Motional EMF

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Any change in magnetic flux induces an emf opposing that change is the process known as induction. The motion is one of the major causes of the process of induction. For example, we can say that a magnet which has moved toward a coil induces an emf and a coil which has moved toward a magnet produces a similar emf. In this section, we will discuss motion in a magnetic field stationary relative to the planet Earth producing what is loosely known as motional emf. There is one situation where we can say there is a motion that generally occurs, called the Hall effect and has already been examined. The moving charges which are moving in a magnetic field experience the magnetic force denoted by F = qvB sinθ.

What is Motional EMF?

The charge which we are talking about in opposite directions and produces an emf = Bℓv. We can generally see that the Hall effect has applications which include measurements that are of symbols which are B and v. We will also notice now that the Hall effect is one aspect of the broader phenomenon of induction and we will conclude that motional emf can be used as a power source. Here we should consider the situation of a rod moving at a speed v along a pair of conducting rails which are separated by a distance denoted by symbol ℓ in a uniform magnetic field B. The rails which are stationary relative to B and are connected to a stationary resistor denoted by R.


Motional Electron Motive Force

The resistor generally could be anything from a light bulb to even a voltmeter. Let us consider the area enclosed by the moving rod, rails, and resistor. The letter B which we know is perpendicular to this area and we can say that the area is increasing as the rod moves. Thus, here we notice that the magnetic flux generally enclosed by the rails and the rod and resistor is increasing. When the term changes then generally an emf is induced according to Faraday’s law of induction.

Here again, we see that to find the magnitude of emf induced along the moving rod uses the law of Faraday of induction without the sign:

denoted as emf=NΔ/ΦΔ

Here and below also the term “emf” is the magnitude of the emf. In this equation which we have learnt the equation N = 1 and the flux denoted by Φ = BA cos θ. We have already seen a symbol denoted by letter or symbol θ = 0º and cos θ = 1 since B is perpendicular to A. Now the symbol ΔΦ = Δ(BA) = BΔA since B is uniform. We can see that the area swept out by the rod is ΔA = ℓΔx. 

Lenz’s Law

To find the direction of the induced field the direction of the current and the polarity of the induced emf we apply the law of Lenz’s The term Flux is increasing too since the area enclosed is increasing. Motional emf also occurs if the magnetic field that moves and the rod or other object is stationary relative to the planet Earth or we can say some observer. We have also seen an example of this in the situation where a moving magnet induces an emf in a stationary coil. It is the relative motion that is important. What is emerging in these observations that we have already seen is a connection between magnetic and electric fields. A moving magnetic field generally produces an electric field seen through its induced emf. We already have seen in our article a moving electric field which generally produces a magnetic field moving charge that generally implies moving electric field and moving charge which produces a magnetic field.

Calculating Motional Electron Motive Force

The emf of earth’s weak field magnetic is not ordinarily very large or we would notice voltage that along the rod of metal such as a screwdriver during ordinary motions. For example, we can say that a simple calculation of the motional emf of a 1 m rod that is moving at 3.0 m/s perpendicular to the planets earth’s field gives emf = Bℓv = (5.0 × 10−5 T)(1.0 m)(3.0 m/s) = 150 μV. 

We can say that there is a spectacular exception, however. In 1996 and 1992 attempts were made with the space shuttle to create large motions that have EMFs. A tethered Satellite which was to be let out on a length of 20 km of wire to create a 5 kV emf by moving at a speed orbital through the field of the planet Earth’s. To complete the circuit the stationary ionosphere was to supply a return path for the current to flow. The ionosphere rarefied and we can say it is the partially ionized atmosphere at orbital altitudes. It could be said to conduct because of the ionization. The ionosphere that generally serves the same function as the stationary rails and the connecting resistor without which there would not be a complete circuit.

FAQ (Frequently Asked Questions)

Q1. What is Motional EMF?

Ans: An emf induced by motion the relative to a magnetic field B is known as a motional emf, and is given by emf = Bℓv denoted as B,ℓ, and v perpendicular where we can say that symbol ℓ is the length of the object moving at speed v relative to the field.

Q2. How is Motional EMF Represented?

Ans: An emf induced by motion relative to a magnetic field is known as a motional emf. This is represented by the equation emf = LvB where L is the length of the object moving at speed denoted by symbol v relative to the strength of the magnetic field B.

Q3. What is self-induced EMF?

Ans: Self-induced emf is the e.m.f generally induced in the coil due to the change of flux produced by linking it with its own turns. This phenomenon which we have seen of the self-induced emf can be further understood by the following example provided above. 

Q4. Why is Induced EMF Called Back EMF?

Ans: We can determine the induced emf in a loop at any time interval rotating in this motion itself at a constant rate in a magnetic field. We can see that rotating coils that generally have an induced emf in motors. This is known back emf because it opposes the emf input to the motor.