WEBVTT 00:00:00.500 --> 00:00:03.976 Welcome to GCSE Edexcel Science revision. 00:00:04.126 --> 00:00:06.372 Unit C P 4: Waves. 00:00:09.033 --> 00:00:17.044 Waves transfer energy and information from one place to another without transferring matter overall (no net transfer of matter). 00:00:17.194 --> 00:00:22.601 In mechanical waves, particles vibrate back and forth (oscillate) around their rest positions. 00:00:25.267 --> 00:00:31.046 A floating cork bobs as water ripples pass; it does not travel across the tank with every crest. 00:00:31.196 --> 00:00:35.086 Sound can travel through air while the air particles vibrate locally. 00:00:37.767 --> 00:00:44.261 In a transverse wave, vibrations are at right angles (perpendicular) to the direction of energy transfer. 00:00:44.411 --> 00:00:49.636 Examples include electromagnetic waves, waves on a string and seismic S waves. 00:00:49.786 --> 00:00:52.197 The two directions are perpendicular. 00:00:56.867 --> 00:01:03.425 In a longitudinal wave, vibrations are back and forth along the direction of energy transfer (parallel to it). 00:01:03.575 --> 00:01:06.943 Sound in air and seismic P waves are longitudinal. 00:01:09.600 --> 00:01:16.805 Longitudinal waves contain compressions, where particles are closer together, and rarefactions, where they are more spread out. 00:01:16.955 --> 00:01:20.291 Particles vibrate parallel to the propagation direction. 00:01:24.967 --> 00:01:30.075 At GCSE, surface water ripples are represented as transverse waves. 00:01:30.225 --> 00:01:35.664 Real surface-water particle motion is more complex than a simple up-and-down line. 00:01:38.333 --> 00:01:42.824 Sound needs a material to travel through, such as air, water or a solid. 00:01:42.974 --> 00:01:47.095 This material is called a medium, so sound cannot cross a vacuum. 00:01:47.245 --> 00:01:50.409 Electromagnetic waves can travel through a vacuum. 00:01:53.067 --> 00:01:59.464 Wavelength Lambda is the distance between two corresponding points on neighbouring waves: points at the same stage, 00:01:59.614 --> 00:02:03.407 such as one crest to the next crest or one compression to the next. 00:02:03.557 --> 00:02:05.371 It is measured in metres. 00:02:08.033 --> 00:02:14.326 Amplitude is the greatest distance a point on a wave moves from its rest position (its maximum displacement). 00:02:14.476 --> 00:02:18.977 Measure from the rest position to a crest or trough, not from crest to trough. 00:02:19.127 --> 00:02:23.905 Amplitude is measured from the rest position; wavelength joins matching points. 00:02:28.567 --> 00:02:33.643 Frequency F is the number of complete waves (cycles) passing a point each second. 00:02:33.793 --> 00:02:35.543 It is measured in hertz. 00:02:35.693 --> 00:02:40.428 A frequency of 5 hertz means five complete waves, or cycles, per second. 00:02:43.100 --> 00:02:47.814 Period T is the time for one complete wave cycle, measured in seconds. 00:02:47.964 --> 00:02:52.775 T equals one divided by F: a higher frequency means a shorter period. 00:02:55.433 --> 00:02:59.776 Wave speed is how fast the wave travels, measured in metres per second. 00:02:59.926 --> 00:03:05.503 A wavefront joins points at the same stage of a wave, such as the points along a ripple crest. 00:03:08.167 --> 00:03:13.788 Use V equals F times lambda for all waves: speed equals frequency times wavelength. 00:03:13.938 --> 00:03:19.740 Rearrange to F equals V divided by lambda or lambda equals V divided by F. 00:03:22.400 --> 00:03:28.032 A wave of frequency 4 hertz and wavelength 0.5 metres travels at 2 metres per second. 00:03:28.182 --> 00:03:33.119 Convert centimetres or millimetres to metres before using S I equations. 00:03:35.800 --> 00:03:40.420 You can also use V equals distance divided by time for a travelling pulse. 00:03:40.570 --> 00:03:46.776 Do not confuse the speed of a pulse with the sideways or up-and-down speed of the vibrating particles. 00:03:49.433 --> 00:03:53.089 In a ripple tank, a vibrating dipper produces water waves. 00:03:53.239 --> 00:04:00.321 Use a lamp and screen to see the wavefronts; measure the spacing across several wavelengths and divide by the number of intervals. 00:04:03.000 --> 00:04:13.217 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. 00:04:13.367 --> 00:04:16.119 Then calculate V equals F times lambda. 00:04:18.800 --> 00:04:23.874 Use shallow, even water and small ripples; reflections from edges can interfere. 00:04:24.024 --> 00:04:28.866 Take repeats and avoid mistaking a moving shadow's scale for the actual wavelength. 00:04:31.533 --> 00:04:36.493 For waves in a solid, a stretched string driven by a vibration generator is suitable. 00:04:36.643 --> 00:04:44.940 Identify adjacent nodes in a stationary-wave pattern: their spacing is half a wavelength, so lambda equals two times node spacing. 00:04:47.600 --> 00:04:50.319 Keep the string tension and length controlled. 00:04:50.469 --> 00:04:58.095 Use the generator frequency and measured wavelength to calculate speed; stationary waves are produced by opposite travelling waves. 00:05:00.767 --> 00:05:06.152 For a pulse on a rope or spring, measure a long known travel distance and time its passage. 00:05:06.302 --> 00:05:11.411 Slow pulses suit a stopwatch; short fast travel needs electronic timing or video. 00:05:14.067 --> 00:05:23.804 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. 00:05:23.954 --> 00:05:28.022 Electronic timing resolves the short delay between the microphones. 00:05:32.700 --> 00:05:40.655 An echo method uses speed equals 2 times distance to reflector divided by echo delay, because the sound travels out and back. 00:05:40.805 --> 00:05:44.935 Longer distances reduce relative timing error; repeat safely. 00:05:47.600 --> 00:05:51.119 Choose equipment with sufficient time and distance resolution. 00:05:51.269 --> 00:05:56.625 State measured quantities, units, controls, repeats and likely sources of uncertainty. 00:05:59.300 --> 00:06:04.142 Refraction happens when a wave crosses into a region where it travels at a different speed. 00:06:04.292 --> 00:06:08.710 Unless it meets the boundary along the normal, it also changes direction. 00:06:11.367 --> 00:06:15.389 The boundary between two materials (media) is called the interface. 00:06:15.539 --> 00:06:19.369 The normal is an imaginary line at right angles to the boundary. 00:06:19.519 --> 00:06:22.524 Measure angles from the normal, not from the surface. 00:06:25.200 --> 00:06:29.863 When a wave slows on entering a medium at an angle, it bends towards the normal. 00:06:30.013 --> 00:06:32.728 When it speeds up, it bends away from the normal. 00:06:32.878 --> 00:06:37.050 Angles are measured from the normal; at normal incidence there is no bend. 00:06:41.700 --> 00:06:49.836 A wave meeting a boundary straight along the normal (normal incidence) does not change direction, although its speed and wavelength can change. 00:06:49.986 --> 00:06:54.375 Its frequency stays the same because the source still vibrates at the same rate. 00:06:57.033 --> 00:07:03.027 Since V equals F times lambda and frequency is unchanged, a lower speed means a shorter wavelength. 00:07:03.177 --> 00:07:06.589 Shallower water slows ripples compared with deeper water. 00:07:09.267 --> 00:07:15.145 Materials can reflect, transmit, absorb or refract waves, with effects depending on wavelength. 00:07:15.295 --> 00:07:18.862 A transmitted wave need not keep the same speed as before. 00:07:21.533 --> 00:07:23.197 That completes Waves. 00:07:23.347 --> 00:07:26.693 Revisit the notes and test yourself on the revision website.