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Concept module

Radioactivity and Half-Life

Use one compact decay bench to see why each nucleus decays unpredictably, why large samples still follow a regular half-life curve, and how to read remaining-count graphs honestly.

Interactive lab

Keep the stage, graph, and immediate control feedback in one working view.

Time

0.00 s / 12.0 sLivePause to inspect a specific moment, then step or scrub through it.

0.00 s12.0 s

Radioactivity and Half-Life

A bounded sample tray, expected-decay bars, and linked time graphs keep single-event chance and large-sample regularity on one honest bench.

Live setup

Graphs

Switch graph views without breaking the live stage and time link.

Remaining nuclei vs time

Shows the stepped live count and the smooth expectation together so integer decays and exponential regularity stay aligned.

time (s): 0 to 12remaining nuclei: 0 to 128

Actual remainingExpected remaining

Hover or scrub to link the graph back to the stage.time (s) / remaining nuclei

Controls

Adjust the physical parameters and watch the motion respond.

Sample size

N_{0}

64 nuclei

Set how many nuclei start in the tray so you can compare small-sample noise with large-sample regularity.

Half-life

T_{1/2}

2.4 s

Set the half-life that defines the equal-interval halving law on the time axis.

More tools

Secondary controls, alternate presets, and less-used toggles stay nearby without crowding the main bench.

Show

More presets

Presets

Predict -> manipulate -> observe

Keep the active prompt next to the controls so each change has an immediate visible consequence.

Try thisPrompt 1 of 3

Small samples can wander noticeably above or below the expectation, but a larger sample hugs the same curve more closely because many independent decays average together.

Try this

Compare Small noisy sample with Large smooth sample at the same half-life.

Equation map

See each variable before you move it.

Select a symbol to highlight the matching control and the graph or overlay it most directly changes.

N_{0}

Sample size

64 nuclei

Changes how noisy the actual tray looks around the same expectation curve. Small samples wander more, while large samples look steadier.

Graph: Remaining nuclei vs timeGraph: Remaining fraction vs timeOverlay: Sample versus expectationOverlay: Single versus sampleOverlay: Recent decays

Equations in play

Choose an equation to sync the active symbol, control highlight, and related graph mapping.

More tools

Detailed noticing prompts, guided overlays, and challenge tasks stay available without taking over the main bench.

Hide

What to notice

Use one prompt at a time so the tray, the readout card, and the time graphs stay on the same compact decay bench.

Try thisPrompt 1 of 3

Graph: Remaining nuclei vs time

Small samples can wander noticeably above or below the expectation, but a larger sample hugs the same curve more closely because many independent decays average together.

Try this

Compare Small noisy sample with Large smooth sample at the same half-life.

Why it matters

It links probability to regularity without pretending the sample must always match the curve exactly.

Control: Sample sizeGraph: Remaining nuclei vs timeGraph: Remaining fraction vs timeOverlay: Sample versus expectationOverlay: Recent decays

Guided overlays

Focus one overlay at a time to see what it represents and what to notice in the live motion.

4 visible

Overlay focus

Recent decays

Highlights nuclei that decayed only recently instead of pretending the tray changes smoothly everywhere at once.

What to notice

The tray changes one nucleus at a time even when the graph of expectation is smooth.

Why it matters

It keeps the single-event story visible and prevents the page from faking continuous matter loss.

Control: Sample sizeGraph: Remaining nuclei vs timeEquation

N_{0}

Challenge mode

Use the same bounded decay bench for compact half-life and sample-noise targets instead of widening into a full nuclear-physics system.

0/2 solved

TargetCore

5 of 8 checks

Land on the one-half-life checkpoint

Starting from Class-lab sample, scrub to about one half-life so the expectation is halved while the live tray stays slightly below it.

Graph-linkedGuided start2 hints

Suggested start

Use the half-life markers and the dashed expectation together, then scrub the time rail.

Matched

Open the Remaining nuclei vs time graph.

Remaining nuclei vs time

Matched

Keep the Sample versus expectation visible.

Matched

Keep the Half-life markers visible.

Matched

Keep sample size between 64 nuclei and 64 nuclei.

64 nuclei

Matched

Keep half life seconds between 2.35 s and 2.45 s.

2.4 s

Pending

Keep elapsed half lives between 0.98 and 1.02.

Pending

Keep expected remaining count between 31.6 nuclei and 32.4 nuclei.

64 nuclei

Pending

Keep actual remaining count between 26.5 nuclei and 28.5 nuclei.

64 nuclei

The checklist updates from the live simulation state, active graph, overlays, inspect time, and compare setup.

At t = 0 s, 64 of 64 nuclei remain while the expectation is about 64. The half-life is 2.4 s, so one nucleus has survival probability 100% by this time. Because many nuclei are decaying independently, the stepped live count stays close to the smooth expectation.

Equation detailsDeeper interpretation, notes, and worked variable context.

Half-life law

Each equal half-life interval multiplies the expected count by one half.

N (t) = N_{0} (\frac{1}{2})^{t / T_{1/2}}

N_{0}

Sample size 64 nuclei

T_{1/2}

Half-life 2.4 s

Exponential form

The half-life law is the same exponential decay law written with a decay constant.

N (t) = N_{0} e^{- λ t}, λ = \frac{ln 2}{T _{1/2}}

T_{1/2}

Half-life 2.4 s

Single-nucleus survival probability

One nucleus survives probabilistically, but the same decay constant links the single-event chance to the sample trend.

P (survive to t) = e^{- λ t}

T_{1/2}

Half-life 2.4 s

Progress

Not startedMastery: NewLocal-first

Start exploring and Open Model Lab will keep this concept's progress on this browser first. Challenge mode has 2 compact tasks ready. No finished quick test, solved challenge, or completion mark is saved yet.

Let the live model runChange one real controlOpen What to notice

Try this setup

Copy the live bench state and reopen this concept with the same controls, graph, overlays, and compare context.

Stable links

Starter track

Step 5 of 50 / 5 complete

Modern Physics

Earlier steps still set up Radioactivity and Half-Life.

1. Photoelectric Effect2. Atomic Spectra3. de Broglie Matter Waves4. Bohr Model+1 more steps

Previous step: Bohr Model.

Start from track beginning

Short explanation

What the system is doing

Radioactivity is the bounded decay case where single events are genuinely probabilistic, but large samples still produce a steady and predictable curve. A nucleus does not carry a countdown clock to a personal half-life. Instead, each nucleus has the same chance to survive each interval, and the sample-level pattern becomes regular only when many independent yes-or-no events are averaged together.

This page keeps one compact bench with a bounded sample tray, two linked time graphs, and one readout card. Sample size, half-life, graph hover previews, compare mode, worked examples, quick tests, overlays, and challenge checks all stay tied to that same honest decay state instead of splitting into a separate probability page and a separate curve page.

Key ideas

01Half-life means the expected sample count is multiplied by one half over each equal half-life interval. It does not mean each nucleus survives exactly half that long.

02For one nucleus, decay is a probabilistic yes-or-no event. For many nuclei, those independent events average into a smoother exponential trend.

03The decay curve should step downward for an actual sample and stay smooth only for the expectation. Both views belong on the same bench.

04An honest decay graph approaches zero without becoming negative, and the sample can wander above or below the expectation while still following the same law.

Live half-life checks

Solve the exact state on screen.

Use the current sample and half-life directly from the live bench. The same tray, graphs, and readout card drive each worked result.

Live valuesFollowing current parameters

For a sample of 64 nuclei with half-life 2.4 s, what expected count belongs at time 0 s?

N_{0}

Starting sample

64 nuclei

T_{1/2}

Half-life

2.4 s s

t

Elapsed time

0 s s

N_{exp}

Expected remaining

64 nuclei

1. Count equal half-life intervals

The live time is 0 s, which is 0 half-lives of 2.4 s each.

2. Apply the half-life law to the expectation

That means the expected surviving fraction is about 100%, so the sample expectation is 64 nuclei.

3. Compare expectation with the live sample

The tray currently shows 64 nuclei still present, so the sample is being compared with an expectation instead of being forced to equal it exactly.

Expected benchmark

\(N_{\mathrm{exp}} \approx 64\) nuclei at \(t = 0 s\)

The expected count comes from repeating the same fractional halving law across equal half-life intervals rather than subtracting a fixed number each second.

Chance-versus-curve checkpoint

A graph shows a smooth curve halving from 80 to 40 to 20 while the sample tray drops in uneven steps. Which description is the most honest?

Prediction prompt

Answer from the difference between expectation and individual decays.

Check your reasoning

The smooth curve is the expected sample trend, while the tray shows the actual integer nuclei that happened to survive. Both are correct views of the same probabilistic decay law.

If the tray matched a perfectly smooth curve, the page would be hiding the fact that nuclei decay one by one. If the page showed only random dots with no expectation, it would hide the regular large-sample law.

Common misconception

A half-life of 2 seconds means every nucleus lives for 2 seconds and then exactly half of them disappear on schedule.

Half-life is a sample-level expectation, not a personal timer for each nucleus. One nucleus may decay early, late, or not yet at all.

The smooth exponential curve is the expected trend for many independent decays. The live sample count stays as integer nuclei and should fall in steps.

Quick test

Reasoning

Question 1 of 4

Answer from the live decay bench, not from a fake countdown picture.

What is the most honest meaning of a 2-second half-life on this page?

Choose one answer to reveal feedback, then test the idea in the live system if a guided example is available.

Accessible description

The simulation shows a bounded sample tray of nuclei on the left and an actual-versus-expected summary on the right. Each nucleus is either still present or marked as decayed, and optional cues can highlight recently decayed nuclei, equal half-life checkpoints, and one-nucleus survival language.

The readout card summarizes sample size, half-life, elapsed time, expected remaining count, actual remaining count, remaining fraction, deviation from the expectation, and the single-nucleus survival probability at the current time.

Graph summary

The remaining-count graph plots time against both the stepped actual count and the smooth expected count. The remaining-fraction graph shows the same story normalized by the starting sample so different sample sizes can be compared without changing the underlying half-life law.

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These suggestions come from the concept registry, so the reason label reflects either curated guidance or the fallback progression logic.

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