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Does time-varying magnetic field induce time varying-electric field?

Physics Asked by I am the hope of the Universe on December 19, 2020

As we all know that Faraday’s law states that the EMF is also given by the rate of change of the magnetic flux:
$$text{emf} = -N frac{dPhi}{dt}$$
So if we are applying a time-varying magnetic field(let $dB/dt =$ constant) on a stationary conducting coil then induced electric field across the coil work as a driving force to induce a current in that coil. According to the above formula, induced emf in a coil will be constant if $dB/dt =$ constant, But if the induced electric field is time-varying then induced emf also be time-varying? isn’t it?
What I want to say is that I learned somewhere in the past that Statement: "A time-varying electric field can not exist without a corresponding time-varying magnetic field and vice versa", But According to Faraday’s law, a linear time-varying magnetic field induces a static electric field, So, is that mean the above statement is wrong?
Or in other words,
Understand with 3 statements written below-

(1) Linearly time-varying electric field {i.e. $dE/dt =$ constant} is capable of inducing static Magnetic field only (not capable of inducing dynamic magnetic field).

(2) Linearly time-varying magnetic field {i.e. $dB/dt =$ constant} is capable of inducing a static electric field only (not capable of inducing dynamic electric field)

(3) "A time-varying electric field can not exist without a corresponding time-varying magnetic field and vice versa"
So, statements (1) and (2) can be understood and verified by

Faraday-Maxwell equation $$oint Ecdot dl = – frac{dPhi}{dt}$$ Where $Phi =$ magnetic flux, verifies the statement (2), and

Ampere-Maxwell equation $$oint B.ds = mu_0I + mu_0epsilon_0 frac{dPhi}{dt}$$ Where $Phi=$ electric flux , verifies the statement (1).
But if statement (3) is Correct then it violates the other two,
Please tell me, about the validation of the 3rd statement.

2 Answers

None of the three claims are correct.

  1. A dynamic electric field can obviously exist without $frac{dmathbf{B}}{dt}$ being non-zero. In fact, it can exist without even $mathbf{B}$ being non-zero. The Faraday-Maxwell equation only implies that the curl of the electric field would be zero without a magnetic field. A dynamic electric field can exist without a magnetic field if the current density is non-zero as can be seen by the Ampere-Maxwell equation. For an explicit counter-example, see this post and Section $18.2$ from the link therein.

  2. A dynamic magnetic field can obviously exist without $frac{dmathbf{E}}{dt}$ being non-zero. A dynamic magnetic field simply requires the curl of the electric field to be non-zero, as can be seen by the Faraday-Maxwell equation.

  3. The third claim is doubly incorrect for it's simply the intersection of the first two claims.

Correct answer by Dvij D.C. on December 19, 2020

It depends on how magnetic field $B$ or magnetic flux $Phi$ varies with time $t$ i.e. linearly varying or non-linearly varying with time $t$

Case-1: If the magnetic field $B$ is varying linearly with time i.e. $B=at+b$ (Assuming area of coil $A$ is constant with time $t$) then $$frac{dPhi}{dt}=frac{d(Bcdot A)}{dt}=Afrac{dB}{dt}=aA=text{constant}implies text{emf}=text{constant}$$ Thus a magnetic field varying linearly with time $t$ induces a constant electric field $E$ as the induced emf is constant.
Case-2: If the magnetic field $B$ is varying non-linearly with time say $B=at^2+bt+c$ (it may also be a sinusoidal function $B=asin(omega t)$ of time $t$) then $$frac{dPhi}{dt}=frac{d(Bcdot A)}{dt}=Afrac{dB}{dt}=A(2at+b)ne text{constant}implies text{emf}ne text{constant}$$ Thus a magnetic field varying non-linearly with time $t$ will induce a time-varying electric field $E$ as the induced emf is time-varying i.e. $text{emf}=f(t)$.

Answered by Harish Chandra Rajpoot on December 19, 2020

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