AS edexcel physics: Tricky past paper questions(and answer)

June 2009 past paper

Note: Question 1a is straightforward, but the information is needed for partb, which is the tricky part.

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Question 1. The length of the zigzag pattern is eight 2.5 cm strips and thecross-sectional area of the iron wire is 9.0 × 10^–8 m2. The resistivity of ironis 9.9 × 10^–8 # m.

a) Show that the total resistance of the strain gauge is about 0.2#.

Answer: R = #l/A

9.9 × 10^–8 x 0.2 / 9.0 × 10^–8 = 0.22#

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b) The length of the zigzag pattern, when under strain, increases to 2.51cm. Calculate the increase in resistance of the wire in the gauge.

Answer: R(new) = (#l^2 x R1) - R1

((2.51^2-2.5^2) x 0.22) - 0.22 = 1.76x10^-3#

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Question 2. A wire of length l and cross-sectional area A is stretched.Assuming the volume V of the wire remains constant

V = lA = constant Show that the resistance of the wire is directly proportionalto l^2.

Answer: R # l^2

V = lA so A = V/l

R = #l/A

R = #l 2/ V

Here is how I calculated this: Rxl2.jpg

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Question 3. A loudspeaker is connected to a signal generator. Amicrophone is connected to an oscilloscope. Sound waves reach themicrophone directly from the loudspeaker and after reflection from the metalplate.

a) As the microphone is moved towards the loudspeaker, the amplitude ofthe wave displayed on the oscilloscope varies through a series of maximaand minima. Explain why the minima never have a zero value.

Answer: The waves never fully cancel out (due to slightly differentamplitudes)

b) As the microphone is moved towards the metal plate, the amplitudes atthe minima gradually decrease. Suggest why this happens. (Standing wavesquestion).

Answer: Energy is lost when the waves are reflected. As the microphonemoves towards the plate, the path difference decreases, and the amplitudesget more similar.

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Additional question

Question 4. Does blue light refract more than red light. Why?

Answer: Blue light refracts more than red light, because blue light has ahigher frequency, so it slows down more when it enters the glass.