WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.402 Unit S P 4: Waves. 00:00:09.067 --> 00:00:17.077 Waves transfer energy and information from one place to another without transferring matter overall (no net transfer of matter). 00:00:17.227 --> 00:00:22.634 In mechanical waves, particles vibrate back and forth (oscillate) around their rest positions. 00:00:25.300 --> 00:00:31.079 A floating cork bobs as water ripples pass; it does not travel across the tank with every crest. 00:00:31.229 --> 00:00:35.119 Sound can travel through air while the air particles vibrate locally. 00:00:37.800 --> 00:00:44.295 In a transverse wave, vibrations are at right angles (perpendicular) to the direction of energy transfer. 00:00:44.445 --> 00:00:49.669 Examples include electromagnetic waves, waves on a string and seismic S waves. 00:00:49.819 --> 00:00:52.230 The two directions are perpendicular. 00:00:56.900 --> 00:01:03.459 In a longitudinal wave, vibrations are back and forth along the direction of energy transfer (parallel to it). 00:01:03.609 --> 00:01:06.977 Sound in air and seismic P waves are longitudinal. 00:01:09.633 --> 00:01:16.838 Longitudinal waves contain compressions, where particles are closer together, and rarefactions, where they are more spread out. 00:01:16.988 --> 00:01:20.325 Particles vibrate parallel to the propagation direction. 00:01:25.000 --> 00:01:30.108 At GCSE, surface water ripples are represented as transverse waves. 00:01:30.258 --> 00:01:35.697 Real surface-water particle motion is more complex than a simple up-and-down line. 00:01:38.367 --> 00:01:42.857 Sound needs a material to travel through, such as air, water or a solid. 00:01:43.007 --> 00:01:47.128 This material is called a medium, so sound cannot cross a vacuum. 00:01:47.278 --> 00:01:50.443 Electromagnetic waves can travel through a vacuum. 00:01:53.100 --> 00:01:59.497 Wavelength Lambda is the distance between two corresponding points on neighbouring waves: points at the same stage, 00:01:59.647 --> 00:02:03.441 such as one crest to the next crest or one compression to the next. 00:02:03.591 --> 00:02:05.404 It is measured in metres. 00:02:08.067 --> 00:02:14.359 Amplitude is the greatest distance a point on a wave moves from its rest position (its maximum displacement). 00:02:14.509 --> 00:02:19.010 Measure from the rest position to a crest or trough, not from crest to trough. 00:02:19.160 --> 00:02:23.938 Amplitude is measured from the rest position; wavelength joins matching points. 00:02:28.600 --> 00:02:33.677 Frequency F is the number of complete waves (cycles) passing a point each second. 00:02:33.827 --> 00:02:35.576 It is measured in hertz. 00:02:35.726 --> 00:02:40.461 A frequency of 5 hertz means five complete waves, or cycles, per second. 00:02:43.133 --> 00:02:47.847 Period T is the time for one complete wave cycle, measured in seconds. 00:02:47.997 --> 00:02:52.808 T equals one divided by F: a higher frequency means a shorter period. 00:02:55.467 --> 00:02:59.809 Wave speed is how fast the wave travels, measured in metres per second. 00:02:59.959 --> 00:03:05.537 A wavefront joins points at the same stage of a wave, such as the points along a ripple crest. 00:03:08.200 --> 00:03:13.821 Use V equals F times lambda for all waves: speed equals frequency times wavelength. 00:03:13.971 --> 00:03:19.773 Rearrange to F equals V divided by lambda or lambda equals V divided by F. 00:03:22.433 --> 00:03:28.065 A wave of frequency 4 hertz and wavelength 0.5 metres travels at 2 metres per second. 00:03:28.215 --> 00:03:33.153 Convert centimetres or millimetres to metres before using S I equations. 00:03:35.833 --> 00:03:40.453 You can also use V equals distance divided by time for a travelling pulse. 00:03:40.603 --> 00:03:46.809 Do not confuse the speed of a pulse with the sideways or up-and-down speed of the vibrating particles. 00:03:49.467 --> 00:03:53.122 In a ripple tank, a vibrating dipper produces water waves. 00:03:53.272 --> 00:04:00.354 Use a lamp and screen to see the wavefronts; measure the spacing across several wavelengths and divide by the number of intervals. 00:04:03.033 --> 00:04:13.250 Find the frequency from the dipper setting or count its complete vibrations (oscillations) over a measured time: frequency equals number of oscillations divided by time. 00:04:13.400 --> 00:04:16.152 Then calculate V equals F times lambda. 00:04:18.833 --> 00:04:23.907 Use shallow, even water and small ripples; reflections from edges can interfere. 00:04:24.057 --> 00:04:28.899 Take repeats and avoid mistaking a moving shadow's scale for the actual wavelength. 00:04:31.567 --> 00:04:36.527 For waves in a solid, a stretched string driven by a vibration generator is suitable. 00:04:36.677 --> 00:04:44.973 Identify adjacent nodes in a stationary-wave pattern: their spacing is half a wavelength, so lambda equals two times node spacing. 00:04:47.633 --> 00:04:50.353 Keep the string tension and length controlled. 00:04:50.503 --> 00:04:58.128 Use the generator frequency and measured wavelength to calculate speed; stationary waves are produced by opposite travelling waves. 00:05:00.800 --> 00:05:06.185 For a pulse on a rope or spring, measure a long known travel distance and time its passage. 00:05:06.335 --> 00:05:11.445 Slow pulses suit a stopwatch; short fast travel needs electronic timing or video. 00:05:14.100 --> 00:05:23.837 To measure sound speed, use two microphones separated by a measured distance and an oscilloscope or data logger to measure the arrival-time difference of a sharp sound. 00:05:23.987 --> 00:05:28.055 Electronic timing resolves the short delay between the microphones. 00:05:32.733 --> 00:05:40.689 An echo method uses speed equals 2 times distance to reflector divided by echo delay, because the sound travels out and back. 00:05:40.839 --> 00:05:44.968 Longer distances reduce relative timing error; repeat safely. 00:05:47.633 --> 00:05:51.152 Choose equipment with sufficient time and distance resolution. 00:05:51.302 --> 00:05:56.658 State measured quantities, units, controls, repeats and likely sources of uncertainty. 00:05:59.333 --> 00:06:04.175 Refraction happens when a wave crosses into a region where it travels at a different speed. 00:06:04.325 --> 00:06:08.743 Unless it meets the boundary along the normal, it also changes direction. 00:06:11.400 --> 00:06:15.423 The boundary between two materials (media) is called the interface. 00:06:15.573 --> 00:06:19.402 The normal is an imaginary line at right angles to the boundary. 00:06:19.552 --> 00:06:22.557 Measure angles from the normal, not from the surface. 00:06:25.233 --> 00:06:29.896 When a wave slows on entering a medium at an angle, it bends towards the normal. 00:06:30.046 --> 00:06:32.761 When it speeds up, it bends away from the normal. 00:06:32.911 --> 00:06:37.083 Angles are measured from the normal; at normal incidence there is no bend. 00:06:41.733 --> 00:06:49.869 A wave meeting a boundary straight along the normal (normal incidence) does not change direction, although its speed and wavelength can change. 00:06:50.019 --> 00:06:54.409 Its frequency stays the same because the source still vibrates at the same rate. 00:06:57.067 --> 00:07:03.061 Since V equals F times lambda and frequency is unchanged, a lower speed means a shorter wavelength. 00:07:03.211 --> 00:07:06.623 Shallower water slows ripples compared with deeper water. 00:07:09.300 --> 00:07:15.178 Materials can reflect, transmit, absorb or refract waves, with effects depending on wavelength. 00:07:15.328 --> 00:07:18.895 A transmitted wave need not keep the same speed as before. 00:07:21.567 --> 00:07:31.847 At a material boundary a wave can be reflected back, transmitted into the next material, refracted as its speed changes, or absorbed with energy transferred to the material. 00:07:31.997 --> 00:07:34.195 These processes can occur together. 00:07:36.867 --> 00:07:42.851 The frequency of a transmitted sound wave is set by the source and stays the same when it crosses a boundary. 00:07:43.001 --> 00:07:48.663 If its speed changes, wavelength changes in proportion because V equals F times lambda. 00:07:51.333 --> 00:07:55.053 (Higher tier) An echo travels to a reflecting object and back. 00:07:55.203 --> 00:08:04.876 One-way distance equals wave speed times round-trip time divided by 2; omitting the factor of two gives twice the correct distance (Higher tier calculation). 00:08:05.026 --> 00:08:08.437 The time interval for an echo corresponds to two journeys. 00:08:13.100 --> 00:08:18.933 (Higher tier) Ultrasound is sound above 20000 hertz; infrasound is below 20 hertz. 00:08:19.083 --> 00:08:24.331 These definitions and the applications below are Higher-tier content in this specification. 00:08:27.000 --> 00:08:34.701 (Higher tier) Ultrasound can be reflected at tissue boundaries to make a fetal image and can locate underwater objects with sonar. 00:08:34.851 --> 00:08:41.485 Different boundaries produce different reflected signals; ultrasound is not an ionising electromagnetic wave. 00:08:44.167 --> 00:08:50.374 (Higher tier) Infrasound travels long distances and can monitor events such as volcanic activity. 00:08:50.524 --> 00:08:59.183 Analysis of seismic waves helps investigate Earth’s interior: their reflection, refraction and transmission reveal changes in materials. 00:09:01.833 --> 00:09:05.372 (Higher tier) Sound vibrations make the eardrum vibrate. 00:09:05.522 --> 00:09:09.843 Three small bones (the ossicles) pass the vibrations to the inner ear. 00:09:09.993 --> 00:09:15.111 The cochlea converts them into electrical signals, which travel along the auditory nerve. 00:09:15.261 --> 00:09:20.183 The ear converts mechanical vibrations into signals carried by the auditory nerve. 00:09:24.833 --> 00:09:32.331 (Higher tier) The ear is sensitive to a limited range, approximately 20 hertz to 20000 hertz for a young healthy person. 00:09:32.481 --> 00:09:40.363 Age and damage can reduce this range; equal sound intensities at different frequencies are not necessarily heard equally loudly. 00:09:43.033 --> 00:09:48.015 (Higher tier) In a microphone, air vibrations move a thin surface called a diaphragm. 00:09:48.165 --> 00:09:52.106 In a loudspeaker, the diaphragm vibrates and makes the air vibrate. 00:09:52.256 --> 00:09:58.347 These parts respond better to some frequencies than others, giving the device a limited frequency response. 00:10:01.000 --> 00:10:07.271 (Higher tier) Protecting hearing involves limiting exposure to loud sound and using appropriate protection. 00:10:07.421 --> 00:10:14.545 Frequency is related to pitch, while loudness depends on intensity and the ear’s response; they are different quantities. 00:10:17.200 --> 00:10:18.864 That completes Waves. 00:10:19.014 --> 00:10:22.359 Revisit the notes and test yourself on the revision website.