/* Circuit Practice: a physics lab worksheet. Cool blue-white paper, schematics drawn in dark ink on a faint
   engineering-pad grid, a cobalt accent like the blue of a lab bench power supply, and the given values of each
   V-I-R-P table on pale amber, the way a teacher highlights the numbers you start from. Geometric headings like
   the lettering on a meter panel. Print: black only, the schematic at a fixed scale, a problem never split. */
[hidden] { display: none !important; }
.nw { white-space: nowrap; }
.sr { position: absolute; width: 1px; height: 1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; }
:root {
  --bg: #eef1f6;
  --paper: #ffffff;
  --grid: #e3e9f3;
  --ink: #18202e;
  --muted: #4d5869;
  --line: #d2d9e4;
  --edge: #8a96a8;
  --accent: #1f54a8;
  --accent-ink: #ffffff;
  --accent-soft: #e2ebf8;
  --given: #fff3cf;
  --given-ink: #5a4300;
  --draw: #141a24;
  --ok: #17663a;
  --ok-soft: #dff1e6;
  --close: #7a5000;
  --close-soft: #f7ebcf;
  --bad: #a1251d;
  --bad-soft: #f9e1de;
  --info: #24527a;
  --focus: #c25e00;
  --head: "Avenir Next", "Futura", "Century Gothic", "Segoe UI", "Trebuchet MS", sans-serif;
  --sans: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  --mono: ui-monospace, "SF Mono", Menlo, Consolas, "DejaVu Sans Mono", monospace;
  --r: 4px;
  color-scheme: light dark;
}
@media (prefers-color-scheme: dark) {
  :root {
    --bg: #11151d;
    --paper: #181e29;
    --grid: #202838;
    --ink: #e5e9f0;
    --muted: #aab4c4;
    --line: #2c3546;
    --edge: #5f6c81;
    --accent: #8db4ff;
    --accent-ink: #0b1a33;
    --accent-soft: #1b2a44;
    --given: #3a3115;
    --given-ink: #f3d98b;
    --draw: #eef2f8;
    --ok: #8fdcaa;
    --ok-soft: #14301f;
    --close: #f0c56b;
    --close-soft: #352a12;
    --bad: #ff9d90;
    --bad-soft: #3a1c18;
    --info: #8ec3ea;
    --focus: #ffb366;
  }
}
* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body { margin: 0; background: var(--bg); color: var(--ink); font: 17px/1.55 var(--sans); }
.wrap { max-width: 1040px; margin: 0 auto; padding: 0 16px; }
a { color: var(--accent); text-underline-offset: 2px; }
.skip { position: absolute; left: -999px; top: 8px; background: var(--paper); padding: 8px 12px; z-index: 10; }
.skip:focus { left: 8px; }
:focus-visible { outline: 3px solid var(--focus); outline-offset: 2px; }

.hero { padding: 22px 0 4px; max-width: 760px; }
h1 { font: 700 clamp(1.65rem, 4.4vw, 2.45rem)/1.12 var(--head); margin: 0 0 8px; letter-spacing: -0.01em; }
.lede { margin: 0; color: var(--muted); max-width: 62ch; }
h2 { font: 700 1.3rem/1.25 var(--head); margin: 0 0 10px; }

/* setup */
.setup { display: flex; flex-wrap: wrap; align-items: flex-end; gap: 10px 12px; padding: 14px 0 10px; margin-top: 8px; border: 1px solid var(--line); border-radius: var(--r); padding-left: 12px; padding-right: 12px; }
.setup label, .print-row label { display: flex; flex-direction: column; gap: 3px; font-size: 1rem; font-weight: 650; color: var(--muted); }
select, input { font: inherit; font-size: 16px; color: var(--ink); background: var(--paper); border: 1px solid var(--edge); border-radius: var(--r); min-height: 44px; padding: 6px 10px; }
select { max-width: 100%; }
.btn { font: inherit; font-weight: 650; min-height: 44px; padding: 8px 18px; border-radius: var(--r); border: 1px solid var(--accent); background: var(--paper); color: var(--accent); cursor: pointer; }
.btn.primary { background: var(--accent); color: var(--accent-ink); }
.btn.small { padding: 6px 14px; }
.btn:hover { filter: brightness(0.96); }
.link { font: inherit; background: none; border: 0; padding: 10px 0; min-height: 44px; color: var(--accent); text-decoration: underline; text-underline-offset: 2px; cursor: pointer; }

.opts-d { border-top: 1px solid var(--line); border-bottom: 1px solid var(--line); }
.opts-d > summary { cursor: pointer; padding: 10px 0; min-height: 44px; font-weight: 650; display: flex; align-items: center; }
.opts-body { padding: 0 0 12px; }
.seg { border: 0; padding: 0; margin: 0 0 10px; display: flex; flex-wrap: wrap; gap: 6px; }
.seg legend { font-size: 1rem; font-weight: 650; color: var(--muted); margin-bottom: 4px; padding: 0; }
.sg { position: relative; }
.sg input { position: absolute; opacity: 0; width: 1px; height: 1px; }
.sg span { display: inline-flex; align-items: center; min-height: 44px; padding: 6px 14px; border: 1px solid var(--edge); border-radius: var(--r); background: var(--paper); cursor: pointer; font-weight: 600; }
.sg input:checked + span { background: var(--accent); color: var(--accent-ink); border-color: var(--accent); }
.sg input:focus-visible + span { outline: 3px solid var(--focus); outline-offset: 2px; }
.aq { display: flex; align-items: center; gap: 10px; min-height: 44px; font-weight: 600; }
.aq input { min-height: 0; width: 20px; height: 20px; accent-color: var(--accent); }
.rl { margin: 6px 0 0; padding-left: 1.3em; max-width: 78ch; font-size: 0.95rem; }
.rl li { margin: 0 0 6px; }
.rl .src { color: var(--muted); font-size: 0.88rem; display: block; }

.note { margin: 12px 0; padding: 10px 12px; border: 1px solid var(--close); background: var(--close-soft); overflow-wrap: anywhere; border-radius: var(--r); }
.note.info { border-color: var(--info); background: var(--paper); }
.note .link { min-height: 0; padding: 0; font-weight: 600; }

/* the worksheet */
.sheet { background: var(--paper); border-top: 6px solid var(--accent); box-shadow: 0 1px 3px rgba(20, 30, 50, 0.12); margin: 16px 0 8px; padding: 14px 18px 10px; }
.sheet:focus { outline: 2px solid var(--accent); }
.sheet-head { display: flex; justify-content: space-between; align-items: baseline; flex-wrap: wrap; gap: 4px 14px; }
.sheet-head h2 { margin: 0; }
.code { font-family: var(--mono); letter-spacing: 0.08em; color: var(--accent); }
.score { margin: 0; color: var(--muted); font-size: 0.95rem; }
.sline { margin: 4px 0 0; color: var(--muted); font-size: 0.92rem; }
.qs { list-style: none; margin: 0; padding: 0; }
.q { border-top: 1px solid var(--line); padding: 16px 0 10px; margin-top: 10px; }
.q h3 { font: 700 1.12rem/1.3 var(--head); margin: 0 0 6px; display: flex; gap: 10px; align-items: baseline; }
.qn { font-family: var(--mono); color: var(--accent); font-weight: 700; }
.scen { margin: 0 0 10px; max-width: 70ch; }
.qbody { display: grid; gap: 12px 24px; grid-template-columns: minmax(0, 1fr); }
@media (min-width: 860px) { .qbody { grid-template-columns: minmax(0, 1.05fr) minmax(0, 1fr); align-items: start; } }
.fig { background-color: var(--paper); background-image: linear-gradient(var(--grid) 1px, transparent 1px), linear-gradient(90deg, var(--grid) 1px, transparent 1px); background-size: 12px 12px; border: 1px solid var(--line); border-radius: var(--r); padding: 6px; }
.fig svg { display: block; width: 100%; height: auto; margin: 0 auto; }
.fig.zoomed { overflow-x: auto; }
.fighead { display: flex; justify-content: flex-end; margin-bottom: 4px; }
.zoom { min-height: 40px; padding: 4px 12px; white-space: nowrap; }

/* schematic ink */
svg.circ .w, svg.circ line, svg.circ path { stroke: var(--draw); stroke-width: 1.6; fill: none; stroke-linecap: square; }
svg.circ .bd { stroke: var(--draw); stroke-width: 1.6; fill: none; }
svg.circ rect.bd, svg.circ circle.bd { fill: var(--paper); }
svg.circ .zz { stroke-linejoin: miter; stroke-linecap: butt; }
svg.circ .pl { stroke-width: 1.6; stroke-linecap: butt; }
svg.circ .pl.th { stroke-width: 3.6; }
svg.circ .dash { stroke-dasharray: 2 3; }
svg.circ .dot, svg.circ .ah { fill: var(--draw); stroke: none; }
svg.circ .cellbox { fill: none; stroke: var(--muted); stroke-width: 1; stroke-dasharray: 4 3; }
svg.circ text { fill: var(--draw); font: 11px Arial, "Helvetica Neue", var(--sans); text-anchor: middle; stroke: none; }
svg.circ text.nm { font-weight: 700; }
svg.circ text.ml { font-weight: 700; font-size: 12px; }
svg.circ.red .vl { fill: var(--accent); }

/* the V-I-R-P table */
.virp { border-collapse: collapse; width: 100%; font-size: 0.95rem; table-layout: fixed; }
.virp th, .virp td { border: 1px solid var(--line); padding: 3px; text-align: center; }
.virp thead th { background: var(--accent-soft); font-family: var(--head); font-weight: 700; }
.virp thead th small { display: block; font-family: var(--sans); font-weight: 400; color: var(--muted); font-size: 0.78rem; }
.virp tbody th { font-weight: 700; width: 3.6rem; }
.virp th.rowh { width: 3.6rem; }
.virp .gv { background: var(--given); color: var(--given-ink); font-size: 0.92rem; font-weight: 650; }
.virp .na { color: var(--muted); font-size: 0.82rem; }
.virp tr.tot th, .virp tr.tot td { border-top: 2px solid var(--ink); }
.virp input { width: 100%; min-width: 0; min-height: 40px; padding: 4px 5px; font-family: var(--mono); font-size: 16px; text-align: center; border-color: var(--edge); }
.virp td.is-right input { border-color: var(--ok); box-shadow: inset 0 0 0 1px var(--ok); background: var(--ok-soft); }
.virp td.is-wrong input, .virp td.is-unread input { border-color: var(--bad); box-shadow: inset 0 0 0 1px var(--bad); background: var(--bad-soft); }
.virp td.is-close input { border-color: var(--close); box-shadow: inset 0 0 0 1px var(--close); background: var(--close-soft); }
.virp td.ans { font-family: var(--mono); font-size: 0.88rem; }
.tnotes { list-style: none; margin: 6px 0 0; padding: 0; }
.tnotes li { margin: 4px 0 0; padding: 5px 9px; border-radius: var(--r); font-size: 0.92rem; }

.qparts { display: grid; grid-template-columns: repeat(auto-fill, minmax(min(100%, 17rem), 1fr)); gap: 10px 20px; margin-top: 12px; }
.part { margin: 0; border: 0; padding: 0; min-width: 0; }
.part.ch { grid-column: 1 / -1; }
.prompt, .part legend { display: block; font-size: 0.95rem; font-weight: 600; margin-bottom: 3px; padding: 0; }
.ain { display: inline-flex; align-items: center; gap: 6px; }
.ain input { width: 8.5em; font-family: var(--mono); }
.unit { color: var(--muted); min-width: 1.6em; }
.part.is-right .ain input { border-color: var(--ok); box-shadow: inset 0 0 0 1px var(--ok); }
.part.is-wrong .ain input, .part.is-unread .ain input { border-color: var(--bad); box-shadow: inset 0 0 0 1px var(--bad); }
.part.is-close .ain input { border-color: var(--close); box-shadow: inset 0 0 0 1px var(--close); }
.opts { display: flex; flex-wrap: wrap; gap: 6px; }
.opt { display: flex; gap: 8px; align-items: center; min-height: 44px; padding: 8px 12px; border: 1px solid var(--line); border-radius: var(--r); background: var(--paper); cursor: pointer; }
.opt input { margin: 0; min-height: 0; width: 18px; height: 18px; accent-color: var(--accent); flex: none; }
.opt:has(input:checked) { border-color: var(--accent); box-shadow: inset 0 0 0 2px var(--accent); }
.fbwrap:empty { display: none; }
.fb { margin: 6px 0 0; padding: 6px 10px; border-radius: var(--r); max-width: 64ch; font-size: 0.95rem; }
.fb.right { background: var(--ok-soft); color: var(--ok); }
.fb.close { background: var(--close-soft); color: var(--close); }
.fb.wrong, .fb.unread { background: var(--bad-soft); color: var(--bad); }
.qfoot { padding: 12px 0 0; }
.work > summary { cursor: pointer; color: var(--accent); font-weight: 650; padding: 10px 0; min-height: 44px; display: flex; align-items: center; }
.keybox { padding: 4px 0; }
.keybox h4 { font: 700 1rem var(--head); margin: 10px 0 4px; }
.keyfigs { max-width: 560px; display: grid; gap: 10px; grid-template-columns: repeat(auto-fit, minmax(min(100%, 15rem), 1fr)); }
.keyfigs .fig svg { max-height: 300px; }
.cap { font-size: 0.88rem; color: var(--muted); margin: 4px 0 0; }
.kt { border-collapse: collapse; width: 100%; font-size: 0.93rem; margin: 6px 0 10px; }
.kt th, .kt td { border-top: 1px solid var(--line); padding: 6px 8px 6px 0; text-align: left; vertical-align: top; }
.kt th { font-weight: 600; width: 38%; }
.kt .ka { font-family: var(--mono); font-weight: 700; white-space: nowrap; }
.kt .kw { color: var(--muted); overflow-wrap: anywhere; }
@media (max-width: 640px) {
  .kt tr { display: grid; grid-template-columns: 1fr auto; column-gap: 12px; border-top: 1px solid var(--line); padding: 6px 0; }
  .kt th, .kt td { border: 0; padding: 2px 0; width: auto; }
  .kt .kw { grid-column: 1 / -1; font-size: 0.88rem; }
  .kt .ka { white-space: normal; }
}
.steps { margin: 0 0 10px; padding-left: 1.3em; font-size: 0.95rem; }
.steps li { margin-bottom: 5px; }
.sheet-foot { display: flex; align-items: center; flex-wrap: wrap; gap: 6px 16px; padding: 14px 0 4px; border-top: 1px solid var(--line); margin-top: 10px; }
.hint { color: var(--muted); font-size: 0.9rem; }
.ask { display: flex; align-items: center; flex-wrap: wrap; gap: 8px; }

.share { padding: 8px 0 0; }
.print-row { display: flex; flex-wrap: wrap; align-items: flex-end; gap: 10px 12px; }
.share-out label { display: flex; flex-direction: column; gap: 3px; font-size: 0.9rem; font-weight: 600; margin-top: 8px; }
.share-out input { width: 100%; }
.recent { border-top: 1px solid var(--line); margin-top: 12px; }
.recent summary { cursor: pointer; padding: 11px 0; font-weight: 600; min-height: 44px; }
.recent ul { list-style: none; margin: 0; padding: 0; }
.recent li { display: flex; justify-content: space-between; align-items: center; gap: 10px; border-top: 1px solid var(--line); padding: 4px 0; }
.recent small { display: block; color: var(--muted); }

.toast { position: fixed; left: 50%; bottom: 16px; transform: translate(-50%, 160%); background: var(--ink); color: var(--bg); padding: 10px 14px; border-radius: var(--r); max-width: calc(100% - 32px); transition: transform 0.2s; pointer-events: none; z-index: 20; }
.toast.on { transform: translate(-50%, 0); }
@media (prefers-reduced-motion: reduce) { .toast { transition: none; } }

.content { padding: 28px 0 8px; max-width: 760px; }
.content h2 { margin-top: 26px; }
.content p, .levels { max-width: 70ch; }
.levels { padding-left: 1.3em; }
.faq details { border-bottom: 1px solid var(--line); }
.faq summary { cursor: pointer; padding: 11px 0; font-weight: 600; min-height: 44px; }
.faq details p { margin: 0 0 12px; }
.sources { font-size: 0.92rem; color: var(--muted); overflow-wrap: anywhere; margin-top: 22px; }
footer { border-top: 1px solid var(--line); margin-top: 20px; padding: 16px 0 28px; color: var(--muted); font-size: 0.92rem; }
.indep { margin: 0 0 10px; }
.sb-footer p { margin: 6px 0; }
.sb-more { line-height: 1.8; }
.sb-footer a { color: var(--ink); }
@media (max-width: 600px) {
  .sheet { margin: 14px -8px 8px; padding: 12px 10px 8px; }
  .setup .btn { flex: 1 1 auto; }
  .virp { font-size: 0.85rem; }
  .virp tbody th, .virp th.rowh { width: 2.7rem; }
  .virp input { padding: 4px 2px; }
}

/* ---------- print ---------- */
.print-root { display: none; }
@page { margin: 10mm 12mm; }
@media print {
  :root { --ink: #000; --muted: #333; --line: #888; --paper: #fff; --edge: #555; --draw: #000; --accent: #000; --given: #fff; --given-ink: #000; }
  body { background: #fff; color: #000; font: 10pt/1.35 Arial, var(--sans); }
  main, footer, .toast, .skip { display: none !important; }
  .print-root { display: block; }
  svg { print-color-adjust: exact; -webkit-print-color-adjust: exact; }
  table.pt { width: 100%; border-collapse: collapse; }
  table.pt > tbody > tr > td, table.pt > thead > tr > td { padding: 0; }
  .prun { font: 8pt Arial, sans-serif; color: #444; text-align: right; padding-bottom: 3pt !important; }
  section.psheet, section.pkey { break-before: page; page-break-before: always; }
  section:first-child { break-before: auto; page-break-before: auto; }
  .ph { border-bottom: 1.5pt solid #000; margin: 0 0 4pt; break-after: avoid; page-break-after: avoid; }
  .ph h2 { font: 700 14pt var(--head); margin: 0; }
  .ph p { margin: 0 0 3pt; font-size: 9pt; }
  .pmeta { display: flex; justify-content: space-between; gap: 12pt; margin: 4pt 0 6pt; }
  .prule { font-size: 9pt; margin: 0 0 6pt; }
  .pqs { list-style: none; margin: 0; padding: 0; }
  .pq { break-inside: avoid; page-break-inside: avoid; margin: 0 0 8pt; padding-top: 5pt; border-top: 0.75pt solid #000; }
  .pq h3 { font: 700 11pt var(--head); margin: 0 0 2pt; }
  .pscen { margin: 0 0 4pt; font-size: 10pt; }
  .pbody { display: flex; gap: 6mm; align-items: flex-start; }
  .pfig { flex: none; }
  .pfig svg { display: block; max-width: 100%; height: auto; }
  .pright { flex: 1 1 0; min-width: 0; }
  svg.circ text { fill: #000; }
  table.pvirp { width: 100%; border-collapse: collapse; font-size: 9pt; table-layout: fixed; }
  table.pvirp th, table.pvirp td { border: 0.6pt solid #000; padding: 1.5pt 3pt; text-align: center; height: 7mm; }
  .pkey table.pvirp th, .pkey table.pvirp td { height: auto; }
  table.pvirp thead th { height: auto; font-weight: 700; }
  table.pvirp tbody th { width: 11mm; }
  table.pvirp .gv { font-weight: 700; }
  table.pvirp .na { font-size: 7.5pt; color: #333; }
  table.pvirp tr.tot th, table.pvirp tr.tot td { border-top: 1.4pt solid #000; }
  .pparts { list-style: none; margin: 4pt 0 0; padding: 0; display: grid; grid-template-columns: 1fr 1fr; gap: 3pt 12pt; }
  .pp { display: flex; align-items: flex-end; gap: 4pt; font-size: 9.5pt; min-height: 7mm; }
  .pp.wide { grid-column: 1 / -1; flex-wrap: wrap; }
  .pline { flex: 1 1 16mm; border-bottom: 0.5pt solid #555; min-width: 16mm; height: 1em; }
  .popts { display: inline-flex; flex-wrap: wrap; gap: 2pt 10pt; }
  .pkfigs { display: flex; gap: 8mm; align-items: flex-end; flex-wrap: wrap; margin-bottom: 3pt; }
  .pkfigs p { font-size: 8pt; margin: 1pt 0 0; }
  .pkrow { display: flex; gap: 5mm; align-items: flex-start; }
  .pkrow > * { flex: 1 1 0; min-width: 0; }
  .pkrow .pkv { flex: 1.3 1 0; }
  .pkrow.stack { display: block; } /* a long question (the AQA comparison) goes under the table, not in a narrow column */
  .pkrow.stack table.pkt { margin-top: 4pt; }
  table.pkt { margin-top: 0; }
  table.pkt { width: 100%; border-collapse: collapse; font-size: 8.5pt; margin-top: 4pt; }
  table.pkt td { border: 0.5pt solid #555; padding: 1.5pt 3pt; vertical-align: top; }
  .pkq { width: 30%; font-weight: 700; }
  .pka { width: 18%; font-weight: 700; }
  .pkw { font-size: 8pt; }
  .psteps { margin: 4pt 0 0; padding-left: 12pt; font-size: 8pt; line-height: 1.3; }
  .psteps li { margin: 0 0 1.5pt; }
}