A community centre replaces an old electric kettle with a newer model. Fig. 1 shows an energy transfer diagram for the new kettle when it heats 1.0 kg of wa...
Question 1 Report
A community centre replaces an old electric kettle with a newer model. Fig. 1 shows an energy transfer diagram for the new kettle when it heats 1.0 kg of water from 20 degrees C to 100 degrees C. The useful energy warms the water. Some energy is transferred to the kettle casing and to the surrounding air.
The kettle is plugged into a 230 V supply. Its power rating is 2000 W. The centre manager wants to explain why keeping the lid closed saves energy and reduces the time needed to boil water.
(a) Complete the energy transfer statement: electrical energy is transferred mainly to __________ energy in the water. [1]
(b) Use Fig. 1 to calculate the efficiency of the kettle. [3]
(c) Use the power rating to calculate the energy supplied in 180 s. [2]
(d) Explain two ways in which a closed lid reduces unwanted energy transfers. [4]
Answer Details
(a) Electrical energy is transferred mainly to thermal, or internal, energy in the water [1].
(b) The useful energy warming the water is 320 kJ [1], while total electrical energy supplied is 400 kJ. Therefore:
\[\text{efficiency}=\frac{\text{useful energy output}}{\text{total energy input}}=\frac{320}{400}=0.80=80\%\]
This gains the substitution [1] and answer 0.80 or 80% [1].
(c) Use 0a\(E=Pt\) [1].
\[E=2000\ \mathrm{W}\times180\ \mathrm{s}=360000\ \mathrm{J}=360\ \mathrm{kJ}\]
Therefore the supplied energy is 360 000 J, or 360 kJ [1].
(d) A closed lid reduces convection because warm air or steam cannot rise away as easily [1]. It also reduces evaporation [1]. Evaporation requires energy, including latent heat [1], so less energy is removed from the water. The lid also reduces conduction or convection from the water surface to cooler air [1].
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