Anyone who takes Physics A2 OCR will know that although the topics you learn about are a big contribution to the final mark of the G495 exam (where you can find the revision notes here), they only account for 60% of all the marks. The last 40% comes from something you can prepare for. You are giving a green paper entitled ‘A2 GCE PHYSICS B (ADVANCING PHYSICS) G495/01 Field and Particle Pictures ADVANCE NOTICE’. Within this booklet is an article which will come up in the Section C of your exam. You can prepare for 40% of your exam this easily. For this reason, here are some revision notes for the year June/May 2013 for the article called ‘Fiddles of the Future’. Feel free to skip to the parts most relevant to you.
If you haven’t got the article, you can find it here.
When analysing this article, I will quote using line numbers and will try to go into as much detail as possible. At the bottom of this article are pictures of my annotations too if that helps.
Fiddles of the Future
- Line 2 talks about ‘pick up devices’. These pick up the vibrations on electric violins.
- Line 5-6 notes that electric violins have the advantage of producing notes that can be ‘electronically amplified’. This means the note’s amplitude can be made to be larger (the peaks and troughs of the voltage will be greater: greater p.d.). This also provides enough fuel for an argument of analogue vs digital.The electronic violin will amplify the noise (but it can remove the noise). When analogue sounds are amplified, it also amplifies the noise which is not good because it cannot be removed making the noise sound distorted.
- Line 14-15 mentions how the ‘frequency of the note depends upon the length of a string’. The frequency depends on length and density of the string (density = mass / volume). The frequency is the number of waves per second. The amplitude is also effected by these factors but more importantly by the tension (force the string is pulled at).
- Line 22 mentions the ‘fundamental standing wave for that length (of string)’. The fundamental standing wave is the lowest frequency standing wave you can produce given the length of the string.
- The diagram below this paragraph shows a standing wave on a bowed string. At the bridge and the nut or player’s finger are nodes. The anti-node is in the middle of the standing wave. If the length of the standing wave shown is L. Then L = λ / 2. The wavelength of the fundamental frequency is 2 x L.
- Line 27 features the sentence, ‘vibrating strings themselves disturb the air surrounding them only very little’. This makes clear that the sound from violins do not come from the air vibrating but the body of the violin vibrating from the vibrations being carried to the body of the violin via the bridge and sound post.
- Line 29-30 states, ‘The vibrating violin body can also cause the air to resonate’. Resonance occurs when the driving frequency = natural frequency. When this happens, the amplitude of the vibrating sound waves increase because the energy going into the system is more than the energy going out from resistive forces. The max amplitude of the sound waves is when the energy going in is the same as the energy going out.
- The next diagram simply shows how the bridge is internally connected to the body of the violin to transfer the vibrations to the body.
- ‘Chladni Patterns’ are mentioned on line 36. The next diagram shows the types of Chladni Patterns produced on violins. The image to the left shows a surface vibrating with white sand on it. The areas where there is white sand is where there are nodes. The area without white sand are the anti-nodes.The image to the left is a metal plate.
- The image to the right of the back of a violin shows a similar picture. This time, the black sand is where the nodes are and the anti-nodes is where there isn’t any black sand. This could make clear that there may be a picture in the exam such as one like these where you are asked to label the nodes.
- Line 41 mentions ’employing lasers and holography’. Although the article is not about holography, it relies on two beams of light. This could be to do with interference (E = hf too) where Young’s double slit experiment might be useful to know about.
Amplifying the Sound
- The image (Fig 6: magnetic (induction) pickup for one steel string) tells us lots about how an electronic violin works. Below the string is a permanent magnet that is wrapped in a coil of wire. The wire is then fed to an amplifier to amplify the sound.
- When the violin’s steel string is no vibrating, the rate of change in flux is 0 because nothing is happening.
- However, when the steel string starts vibrating due to a note being played by the violinist, the steel string oscillates towards and further away from the permanent magnet. Steel has a higher permeability than air. Therefore, from the steel string moving closer and further away from the magnet, the whole permeance of the magnetic circuit is changing. If the permeance of the circuit is changing, the amount of flux flowing is also changing (higher the permeance, the more flux).Therefore, we have ourselves a rate of change of flux! This induces an EMF in the coil of wire which the amplifier picks up and converts to a specific frequency – the same frequency originally being played.
- If the above point was confusing, think that when the steel string is vibrating and is closest to the magnet, some of the flux from the magnetic circuit goes through the steel string and not the air (like it was doing). This means the permeance of the circuit has increased as air has a low permeability compared to steel. The flux increases int he circuit from this which increases the flux linkage. Therefore, because the string is oscillating back and forth, there is a rate of change of flux which induces an EMF in the coil.
- Flux (Φ) = Permeance (Λ) x Number of turns in a coil (N) x Current (I) – The factor that has caused the flux to increase and decrease is the steel vibrating changing the permeance of the circuit.
- Permeance (Λ) = ( Permeability (µ) x Area (m²) ) / Length (L) – Permeance of the circuit is changing because of the permeability changing (increasing when steel string is closest to magnet and decreasing when steel string is furthest from magnet (where air’s permeability which is replacing the steel string is low).
- Flux Linkage (Φ) = Flux (Φ) x Number or turns in a coil (N) – The Flux linkage is changing due to the flux changing.
- EMF = -N x dΦ/dt – The rate of change in flux of the circuit creates different EMF (different vibrations from the steel string create different rates of flux change inducing different EMFs to the amplifier).
- Line 53 – ‘The permanent magnet produces a flux linkage in the coil’. With flux linkage = flux x number of turns in the coil, we know that the flux linkage changes from a changing flux caused by a changing permeance.
- Lines 54 – 55 – ‘the steel string contributes to the permeance too, slightly changing the strength of the magnet’. If the air in the magnetic circuit is replaced with steel from the steel string vibrating closer to the magnet, the permeance increases.
- Lines 56 – 57 – ‘the steel string vibrated and moves to the relative pickup, the total permeance of the magnetic circuit changes’. The air gap has reduced/increased changing the permeance.
- Line 57 – ‘This emf produces a current’. An EMF in the coil is created from the rate of change of flux (EMF = -N x dΦ/dt). As the coil is connected to the amplifier and is a complete electrical circuit, the EMF produces a current through the coil. The current will be alternating at the same frequency as the string is oscillating at.
- A benefit of using a piezoelectric pickup is that an acoustic violin can be amplified without any noise. The pickup picks up the vibrations from the violin and turns it into electricity to go to an amplifier. If you wanted to amplify an acoustic violin with a microphone for example, the microphone would pick up noise too. It has a natural sound production.
- Line 64 – ‘sensors from strain gauges to accelerometers’. This is crying out for a question on sensitivity and resolution. Sensitivity is the change in output / change in input. Therefore, if there is a 5 degrees temperature input change that causes a 10V output change, the sensitivity would be 10/5 = 2 V/degrees. The resolution is the smallest possible value the instrument can measure accurately.
- Range – highest value – lowest value.
- Spread – range / 2.
- Mean – sum of all values / number of values there are.
- Outliers – Mean plus or minus 2 X spread. If it falls within the values, it is not an outlier.
- Uncertainty – spread / mean.
The last paragraph of the whole article has a link to the frequency spectra:
- To remember what order comes which frequency, think of ‘Reading Music Is Very Unsatisfactory for Xylophones and Glockenspiels‘. Radio Microwaves Infra-red Visible Ultraviolet X-Ray Gamma.
- Lines 73 – 74 – ‘the timbre of sound, depends on the presence of higher frequencies above the dominant frequency’. The dominant frequency of highest amplitude.