Keeping tea hot
Pour tea at 90 °C into an open cup, a covered cup or a vacuum flask in a 25 °C room, and follow its temperature for an hour to see which keeps it hot and why.
Core concept
Hot things cool faster the hotter they are than the room: T = room + (start − room) × e^(−kt); a lid and a vacuum flask cut heat loss, giving a much smaller k
Topic: Science around us
A cup of hot tea loses heat to the cooler room: by conduction through the cup, by convection as warm air and steam rise from the top, and by radiation. The bigger the difference between the tea and the room, the faster it cools, so the temperature: How hot or cold something is, measured in degrees Celsius (°C). All key words falls quickly at first and then more and more slowly. Newton's law of cooling describes this: T = room + (start − room) × e^(−kt). The number k says how leaky the container is. A lid stops steam and warm air carrying heat away. A vacuum flask has two glass walls with a vacuum between them, which stops conduction and convection, and shiny walls that reflect radiation, so its k is tiny.
What you will learn
- Describe how a hot drink cools over time.
- Explain how a lid and a vacuum flask reduce heat loss.
- Use Newton's law of cooling to find a temperature.
Before you start
- Know the three ways heat moves: conduction, convection and radiation.
Materials
- Two cups, a saucer for a lid and a vacuum flask
- A kitchen thermometer
Equipment
- None
Procedure
- Record the open cup after 10 minutes.
- Record the open cup after 30 minutes, then a covered cup after 30 minutes.
- Record the flask after 60 minutes.
How this simulation is simplified
- The tea starts at 90 °C in a room at 25 °C.
- Cooling constants k: open cup 0.05, covered cup 0.03, vacuum flask 0.005 per minute.