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John Parkinson John Parkinson ©©
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THE AURORA
The Aurora Borealis, the Northern Lights and the Aurora Australis, the Southern lights are phenomena
resulting from the interaction between Solar Wind and the Earth’s magnetic field.
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N S
Lines of magnetic flux, Φ
Magnetic flux density, B, measures the strength of the magnetic field.
B = number of lines of flux per square metre
AB
UNITS are Webers per square metre Wb m-2 or TESLAS (T)
B IS A VECTOR QUANTITY
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N S
Lines of magnetic flux, Φ
AB
A
Total magnetic flux through area A is given by:
Φ = B A webers, when the area is perpendicular to the field
Aθ
Φ = B A sinθ
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geographic north
geographic north
magnetic north
s
N
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1820Professor Oersted was demonstrating an experiment for students when he accidentally discovered that a compass needle moved when it was close to a wire connected to a battery.
Professor Hans Christian Oersted
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B
I
8
I
B
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RIGHT HAND GRIP RULE:
I
10
current outwards current inwards
r
B
r
IB
20
-
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length l
IN turns
In the middle
l
NIB 0
at the end l
NIB
20
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N S
Interaction between a magnet and the field due to a current
N S
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N S
F
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FLEMING’S LEFT HAND MOTOR RULE
FOREFINGER = FIELD
THUMB = MOTION
SECOND FINGER = CURRENT
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FLEMING’S LEFT HAND MOTOR RULE
FOREFINGER = FIELD
THUMB = MOTION
SECOND FINGER = CURRENT
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N S
I
B
F
F = B I lWhere l = length of conductor in the field
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S N
HAIR SPRINGS CONTROL MOVEMENT AND ALLOW CURRENT TO ENTER AND LEAVE THE COIL
RADIAL SOFT IRON POLE PIECES
COIL
PERMANENT MAGNET
MOVING COIL
METER
SOFT IRON CYLINDER
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S N+
+
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N
A
B
D
C
I
S
brushes
EVERY TIME THE COIL PASSES THROUGH THE VERTICAL POSITION, THE COMMUTATOR REVERSES THE CURENT IN THE COIL, IN ORDER TO
SUSTAIN THE ROTATION
DC MOTORC
A
B
SN
D
I
Split ring commutator
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LOUDSPEAKER
N
S
N
Solenoid on paper cone is forced to vibrate - left hand rule
magnet
S
N
N N
N
magnet
AC signalcoil
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Force experienced by a charged particle (charge Q) moving with a velocity, v in a magnetic field of Flux density, B
+Q
B
I
A
B
l
Particle travels l metres from A to B in t seconds
F
F = B I l
t
QI
t
BQlF
but vt
l
F = B Q v
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+ I
FF THE FORCE IS ALWAYS
PERPENDICULAR TO THE MOTION
THE PARTICLE MUST MOVE
ALONG AN ARC OF A CIRCLE
r
mvBQv
2
BQ
mvrarcofradius ,
+
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Frequency of Rotation, f
x x x x x x x x x x x xx x x x x x x x x x x xx 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
B inwards
+
r
mvBQv
2
rfT
rv
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r
mvBQ
r
frmBQ
2
m
BQf
2
The frequency is independent of the radius and the velocity
This is the principle of the CYCLOTRON
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High frequency square wave to accelerate charge across the gap
CYCLOTRON
+High frequency square wavem
BQf
2