Black-body radiation: temperature and colour sim
Turn up the supply to a lamp and watch its filament go from giving out only infrared to dull red, orange, yellow and yellowish white while its emission curve's peak moves to shorter wavelengths and its total power climbs; then warm a black or shiny can in a lamp's light until it absorbs as fast as it gives out, and see the same balance set the Earth's temperature.
Electromagnetic spectrum: black-body radiation · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Model. A 12 V, 24 W filament lamp on a variable supply. At a steady temperature the electrical power going in equals the power given out: V² ÷ R = ε σ A (T⁴ − T₀⁴) + G (T − T₀). The filament radiates as a grey body (ε = 0.35, chosen) and loses a chosen 10 % of its power at 12 V through its leads and the gas; its resistance rises as R = R₀ (T ÷ 293 K)^1.2 (from memory). Its area (17.7 mm²) and cold resistance (0.40 Ω) follow from the rating: 2800 K and 2.00 A at 12 V. The curve is Planck's law, its peak b ÷ T (Wien) and its area σT⁴ (Stefan–Boltzmann), all beyond the syllabus. The colour is worked out from the curve with the CIE 1931 colour-matching functions (Wyman–Sloan–Shirley fit, from memory) and shown as sRGB; its brightness is squeezed to fit a screen (none below 800 K, full at 2800 K, chosen). The detector's reading is taken as proportional to the power radiated (chosen calibration). The can: 50 g of water in a 30 g aluminium can, 66 mm across and 88 mm tall; the light is 500 W/m² at 20 cm, falling as 1 ÷ d² (chosen); it absorbs and emits a share 0.95 (matt black) or 0.1 (shiny), and loses heat to the air by convection with h = 5 W/m² °C (chosen). Readings carry seeded scatter and are read to the instrument: 0.01 V, 0.01 A, 0.1 mV, thermometer 0.5 °C. Time moves only when you press a button.
- The part to watch: the lamp's bulb with its filament, or the can in the lamp's light
- Variable power supply, radiation detector and meter, hand spectroscope, retort stands, cork mat, thermometer, lamp in a reflector, metre rule, stopwatch
Use it in your lessons
- Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
- Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
- Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.
Free guide: 12 practicals as sims, and how to run one in a lesson →
Need a version for your exact set-up? Have one made: US$49 a figure in an order of 5.
More physics sims
- Circular motion: the whirling bung simWhirl a rubber bung on a string: set its speed, radius and mass and read the pull on a newton-meter, let go and watch it leave along the tangent, then hang slotted masses on the string, whirl until the paper clip holds steady, time ten turns and plot F against v².
- Convection in water and air simDrag a Bunsen burner or an immersion heater and a potassium permanganate crystal around a beaker of water and watch the purple trace the convection loop (or stay put when the water is heated at the top), then pull smoke through a candle-heated chimney box and drag the sun to turn a sea breeze into a land breeze.
- Cooling curve of a pure substance simMelt stearic acid (or paraffin wax) in a hot water bath, drag the boiling tube into an air jacket, click the stopwatch on 30 s at a time and read the thermometer at eye level; your readings make a cooling curve with a flat part at the freezing point, which you read off and explain (the wax, a mixture, has none).
- Density: a regular block, an irregular stone and a liquid simMeasure a block with a rule, find a stone's volume by displacement, weigh a liquid in a cylinder, and work out ρ = m ÷ V for each.
- Digital calipers: outside, inside and depth simClose the jaws on a rod, open them in a pipe, push the depth rod into a hole, find and correct the zero error, and choose between a rule, calipers and a micrometer.
- Digital micrometer screw gauge: zero it, then measure a wire simFind and correct a positive or negative zero error, close the jaws on a wire with the ratchet, measure the diameter at three places, average, and work out A = πd²/4.