PHY_10_64_V1_P_Faraday law of induction

10.4.2.1 – to explain the occurrence of electromotive force when changing the magnetic flux;
10.4.2.2 – to explain the Lentz's law;

Lesson Objectives

Course of the lesson

Just Do and Observe!

Faraday’s Law of Induction; Lenz’s Law

The induced emf in a wire loop is proportional to the rate of change of magnetic flux through the loop.
Magnetic flux:

(on formula sheet)

Unit of magnetic flux: weber, Wb.
1 Wb = 1 T·m2

This drawing shows the variables in the flux equation:

Faraday’s Law of Induction; Lenz’s Law

Faraday’s law of induction:

[1 loop]
(on formula sheet)

[N loops]

Faraday’s Law of Induction; Lenz’s Law

The minus sign gives the direction of the induced Emf:
An induced Emf always gives rise to a current whose magnetic field opposes the original change in flux (Lenz’s Law).

Faraday’s Law of Induction; Lenz’s Law

Induced EMF

Therefore, a changing magnetic field induces an Emf.
(Faraday’s experiment used a magnetic field that was changing because the current producing it was changing;
the previous graphic shows a magnetic field that is changing because the magnet is moving.)

Magnetic flux will change if the area of the loop changes:

Faraday’s Law of Induction; Lenz’s Law

Magnetic flux will change if the angle between the loop and the field changes:

Faraday’s Law of Induction; Lenz’s Law

Problem Solving: Lenz’s Law
Determine whether the magnetic flux is increasing, decreasing, or unchanged.
The magnetic field due to the induced current points in the opposite direction to the original field if the flux is increasing; in the same direction if it is decreasing; and is zero if the flux is not changing.
Use the right-hand rule to determine the direction of the current.
Remember that the external field and the field due to the induced current are different.

Faraday’s Law of Induction; Lenz’s Law