Question

Which of the following laws gives the direction of induced emf?

• A. Lenz's law
• B. Fleming's left-hand rule
• C. Ohm's law
• D. Faraday's law of electromagnetic induction

Answer 1

The correct option is A

Lenz's law

Explanation for the correct option: option A. Lenz's law:

1. Lenz’s law states that the polarity of induced emf is such that it tends to produce a current which opposes the change in magnetic flux that produced it.
2. Lenz's law predicts the direction of an e.m.f. caused by a changing magnetic field through a wire loop or coil.
3. To establish current flow, the external source of the electromagnetic field must perform some work to overcome this opposition.
4. Mathematically, it can be expressed as $E=-N\frac{d\varphi }{dt}$, where $d\varphi$is the change in magnetic flux, $dt$ is the change in time, $N$ is the number of loops, and $E$ is the induced emf.
5. The negative sign in the formula represents this effect. Thus, the negative sign indicates that the direction of the induced emf and the change in the direction of magnetic fields have opposite signs.
6. Electromagnetic brakes and induction cooktops use Lenz's law. It is also used in electric generators and alternating current generators.

An illustrative diagram to show the direction of induced emf is given below.

Hence, option (A) is correct.

Explanation for the incorrect options:

1. According to Fleming's left-hand rule, if we place our thumb, forefinger, and middle finger of the left-hand perpendicular to each other, the thumb points toward the magnetic force, the forefinger points towards the magnetic field, and the middle finger points towards the current direction.
2. Ohm’s law states that the voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions and temperatures remain constant.
3. Faraday’s laws of electromagnetic induction consist of two laws. The first law describes the induction of emf in a conductor and the second law quantifies the emf produced in the conductor.
4. Here, options (B), (C), and (D) are incorrect.

Hence, option (A) is correct.