TABULA/samples/boiling a kettle

From J Wiki
Jump to navigation Jump to search

This is SAMPLE8. To work with this sample, enter $8

Boiling a kettle
  ┌ ┌ {1}        0.997 kg     water mass in kettle
  │ ├ {2}          997 kg/m³  density of pure water=
  │ └>{3}            1 l      water vol in kettle
  ├   {4}       80.008 K      temperature rise
  ├   {5}            1 sht.w  specific heat of water=
┌ └>  {6}      3.337E5 J      heat energy needed
│   ┌ {7}         1800 W      kettle wattage=
│   ├ {8}        3.654 min    time to boil
├ ┌ └>{9}      3.946E5 J      energy input
└>├   {10}   60843.132 J      energy wasted
  └>  {11}      15.419 %      percentage wasted


After room heating and air-conditioning, the electric kettle is the biggest consumer of electricity in the kitchen.

The owner of a cheap kettle wants to know how much electricity it is wasting. Will he waste less if he insulates the kettle with bubble-wrap?

Now in theory all the electrical energy supplied to the kettle is converted to heat which passes into the water inside it. But when the boiling water is put to use, some heat energy remains behind in the body of the kettle. Insulation won't help here. Also heat is lost by radiation and convection whilst the water is heating up. Insulation will reduce this loss.

This is why a kettle is not a 100% efficient converter of electrical energy into heat energy.

The t-table was written to support a "kitchen table" experiment to determine the efficiency of an actual kettle. Swaddling the kettle with bubble-wrap made no significant difference to the time it took to boil. However it did make a noticeable difference to the rate of cooling once the kettle had boiled and had switched itself off.

Conclusion: bubble-wrap yields little benefit, provided the boiled water is used promptly, and all of it is used. The biggest waste of electricity arises from the boiled water which is not used, but allowed to cool back down to room temperature.

Try it out

  1. Find out the kettle wattage (it will be shown on the kettle) and insert it into item {7}.
  2. Put one litre of cold water (of known temperature) in the kettle.
    An easy way to estimate the water temperature is to fill the kettle from the faucet to the 1 litre mark, let it stand for an hour or two to reach room temperature, then read the room temperature from the wall thermometer.
  3. Subtract the room temperature from 100°C and insert the result in item {4}.
    This is the temperature rise that must take place to boil the water.
  4. Switch the kettle on and time how long it takes to boil. Insert the time in item {8}.
  5. Item {9} shows the electrical energy used, and item {11} shows what percentage of {9} is wasted.

Build this t-table


Suggested Improvements

  1. Weigh the water and ignore the volume (item {3}), which plays no role in the energy calculation.
    Today's kitchen scales are accurate to ±1 gram (±0.1% of 1 kg), whereas the graduation marks on a kettle can probably only be read to ±1% accuracy.
  2. The t-table will work for any weight of water that covers the heating element. There is no need to fill the kettle with exactly 1 litre.
  3. Calculate the required temperature rise {4} from the room temperature.
    Create a new item to accept the room temperature, in whatever scale the wall thermometer uses (some use °F). TABULA will automatically handle the temperature conversion.
  4. Calculate the kettle's energy efficiency instead of the percentage of electricity wasted.
    HINT: calculate the ratio {6}/{9}.
  5. Repeat the experiment using a microwave oven to boil a cupful of water. Doesn't all the microwave energy heat up the water, not the cup or the oven?
    Ah… but what percentage of electricity does the oven convert into microwaves? A good oven might manage 50%.