Core Practical: thermal radiation absorption/emission by surfaces

Physics · Thermal radiation

Plan it like a scientist

Two investigations, one question each time: what do you change, what do you measure, and what must stay the same so that only the surface makes the difference? Pick a quantity, then pick its job. The small print under each one tells you which investigation it belongs to.

13 of 13 still to sort.

Predict, then check

Commit to an answer first. This one is the surprise of the whole lesson.

A Leslie cube is full of hot water, so every face is at the same temperature. You point an infrared thermometer at each face in turn. What do you predict?

What you need to know

• Everything emits and absorbs infrared, whatever its temperature, and hotter objects emit more in a set time.
• Absorption test: change the intensity of black ink on a card sleeve, and measure the temperature rise in a fixed time.
• Control variables stay fixed: heater power, type of ink, area of card, angle of card and distance from the heater.
• Have a goDev says: 'I'll hold the black sleeve closer to the heater, so it heats up faster and the ink wins.' Why does that ruin the test?

  Distance is a control variable, so it has to stay the same.

  If the distance changes as well as the ink, you cannot tell which one caused the bigger rise.

• Emission test: change the surface (colour or finish) wrapped round identical containers of hot water, and measure the temperature fall in a set time.
• Control the water's starting temperature, its mass, the container, and cooling by conduction and evaporation.
• Record the starting and final temperatures, then calculate the rise as the difference; repeats help spot anomalous results.
• Have a goOne sleeve starts at 22 °C and ends at 31 °C. What is its temperature rise?

  9 °C

  The rise is the final temperature minus the starting temperature, so 31 − 22 = 9 °C.

• A slightly different angle or distance each time is random error; a fault shifting every reading equally is systematic error.
• Have a goA faulty thermometer reads 2 °C too high every single time. Random or systematic error?

  Systematic error.

  Every reading is out by the same amount; random error is the kind that varies, like a wobbling angle.

• A Leslie cube is a hollow metal cube you fill with hot water, with each side a different colour or finish.
• A body's temperature depends on the rates at which it absorbs and emits radiation.
• Good absorbers of infrared are good emitters too; shiny metal is poor at both, and white paint is good at both.

The big picture

A surface that soaks up infrared well also gives it out well. You can show that with a fair test: change only the surface, measure the temperature change, and hold everything else the same.

⚠ Watch out

Judging a surface by colour alone. White paint is a good absorber and a good emitter of infrared, so colour on its own is not a reliable guide.

Memory hook

Dark and dull take in well and give out well. Shiny does neither. White paint is the plot twist: pale, but good at both.

Check yourself

A friend says: 'White things are poor emitters of infrared because they are not dark.' Write two sentences to put them right, then check them against the grid.