Level 3 Science, 2004

Transcription

1 For Supervisor s Level 3 Science, Describe properties and applications selected from EMR, radioactive decay, sound and ultrasound Credits: Four 2.00 pm Wednesday 17 November 2004 Check that the National Student Number (NSN) on your admission slip is the same as the number at the top of this page. You should answer ALL the questions in this booklet. Show ALL working. If you need more space for any answer, use the page provided at the back of this booklet and clearly number the question. Check that this booklet has pages 2 11 in the correct order and that none of these pages is blank. YOU MUST HAND THIS BOOKLET TO THE SUPERVISOR AT THE END OF THE EXAMINATION. Achievement Criteria Achievement Describe properties and applications selected from EMR, radioactive decay, sound and ultrasound. For Achievement with Merit Explain properties and applications selected from EMR, radioactive decay, sound and ultrasound. Overall Level of Performance Achievement with Excellence Discuss properties and applications selected from EMR, radioactive decay, sound and ultrasound New Zealand Qualifi cations Authority, 2004 All rights reserved. No part of this publication may be reproduced by any means without the prior permission of the New Zealand Qualifi cations Au thor i ty.

2 2 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 λ v = f λ v = 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 = 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 ms 1. d t A TV and radio transmitter Robin s House 150 km Source: UB Science paper 1997, p8 (a) Describe what is meant by the term wavelength. Draw a diagram if this will help your answer.

3 3 (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. (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.

4 4 (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.

5 5 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. Pressure Time (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.

6 6 (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.

7 7 QUESTION THREE: RADIOACTIVE DECAY Photograph 1: A smoke detector (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 Am He + Np + energy 2 x y (i) State the values of x and y in the above equation. x = y = (ii) Explain why the charged particles used in the smoke detector are not harmful to humans unless ingested, ie taken internally.

8 8 (iii) Discuss how 2 4 He ionises the air inside the smoke detector.

9 9 (b) Carbon Dating Fossils are dated using the radioactive decay of carbon-14. Carbon-14 decays with a half-life of years. (i) Describe what is meant by the term half-life. The following graph shows how the radioactivity of 1.0 g of processed carbon decreases. 15 radioactive decay (counts per minute) age (years) 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.

10 10 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.

11 11 Extra paper for continuation of answers if required. Clearly number the question. Question number