# Search results

1. ### Hacked pulsed laser driver using Arduino

Try driving the laser with the transistor circuit on its own without the Arduino first and get that working. Lasers are dangerous to eyes period. No? Overdriving an diode is likely just to burn the thing out. Unless you are already staring at the laser I don’t see how this could be a...
2. ### Rod resting on a cylinder, find mu

Sorry I forgot to add the file.
3. ### Rod resting on a cylinder, find mu

Thanks. I have another answer that someone else has done. It looks like they are doing the right thing with what looks like possibly a good free body diagram. However, at the start of their answer they just seem to pull out of nowhere 1/2 NRC - I can’t see what the justification is for that...
4. ### Rod resting on a cylinder, find mu

My solution is on the files. The basic approac was the take moments about A. Then find the reaction at C. Use the reaction at C to find the friction between rod and cylinder. I take it to mean from the question that the friction between the rod and plane is equal to the friction between...
5. ### Index laws question

Thank you confirming that.
6. ### Index laws question

So then 0.5^x becomes 2^-x? So the two functions are effectively 2^x and 2^-x and because of this they are reflections of each other in the Y - axis. I don't know what else the teacher might be expecting but I suspect that the answer the teacher is looking for. What do you reckon? The...
7. ### Index laws question

Homework Statement we are asked to compare the graphs of 0.5^x with 2^x using index laws Homework Equations I am not sure how the regular index laws apply The Attempt at a Solution All I can say at the moment is the the graph of 2^x is a reflection of 0.5^x in the Y axis. I don't know...
8. ### A question about ionic bonding

Ok - that makes more sense now. Thanks
9. ### A question about ionic bonding

Yes - I have done questions about the Born-Haber cycle and I do appreciate the arguments about screening. I still struggling though to understand from the point of view of energy why the electron is actually transferred say from the sodium atom to the chlorine atom. Because what we are...
10. ### A question about ionic bonding

I am sure this question will have been asked elsewhere, so please forgive me if it has. My question involves the transfer of electrons in ionic bonding. We are told that a metal will give up its outer most electron because by doing so it will gain the electron configuration of a noble gas...
11. ### Statistics Permutations and Combinations

Perok - that was what I was trying to do. I think I can see now that the answer to that question is 11P7 x 2 and then we subtract that from the answer to part a) Thanks
12. ### Statistics Permutations and Combinations

Thanks - sorry about the bold font. It just picked it up from the template - I should have gone back to change it. I actually found that the way I did it got a correct answer. I had made a mistake in my calculation for part a) I will try your method also. The make scheme just said 11P7 x 2...
13. ### Statistics Permutations and Combinations

Homework Statement The back row of a cinema has 12 seats, all of which are empty. A group of 8 people including Mary and Francis, sit in this row. Find the number of ways they can sit in these 12 seats if a) There are no restrictions b) Mary and France's do not sit in seats which are next to...
14. ### Probability Question -- probability that no member will receive more than one of the medals

So the first factor of 15/15 which you said is not really necessary is just saying that there is a 15/15 or probability of 1 that one of the 15 will get the first meddle - which is trivial. Thanks
15. ### Probability Question -- probability that no member will receive more than one of the medals

Thank you - I think you have explained it very well and in the simplest terms.
16. ### Probability Question -- probability that no member will receive more than one of the medals

Homework Statement There are 15 members of a maths club. There are 4 different medals to be randomly given to the members of the club. What is the probability that no member will receive more than one of the medals. Homework Equations Try to find the number of combinations where no member...
17. ### Estimates of Percentage Errors in Cosomology

Thanks - yes I am coming to the same conclusion - that what they are looking for is 5%. It seems to me though at really daft question - even possibly a fake question! Thanks for your help. It is very appreciated.
18. ### Estimates of Percentage Errors in Cosomology

The equation I have given is one I have found which is said to be the standard equation for confidence levels. $$\mu = real value$$ $$\bar X = average experimental value$$ n = sample number Z* = confidence level the funny sigma means standard deviation
19. ### Estimates of Percentage Errors in Cosomology

Thanks for looking at this. When they say as discrepant as ours they mean that our value of 1.15 versus the real value of 1.0 is considered discrepant. So they are asking what percentage of other observers who make the measurement are likely to come up with measurements that are as far away...
20. ### Estimates of Percentage Errors in Cosomology

Homework Statement Suppose that there is some hypothetical cosmological parameter called C, and suppose that the real value of C is C(real) = 1. Also suppose that everyone in the universe is able to measure C. Our value of C is C(ours) = 1.15 +/- .15 at confidence level 95 % Make an...
21. ### I Variations in the local value of Hubble constant

The value found for Ho by direct measurements of Type 1a supernovae is significantly higher than the value which is inferred by the Planck data. The suggestion is that what we are measuring is a local value for Ho which might be higher than the global value because we are in a cosmic void - as...
22. ### I Variations in the local value of Hubble constant

Not sure what you mean there Chronos. The Plank data seem to show a difference between the measured value of Ho and the value inferred from the CMB data. The former being about 1.3 times greater than the latter. One suggestion is that this discrepancy is due to us occupying a region of...
23. ### I Variations in the local value of Hubble constant

It is widely accepted that the expansion rate is accelerating but wouldn't a local underdensity give the same appearance in terms of the measured local value for Ho?
24. ### I Variations in the local value of Hubble constant

Thanks - I have read that paper. I am just looking whatever simple explanation of why a local underdensity would result in a high expansion rate. At the moment I have got that Ohmega m is inversely proportional to H squared but I am looking for a some kind of explanation for why this is the case.
25. ### I Variations in the local value of Hubble constant

I have been reading in various sources that the directly measured value of Ho is high than the global value inferred by precise measurements of the CMB. The local value seems to be around 73 km/s/Mpc where as the global value inferred, assuming a flat Lamda CDM model is about 67 km/s/Mpc...
26. ### I Cosmological Paramaters

Thanks. So it is accepted then the Planck data is the most accurate to date? The reason I ask this is because I had been wading through so much, which is why I was using the word accepted, meaning the it is not contested that the Planck data is the most accurate. Any way I will use this...
27. ### I Cosmological Paramaters

Cosmological paramaters in the cold dark matter Lamda concordance model. I have been trying to find the most uptodate values for Ho , Ohmega m and Ohmega Lamgad I was wondering if anyone knows where I might find the most reliably, widely accepted values for these values that are currently...
28. ### I Lyman Break drop-out technique

Hmmm - that's interesting. I need to do more research about this, but most of the papers I have come across just assume one already knows all about the technique and not how it actually works. Thanks
29. ### I Lyman Break drop-out technique

Hi I have a question which is about the lyman break drop-out technique for identifying high red shift galaxies. I am basically trying to understand how the technique works. My current understanding is that the drop-out technique makes use of the fact that very high red shift galaxies...
30. ### B Black hole mass and sigma (velocity dispersion)

I am pretty sure that orbital speed is not constant regardless of dark matter. Velocity dispersion is how much the velocities of different stars differ from one another. It shows up as Doppler shifts, blue and red, which mean invidiual spectral lines become broader. Supermassive blackholes...