- Mugs made of different materials, like ceramic, enamel, plastic and cardboard (don’t use glass – heat can break it!)
- Freshly boiled water
- A heat-resistant measuring jug
- A thermometer that you can put in liquids that goes up to at least 100 °C (a candy thermometer is ideal)
- A stopwatch
- A pen and paper
- Pieces of cardboard to use as lids for the mugs
- A vacuum-insulated mug – often called a Thermos (optional)
What to do
Note: if measuring the temperature takes too long for each mug, consider doing the experiment with one mug at a time and leaving the thermometer in the water.
- Arrange your mugs on a flat, heat-resistant surface.
- Note the shape of each mug and what it is made of. Make a prediction: which mug will keep water hot for the longest time?
- Boil some water and use the measuring jug to pour 200–250 mL of water into each mug. Try to put the same amount in each mug.
- Measure and record the temperature of the water when you first put it into each mug.
- Check the temperature at regular intervals (such as every 2 minutes) and write down your results. Notice the temperature changing. When does the water stop cooling down?
- Repeat steps 3 to 5, except place a lid over each mug.
- How do your results compare with those from when you didn’t use a lid?
- If you have one, try the experiment with a vacuum-insulated (Thermos) mug. How much of a difference does it make to the results?
Questions to ask
Which mug of water will cool down the fastest? Why do you think that?
How long do you think it will take the water to cool down for each type of mug?
How is the heat escaping the mugs?
Do you think the water will get colder than the air in the room? Why or why not?
When something is hotter, it has more energy than when it is cool. The laws of thermodynamics explain how energy in the form of heat moves around.
The second law of thermodynamics tells us that heat energy travels from warmer areas to cooler ones. The hot water in our mug will release heat energy until it reaches equilibrium. That is, until it reaches the same temperature as the environment around it.
Heat can be transferred in 3 ways: convection, conduction and radiation.
In liquids and gases, heat travels by convection. Convection is the movement of hot gas or liquid – the energy is moved because the hot material itself moves. Hot air or liquid rises, cooler air or liquid sinks (because it is denser) and so the hot material moves.
For solid materials, and some liquids, heat can travel by conduction. Conduction is when heat is transferred by collisions between molecules. The molecules pass the energy from one to the next, but do not move themselves. An example of conduction is a frying pan heating up when it’s put on a hot stove. Some materials conduct heat better than others. Choosing the right material for your mug – one that does not conduct heat very well – can keep the water hotter for longer.
Radiation is when heat moves as electromagnetic radiation (like light or heat from the sun).
Insulation slows the rate of heat transfer.
The lid prevents hot air above the mug from escaping – it reduces convection.
Thick walls that don’t conduct heat very well reduce conduction.
A shiny surface inside the cup can help reflect radiated heat back into the liquid.
Did you know
Insulation in our homes reduces energy costs and makes our homes more environmentally friendly. The heat inside our homes will flow to unheated areas like attics, garages, basements and the outdoors through convection, just like heat from your mug. By using insulation to slow the rate of heat transfer, we can save on energy costs and help the environment!