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Standing Waves or Stationary Waves

Q1.

Below is a diagram of a microwave oven.

(a) Mario wants to use the stationary waves formed in the microwave oven to measure the frequency of the microwaves emitted by the transmitter.

Suggest how stationary waves are formed in the microwave oven.

The waves are reflected from the oven wall and superpose/interfere with transmitted/incident waves travelling in the opposite direction.

[2 marks]

(b) Mario removes the turntable and places a bar of chocolate on the floor of the oven. He then switches the oven on for about one minute.

When the chocolate is removed he observes that there are three small patches of melted chocolate with unmelted chocolate between them.

Here is a diagram of the chocolate bar:

(i) Suggest why the chocolate only melts in the positions shown.

The energy/amplitude of the standing wave is at a maximum at the antinodes of the standing wave - halfway between these antinodes you would find the nodes - there no energy would be transferred at all.

The maximum chocolate melt occurs at the antinodes of the microwave as this is where maximum energy transfer occurs.

[2 marks]

(ii) Calculate the frequency of the microwaves used by the oven.

Distance between antinodes = 5.9 cm = 0.059 m

Wavelength λ = 2 x 0.059 m = 0.118 m

c = 3.0 x 108 m/s

c = fλ

f = c/λ

f = 3.0 x 108/0.118

f = 2.5 GHz (or x 109 Hz)

[5 marks]

(c) Explain why most microwave ovens contain a rotating turntable on which the food is placed during cooking.

By rotating the turntable the position of antinode/maximum energy/maximum amplitude/nodes in food continually changes enabling the heat transfer to occur more evenly across the food.

However there are still areas that get more energy than others - that is why you are told to stir and/or shake part of the way through cooking!

[1 mark]

(Total 10 marks)