A microwave cooker uses electromagnetic waves of frequency 2450 MHz.

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