Question 2
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]
Reference: Past Exam Paper – June 2014 Paper 42 Q3
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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