Have you ever wondered how much energy it takes to heat a kettle? the amount of energy transferred will depend on three things. can you think about what they might be?
- The amount of water
- The rise in temperature
- The specific heat capacity of water
You may not have heard of specific heat capacity, this is the amount of energy needed to heat one kilogram of water by one Celsius. the value is specific to water. hence the name of these three quantities can be connected by a formula. Delta e equals MC. Delta theta or change in thermal energy equals mass times specific heat capacity times temperature change. where Delta e is measured in joules M in kilograms C in joules per kilogram per degree Celsius and Delta Theta in Celsius.
Water has a specific heat capacity of four-point one eight kilojoules per kilogram per degree Celsius. this means it needs four point one eight kilojoules to heat one kilogram up by one degree Celsius. we can use the equation Delta e equals MC delta theta to work out the energy needed to boil a kettle of water.
If we take zero point five kilograms of water or half a liter at 18 degrees Celsius and heat it to boiling or 100 degrees Celsius then the energy used equals 0.5 x 4.18 x 82 which equals 171.380 kilojoules. this assumes that there is no loss of energy during heating. which of course there will be heat and sound will be lost to the surroundings.
If this kettle took 80 seconds to boil then we can begin to estimate the power of the kettle. Power is the rate of energy transfer. power equals energy over time. so we get 171.38 over 80 equals 2.14 kilojoules per second or 2.14 kilowatts.
This answer will be slightly lower than the actual power due to the kettle being uninsulated. this water boiler holds ten point five liters of water. how much energy is transferred if the water at fourteen degrees Celsius is heated to boiling point?
While you work it out you get 3,770 4.5 kilojoules or 3.77 5 mega joules.
Different materials have their own specific heat capacity. this has consequences in our everyday life. that we may not be aware of most of us probably cook with water more often than we do with oil. imagine that we are heating pans of water and oil over identical heat sources. The specific heat capacity of water is 4.18 KJ/kg degree Celsius and the specific heat capacity of olive oil is 1.97 KJ/kg degree Celsius.
What temperature rise would you get for each?
If one kilogram of each had 40 kilojoules of energy transferred. if we rearrange the equation delta theta is the temperature rise for water we get 9.5 degrees celsius and for olive oil, we get 20.3 degrees celsius. Oils have a less specific heat capacity than water. so heat up much quicker when exposed to an identical heat source. this is why we are often surprised by how fast a pan of oil heats up when we are cooking. as shown when heated under identical conditions the same mass of oil will heat up about twice as fast as water will reaching 200 degrees Celsius in the time it takes water to boil.
You might want to pause the article to take in the data shown here. which shows the specific heat capacities of a range of substances. notice how the value for metals gets lower as the atomic mass increases and that materials made up of lighter elements have a much higher figure.