90732
Level 3 Science, 2004
90732 Describe properties and applications selected from
EMR, radioactive decay, sound and ultrasound
Credits: Four
You should answer ALL the questions in this booklet.
Show ALL working.
Achievement Criteria
For Assessor’s use only
Achievement
Achievement
with Merit
Achievement
with Excellence
Describe properties and
applications selected from EMR,
radioactive decay, sound and
ultrasound.
Explain properties and
applications selected from EMR,
radioactive decay, sound and
ultrasound.
Discuss properties and
applications selected from EMR,
radioactive decay, sound and
ultrasound
Overall Level of Performance
© New Zealand Qualifications Authority, 2004
All rights reserved. No part of this publication may be reproduced by any means without the prior permission of the New Zealand Qualifications Authority.
90732, L3 Science, 2004
Page 1
You are advised to spend 45 minutes answering the questions in this booklet.
The following equations may be of use while answering this paper:
c f
v f
v
d
t
QUESTION ONE: ELECTROMAGNETIC RADIATION
The diagram below (not to scale) shows a radio transmitter (A) situated on the opposite side of a hill
from a small town. The transmitter broadcasts AM radio programmes on a frequency of 935 kHz
(1 kHz = 1 × 103 Hz). The radio waves are picked up by Robin’s stereo in his house. The house is
150 km away from the transmitter, as shown below.
The speed of radio waves in air is 3.0 × 108 ms–1.
[For copyright reasons, this resource cannot be reproduced here. See
below.]
Source: UB Science paper 1997, p8
(a)
Describe what is meant by the term wavelength. Draw a diagram if this will help your answer.
(b)
Calculate the wavelength of the radio waves emitted by the transmitter. State your answer with
appropriate units and rounded to a sensible number of significant figures.
90732, L3 Science, 2004
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(c)
To boost the signal, another transmitter is placed beside transmitter A. Both transmitters are
exactly the same distance from Robin’s house. The waves from each transmitter have the same
amplitude, frequency and phase.
Explain why the aerial in Robin’s house detects a radio wave that has twice the amplitude of
each transmitted wave. Draw a diagram if this will help your answer.
(d)
The transmitter at A also transmits FM waves. These waves typically have a wavelength of
3.0 m. Robin gets poor reception of these FM waves.
Discuss why Robin’s house can receive the AM radio waves from the transmitter better than the
higher frequency FM waves.
QUESTION TWO: SOUND AND ULTRASOUND
(a)
A car horn is sounded and a graph is plotted of pressure vs. time for the sound wave produced.
(i)
On the graph above clearly mark and label the amplitude of the wave shown.
(ii)
A second, louder horn is sounded at the same pitch as the first. On the graph above draw
the sound wave from the second horn.
(iii)
A cliff is 75 m away. Given that the speed of sound is 300 ms–1, how long does it take for
the people in the car to hear the echo of the horn? State your answer with the appropriate
unit.
90732, L3 Science, 2004
Page 3
(b)
On a fishing trip, a fish finder is used to help locate schools of fish by echolocation.
The fish finder operates by sending ultrasonic wave pulses into the water and receiving any
echoes of these waves as they reflect off solid objects, such as the lake bottom, rocks or fish.
[For copyright reasons, this resource cannot be reproduced here. See below.]
Adapted from: P. Howison, Form 7 Physics Study Guide, ESA Publications (NZ Ltd), Auckland, 1999
Discuss why the echo received back from the rocky bottom of the lake would be stronger than
the echo received from a school of fish at the same depth.
90732, L3 Science, 2004
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QUESTION THREE: RADIOACTIVE DECAY
(a)
A common smoke detector used in homes contains a radioactive material called americium-241.
The radioactive material decays, releasing radiation that ionises the air inside the smoke
detector. The following equation shows the decay of americium-241.
241
95
(i)
4
x
2
y
Am  He  Np energy
State the values of x and y in the above equation.
x=
y=
(b)
(ii)
Explain why the charged particles used in the smoke detector are not harmful to humans
unless ingested, ie taken internally.
(iii)
Discuss how He ionises the air inside the smoke detector.
2
4
Carbon Dating
Fossils are dated using the radioactive decay of carbon-14. Carbon-14 decays with a half-life of
5 730 years.
(i)
Describe what is meant by the term half-life.
90732, L3 Science, 2004
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The following graph shows how the radioactivity of 1.0 g of processed carbon decreases.
1.0 g of a similarly processed sample of carbon obtained from a fossil had an activity of 5.0
counts per minute.
(ii)
Use the graph to estimate the age of the fossil. Give the appropriate unit.
The half-life of an unknown material was investigated using a Geiger counter. The initial count
rate was 156 per second. After 7 minutes the count rate fell to 39 per second.
(iii)
Calculate the half-life of the material.
(iv)
Explain why it is not possible to predict when a particular atom in a radioactive sample will
decay.
90732, L3 Science, 2004
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