Electromagnetic
Induction
Physics
La Cañada High School
Dr.E
• Electromagnetic Induction
• Motors/Generators
• Faraday’s Law
• Transformers
• Power Transmission
• Solenoids
• MRI
http://science.howstuffworks.com/motor4.htm
The Magnetic Field
• The electric field proves a useful concept to
explain the effects of charge at a distance
(explains how one charge knows another
charge is there).
• Stationary charges produce only an electric
field
• Moving charges (in flux) produce both an
electric field and a magnetic field
• Electromagnetic Induction
•Motors/Generators
http://science.howstuffworks.com/motor4.htm
• Faraday’s Law
• Transformers
• Power Transmission
• Solenoids
• MRI
Motors
• Motors work due to two major
principles
1. Opposite poles attract while
like poles repel
2. Current running through a
coiled wire creates a magnet
Theory
Mechanics
S
N
S
• Electromagnetic Induction
• Motors/Generators
http://science.howstuffworks.com/motor4.htm
•Faraday’s Law
• Transformers
• Power Transmission
• Solenoids
• MRI
Faraday’s Law of Induction
The voltage induced in a coil
is proportional to the number
of coils times the magnetic
flux (rate at which the
magnetic field changes)
Wrap Rule to find Magnetic Field
1. Wrap your fingers in the direction of the current
2. The magnetic field points in the direction of the thumb (to
the left)
3. Since the field lines leave the left end of solenoid, the left
end is the North pole
N
• An ammeter is connected in a circuit of a conducting
loop
• When a bar magnet is moved closer to, or farther from,
the loop, an electromotive force (emf) is induced the
loop
• The ammeter indicates currents in different directions
depending on the relative motion of magnet and loop
• When the magnet stops moving, the current returns to
zero as indicated by the ammeter
• Electromagnetic Induction
• Motors/Generators
• Faraday’s Law
http://science.howstuffworks.com/motor4.htm
•Transformers
• Power Transmission
• Solenoids
• MRI
Primary Voltage
Secondary Voltage
# of 1o turns
# of 2o turns
(Power IN)
(Power OUT)
(Voltage x Current)primary
(Voltage x Current)secondary
• Electromagnetic Induction
• Motors/Generators
• Faraday’s Law
• Transformers
http://science.howstuffworks.com/motor4.htm
•Power Transmission
• Solenoids
• MRI
Power Transmission
Plant
6000 V and 20Amps
Local and Homes
120 V and 100 Amps
Low Voltage Wires
2200 V and 54 Amps
High Voltage Wires in Town
120,000 V and 1 Amp
Ultra High Voltage
400,000 V and 0.3 Amps
• Electromagnetic Induction
• Motors/Generators
• Faraday’s Law
• Transformers
• Power Transmission
http://science.howstuffworks.com/motor4.htm
•Solenoids
• MRI
Solenoids
The magnetic field of a solenoid is essentially
identical to that of a bar magnet.
The big difference is that we can turn the solenoid
on and off ! It attracts/repels other permanent
magnets; it attracts ferromagnets, etc.
Solenoid Applications
Digital [on/off]:
– Doorbells
Magnet off  plunger held in place by spring
Magnet on  plunger expelled  strikes bell
– Power door locks
– Magnetic cranes
– Electronic Switch “relay”
Close switch
 current
 magnetic field pulls in plunger
 closes larger circuit
Advantage:
A small current can be used
to switch a much larger one
– Starter in washer/dryer, car
ignition, …
Solenoid Applications
Analog (deflection α I ):
– Variable A/C valves
– Speakers
Solenoids are everywhere!
In fact, a typical car has over 20 solenoids!
• Electromagnetic Induction
• Motors/Generators
• Faraday’s Law
• Transformers
• Power Transmission
• Solenoids
http://science.howstuffworks.com/motor4.htm
•MRI
Thanks to
MRI / NMR
If we “bathe” the protons in radio waves at a particular
frequency, the protons can flip back and forth.
If we detect this flipping  hydrogen!
The presence of other molecules can partially shield the
applied magnetic field, thus changing the resonant
frequency (“chemical shift”).
Looking at what the resonant frequency is  what molecules are
nearby.
If a strong magnetic field gradient is produced across the
sample, can look at individual slices, with ~millimeter
spatial resolution.
B
Small B
low freq.
Bigger B
high freq.
Signal at the right frequency only from this slice!
Bibliography
1. Magnetism: Examples of Magnetic Field Calculations, Innovations in Undergraduate
Physics Education at Illinois @ online.physics.uiuc.edu/courses/phys112/spring04/
Lectures/Lect15.ppt, 4/17/04