Q#2 (Past Exam Paper – June 2014 Paper 42 Q3)
A microwave cooker uses electromagnetic waves of frequency 2450 MHz.
The microwaves warm the food in the cooker by causing water molecules in the food to oscillate with a large amplitude at the frequency of the microwaves.
(a) State the name given to this phenomenon. [1]
(b) The effective microwave power of the cooker is 750 W.
The temperature of a mass of 280 g of water rises from 25 °C to 98 °C in a time of 2.0 minutes.
Calculate a value for the specific heat capacity of the water. [3]
(c) The value of the specific heat capacity determined from the data in (b) is greater than the accepted value.
A student gives as the reason for this difference: ‘heat lost to the surroundings’.
Suggest, in more detail than that given by the student, a possible reason for the difference. [1]
Solution:
(a) Resonance
(b)
{When there is a change in temperature,
Heat (energy) = mcΔθ
But Power = Energy / time
so, Energy = Power × Time = Pt}
Pt = mcΔθ
{Time should be in seconds and mass in kilogram.}
750 × 2×60 = 0.28 × c × (98 – 25)
Specific capacity of water, c = 4400 J kg-1 K-1
(c) Example:
There is some microwave leakage from the cooker
The container for the water is also heated
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