Fig. 1 shows a student testing an electric kettle used in a school kitchen. The kettle is connected to a 230 V supply and takes a current of 8.0 A. It heats...
Question 1 Report
Fig. 1 shows a student testing an electric kettle used in a school kitchen. The kettle is connected to a 230 V supply and takes a current of 8.0 A. It heats water for 150 s. The student assumes that the power remains constant during this time. Some energy also heats the metal case and the surrounding air.
(a) State the energy store that decreases in the kettle element when it is switched on. [1]
(b) Calculate the electrical energy supplied to the kettle in 150 s. [3]
(c) Explain why the energy transferred to the water is less than the electrical energy supplied. [2]
Answer Details
(a) The supplied mark scheme is not fully consistent with the wording: when a kettle element is switched on, its internal (thermal) energy store normally increases, rather than decreases. The intended accepted idea appears to be that electrical energy is transferred from the supply to the element, with the element's internal energy store increasing while it heats. [1]
(b) First calculate the electrical power:
\[P=VI=230\text{ V}\times8.0\text{ A}=1840\text{ W}\]
Then use \(E=Pt\):
\[E=1840\text{ W}\times150\text{ s}=276000\text{ J}\]
\[E=2.76\times10^5\text{ J}\]
[3]
(c) Not all the supplied electrical energy heats the water. Some energy heats the metal kettle case, and some is transferred by heating to the surrounding air. These transfers reduce the energy gained by the water. [2]
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