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`“Life is like riding a bicycle.  To keep your balance you must keep moving.”–Albert Einstein`

## Sunday, January 13, 2008

### AP Physics B- Free Response Question (for practice) on Interference

Let us discuss a free-response question of the type you can expect in the section ‘interference’.

At the moment I just give you the question for you to try yourself. (I’ll give you the answer in the next post). Here is the question:

Your teacher suggests to you that an interference method could be used to measure the frequency of the sound emitted by two small identical speakers S1 and S2 arranged horizontally with a separation of 0.8 m (or any suitable distance) as shown. The speakers are connected in series to a sine wave generator of constant output frequency. Your teacher moves the probe (microphone) of a sound level meter along the horizontal line AB parallel to the line S1 S2 and demonstrates to you that at O, equidistant from S1 and S2, the intensity of sound is maximum and it varies between maximum and minimum when the probe is moved along OA and OB.

(a) Explain briefly what measurements you will make to enable you to calculate the frequency of the sound emitted by the speakers.

(b) If the distance between the speakers is increased, what quantity you measure is affected and how?

(c) What equations you will use for calculating the frequency? Explain the reason for your answer to (b) based on the relevant equation.

(d) If the distance between the 2nd maximum on one side of O and the 3rd maximum on the other side of O is 1.725 m, calculate the frequency of the sound emitted by the speakers, assuming that the speed of sound at the laboratory temperature is 345 ms–1.

(e) Your teacher asks you to reverse the connection to the terminals of one of the speakers. What change, if any, will this produce in the location of the maxima and minima? Give reason for your answer.

Try to answer the above question which carries 15 points. I’ll be back with the answer shortly.