Science 7–10 · Year 8
Radiation: matt black against shiny cans heating under a lamp and cooling in air
Physical sciences — Change, content group Energy transfers (NSW Stage 4 focus area)
School laboratory, not for home
In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
The idea
Dull dark surfaces absorb and emit radiated energy faster than shiny light ones, so a black can warms faster under a lamp and cools faster afterwards.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- hot lamp and scalds from 70 degrees C water
- glass lamp breaking if splashed
- cardboard lids or probe leads scorching close to the lamp
Controls
- teacher pours hot water
- lamp on a stable stand, not touched while on
- keep water away from the lamp
- lids and leads kept at least the lamp maker's stated distance from the lamp, which is never left on unattended
Note
Heat or electrical energy is involved. Complete the school's risk assessment for the activity before the lesson, using CSIS 1.7 (Risk assessment – a pre-requisite for risk control) from the department's Chemical Safety in Schools package (2021 Technical Update), which the NSW Department of Education Science safety and compliance page names for risk assessment advice.
What you need
- identical aluminium drink cans, one painted matt black and one wrapped smoothly in shiny aluminium foil (a printed drink can is coated with ink and lacquer, which radiate almost as well as paint, so it is not a shiny surface for this test), or a third painted white, 2 or 3
- digital thermometers or temperature probes, 2 or 3
- cardboard lids with a hole for the probe, 2 or 3
- 150 W infrared heat lamp on a stand, 1
- kettle and measuring cylinder, 1 each
- stopwatch, 1
How to do it
- Heating: fill each can with 200 mL of room-temperature water, fit the lids and probes, and place the cans side by side facing the lamp equally, 20 cm from it or at the lamp maker's stated minimum distance from combustible material if that is greater (the cardboard lids and probe leads are combustible).
- Switch on the lamp and record each temperature every minute for 15 minutes.
- Cooling: fill each can with 200 mL of water at 70 degrees C from the kettle, fit lids and probes, and stand them apart in still air away from the lamp.
- Record each temperature every minute for 15 minutes.
- Plot temperature against time for both cans on one axis for each part and compare the gradients over the first 5 minutes.
What you should see
Under the lamp the black can warms faster than the foil-wrapped can, so its temperature-time line is steeper. In cooling from 70 degrees C the black can also cools faster, because a matt black surface emits radiation better than shiny foil. Both cans also lose energy to the air by convection, which is why the lids, the still air and the equal starting temperatures matter. The learner knows it worked when the black can's line is steeper in both graphs.
What changes
- What you change
- surface finish of the can
- What you measure
- temperature change of the water over 15 minutes (degrees C)
- What you keep the same
- same volume and starting temperature of water
- same distance and orientation to the lamp
- lids on both cans
- still air
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Shiny surfaces are better emitters because they look bright.
- Black surfaces absorb heat but do not radiate it.
- Radiation needs air to carry it.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC4-CHG-01SC4-WS-04SC4-WS-05SC4-WS-06
- Australian Curriculum v9AC9S8U05AC9S8I02AC9S8I04AC9S8I05
Sources
The pages the author read to write this activity.