Question

Direction of induced emf can be found from
A) Lenz law
B) Laplace law
C) Fleming law
D) None of the above

Answer 1

Hint: When a moving charge passes through a magnetic field, an emf is induced in the charge. The direction of induced emf is perpendicular to the direction of magnetic field as well as the direction of velocity of the charge. This direction can be determined by right-hand rule.

Complete answer:
When a charge is made to move in a magnetic field, an emf is induced in the charge. The direction of this induced emf is perpendicular to both the direction of magnetic field and the direction of motion of the charge. We can easily determine the direction of induced emf with the help of Fleming’s right-hand rule.
Now, let us understand Fleming’s right-hand rule. The right hand is held in such a way that the thumb finger, index finger and the middle finger are mutually perpendicular to each other as shown in the following figure. Fleming’s right-hand rule suggests that: If the thumb finger is pointed in the direction of motion or velocity of the charge, and if the index finger is pointed in the direction of magnetic field, then, the middle finger represents the direction of induced emf of the charge. The figure given below explains the same clearly.

Therefore, the direction of induced emf is given by Fleming’s law .

The correct answer to be marked is option C.

Additional information:
When a moving charge encounters a magnetic field, an emf is induced in the charge. This is due to Faraday’s law of induction, which says that induced emf is equal to the rate of change of magnetic flux. Faraday’s law of induction is given by
$e=-\dfrac{d\Phi }{dt}$
where
$e$ is the induced emf
$\dfrac{d\Phi }{dt}$ is the rate of change of magnetic flux

Note:
Students need not get confused with Lenz’s law for this question. Lenz’s law suggests that the direction of induced emf opposes the change in magnetic flux. The negative sign in Faraday’s law can be related to this law. Lenz’s law gives the direction of induced emf with respect to the change in magnetic flux but Fleming’s law gives the direction of induced emf more accurately. But if the question talks about the change in magnetic flux, students may have to go with Lenz’s law. Students can easily rule out option B too, because Laplace law talks about the relationship between pressure and volume in spheres.