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## Physics library

### Course: Physics library>Unit 13

Lesson 4: Magnetic flux and Faraday's law

Example calculating induced current based on change in flux.

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• here we had a resistance of 2 ohms but what if it has no resistance i.e. superconductor? • If we had a resistance of 0 , there would be a potential drop of zero .
So if the potential drop is zero (assuming a circuit with a battery and of course 0 resistance) then the battery would work forever .
Going to the basic definition of potential difference
Work done per unit charge
The work done to bring any amount of charge is 0 so the current would be infinite .
But this is practically not possible because
R=P l/a
Where p(rho) is the resistivity of the material
l is the length
A is the area of cross section
So for 0 resistance the resistivity(p) should be 0 which is not possible because all materials have electrons in them ( that is obvious for flow of current ) and the repulsion of electrons cause resistance and all materials have positive charge ( protons ) that attract the electrons and don't give away their electrons and so it creates an obstruction for the electrons .
Or for 0 resistance the length should be zero that is obviously not possible.
Hope u understood . :)
• In video we calculated the voltage in ring. And this voltage is between which points? If we would have ring with small resistance and resistor, then the voltage would be between ends of resistor. What if we have ring with constant "density" of resistance? • At , I do not understand how you find the direction the current flows. IF you do the right-hand rule for both of the possibilities, don't you get the same result. With each current, at one point, it creates magnetic field in the direction of magnetic flux and opposite direction of magnetic flux. If someone could explain this, it would be great. Thanks! • This may be a point of confusion for others too!

With each current, try the right hand grip rule, only with your fist open and your palm in the direction of the field.
You will find that you can trace the loop with your thumb, all the while your palm in the same direction. This means the current will also move in the same direction throughout.

Can't understand? Search "Right Hand Palm Rule".
• What is the difference between emf, voltage, and potential difference? Or do they all mean the same?  • Hello Shrey,

Nothing in life is free!

Lenz's law tells us that the induced current will be opposite to that which caused it.

It may help to consider what would happen if the sign was reversed. In this fictitious environment the induced current would add to the magnetic field causing more current and so on forever increasing. You could say this is a violation of the laws of thermodynamics...

Regards,

APD
• Why do we take only the negative sign to represent direction in the faraday-lenz's law? Why not positive? • Because, the induced e.m.f always opposes any change in magnetic flux associated with the coil.
It happens because, when anyone pushed the north pole of a bar magnet towards the coil, at the upper face of the coil, the current is anti-clockwise producing north pole on that face. So, there acts a repulsive force between the north pole of the magnet and north pole of the coil. As a result, we need to do more work to move the magnet and to change the flux.
The same logic is applied for south pole.
• for any given problem is the direction of the current always clockwise since the voltage is negative ? • At , when we are try to figure out the direction of the induced current are we referring to the conventional current or the actual electron flow? This becomes a point of confusion in a number of my homework problems for my AP class, do physicists generally stick to conventional current for all problems having to do with inducing a current?   