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2.3.6Thermal Physics

Investigating IR Radiation — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 10 parts Lesson 45 of 45 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

2.3.6 Investigating IR Radiation — IGCSE PHYSICS notes: Plan and interpret experiments comparing the emission and absorption of infrared radiation by different surfaces.Aim — investigate how the infrared radiation absorbed or radiated by a surface depends on the nature of the sur…

  • Independent variable
  • Control variables
  • Leslie cube
  • All objects emit
  • Conduction and convection
  • Thermometer hole

Part 1 of 8Aim and variables

Aim
  • Aim investigate how the infrared radiation absorbed or radiated by a surface depends on the nature of the surface
Independent variable
  • Independent variable surface colour and texture
Dependent variable
  • Dependent variable temperature
Control variables
  • Control variables identical flasks except for colour, same amount of hot water, same starting temperature, same time interval, same room temperature

Part 2 of 8Equipment

EquipmentPurpose
Heatproof matProtect the bench
StopwatchTime the cooling
KettleBoil water
Four thermometersMeasure the water temperature in each flask
Flasks painted black, dull grey, white and silverCompare cooling of different surfaces

Part 3 of 8Method for emission

1
Set up four identical flasks painted black, dull grey, white and silver.
2
Fill each with the same volume of hot water at the same starting temperature.
3
Record the starting temperature.
4
Record the temperature of each flask at regular intervals, for example every 30 s for 10 minutes.
5
The flask whose temperature falls fastest has the best-emitting surface.

Part 4 of 8Method for absorption

1
Take two identical metal plates, one matt black and one shiny silver.
2
Place them the same distance from a radiant heater with a thermometer or blob of wax on the back of each.
3
Switch on the heater and start the stopwatch.
4
The black plate shows the bigger temperature rise, or its wax melts first.
  • Leslie cube
    • alternative for emission, a hollow cube with faces of different finishes filled with hot water
    • An infrared detector at equal distances gives the highest reading for matt black
    • Shiny silver gives the lowest reading

Part 5 of 8Analysis

Analysis
All objects emit
  • All objects emit hotter objects emit more infrared
Emission depends on
  • Temperature of the body
  • Surface area of the body
  • Colour of the surface
Conduction and convection
  • Conduction and convection cause most of the loss but are equal for each flask, so any difference is due to radiation
Graph
  • Graph plot temperature on the y-axis against time on the x-axis and draw curves of best fit
  • A steeper initial gradient means a better emitter
Findings
  • Findings dull black is the best emitter and absorber, shiny silver the poorest emitter and absorber and the best reflector

Part 6 of 8Errors and improvements

Thermometer hole
  • Thermometer hole keep it small, otherwise energy escapes through it
Parallax
  • Parallax read the thermometer at eye level
Data logger
  • Data logger gives more accurate readings with a digital thermometer
Random errors
  • Random errors take repeat readings and avoid timing errors from starting the stopwatch in pairs
Lids
  • Lids reduce loss by evaporation and convection so the main difference is radiation
Shielding
  • Shielding protect a detector from other sources such as the Sun

Part 7 of 8Safety

Goggles
  • Goggles wear them when filling flasks with boiling water
Electrics
  • Electrics keep water away from electrical equipment
Burns
  • Burns run any burn under cold water for at least 5 minutes
Kettle
  • Kettle do not overfill it
Positioning
  • Positioning keep equipment in the middle of the desk and stand to react to spills

Part 8 of 8More from the specification

1Radiant absorption by matte dark solids
  • Radiant absorption by matte dark solids non-reflective black painted solids absorb incident thermal infrared radiation far more rapidly than polished metallic solids.
2Infrared Radiation and Surface Emission
  • Thermal energy is transferred through infrared radiation without requiring a physical medium;
    • matte black surfaces are superior emitters and absorbers of radiant heat compared to shiny, silvered tiles.
3Thermal Convection in Liquids and Gases
  • When a liquid or gas is heated, warmer fluid expands, becomes less dense,
  • and rises while cooler fluid sinks, setting up convection currents that drive bulk heat transfer.
4Terrestrial Energy Balance and Greenhouse Effect
  • Solar radiation warms Earth's surface,
  • which re-radiates thermal infrared wavelengths into space;
    • greenhouse gases in the atmosphere absorb and re-emit this energy, trapping heat.
5Thermal equilibrium
  • Constant temperature because energy is absorbed and lost at equal rates
6Potassium permanganate
  • A purple compound used to show convection currents in water.
7Liquid
  • State with a fixed volume that takes the shape of its container.
8Lattice
  • The regular arrangement of atoms in a solid.
9Solid
  • State with a fixed shape and a fixed volume.
10Vibration
  • A repeated backwards and forwards movement.
11Energy transfer
  • Movement of energy from one store to another.
12Energy transferred
  • W = V Q, in joules.
    What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)
13Greenhouse effect
  • Warming of the Earth because gases absorb and re-emit its infrared radiation
14Lattice vibrations
  • Vibrations of atoms or ions about fixed positions that pass energy on

Key terms

TermMeaning
EmitterSurface that gives out infrared radiation
AbsorberSurface that takes in infrared radiation
Independent variableThe variable deliberately changed
Dependent variableThe variable that is measured
Leslie cubeHollow cube with faces of different finishes used to compare emission

Avoid these mistakes

1
Mixing up emission and absorption experiments — an emission experiment measures cooling, an absorption experiment measures heating.
2
Saying a larger surface area changes the total energy to lose — it increases only the rate of emission.
3
Not controlling the starting temperature and volume of water — the test is then unfair.
4
Assuming conduction and convection differ between flasks — they are equal, so any difference is due to radiation.

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