Heating and cooling curves
Practical Activity for 14-16
To introduce ideas of energy transfer by heating and thermal capacity.
Apparatus and Materials
For each student group
- Datalogger with temperature sensor
- 1 litre beaker
- 250 ml beaker
- Insulating jacket
- Immersion heater
- 1 kg metal block (e.g. aluminium) with bores drilled for heater and temperature sensor
- Electric kettle or Bunsen burner to heat water rapidly
- Mug(s), ceramic OPTIONAL
- Cup(s), paper, polystyrene and plastic, with lids if possible OPTIONAL
- Different insulating materials (e.g. expanded polystyrene, newspaper, wool) OPTIONAL
- Instant coffee and tea bags OPTIONAL
Health & Safety and Technical Notes
An electric kettle is a much safer source of hot water than a Bunsen burner, tripod and gauze. However, immersion heaters also get hot and must be handled with care.
There are a number of things you can do with just temperature sensors.
- Cooling curves. Fill a beaker with hot water from a kettle. Record its temperature once a second for a few minutes. If possible, produce a graph directly.
- Compare cooling curves for beakers with different insulation, lids etc. Start each with water at the same temperature and record information from several sensors on the same graph.
- Heating curves. Place sensors and heaters in beakers with 1 litre water and 250 ml water, and a 1 kg metal block. Start the heaters at the same time and with the same voltage and record the temperature-time graphs, all on the same display.
- These activities are excellent to emphasize the value of datalogging as the display is much easier to read than normal thermometers. Readings can be taken more often and with less chance of recording errors. Suitable software can produce an immediate graphical display to confirm that the data are being collected correctly. Specific teaching points:
- This experiment can be used to calculate cooling rates in °C per second. The flattening curve shows that the rate of decrease of temperature is lower as the temperature falls.
- Without being quantitative, cooling curves which are produced live provide at-a-glance evidence for the effectiveness of different insulations.
- Comparing different masses of the same material (water is easiest) shows how the same amount of energy transferred causes different changes in temperature that depends on the mass. This is an introduction to thermal capacity and to the difference between energy transferred and temperature.
- Comparing the different materials (but same mass) is a further step on this road. The temperature of the aluminium will rise much more quickly than the 1 kg of water (1 litre). This is also partly because it will dissipate energy more slowly: it will take longer for energy to be transferred to the surface of the aluminium by conduction, and then be transferred to the surroundings by radiation, compared with time for convection currents to be set up in water. Hence the ratio of the rate of temperature rises is not the same as the ratio of the specific thermal capacities.
- If you want to use these methods to measure specific thermal capacities, then you need to ensure that you minimize energy dissipated to the surroundings with good thermal insulation.
- How Science Works extension: You could either set students a structured investigation and then follow with questions based on this or offer an open-ended investigation.
- Students could:
- identify and select the variables that they wish to measure and control
- produce their own experimental procedure, including the selection of appropriate time intervals.
- The amount of guidance given will very much depend on your students’ level of confidence and skills with designing their own experiments.
- Some groups could be set a very open-ended brief, ‘investigate cooling’. With others you might set the investigation in a real world context, suggesting some of the possible variables e.g. you could tell them that they are to investigate which is better to keep a cup of hot coffee warm for longest - a ceramic mug, a paper or a polystyrene cup? Most takeaway coffee cups have a lid, so this could be extended to investigating how effective the lid is at reducing energy dissipation. More advanced students could investigate whether tea and coffee behave in exactly the same way as water.
- Collecting data for cooling curves for cups of different materials is relatively straightforward, so students need only minimal guidance in the specifics of what they are to do.