Heat Transfer from Kettle to Water

Now the kettle has been filled, the water needs boiling.

Boiling water involves the conversion of electrical energy to thermal energy via the heating element in the kettle. This heat energy is the transferred to the water in the kettle, which is in contact with the heating element. The rate of energy transfer, is given as the power rating in watts on most household appliances. The power rating of kettles varies depending on their design, but most operate around 1800 to 2200W.

Ideally the kettle would be adiabatic (no heat transfer between it and its surroundings), however this would require the kettle to be sealed to stop any steam escaping and taking heat with it. As a result, the kettle would become pressurised and therefore potentially liable to explosion/escape of high-pressure steam upon failure which could cause serious harm to its operator. It would also require the kettle to be significantly more complex in its design to withstand high pressures, which would in turn increase its cost of manufacture and therefore also to the user.

The kettle can be considered as a system as in the following:

 In reality, electrical work done by the heating element heats the water, but some of the heat is lost through the spout and by conduction through the walls of the kettle.

The specific heat capacity (the heat energy required to heat 1kg of a substance up by 1ᵒC) at fixed pressure, , of water is required to calculate the amount of heat required to bring the temperature of the water up to 100ᵒC.

Assuming the 250ml (≈0.25kg) of water is at room temperature (20ᵒC) the theoretical amount of energy required to heat the water to 100ᵒC is found by:

Given the following details, the theoretical time required to boil the kettle can be calculated:

The efficiency of the kettle can also be calculated:

These calculations include several assumptions:

·         cp is constant:

Cp actually varies as temperature varies. As a result,  would not be as simple as and is far more complex in reality although taking it as a constant between 20 and 100ᵒC is a reasonable approximation.

·         No energy contributes to the latent heat of vaporisation for the water:

Some of the work input actually goes towards the latent heat of vaporisation. The energy that is lost as the water that changes phase to steam leaves is accounted for in the Qout, but the energy that contributes to the latent heat of saturated liquid water but does not change temperature of the water is not accounted for.

·         No mass is lost as steam:

When some of the water is converted into steam and rises out of the funnel, mass of water is lost. As a result, the mass is not constant and is in fact a function of time throughout the heating process

Now you've learnt about energy transfer, click the link below to explore the linked practical experiment into finding the cp of tea:

The Practical Experiment