Superconductivity defies Ohms Law?

Question

Ohms Law states that
1 ohm = 1V/1A
(V= volt, A=ampere)
But in superconductivity, resistance is zero. But then, how can V be zero, if current is passed through a superconductor? Whatever other numbers V and A are made, the division cannot be equal to zero. Then, are superconductors an exception to ohm's law?

Answer 1

YES, superconductors r an exception to ohms law.

read ohms law again, it is ONLY defined for conductors. Various other substances/devices exist which dont obey ohms law eg thyristor, diodes, transistors, semiconductors, vacuum triodes etc

Answer 2

Ohm's law rarely applies exactly in real life, because resistance usually depends on current. But it is very close for what are called Ohmic conductors.

Contrary to what some have said, the resistance of a superconductor is precisely zero. It has been measured to be lower than about 10^-34 ohms. What this implies is that a current can flow without an associated voltage. This is because the voltage represents the energy the electrons lose by flowing - and in a superconductor they lose none.

It means that you need a current source (like a photocell) rather than a voltage source (like a battery or the mains) to get a current to flow.

Typically, what is done with a superconductor is that a weak link is created in the circuit. This is a bit of superconductor that is kept warm, and so does not have 0 resistance. This allows the current source to drive a current through the superconductor. Once the current is established, the weak link is cooled and the whole circuit superconducts. The current source can then be turned off. The current will flow undiminished forever.

Superconducting magnets working like this hold their field indefinitely provided they are kept cool - its how NMR scanners work.

Answer 3

Actually, there is an upper limit to the current density that a superconductor can carry before the superconductivity effect breaks down. What really happens is that the voltage is forced to be zero. In other words, you CANNOT apply a voltage to a superconductor. (Well, if you try, you will temporarily succeed in applying a small voltage, because the current will be increasing and inductive effects will give you a small voltage. Once the current is established at a constant value, even this small voltage will go away.)

Essentially, connecting a superconductor across a battery, generator, or anything else, will simply short it out completely and force the voltage to zero. Then all the energy will be dissipated across the internal resistance of the battery, generator, or whatever, which will go off with a loud bang and/or burst of flame.

Answer 4

V is zero because there is no energy loss when current flows through a superconductor. And if V= 0, then R= 0. That's why it's called a superconductor; no resistance.

Wikipedia has a very complete explanation under "superconductivity."

Answer 5

Ohm’s law states that V= IR. The energy lost across resistance is given by I^2 R. This energy is lost as heat energy.

In Super conductors R = 0. That implies no loss of energy as heat.

Once, a conductor begins to conduct, a current flows through the circuit, even after the supply is removed.

Consider a close circuit and charges flow across the circuit; since there is no resistance there is no loss of energy and hence there is nonstop flow of charge.

For this no emf is necessary. Only if there is some resistance there will be a voltage drop and hence there will be loss of energy. And one needs an emf to send the charges against the resistance.

In practice as there is small resistance, the current once started will lost for months.

Theoretically there is no end for the passage of charges in super conductors.

Using V= IR, we must take I as constant and when R= 0, V= 0.

Answer 6

Nothing in your question suggests any inconsistency with Ohm's law. Zero is a perfectly possible value for voltage.
If R=0, V=0. Current I can be any value. Ohm's Law states that 1 ohm = 1V/1A, true. It also states that 0 ohm = 0V/any value of current, right?

Answer 7

To add to all the answers. Also remember most voltage sources have an internal resistance too. So appying finite voltage to a super conductor will not result in Infinite or extremely high current.

Answer 8

You're misreading Ohm's law.

Ohm's law states that V=I*R, where V is voltage, I is current, and R is the resistance of the object.

Answer 9

If you investigate you will see that resistance never actually reaches the value of zero, a small number yes, but not zero.

Answer 10

resistance also depends on temperature. right now, in most cases, superconductivity occurs at really low temps. thus the low resistance (near zero).