Magnetism and
Induction
NCEA AS 3.6
Text Chapters: 15,16,17
Electromagnetism

Fields are formed around current
carrying wires
Solenoids
Fields are
formed in
solenoids or
coils.
Induction
If a wire is moved
through a magnetic
field then a voltage
can be induced
across the ends the
wire.
Induction
If the wire is
connected to a circuit
then current will flow.
The direction of
induced current is
determined by a right
hand rule.
Right Hand Slap Rule
v=direction of wire
movement
B=direction of
magnetic field lines
F= force on a positive
charge (ie direction of
current flow)
Induction
The size of this induced voltage is given by:




V=BvL
(B=mag field strength,
v=velocity of movement,
L=length of wire in field)
This is known as Faraday’s Law
Induction
The direction of the induced current is
such that it creates an opposing force on
the motion that is causing it.

This is known as Lenz’s Law
Induced
current I
Opposing
Force F=BIL
Direction of
movement
Induction
Induced voltage/current can be made
larger if:




The mag field is stronger
The wire is longer
The movement is faster
(Solenoid has an iron core)
Induction
Induction can also occur if it is the magnetic
field that is moved, rather than the wire.
Magnetic Flux
The magnetic field in a
circuit is measured as
magnetic flux Φ
Φ= BxA


B = mag field strength
A = area perpendicular to
field
The unit for flux is the
Weber Wb
Magnetic Flux
A useful analogy is
using a net to catch
whitebait in a
stream….
If you don’t hold the
net straight up and
down, you don’t catch
many whitebait!!
Faraday’s Law
According to Faraday’s Law, V=BvL
The area of the loop in the field is zero…..
L
Speed v
Faraday’s Law
Some time t later, A
has changed by
 ΔA = (vt x L)
ΔΦ = B x ΔA
ΔΦ = B x v x L x t
But V=BvL
So ΔΦ = V x t
L
A
v
Faraday’s Law (again)
Another way to look at Faraday’s Law is that
the induced voltage in a circuit is determined
by the rate of change of flux
The negative
sign is a
reminder of
Lenz’s Law

V 
t
Generators
Rather than sliding a loop through a field, it is
easier to spin it.
This is how a generator works
Generators
When the coil is
horizontal, the
induced current is
maximum, as the
coil is cutting
across the field
lines at right
angles as it
moves.
Generators
When the coil is
vertical, the
induced current
is zero, as the
coil is moving
parallel to the
magnetic field
lines
Generators
If we start timing from when the coil is vertical,
then at t=0, Φ= BxA
B
N
A
S
Generators
If the coil rotates with speed ω, then after
time t the coil will have turned through
angle θ = ωt
The flux will now be Φ= Bcosωt x A
B
θ
N
S
A
Generators
Faraday’s Law says:
d
V 
dt
d ( BA cos t )
V 
dt
V  BA  sin t
For a coil of N turns:
V  BAN sin t
Generators
The formula for alternating generator voltage is
often written as:
V  Vmax sin t
Where Vmax=BANω
This produces a voltage-time graph that looks
like a sine curve
NB. Similarities to SHM!!
Generators
To generate A.C, slip rings are used…
Generators
To generate D.C, split rings are used.