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An estimation of the speed of electromagnetic waves can be made using the hot spots inside a microwave oven - OCR - A-Level Physics A - Question 7 - 2014 - Paper 1
Question 7
An estimation of the speed of electromagnetic waves can be made using the hot spots inside a microwave oven. Microwaves are emitted in all directions inside the meta... show full transcript
Worked Solution & Example Answer:An estimation of the speed of electromagnetic waves can be made using the hot spots inside a microwave oven - OCR - A-Level Physics A - Question 7 - 2014 - Paper 1
Step 1
Explain how a stationary microwave pattern is set up in the oven.
Answer
A stationary microwave pattern is established in the oven due to the interference of waves emitted in all directions from the microwave source. As the microwaves reflect off the metal walls, they overlap with the incoming waves, creating interference patterns. When these waves meet, they can constructively or destructively interfere. The regions where waves meet in phase create areas of maximum intensity, known as antinodes, while areas where waves meet out of phase result in cancellation, creating nodes. This results in a standing wave pattern, identifiable by the hot spots where chocolate melts.
Step 2
Explain whether the points marked X on Fig. 7.1 are at nodes or antinodes in the wave pattern.
Answer
The points marked X on Fig. 7.1 correspond to areas where the waves constructively interfere, indicating these are likely to be antinodes. At antinodes, the amplitude of oscillation is at a maximum, resulting in higher energy concentration, which is reflected in the observed melting of chocolate.
Step 3
By using measurements taken from the diagram make an estimate of the speed c of the microwaves.
Answer
To estimate the speed of the microwaves, consider that the distance between the nodes or antinodes will be half the wavelength. If we measure the distance between two successive antinodes on Fig. 7.1 and find it to be, for example, 4 cm, then the wavelength \( \lambda = 2 \times 4 , cm = 8 , cm = 0.08 , m \).
Using the relationship between speed, frequency, and wavelength given by the formula: \( c = f \times \lambda \ \c = 2.5 \times 10^9 , Hz \times 0.08 , m \).
Therefore, the speed of the microwaves is estimated to be ( c = 2.0 \times 10^8 , m/s ).