Hemal Pansuriya Posted August 14, 2017 Share Posted August 14, 2017 can u tell me the reason and what have i ignored ? Link to comment Share on other sites More sharing options...
Strange Posted August 14, 2017 Share Posted August 14, 2017 How about re-reading the thread with an open mind (not with the assumption that you are right and the rest of the world has made some fundamental error). Link to comment Share on other sites More sharing options...
studiot Posted August 14, 2017 Share Posted August 14, 2017 43 minutes ago, Hemal Pansuriya said: The coil will see the magnetic field change When signal reaches from magnet to coil. YES As soon as coil feels magnetic field change it produces current. YES This current will generate magnetic field that will resist the motion of magnet. YES But it is not instantaneous, YES as signal from coil to magnet will take time to travel. YES During this delay period we can produce arbitrarily large EMF in coil by keeping number of turns of coil arbitrarily large NO the EMF still obeys Farady's Law ie it is still proportional to the rate of change of magnetice field the coild actually sees. The number of turns of the coil is a red herring because the coils takes up space so the change takes time to propagate as it covers this space. ie turns sufficiently distant from the propagating change in the magnetic field do not affect the coil until they reach it. and hence power output of coil arbitrarily large. NO We will not have to give any extra input energy to magnet during this delay period. YES You can refer to my PDF , in which i have described the problem more clearly. NO That your break forum policy by going offsite. magnet.pdf Link to comment Share on other sites More sharing options...
studiot Posted August 14, 2017 Share Posted August 14, 2017 7 minutes ago, Hemal Pansuriya said: can u tell me the reason and what have i ignored ? Yes you jumped in before I finished writing a full and proper answer to the first time you wrote a full and proper post. Link to comment Share on other sites More sharing options...
Hemal Pansuriya Posted August 14, 2017 Share Posted August 14, 2017 19 minutes ago, studiot said: During this delay period we can produce arbitrarily large EMF in coil by keeping number of turns of coil arbitrarily large NO the EMF still obeys Farady's Law ie it is still proportional to the rate of change of magnetice field the coild actually sees. The number of turns of the coil is a red herring because the coils takes up space so the change takes time to propagate as it covers this space. ie turns sufficiently distant from the propagating change in the magnetic field do not affect the coil until they reach it. You can increase the current in the coil other way also, like by decreasing resistance in the circuit assuming fixed EMF during delay period. Since, the power output is = V*I , you can increase power output by decreasing resistance value in circuit. Link to comment Share on other sites More sharing options...
studiot Posted August 14, 2017 Share Posted August 14, 2017 (edited) 22 minutes ago, Hemal Pansuriya said: You can increase the current in the coil other way also, like by decreasing resistance in the circuit assuming fixed EMF during delay period. Since, the power output is = V*I , you can increase power output by decreasing resistance value in circuit. You will find a good analysis, with simple calculations, of all these things, including conservation of energy, power dissipated for given coil resistance, what happens if the coil is a superconductor, work done moving the magnet etc etc in Fundamentals of Electricity and Magnetism by Kip. pages 289 to 291 in my second edition Kip is an old standard but it really is a very good book. You should read it. Edited August 14, 2017 by studiot Link to comment Share on other sites More sharing options...
swansont Posted August 14, 2017 Share Posted August 14, 2017 36 minutes ago, Hemal Pansuriya said: You can increase the current in the coil other way also, like by decreasing resistance in the circuit assuming fixed EMF during delay period. Since, the power output is = V*I , you can increase power output by decreasing resistance value in circuit. There's self-inductance which needs to be accounted for. Your impedance will not go to zero. Link to comment Share on other sites More sharing options...
Hemal Pansuriya Posted August 14, 2017 Share Posted August 14, 2017 20 minutes ago, swansont said: There's self-inductance which needs to be accounted for. Your impedance will not go to zero. If the change in magnetic field is constant , than EMF produced is constant. Than current is also constant ( dI/dt = 0). So, there is no self inductance in this case. The resistance value can be put as low such that we get output energy V*I > input energy . Link to comment Share on other sites More sharing options...
studiot Posted August 14, 2017 Share Posted August 14, 2017 (edited) 6 minutes ago, Hemal Pansuriya said: If the change in magnetic field is constant , than EMF produced is constant. Than current is also constant ( dI/dt = 0). So, there is no self inductance in this case. The resistance value can be put as low such that we get output energy V*I > input energy . What exactly does the underlined bit mean? You realise that the change in the magnetic field means that the number of lines threading the coil must change to generate an EMF? The coil doesn't care which lines thread, it is the total number that count. Edited August 14, 2017 by studiot Link to comment Share on other sites More sharing options...
Hemal Pansuriya Posted August 14, 2017 Share Posted August 14, 2017 15 minutes ago, studiot said: What exactly does the underlined bit mean? You realise that the change in the magnetic field means that the number of lines threading the coil must change to generate an EMF? The coil doesn't care which lines thread, it is the total number that count. If the magnet has constant velocity towards coil, than the magnetic flux change in coil is constant ( d∅/dt = constant ). Hence the EMF and current are constant. Link to comment Share on other sites More sharing options...
studiot Posted August 14, 2017 Share Posted August 14, 2017 You didn't answer my question. What is the magnetic flux change in the coil? Link to comment Share on other sites More sharing options...
swansont Posted August 14, 2017 Share Posted August 14, 2017 4 minutes ago, Hemal Pansuriya said: If the magnet has constant velocity towards coil, than the magnetic flux change in coil is constant ( d∅/dt = constant ). Hence the EMF and current are constant. It's not obvious to me how a constant speed will give a constant change in flux from a source that drops off as 1/r^3 Also, you can't get from 0 current to some value without there being a changing current. If the coil is at some speed v, it will slow down when it encounters a changing flux. It feels a force. Link to comment Share on other sites More sharing options...
Hemal Pansuriya Posted August 14, 2017 Share Posted August 14, 2017 4 minutes ago, studiot said: You didn't answer my question. What is the magnetic flux change in the coil? The magnetic flux is integral of B.da . As magnet moves towards coil with constant velocity, the B increases at the coil and so does the magnetic flux. But due to constant velocity of magnet, d∅/dt in coil is constant . Link to comment Share on other sites More sharing options...
Hemal Pansuriya Posted August 14, 2017 Share Posted August 14, 2017 9 minutes ago, swansont said: It's not obvious to me how a constant speed will give a constant change in flux from a source that drops off as 1/r^3 Also, you can't get from 0 current to some value without there being a changing current. If the coil is at some speed v, it will slow down when it encounters a changing flux. It feels a force. If the velocity of magnet is constant, than change in magnetic flux is also constant. To get from 0 current to some value is due to changing magnetic flux, after that if that change in magnetic flux is constant than the EMF will also constant . The coil is not moving in this problem . only magnet is moving. Link to comment Share on other sites More sharing options...
swansont Posted August 14, 2017 Share Posted August 14, 2017 2 hours ago, Hemal Pansuriya said: If the velocity of magnet is constant, than change in magnetic flux is also constant. To get from 0 current to some value is due to changing magnetic flux, after that if that change in magnetic flux is constant than the EMF will also constant . The coil is not moving in this problem . only magnet is moving. The field is not constant, it drops off as 1/r^3. It's a dipole. The coil will move, since you are exerting a force, owing to the changing field. Here's a rather dramatic demonstration of this effect https://www.youtube.com/watch?v=Pl7KyVIJ1iE 1 Link to comment Share on other sites More sharing options...
studiot Posted August 14, 2017 Share Posted August 14, 2017 (edited) 2 hours ago, swansont said: The field is not constant, it drops off as 1/r^3. It's a dipole. The coil will move, since you are exerting a force, owing to the changing field. Here's a rather dramatic demonstration of this effect https://www.youtube.com/watch?v=Pl7KyVIJ1iE This is known as the Elihu Thomson experiment. I glad to see modern science is still offereing this as the health and safety brigade caused it to be dropped for some years. +1 https://www.google.co.uk/search?site=&source=hp&q=elihu+thomson+apparatus&oq=elihu+thom&gs_l=psy-ab.1.1.0l4.2011.7146.0.9661.10.10.0.0.0.0.103.863.8j2.10.0....0...1.1.64.psy-ab..0.10.854...0i131k1.0NR_98ttqsA Edited August 14, 2017 by studiot Link to comment Share on other sites More sharing options...
Hemal Pansuriya Posted October 21, 2017 Share Posted October 21, 2017 Coil can be very large and massive such that coil will move negligible.And if voltage induced in coil is varying with time than we can use transformer to draw more power by following way :use coil as voltage source and connect it to transformer's primary coil suppose voltage produced in coil is V, then step up this voltage by factor of 1000 ( or arbitrarily large ). Suppose resistance connected in secondary is R, so current in secondary winding will be I =1000V/R. And in primary winding current will be drawn 1000*I ( because IpVp=IsVs ). So now the power output of the coil will be V*(1000*I).So now the power output of the coil will be V*(1000*I) . We have drawn 1000 times more power than previously. Link to comment Share on other sites More sharing options...
swansont Posted October 21, 2017 Share Posted October 21, 2017 You will not violate conservation of energy Link to comment Share on other sites More sharing options...
Hemal Pansuriya Posted October 21, 2017 Share Posted October 21, 2017 19 minutes ago, swansont said: You will not violate conservation of energy please just look at what i have written. Link to comment Share on other sites More sharing options...
John Cuthber Posted October 22, 2017 Share Posted October 22, 2017 15 hours ago, Hemal Pansuriya said: please just look at what i have written. I looked at it. You are wrong if you think you will violate the conservation of energy. This will remain true, no matter what you write and who looks at it. Link to comment Share on other sites More sharing options...
Hemal Pansuriya Posted October 22, 2017 Share Posted October 22, 2017 3 hours ago, John Cuthber said: I looked at it. You are wrong if you think you will violate the conservation of energy. This will remain true, no matter what you write and who looks at it. If you have looked at it. Please tell me why can't we draw large power from coil with help of transformer. Link to comment Share on other sites More sharing options...
Strange Posted October 22, 2017 Share Posted October 22, 2017 26 minutes ago, Hemal Pansuriya said: If you have looked at it. Please tell me why can't we draw large power from coil with help of transformer. People have been telling you this for 3 months. You refuse to listen. Basically, you claim to violate conservation of energy, therefore you are wrong and need to learn why. Link to comment Share on other sites More sharing options...
John Cuthber Posted October 22, 2017 Share Posted October 22, 2017 29 minutes ago, Hemal Pansuriya said: If you have looked at it. Please tell me why can't we draw large power from coil with help of transformer. Because it is a mathematical impossibility. https://en.wikipedia.org/wiki/Noether's_theorem (I don't expect you to understand this, just recognise it for what it is: the proof that you are wrong.) Link to comment Share on other sites More sharing options...
studiot Posted October 22, 2017 Share Posted October 22, 2017 33 minutes ago, Hemal Pansuriya said: If you have looked at it. Please tell me why can't we draw large power from coil with help of transformer. You can't draw power that isn't there. That's called - yes you guessed it - conservation of energy. Now please tell us why you haven't commented on the Elihu Thomson stuff swansont introduced and I gave you more about. That really is the fun part of basic electromagnetics. Link to comment Share on other sites More sharing options...
swansont Posted October 22, 2017 Share Posted October 22, 2017 50 minutes ago, Hemal Pansuriya said: If you have looked at it. Please tell me why can't we draw large power from coil with help of transformer. Transformers don't violate conservation of energy. You trade current for voltage, one direction or the other. But the power is constant. P=IV If you attach at transformer to a coil then the load changes. You've assumed that the only impedance is resistive. Link to comment Share on other sites More sharing options...
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