# Maxwell third equation and its derivation

**Statement**.(a) It states that,whenever magnetic flux linked with a circuit changes then induced electromotive force (emf) is set up in the circuit. This induced emf lasts so long as the change in magnetic flux continues.

(b) The magnitude of induced emf is equal to the rate of change of magnetic flux linked with the circuit.

Therefore induced emf= – dφ_{m}/dt

Where φ_{m}=∫**B.dS** (5)

Here negative sign is because of Lenz’s law which states that the induced emf set up a current in such a direction that the magnetic effect produced by it opposes the cause producing it.

**Also definition of emf states **that emf is the closed line integral of the non-conservative electric field generated by the battery.

That is emf=∫E.dL (6)

Comparing equations (5) and (6), we get

∫E.dL= – ∫_{s} d/d t B.dS (7)

Equation (7) is the **integral form of Maxwell’s third Equation** or Faraday’s law of electromagnetic induction.

Note: You can also read the discussion and derivation of Maxwell first and second equation.

**Differential form :**

Apply Stoke’s theorem to L.H.S. of equations (7) to change line integral to surface integral.

That is ∫ E.dL=∫( ∇ x E).dS

By substituting above equation in equation(7), we get

∫_{s} ( ∇ x E).dS = -∫ d/d t(B.dS)

As two surface integral are equal only when their integrands are equal.

Thus ∇ x E= – dB/ dt (8)

Equation (8) is the **Differential form of Maxwell’s third equation**.

I hope you have understood the significance and derivation of Maxwell’s third equation.

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