Transcript Slide 1

Electricity and Magnetism
Electromagnetic Induction
Mr D. Patterson
Outcomes
• describe and apply the concepts of magnetic
flux and magnetic induction—this will include
applying the relationships:
• describe the production of an induced emf by
the relative motion of a straight conductor in a
magnetic field—this will include applying the
relationship:
• An electric motor
generator
usesuses
electricity
motiontotoproduce
produce
movement
electric current
Simple example
Explanation
• Electric charges cutting across
magnetic field lines will feel a
force
• The conductor must be
moving perpendicular to the
magnetic field
V
Emf
• The separation of charge
is similar to a battery
• An EMF is produced!
V
• No current however as
the “battery” is not
connected to a circuit
Current
• If there is a complete circuit the EMF
will force a current to flow.
V
• This process of generating an electric
current using a magnetic field is called
electromagnetic induction.
• “An EMF is induced in the conductor”
• “The induced current flows through an
external circuit”
Formula
• The EMF induced in a conductor:
•
•
•
•
ε is the EMF (V)
l is the length of the conductor (m)
v is the velocity of the conductor (ms-1)
B is the magnetic flux density (T)
Magnetic Flux
• The magnetic flux is a measure
of the total magnetic field in a
given area
is the magnetic flux (Wb)
B is the magnetic flux density (T)
A is the perpendicular area (m2)
Faraday’s and Lenz’s Law
• Faraday’s Law:
• The induced EMF in a single coil is equal to the
negative rate of change of magnetic flux
through that coil
There are multiple ways to Induce an EMF
Lenz’s Law
• Lenz’s Law:
• An induced current will create a magnetic field
which opposes the change in magnetic field that
created the current
• This is the reason for the negative sign in the
formula
Using the right hand rule with Lenz’s Law
• For a straight conductor: Swap the direction
of the magnetic field to find the induced
current
V
Using the right hand rule with Lenz’s Law
• For a coil: Create a magnetic field inside the
coil which is opposite to the changing
magnetic field
• OR create a magnet inside the coil
whose poles will resist the motion
of the external magnet
Using the right hand rule with Lenz’s Law
• If a magnet is stationary in (or near) the coil,
there is no change in flux, so no induced EMF
or current
Using the right hand rule with Lenz’s Law
• For a coil: Create a magnetic field inside the
coil which is opposite to the changing
magnetic field
• OR create a magnet inside the coil
whose poles will resist the motion
of the external magnet
Size of the induced magnetic field depends on size of the
induced current
N
N
Small
resistance
N
Large
resistance
No resistor
(Infinite resistance)
Example Problem
• A square coil is free to rotate about its central axis
and is perpendicular with an external magnetic
field. The coil rotates 90 deg until it is parallel with
the field in 12.0 ms. The external magnetic field
has a flux density of 850 mT.
+
A
-
• a) What is the initial magnetic flux passing through
the coil?
• b) What is the final magnetic flux passing through
the coil?
• c) What is the average EMF produced from the
rotation?
• d) Will the reading on the ammeter be positive or
negative?
8.50 cm