Physics. Waves and Radiation. N4/5 Summary Notes. Wave Characteristics

Transcription

1 Physics Waves and Radiation Wave Characteristics N4/5 Summary Notes

2 Content Level 4 SCN 4 20a I have researched new developments in science and can explain how their current of future applications might impact on modern life. Content National 4 Wave characteristics Comparison of longitudinal and transverse waves. Frequency as the number of waves per second. Wavelength and amplitude of transverse waves. Use of numerical or graphical data to determine the frequency of a wave. Use of appropriate relationship between wave speed, frequency and wavelength. Use of appropriate relationship between distance, speed and time for waves. Sound Analysis of sound waveforms including changing amplitude and frequency. Different methods of measurement of speed of sound in air. Sound level measurement including decibel scale. Noise pollution; risks to human hearing and methods of protecting hearing. Applications of sonar and ultrasound. Sound reproduction technologies. Noise cancellation. Content National 5 Wave Parameters and behaviours Energy can be transferred as waves. Determination of frequency, wavelength, amplitude and wave speed for longitudinal and transverse waves. Use of the relationships between wave speed, frequency, wavelength, distance and time Diffraction and practical applications Comparison of long wave and short-wave diffraction Waves and Radiation 2 Content Statements

3 Learning Outcomes Wave Characteristics, parameters and behaviours At National 4 level, by the end of this section you should be able to: Wave Characteristics 1. State the difference between longitudinal and transverse waves and give examples of both. 2. Define frequency as the number of waves per second. 3. Identify and/or calculate the wavelength and amplitude of transverse waves from a diagram. 4. Use numerical or graphical data to determine the frequency of a wave. 5. Carry out calculations using the relationship between wave speed, frequency and wavelength. 6. Carry out calculations using the relationship between distance, speed and time for waves. Additionally, at National 5 level: Wave parameters and behaviours 1. State that energy can be transferred as waves. 2. Determine frequency, wavelength, amplitude and wave speed for longitudinal and transverse waves. 3. Carry out calculations using the relationship between wave speed, frequency, wavelength, distance and time. 4. Describe what is meant by diffraction and give practical applications of the effect. 5. Describe the differences between long wave and short wave diffraction. Waves and Radiation 3 Learning Outcomes

4 Waves All waves transfer energy. All waves involve a movement of energy Water waves Sound waves Light, radio, microwaves (electromagnetic spectrum Transverse Wave The particles move at right angles to the direction of energy transfer. Water and all the waves on the electromagnetic spectrum are transverse waves. Longitudinal Wave The particles move parallel to the direction of energy travel Sound waves are longitudinal waves. Waves and Radiation4/5 4Wave Characteristics, parameters and behaviours Wave PBC P 16, Q 1-4

5 Wave Terms Frequency (f) the number of waves per second (Hertz) Hz Wavelength (λ) the distance from one point on a wave to the corresponding point on the next wave (metres m) Amplitude The height of a wave from middle to top (or middle to bottom) (metres m) Wavespeed (v) The speed of the wave (m/s) Period (T) the time for one complete wave to pass a point. (seconds s) Waves and Radiation4/5 5Wave Characteristics, parameters and behaviours Waves PBC Frequency P 2 3, Q 1 11; Wavelength P 4 6 Q 1-10

6 The Wave Equation v= fλ v = speed of wave (m/s) f = frequency (Hz) λ = wavelength (m) Example 1 Find the speed of a wave with frequency 30Hz and wavelength 25cm v= fλ = 30 x 0.25 = 7.5 m/s Example 2 A sound wave (v = 340m/s) has a frequency of 60Hz. What is its wavelength? v= fλ => λ = v/f = 340/60 =5.7 m Example 3 What is the frequency of a wave with speed 1500m/s and wavelength 20km? Example 4 What is the wavelength of SIBC? (96.2MHz) v= fλ => f = v/λ = 1500/20,000 = 0.075Hz v= fλ => λ = v/f = 3 x 10 8 / 92.6 x 10 6 = 3.24m Waves and Radiation 4 6 Wave Characteristics Waves PBC P 7 11, Q 1 25; P 12-13, Q 1-3; P 14 15, Q Sound P 6-8, Q 1-10

7 Speed, Distance and Time Speed = distance /time speed = metres per second (m/s) Distance = metres (m) Time = seconds (s) Example 1 What is the speed of a sound wave that travels 6800m in 60 seconds? v = d = 6800 = m/s t 60 Example 2 How long does it take a water wave to travel 15m at 0.5 m/s? t = d = 15 = 30s v 0.5 Example 3 The sound of a horn is heard 1190m away, 3.5 seconds after it is sounded. What is the speed of the horn sound in air? v = d = 1190 = 340 m/s t 3.5 Example 4 A boat sounds a foghorn and hears the echo 4seconds later. How far is it from the cliff? Solution 1 d = vt = 4 x 340 = 1360m Sound travel to cliff and back, so actual distance = 1360/2 = 680 m Solution 2 Time to cliff and back = 4s Time to cliff = 4/2 = 2s d = vt = 2 x 340 = 680m Waves and Radiation 4 7 Wave Characteristics Waves PBC P Q 1 9; P 19, Q 1 7. Sound P2, Q 1 6.

8 Diffraction Diffraction is the bending of waves round objects. Long wavelengths diffract more than short wavelengths. You can pick up long wave radio in the valley, but not short wave radio. If the gap between the barriers is less than a wavelength you get circular waves if it is more you get straight wavefronts with curved ends. Waves and Radiation 5 8 Learning Outcomes - Sound

9 Learning Outcomes - Sound At National 4 level, by the end of this section you should be able to: Sound 1. Recognise changes in amplitude and frequency from diagrams of sound waveforms. 2. Describe different methods of measuring speed of sound in air. 3. State that sound level is measured in decibels (db) and give typical examples of readings on the decibel scale. 4. Describe types of noise pollution, the risk it has to human hearing and methods of protecting hearing. 5. Describe applications of sonar and ultrasound. 6. Describe what is meant by noise cancellation and give an example of its use. Waves and Radiation 5 9 Learning Outcomes - Sound

10 Wave diagrams (from oscilloscope) Analysis of Sound Waveforms High frequency - quiet sound Low frequency -quiet Sound High frequency - loud sound Low frequency loud sound Waves and Radiation 4 10 Sound

11 Measuring the Speed of Sound Experiment 1 Measure the distance between the person with the balloon and the person with the stopwatch using a tape measure. Start the timer when the balloon is seen to burst, then stop the timer when it is heard going pop. Speed = distance between people Time on timer Experiment 2 Measure the distance between the two microphones using a metre stick Hit the piece of metal with the hammer. The timer starts when the sound reaches microphone 1 and stops when it reaches microphone 2. Speed = distance between microphones Time on timer {Discussion regarding which is most accurate and how to improve accuracy.} The speed of sound in air 340m/s The speed of light in a vacuum 3 x 10 8 m/s Waves and Radiation 4 11 Sound Sound P 2 Q 1-6

12 Sonar Sonar or echolocation is when sound echoes are used to find the position of objects. It stands for SOund NAvigation Ranging. Bats, dolphins and other animals use this method to navigate or to find prey. Fishing vessels use sonar to monitor the depth of water and to find shoals of fish. A ship uses its sonar to send out a pulse of sound and detects the reflected pulses 0.2 seconds later. If the velocity of sound in water is 1500m/s, how deep is the sea below the ship? Method 1 Method 2 d = vt = 1500 x 0.2 Time for echo = 0.2s = 300m Time to reach seabed = 0.2/2 = 0.1s Actual depth = 300/2 d = vt = 1500 x 0.1 = 150m = 150m The fishermen then pick up a pulse from a shoal of fish 75m below the ship how long does it take the sound to reach the fish? t = d/v = 75/1500 = 0.05s Waves and Radiation 4 12 Sound Sound P 3, Q 1-2

13 Ultrasound Ultrasound is high frequency sound, above the range of human hearing (20,000 Hz). It is used to monitor the growth of unborn babies and can also detect the blood flow through the heart, veins and arteries. Ultrasound does not harm tissue. A layer of jelly is placed between the skin and the transmitter/receiver to prevent false echoes. Ultrasound is used to measure the depth of a baby in its mother s womb. The speed of ultrasound in human tissue is 1500m/s. The time between transmission and the reflected signal is 1 x 10-4 s. a) How far did the sound travel? b) How deep is the part of the baby which caused the reflection? a) d = vt = 1500 x 1 x 10-4 = 0.15m b) Actual depth = 0.15/2 = 0.075m Waves and Radiation 4 13 Sound Sound P 4 5, Q 1-4

14 Sound Level Sound level is measured using a sound level meter. The units for sound level are Decibels(dB) Examples of noise levels 20dB 40dB 60dB 80dB 100dB 120dB 140dB Whisper/ticking of watch/quiet country lane Average house/normal private office Conversational speech Noisy office/electric shaver/alarm clock Passing truck/car horn at 5m/ lawnmower Thunder/fireworks display Above threshold of pain. Jet engine around 40m away Noise What is noise pollution? Unwanted loud or annoying sounds e.g. traffic noise, aircraft landing at airports, building work What noise level can damage hearing? 80dB Exposure to noise above 90dB is limited by law. Protecting hearing Hearing can be protected by wearing ear defenders or ear plugs, or by avoiding loud sounds. You should avoid prolonged exposure to loud sounds (including from headphones) Waves and Radiation 4 14 Sound Sound P 8 Q 1

15 Sound Reproduction Technologies Analogue and Digital Signals Analogue signal Digital signal Speech signal The wave produced by music and speech is an analogue signal. Analogue waves can have any value. MP3 players are digital devices, which means they use digital signals. Digital signals are made from signals that have two values 0 or 1. Analogue to Digital Conversion Sounds are turned into a digital signal by sampling the analogue wave at regular intervals. Sampling the sound wave produces a digital signal that is similar to the original in shape. Waves and Radiation 4 15 Sound

16 Sound Reproduction Technologies Doubling the sampling rate improves the accuracy of the output wave. The higher the sample rate the closer the digital signal sounds to the original analogue signal. Waves and Radiation 4 16 Sound

17 Noise Cancellation Noise cancellation is the removal of unwanted sound. e.g. the sound of the dentist s drill the sound of the engines on a boat or on an aeroplane. How active noise cancellation (ANC) works Two sound waves are added together in a process called destructive interference is added to Sound wave Cancellation wave The two waves cancel out producing silence! A small microphone picks up the background noise, sends the signal through an electronic circuit, which produces the cancellation wave. This is called active noise cancellation because it responds to changes in the background sounds. Waves and Radiation 4 17 Sound

18 Noise Cancellation Noise cancelling headphones. Padded ear cushion - helps block out noise Springy headband - keeps ear cushions on ears Microphone for cancellation electronics The combination of the padded headphones and springy headband give passive noise cancellation. This just blocks sound whether it is there or not. These headphones are useful when travelling, but they are also used by airline pilots to help them hear radio conversations more easily. The technology can allow patients to listen to their MP3 player without hearing the drill when at the dentist. It is also being introduced by some car manufacturers to improve passenger comfort. In military applications personnel have found that there is a reduction in stress and fatigue as well as an improvement in how easy communications are to understand. Waves and Radiation 4 18 Sound