = \\dfrac{\\phi}{h}\\) (at this point \\(E_k = 0\\))<\/p>\n<p>Stopping voltage: \\(E_{k(max)} = eV_s\\)<\/p>\n<h3 id=\"why-wave-theory-fails-threepoint-answer-chain\">Why Wave Theory Fails \u2014 Three-Point Answer Chain<\/h3>\n<table>\n<thead><tr><th>Observation<\/th><th>Wave theory predicts (wrong)<\/th><th>Photon theory explains (correct)<\/th><\/tr><\/thead>\n<tr><td>Threshold frequency exists<\/td><td>Sufficient intensity should eject electrons at any frequency<\/td><td>Single photon energy \\(hf\\) insufficient; more photons don’t help<\/td><\/tr>\n<tr><td>No time delay<\/td><td>Weak light needs time to build up energy<\/td><td>One photon transfers energy to one electron instantly<\/td><\/tr>\n<tr><td>Kinetic energy independent of intensity<\/td><td>Higher intensity \u2192 higher kinetic energy<\/td><td>Intensity increases photon count, not individual photon energy<\/td><\/tr>\n<\/table>\n\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c Must state: single photon energy \\(hf < \\phi\\), so electron cannot escape<\/li>\n<li>\u274c Gradient of stopping voltage vs frequency graph = \\(h\/e\\), not \\(h\\)<\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"topic-2-doubleslit-interference-and-diffraction-grating\">Topic 2: Double-Slit Interference and Diffraction Grating<\/h2>\n<p><strong>Question types:<\/strong> Fringe spacing calculation, maximum order for diffraction grating, effect of changing parameters.<\/p>\n<h3 id=\"doubleslit-fringe-spacing\">Double-Slit Fringe Spacing<\/h3>\n<p>\\[\\Delta y = \\frac{\\lambda L}{d}\\]<\/p>\n<ul>\n<li>\\(\\lambda\\): wavelength, \\(L\\): slit-to-screen distance, \\(d\\): slit separation<\/li>\n<li>Wider fringes: increase \\(\\lambda\\) \/ increase \\(L\\) \/ decrease \\(d\\)<\/li>\n<\/ul>\n<h3 id=\"diffraction-grating\">Diffraction Grating<\/h3>\n<p>\\[d\\sin\\theta = n\\lambda\\]<\/p>\n<ul>\n<li>\\(d\\): grating spacing (if question gives N lines per mm, then \\(d = 1\/N\\) mm)<\/li>\n<li>Maximum order: set \\(\\sin\\theta = 1\\), \\(n_{max} = d\/\\lambda\\) (round down to integer)<\/li>\n<\/ul>\n<h3 id=\"conditions-for-interference\">Conditions for Interference<\/h3>\n<p class=\"numbered-step\"><strong>Coherent<\/strong>: same frequency + constant phase difference<\/p>\n<p class=\"numbered-step\">Same type of wave<\/p>\n<h3 id=\"path-difference-rule\">Path Difference Rule<\/h3>\n<table>\n<thead><tr><th>Path difference<\/th><th>Result<\/th><\/tr><\/thead>\n<tr><td>\\(n\\lambda\\) (whole number of wavelengths)<\/td><td>Constructive interference \u2014 bright fringe<\/td><\/tr>\n<tr><td>\\((n+0.5)\\lambda\\)<\/td><td>Destructive interference \u2014 dark fringe<\/td><\/tr>\n<\/table>\n\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c \\(d\\) = slit separation (centre to centre), not slit width<\/li>\n<li>\u274c For the grating: \\(d\\) is the separation between adjacent slits, not the slit width<\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"topic-3-stationary-standing-waves\">Topic 3: Stationary (Standing) Waves<\/h2>\n<p><strong>Question types:<\/strong> Resonance conditions, counting nodes and antinodes, frequency calculation.<\/p>\n<h3 id=\"resonance-condition\">Resonance Condition<\/h3>\n<p>String with both ends fixed: \\(L = \\dfrac{n\\lambda}{2}\\), \\(n = 1, 2, 3…\\)<\/p>\n<ul>\n<li>Fundamental (\\(n=1\\)): \\(f_1 = \\dfrac{v}{2L}\\)<\/li>\n<li>\\(n\\)th harmonic: \\(f_n = nf_1\\)<\/li>\n<\/ul>\n<h3 id=\"node-vs-antinode\">Node vs Antinode<\/h3>\n<ul>\n<li><strong>Node<\/strong>: point of zero displacement (fixed end)<\/li>\n<li><strong>Antinode<\/strong>: point of maximum displacement<\/li>\n<\/ul>\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c Recount nodes\/antinodes by drawing the wave pattern<\/li>\n<li>\u274c Distance between adjacent nodes = \\(\\lambda\/2\\), not \\(\\lambda\\)<\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"topic-4-dc-circuits-kirchhoffs-laws\">Topic 4: DC Circuits (Kirchhoff’s Laws)<\/h2>\n<p><strong>Question types:<\/strong> Series\/parallel equivalent resistance, complex circuit analysis, internal resistance and terminal voltage.<\/p>\n<h3 id=\"series-and-parallel-rules\">Series and Parallel Rules<\/h3>\n<table>\n<thead><tr><th><\/th><th>Series<\/th><th>Parallel<\/th><\/tr><\/thead>\n<tr><td>Current<\/td><td>Same throughout<\/td><td>Branch currents sum to total<\/td><\/tr>\n<tr><td>Voltage<\/td><td>Sum of p.d.s = total voltage<\/td><td>Same across all branches<\/td><\/tr>\n<tr><td>Resistance<\/td><td>\\(R_{total} = R_1 + R_2 + …\\)<\/td><td>\\(\\dfrac{1}{R_{total}} = \\dfrac{1}{R_1} + \\dfrac{1}{R_2} + …\\)<\/td><\/tr>\n<\/table>\n\n<p>Two resistors in parallel: \\(R_{total} = \\dfrac{R_1 R_2}{R_1 + R_2}\\)<\/p>\n<h3 id=\"internal-resistance-and-terminal-voltage\">Internal Resistance and Terminal Voltage<\/h3>\n<p>\\[\\varepsilon = I(R + r) \\qquad V_{terminal} = \\varepsilon – Ir\\]<\/p>\n<p>Higher current \u2192 lower terminal voltage (more voltage lost across internal resistance).<\/p>\n<h3 id=\"kirchhoffs-laws\">Kirchhoff’s Laws<\/h3>\n<ul>\n<li><strong>KCL<\/strong>: Sum of currents into a junction = sum of currents out<\/li>\n<li><strong>KVL<\/strong>: Sum of e.m.f.s = sum of p.d.s around any closed loop<\/li>\n<\/ul>\n<h3 id=\"potential-divider\">Potential Divider<\/h3>\n<p>\\[V_{out} = \\frac{R_2}{R_1 + R_2} \\times V_{in}\\]<\/p>\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c Parallel resistance is always <strong>less<\/strong> than the smallest individual resistance<\/li>\n<li>\u274c Current through internal resistance = total circuit current<\/li>\n<li>\u274c Terminal voltage < e.m.f. whenever current flows<\/li>\n<\/ul>\n<\/div>\n<hr>\n<section class=\"formula-card\" id=\"topic-5-power-formulae\">\n<h2>Topic 5: Power Formulae<\/h2>\n<p>\\[P = IV = I^2R = \\frac{V^2}{R}\\]<\/p>\n<ul>\n<li>Know \\(I\\) and \\(R\\): use \\(P = I^2R\\)<\/li>\n<li>Know \\(V\\) and \\(R\\): use \\(P = V^2\/R\\)<\/li>\n<li>Series circuit: same current \u2192 larger resistance dissipates more power<\/li>\n<li>Parallel circuit: same voltage \u2192 smaller resistance dissipates more power<\/li>\n<\/ul>\n<hr>\n<h2 id=\"topic-6-basic-wave-properties\">Topic 6: Basic Wave Properties<\/h2>\n<p><strong>Question types:<\/strong> Classify transverse\/longitudinal waves, wave speed calculation, phase difference, polarisation.<\/p>\n<h3 id=\"basic-equations\">Basic Equations<\/h3>\n<p>\\[v = f\\lambda \\qquad T = \\frac{1}{f}\\]<\/p>\n<h3 id=\"transverse-vs-longitudinal\">Transverse vs Longitudinal<\/h3>\n<table>\n<thead><tr><th><\/th><th>Transverse<\/th><th>Longitudinal<\/th><\/tr><\/thead>\n<tr><td>Oscillation direction<\/td><td>Perpendicular to propagation<\/td><td>Parallel to propagation<\/td><\/tr>\n<tr><td>Can be polarised?<\/td><td><strong>Yes<\/strong><\/td><td>No<\/td><\/tr>\n<tr><td>Examples<\/td><td>Light, EM waves, rope<\/td><td>Sound, ultrasound<\/td><\/tr>\n<\/table>\n\n<div class=\"seo-note-box\">\n<p>Polarisation only occurs in transverse waves \u2014 this is evidence that light is a transverse wave.<\/p>\n<\/div>\n<h3 id=\"phase-difference\">Phase Difference<\/h3>\n<p>\\[\\Delta\\phi = \\frac{2\\pi\\,\\Delta x}{\\lambda}\\]<\/p>\n<ul>\n<li>In phase: \\(\\Delta\\phi = 2n\\pi\\) (whole number of wavelengths)<\/li>\n<li>Antiphase: \\(\\Delta\\phi = (2n+1)\\pi\\) (half-integer wavelengths)<\/li>\n<\/ul>\n<h3 id=\"intensity-relationships\">Intensity Relationships<\/h3>\n<p>\\[I \\propto A^2 \\qquad I \\propto \\frac{1}{r^2} \\text{ (point source, spherical spread)}\\]<\/p>\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c Sound is longitudinal \u2014 cannot be polarised<\/li>\n<li>\u274c Wave entering new medium: frequency unchanged; speed and wavelength change<\/li>\n<li>\u274c Path difference \u2192 phase difference: \\(\\Delta\\phi = 2\\pi\\Delta x\/\\lambda\\)<\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"topic-7-refraction-and-total-internal-reflection\">Topic 7: Refraction and Total Internal Reflection<\/h2>\n<p><strong>Question types:<\/strong> Calculate refractive index, critical angle, conditions for TIR, optical fibre principle.<\/p>\n<h3 id=\"snells-law\">Snell’s Law<\/h3>\n<p>\\[n_1\\sin\\theta_1 = n_2\\sin\\theta_2\\]<\/p>\n<p>Air (\\(n\\approx1\\)) \u2192 medium: \\(n = \\dfrac{\\sin i}{\\sin r}\\)<\/p>\n<p>Physical meaning: \\(n = \\dfrac{c}{v}\\) (speed in vacuum \/ speed in medium, \\(n > 1\\))<\/p>\n<h3 id=\"two-conditions-for-tir\">Two Conditions for TIR<\/h3>\n<p class=\"numbered-step\">Light travels from optically <strong>denser<\/strong> to optically <strong>less dense<\/strong> medium<\/p>\n<p class=\"numbered-step\">Angle of incidence \\(\\geq\\) critical angle \\(c\\)<\/p>\n<p>\\[\\sin c = \\frac{1}{n} \\quad \\text{(from medium of index }n\\text{ into air)}\\]<\/p>\n<h3 id=\"optical-fibre-principle\">Optical Fibre Principle<\/h3>\n<p>Light in the glass core hits the core-cladding boundary at an angle > critical angle \u2192 TIR \u2192 light propagates along the fibre without loss.<\/p>\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c TIR occurs when going from <strong>denser to less dense<\/strong> medium \u2014 not the other way<\/li>\n<li>\u274c Formula \\(\\sin c = 1\/n\\) assumes the less dense medium is air (\\(n=1\\))<\/li>\n<li>\u274c Entering denser medium: speed decreases, wavelength decreases, <strong>frequency unchanged<\/strong><\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"topic-8-atomic-energy-levels-and-spectra\">Topic 8: Atomic Energy Levels and Spectra<\/h2>\n<p><strong>Question types:<\/strong> Explain line spectra, calculate photon frequency from energy level diagram, emission vs absorption spectra.<\/p>\n<h3 id=\"photon-energy\">Photon Energy<\/h3>\n<p>\\[E_{photon} = hf = \\frac{hc}{\\lambda} = E_{high} – E_{low}\\]<\/p>\n<h3 id=\"transition-rules\">Transition Rules<\/h3>\n<table>\n<thead><tr><th>Direction<\/th><th>Result<\/th><\/tr><\/thead>\n<tr><td>Electron drops to lower level<\/td><td><strong>Emits<\/strong> a photon<\/td><\/tr>\n<tr><td>Electron rises to higher level<\/td><td><strong>Absorbs<\/strong> a photon of specific frequency<\/td><\/tr>\n<\/table>\n\n<p>Electrons can only occupy discrete energy levels \u2192 only specific frequencies emitted\/absorbed \u2192 <strong>line spectrum<\/strong>.<\/p>\n<h3 id=\"emission-vs-absorption-spectra\">Emission vs Absorption Spectra<\/h3>\n<table>\n<thead><tr><th>Type<\/th><th>Appearance<\/th><th>Cause<\/th><\/tr><\/thead>\n<tr><td>Emission spectrum<\/td><td>Bright lines on dark background<\/td><td>Excited electrons fall to lower level<\/td><\/tr>\n<tr><td>Absorption spectrum<\/td><td>Dark lines on continuous spectrum<\/td><td>Electrons absorb specific frequencies<\/td><\/tr>\n<\/table>\n\n<p>Same frequencies for the same element.<\/p>\n<h3 id=\"ionisation\">Ionisation<\/h3>\n<p>Energy levels are negative values; the ionisation energy = energy needed to move an electron from ground state to \\(E = 0\\).<\/p>\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c Energy levels are <strong>negative<\/strong>; \\(\\Delta E = E_{high} – E_{low}\\) (positive result)<\/li>\n<li>\u274c One photon is absorbed by one electron only \u2014 photons cannot combine<\/li>\n<li>\u274c Each line represents a specific wavelength\/frequency, not a position in space<\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"topic-9-resistivity-and-drift-velocity\">Topic 9: Resistivity and Drift Velocity<\/h2>\n<p><strong>Question types:<\/strong> Calculate wire resistance, derive drift velocity, explain resistance-temperature behaviour.<\/p>\n<h3 id=\"resistivity-formula\">Resistivity Formula<\/h3>\n<p>\\[R = \\frac{\\rho L}{A}\\]<\/p>\n<ul>\n<li>\\(\\rho\\): resistivity (unit: \\(\\Omega\\cdot\\text{m}\\)) \u2014 material property, independent of shape<\/li>\n<li>\\(R \\propto L\\) (longer wire \u2192 greater resistance)<\/li>\n<li>\\(R \\propto 1\/A\\) (larger cross-section \u2192 smaller resistance)<\/li>\n<\/ul>\n<h3 id=\"drift-velocity-formula\">Drift Velocity Formula<\/h3>\n<p>\\[I = nAve\\]<\/p>\n<table>\n<thead><tr><th>Symbol<\/th><th>Meaning<\/th><th>Unit<\/th><\/tr><\/thead>\n<tr><td>\\(n\\)<\/td><td>Number density of charge carriers<\/td><td>m\u207b\u00b3<\/td><\/tr>\n<tr><td>\\(A\\)<\/td><td>Cross-sectional area<\/td><td>m\u00b2<\/td><\/tr>\n<tr><td>\\(v\\)<\/td><td>Drift velocity<\/td><td>m\/s<\/td><\/tr>\n<tr><td>\\(e\\)<\/td><td>Elementary charge \\(1.6\\times10^{-19}\\)<\/td><td>C<\/td><\/tr>\n<\/table>\n\n<div class=\"seo-note-box\">\n<p>Drift velocity is very slow (~\\(10^{-4}\\) m\/s). Light turns on instantly because the electric field propagates at near light speed \u2014 not because electrons move fast.<\/p>\n<\/div>\n<h3 id=\"temperature-effect\">Temperature Effect<\/h3>\n<table>\n<thead><tr><th>Material<\/th><th>As temperature rises<\/th><th>Resistance change<\/th><th>Reason<\/th><\/tr><\/thead>\n<tr><td>Metal conductor<\/td><td>Ions vibrate more, more collisions<\/td><td><strong>Increases<\/strong><\/td><td>\\(n\\) unchanged, but collision frequency rises<\/td><\/tr>\n<tr><td>NTC thermistor (semiconductor)<\/td><td>More electrons gain sufficient energy<\/td><td><strong>Decreases<\/strong><\/td><td>\\(n\\) increases exponentially<\/td><\/tr>\n<\/table>\n\n<div class=\"seo-warning-box\" id=\"common-mistakes\">\n<h3>Common Mistakes<\/h3>\n<ul>\n<li>\u274c Unit of resistivity: \\(\\Omega\\cdot\\text{m}\\), not \\(\\Omega\/\\text{m}\\)<\/li>\n<li>\u274c Resistivity \\(\\rho\\) does not change with dimensions \u2014 it is a material property<\/li>\n<li>\u274c Same current, larger cross-section \u2192 <strong>smaller<\/strong> drift velocity<\/li>\n<\/ul>\n<\/div>\n<hr>\n<h2 id=\"practice-questions\">Practice Questions<\/h2>\n<p><strong>Q1.<\/strong> Work function \\(\\phi = 3.0 \\times 10^{-19}\\) J, light frequency \\(f = 8.0 \\times 10^{14}\\) Hz. Find the maximum kinetic energy of the photoelectron. (\\(h = 6.63 \\times 10^{-34}\\) J s)<\/p>\n<details><summary>Answer<\/summary><div class=\"answer-block\"><p>\\(E_k = hf – \\phi = 6.63\\times10^{-34} \\times 8.0\\times10^{14} – 3.0\\times10^{-19} = \\mathbf{2.3\\times10^{-19}\\ \\text{J}}\\)<\/p><\/div><\/details>\n<hr>\n<p><strong>Q2.<\/strong> Diffraction grating: 400 lines per mm, wavelength 600 nm. What is the maximum order of bright fringe?<\/p>\n<details><summary>Answer<\/summary><div class=\"answer-block\"><p>\\(d = 1\/400\\ \\text{mm} = 2.5\\times10^{-6}\\ \\text{m}\\)<\/p>\n<p>\\(n_{max} = d\/\\lambda = 2.5\\times10^{-6} \/ 600\\times10^{-9} = 4.17\\)<\/p>\n<p>Maximum order \\(= \\mathbf{4}\\) (round down)<\/p><\/div><\/details>\n<hr>\n<p><strong>Q3.<\/strong> \\(6\\ \\Omega\\) and \\(3\\ \\Omega\\) in parallel, then in series with \\(2\\ \\Omega\\). Find total resistance.<\/p>\n<details><summary>Answer<\/summary><div class=\"answer-block\"><p>Parallel: \\(R = \\dfrac{6\\times3}{6+3} = 2\\ \\Omega\\)<\/p>\n<p>Series: \\(R_{total} = 2 + 2 = \\mathbf{4\\ \\Omega}\\)<\/p><\/div><\/details>\n<hr>\n<p><strong>Q4.<\/strong> Glass with refractive index \\(n = 1.5\\). Find the critical angle.<\/p>\n<details><summary>Answer<\/summary><div class=\"answer-block\"><p>\\(\\sin c = \\dfrac{1}{1.5} = 0.667\\)<\/p>\n<p>\\(c = \\sin^{-1}(0.667) = \\mathbf{41.8\u00b0}\\)<\/p><\/div><\/details>\n<hr>\n<p><strong>Q5.<\/strong> Two energy levels in hydrogen differ by \\(\\Delta E = 2.55\\ \\text{eV}\\). Find the wavelength of the emitted photon. (\\(h = 6.63\\times10^{-34}\\) J s, \\(c = 3.00\\times10^8\\) m\/s, \\(1\\ \\text{eV} = 1.6\\times10^{-19}\\ \\text{J}\\))<\/p>\n<details><summary>Answer<\/summary><div class=\"answer-block\"><p>\\(E = 2.55 \\times 1.6\\times10^{-19} = 4.08\\times10^{-19}\\ \\text{J}\\)<\/p>\n<p>\\(\\lambda = \\dfrac{hc}{E} = \\dfrac{6.63\\times10^{-34} \\times 3.00\\times10^8}{4.08\\times10^{-19}} = \\mathbf{4.88\\times10^{-7}\\ \\text{m}}\\) (488 nm \u2014 blue-green)<\/p><\/div><\/details>\n<hr>\n<p><em>Want more? Visit <a href=\"https:\/\/flowxiom.com\">flowxiom.com<\/a><\/em><\/p>\n<footer class=\"site-footer\">\n <p>Free resource by <a href=\"https:\/\/flowxiom.com\">Flowxiom<\/a> \u2014 Edexcel A-level Physics<br>\n High-frequency topics only, covering ~80% of exam marks.<\/p>\n<\/footer>\n<\/body>\n<\/html>\n\n\n\n\n<p><\/p>\n","protected":false},"excerpt":{"rendered":"<p>This guide is a complete “cheat sheet” for Unit 2 Physics. It focuses on the topics that appear most often in exams, such as electricity and wave behavior. Key sections include:<\/p>\n<p>Quantum Physics: Explaining the photoelectric effect and energy levels.<\/p>\n<p>Waves & Optics: Simple rules for double-slit interference, diffraction, and fiber optics.<\/p>\n<p>DC Circuits: Easy-to-follow steps for series\/parallel resistance and Kirchhoff\u2019s Laws.<\/p>\n<p>Materials & Resistance: Understanding resistivity and how temperature affects different components.<\/p>\n<p>Exam Support: A “Common Mistakes” section for every topic to help you keep your marks.<\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5],"tags":[],"class_list":["post-38","post","type-post","status-publish","format-standard","hentry","category-exam-sprint-pack-physics-exam-sprint-pack"],"_links":{"self":[{"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/posts\/38","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/comments?post=38"}],"version-history":[{"count":2,"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/posts\/38\/revisions"}],"predecessor-version":[{"id":40,"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/posts\/38\/revisions\/40"}],"wp:attachment":[{"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/media?parent=38"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/categories?post=38"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/flowxiom.com\/index.php\/wp-json\/wp\/v2\/tags?post=38"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}