What is Motional EMF?
The emf induced in a conducting rod when it moves in a region of magnetic field is known as motional emf. Either showing translator motion or rotatory motions, whenever a conducting rod is moving in a magnetic field, a potential difference is established at the ends of the rod. Directed from the high potential end to the low potential, an electric field appears along with the length of the rod. This EMF is called motional emf.
Motional emf is calculated by using the formula: E=vBl.
Where l denotes the length of the rod, B denotes the magnetic field, and the velocity of the rod perpendicular to the length of the rod is denoted by v.
Following are the factors on which the motional emf depends:
- Magnetic field
- The magnitude of length of rod
- Velocity of rod
The concept of Lorentz force acting on free charge carriers of the conductor is used to understand the concept of motional emf. Consider any random charge q in the conductor AB. If the rod moves with a constant speed v, then in the presence of magnetic field B, the charge also starts moving with the same speed v.
Therefore, the Lorentz force on this charge is given by:
F = qvB
In moving the charge from A to B, the work done can be given by,
W = QBvl
As we know that the work done per unit charge is known as electromotive force.
So, the final equation can be written as: ∈ = wq=Bvl
Important Questions on Motional EMF
1) What is motional emf? State the factors on which the motional emf depends.
The emf induced in a conducting rod when it moves in a region of magnetic field is known as motional emf.
Motional emf is calculated by using the formula: E=vBl.
Where l denotes the length of the rod, B denotes the magnetic field, and the velocity of the rod perpendicular to the length of the rod is denoted by v.
Following are the factors on which the motional emf depends:
- Magnetic field
- The magnitude of length of rod
- Velocity of rod
2) EMF stands for _____, according to Faraday’s law.
a) Electromagnetic friction
b) Electromagnetic field
c) Electromotive friction
d) Electromotive force
Correct Option: (d)
Explanation: Due to the change in the flux linkage of the circuit, the force in any closed circuit is called the electromotive force EMF. This phenomenon is named Faraday’s law.
3) Induced emf and motional emf are exactly the same. Is it true or false?
a) True
b) False
Correct Option: (b)
Explanation: The above statement is false because both of the emfs are not exactly the same. Motional emf comes into play when the changing emf is due to some kind of motion of a conductor in a magnetic field. On the other hand, an induced emf is created whenever there’s a changing magnetic flux through a loop; this phenomenon is known as Faraday’s Law.
4) If an accelerating magnet is moved inside a current-carrying coil, then what will happen to the current inside it?
a) Current increases
b) Current decreases
c) Current remains constant
d) Reverse current flows
Answer: (a)
Explanation: Since an emf is induced by the change in the magnetic field, there should be a current when there is an emf. Due to this reason, the current inside the coil increases when the magnet is moved inside it.
5) The total number of magnetic field lines passing through an area is known as _____.
a) Electric potential
b) EMF
c) Magnetic flux
d) Magnetic flux density
Answer: (c)
Explanation: Magnetic flux is the number of magnetic field lines passing through an area.
6) What is emf?
a) Flux
b) Voltage
c) Force
d) Current
Correct Option: (b)
Explanation: Electromotive force is basically a voltage developed by any source of electrical energy; it is not a force.
7) Faraday’s law of electromagnetic induction says that an emf is induced in a conductor whenever the conductor is _____.
a) Placed perpendicular to the magnetic field
b) Lies in the magnetic field
c) Intersects the magnetic lines of flux
d) Moving parallel to the magnetic field
Correct Option: (c)
Explanation: According to Faraday’s laws of electromagnetic induction, an emf is induced whenever the conductor in a magnetic field intersects the magnetic lines of flux.
8) Which among the following rules is used to determine the direction of induced emf?
a) Right-hand thumb rule
b) Fleming’s right-hand rule
c) Fleming’s left-hand rule
d) None of the above
Correct Option: (b)
Explanation: According to Fleming’s right-hand rule, magnetic flux is pointed by the index finger, motion of the conductor is pointed by the thumb, and induced emf is pointed by the middle finger.
9) What is the relation between the direction of motion of the conductor and the direction of induced emf?
a) Parallel to each other
b) Equal to each other
c) They are not related
d) Mutually perpendicular
Correct Option: (d)
Explanation: Fleming’s right-hand rule says that the motion of the conductor, the induced emf and the magnetic flux are mutually perpendicular to each other.
10) What will be the dissipation factor, when the displacement current density and conduction current density are the same?
a) Infinity
b) Zero
c) Maximum
d) Unity
Correct Option: (d)
Explanation: The dissipation factor is the ratio of conduction current density to displacement current density. So, the dissipation factor will be unity when both quantities are the same.
Practice Questions
1) What is electromotive force?
2) What is magnetic flux density?
3) State Fleming’s right-hand rule.
4) What is induced EMF?
5) What is electric potential?
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