This document discusses Faraday's law of induction and how changing magnetic flux induces an electromotive force (emf) in a closed loop. It explains that the emf produced is directly proportional to the rate of change of magnetic flux through any surface bounded by the loop. The magnetic flux can be changed by changing the magnetic field strength, the area of the coil, or rotating the coil to change the area. Faraday's experiments showed that relative motion between a magnet and coil induces a current in the coil, and changing the direction of motion reverses the direction of the induced current.

1. HSC Physics
9.3 Motors and Generators
Faraday’s Law

2. Flux refers to the number of
field lines passing through
any given area under
investigation
B has a greater magnetic
field strength than A
A has greater flux than B
Φ = BA
Where:
B is magnetic field strength
or flux density
A is the area through which
the lines of flux are passing

3. These field lines into the
page represent a uniform
magnetic field strength
because they are evenly
spaced.
The flux can vary
depending on the area that
is defined.
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X

4. Faraday’s Experiment
Relative motion between a
magnet and a coil induces a
current or emf (voltage) in
the coil.
Changing the direction of
relative motion changes the
direction of the induced
current or emf.
The faster the relative
motion the higher the
induced current or emf.

5. Faraday’s Law
Changing flux through a
closed loop induces a
current or emf (voltage).
The greater the rate of
change of flux, the greater
the current or emf.
The electromotive force
(emf) produced along a
closed path is directly
proportional to the rate of
change of magnetic flux
through any surface
bounded by the path.
𝑒𝑚𝑓 ∝ −𝑁
𝑑Φ
𝑑𝑡
𝑒𝑚𝑓 = −𝑁
𝑑Φ
𝑑𝑡
= −𝑁
ΔΦ
Δ𝑡
= −𝑁
Δ𝐵𝐴
Δ𝑡

6. Changing the Flux
The flux can be changed by:
1. Changing the magnetic
flux density,
2. Changing the
dimensions of the coil
to change the area, and
3. Changing the area by
rotating the coil.
1 2
3

7. Moving a Conductor
If the circuit is not closed,
an emf is produced, but no
current.
Use right hand palm rule
where:
• the thumb is direction of
movement of the
conductor,
• the fingers represent the
magnetic field lines, and
• the force on the charges
is out of the palm of the
hand.