Required practical - Infrared radiation (AQA GCSE Physics): Revision Notes
Required practical - Infrared radiation
What this practical is about
This practical helps you understand how different surfaces give off and take in thermal energy through infrared radiation. You'll discover that the type of surface makes a big difference to how much heat energy it can emit or absorb.
This is a fundamental physics practical that demonstrates the relationship between surface properties and thermal energy transfer. Understanding these principles is essential for many real-world applications, from building design to space technology.
Aim of the practical
The main goal is to investigate how the nature of a surface affects the rate at which it absorbs or radiates thermal energy.
Equipment you'll need
You'll use these items for the experiment:
- Leslie's cube (special cube with four different surfaces)
- Thermometers (to measure temperature)
- Stopwatch (to time the experiment)
- Ruler (to measure distances)
- Clamp, boss and retort stand (to hold equipment)
What is Leslie's cube?
Leslie's cube is a special piece of equipment commonly used in schools. It's a cube-shaped container that has four different types of surfaces:
- Dull black surface
- Shiny black surface
- Silver surface
- White surface
This allows you to compare how different surfaces behave with thermal energy.
Leslie's cube was invented by Sir John Leslie in the early 19th century specifically to demonstrate the principles of thermal radiation. Each surface has been carefully chosen to show maximum contrast in radiation properties.
Method 1: Testing how surfaces radiate thermal energy
This method tests how well different surfaces give off heat:
Worked Method: Testing Radiation
Step 1: Fill Leslie's cube with hot water at a known temperature
Step 2: Place a thermometer a small distance away from each of the four sides
Step 3: Plug the top of the cube with a bung to keep heat in
Step 4: Measure temperature at 10cm from each surface for 5 minutes
Step 5: Take readings every 30 seconds
Safety Warning: Be careful when using hot water as it may scald you. Always handle hot equipment with appropriate care and consider using heat-resistant gloves.
Method 2: Testing how surfaces absorb thermal energy
This method tests how well different surfaces take in heat:
Worked Method: Testing Absorption
Step 1: Fill Leslie's cube with the same volume of cold water at a known temperature
Step 2: Plug the top with a thermometer and bung
Step 3: Heat each side one at a time using a radiant heater from about 10cm away
Step 4: Record temperature changes every 30 seconds over an appropriate time period
Key points for accurate results
Essential Requirements for Accuracy:
- Use the same thermometer for each surface
- Keep the same distance from each surface
- Use the same starting temperature
- Take the same number of readings over the same time period
- Use the same time intervals between readings
These controlled variables are crucial for obtaining valid, comparable results.
Typical results
When you do this practical, you'll get results similar to these:
Sample Results Data
Method 1 (Radiating energy):
- Start temperature: All surfaces at 80°C
- Finish temperature: Dull black (56°C), Shiny black (62°C), Silver (74°C), White (68°C)
Method 2 (Absorbing energy):
- Start temperature: All surfaces at 20°C
- Finish temperature: Dull black (34°C), Shiny black (28°C), Silver (22°C), White (24°C)
Conclusions from typical results
The results show a clear pattern for both emitting and absorbing thermal energy:
Best to worst emitters:
- Dull black - best emitter
- Shiny black - second best
- White - third best
- Silver - worst emitter
Best to worst absorbers:
- Dull black - best absorber
- Shiny black - second best
- White - third best
- Silver - worst absorber
Notice how the ranking is identical for both emission and absorption. This demonstrates an important principle: good emitters are also good absorbers, and poor emitters are also poor absorbers.
Why do surfaces behave differently?
Different surfaces have different properties when it comes to infrared radiation:
- Dark, matt surfaces are good at both emitting and absorbing infrared radiation
- Light, shiny surfaces are poor at both emitting and absorbing infrared radiation
- Silver surfaces reflect most infrared radiation rather than absorbing it
This behaviour is related to the surface's atomic structure and how it interacts with electromagnetic radiation. Dark, rough surfaces have more opportunities to interact with infrared photons, while smooth, reflective surfaces tend to bounce the radiation away.
Remember!
Key Points to Remember:
- Dull black surfaces are the best emitters and absorbers of infrared radiation
- Silver surfaces are the worst emitters and absorbers of infrared radiation
- The same surface that's good at emitting thermal energy is also good at absorbing it
- Safety first - always be careful when using hot water in experiments
- Keep variables constant - same distance, same time intervals, same starting temperature for accurate results