Transcript electromagnetism-100512041524-phpapp02

Effects of electric current
An electric current that flows in a conductor has
a number of effects:
1.
HEATING The friction caused by the current
causes the conductor to heat up. The
greater the current the more heat is
generated.
2.
MAGNETIC EFFECT - A magnetic field is
generated around any conductor when an
electric current flows through it.
Magnetic Effect
A magnetic field is generated around any conductor
when an electric current flows through it.
Electric current
Magnetic
field
Wire with current
coming towards
you
Right Hand Wire
Rule>>>
Wire with current
going away from
you
x
Right Hand Wire Rule
Magnetic Effect
A magnetic field is generated around any conductor
when an electric current flows through it.
Electric current
Wire with current
coming towards
you
Wire with current
going away from
you
x
SOLENOID
A coil generates a very concentrated (strong)
magnetic field in its center.
Increasing the
Electric current number of coils
strengthens the
magnetic field.
N
x
The Right Hand Rule
can also be applied
to a solenoid!
x
S
x
INSIDE THE COIL THE FIELD GOES FROM SOUTH TO NORTH!!!
Right Hand Rule - Solenoid
Solenoid Field

Web Applet Demo>>

Note field same as bar magnet
Inside S  N!!!!!
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Electromagnetic Induction
-
An electrical conductor that is accelerated
through a magnetic field will experience an
INDUCED electrical current according to
Fleming's Right Hand Rule.
Thrust Thumb
Field
Forefinger
Second
Current
N
+
S
Thrust
Faraday’s Law
The size of the induced current is directly proportional to the
rate of change of the magnetic flux linkage.
Electric Field Strength
+
Magnetic field Strength (B) measured
in ……………… (………)

Total number of field lines is
called ………………….. (…….)
measured in ……………….
(……)
 = ……….



 = ………………….. (………)
B = ……………………. (……)
A = ………………. through
which flux passes
-
(…...)
N
(…
….)
S
Thrust (…..)
= no of
………. in
that area.
1 ………….. is the field ……………. when a current of …….. A flows
through a conductor and it experiences a ………….. of …N per
meter of length.
Electric Field Strength
+
Magnetic field Strength (B) measured
in Tesla’s (T)

Total number of field lines is
called magnetic flux ()
measured in Webers (Wb)
 = BA



 = magnetic flux (Wb)
B = magnetic field strength (T)
A = area through which flux
passes
(1A)
(1T)
N
S
Thrust (1N)
= no
lines in
that
area
1 Tesla is the field strength when a current of 1 A flows through a
conductor and it experiences a force of 1N per meter of length.
Faraday’s Law
The induced emf (….) is directly proportional to the
…………. of change of …………………..

  N
t
S
N
The induced emf can be increased by:
• Increasing the …………………
• Using ………………………
• Decreasing the …………. – moving
………..
The size of the induced ……………… is (also) directly
proportional to the ………………………. of the
……………………………………...
Faraday’s Law
The induced emf () is directly proportional to the
rate of change of flux linkage.

  N
t
S
N
The induced emf can be increased by:
• Increasing the number of coils
• Using stronger magnets
• Decreasing the time – moving faster
The size of the induced current is (also) directly proportional to
the rate of change of the magnetic flux linkage.
Electric Guitar
Lenz’s Law

The induced field …….. (………………) is
induced in such a way as to ………………
the ……… causing it.
Lenz’s Law

S
N
S
N
N
S
N
S
The induced field B (S N) is induced in
such a way as to oppose the action
causing it.
THE MOTOR EFFECT
-
+
A current carrying conductor
in a magnetic field
experiences a …………….
The direction is given by
………………………………..
……….
rule.
Second
……….
Thumb
N
S
Forefinger
………..
Magnetic fields ……………………..
Fields in opposite direction
they …………………………
………………… - resulting in a
…………………….. force.
N
X
S
THE MOTOR EFFECT
-
+
A current carrying conductor
in a magnetic field
experiences a force.
The direction is given by
Fleming's Left Hand Motor
Current
rule.
Second
Thrust
Thumb
N
Forefinger
Field
S
Force
Magnetic fields strengthened!
Fields in opposite direction
they cancel out/become
weakened - resulting in a
downward force.
N
X
X
S
THE ELECTRIC MOTOR
current
B
C
N
S
-
A
+
D
THE ELECTRIC MOTOR
current
B
C
Current
Field
Thrust
S
N
A
+
D
-
THE ELECTRIC MOTOR
C
B
current
BB
CC
Current
C
Field
ThrustD
N
B
-
AA
+
D
DD
A
A
S
THE ELECTRIC MOTOR
Mutual Induction
~


MUTUAL INDUCTION is defined as the changing
electric …………………… producing a changing
…………………….. field which can produce a
changing ………………….. in another conductor.
This is the basic principle of the
………………………
Mutual Induction
~


MUTUAL INDUCTION is defined as the changing
electric current producing a changing magnetic
field which can produce a changing current in
another conductor.
This is the basic principle of the
TRANSFORMER
Transformers

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Coils of insulated conducting wire are
wound around a ring of iron constructed of
thin isolated laminations or sheets.
The laminations minimize eddy currents in
the iron.
Eddy currents are circulatory currents
induced in the metal by the changing
magnetic field. These currents produce an
undesirable by-product—heat in the iron.
Energy loss in a transformer can be
reduced by using thinner laminations, very
“soft” (low-carbon) iron and wire with a
larger cross section, or by winding the
primary and secondary circuits with
conductors that have very low resistance.
Transformers used to transmit and
distribute power are commonly 98 to 99
percent efficient.
While eddy currents are a problem in
transformers, they are useful for heating
objects in a vacuum.
Transformers
Transformers


Ideally – all magnetic flux from primary coil links with
secondary coil.
The number of turns of wire is directly proportional to the
voltage in the coil.
Np
Vp
— = —
Ns
Vs
How many turns of wire would you need in a transformer that has
1000 turns in the primary and needs to step down the voltage
from 220V to 9V?
Transformers
Ideally – all magnetic flux from primary coil links with
secondary coil.

From Faraday’s Law:
“The size of the induced current is directly proportional to the
rate of change of the magnetic flux linkage.”
 Therefore:
work done by primary = energy gained by secondary
Since W = QV and Q = It .: W = VIt

Wp = Ws
VpIpt = VsIst
VpIp = VsIs
P p = Ps
Calculate the current in the
secondary coil of the 220V/9V
transformer if the current in the
primary was 1.5A