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P.50
Magnetic effect of a current
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North pole and South Pole
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Like poles repel
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Unlike poles attract
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N
S
Compass needle shows the direction
of magnetic field lines
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Magnetic
field
Magnetic
field lines
N
Magnetic field line
ends at south pole
Magnitude &
direction of
magnetic force
acting on a
north pole
Magnetic field line
starts at north pole
No intersection of magnetic field lines
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N
N
Neutral point
Magnetic forces
are balanced
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I=0A
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Current flows
Magnet affects
compasses
Magnetic field
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Right-hand grip rule
Thumb
Direction of current
Fingers
Direction of magnetic
field lines
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Current flows
out of paper
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Current flows
into paper
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Electric
current
Magnetic
field
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Right-hand
solenoid rule
Fingers
Thumb
Direction of current
Direction of magnetic
field lines
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Iron filings
show the
magnetic
field pattern
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N
S
Solenoid
A long coil with
many turns
Produces the same magnetic
field pattern as a bar magnet
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X : South pole, Y : North pole
(a) Current in wire will produce
Magnetic field to affect the compass
(b) To stop the light from the
bulb by a black box
X : iron, Y : Magnet, Z : Al
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Each part of the wire produces the
magnetic field lines in the same direction
in the central part of the circle
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Methods to strengthen B-field
Increase current
Winding more turns per unit length
Insert a soft iron bar inside the solenoid
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Follow Earth’s magnetic field
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To align the Earth’s
magnetic field
SEarth
NEarth
North pole is produced to
attract South pole of
magnet
N
S
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Cancellation
Into paper
Out of paper
Cancellation
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I
I
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Electromagnet
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Direction of field lines can be Solenoid with
current
reversed or alternate
Electromagnet
On or off
any time
Electromagnet
can be
strengthened
Larger current
Increase number
of turns of coil
Use soft iron core
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Horse-shoe
magnet
Ferromagnetic material
Substance attracted by magnet
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Electromagnetic
Lifter
Pick up heavy
iron objects
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Maglev
Giant electromagnet repels magnetism
induced on the track
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Switch is on
Current flows
Circuit
is broken
Electromagnet
attract metal plate
Hammer strikes
on the bell
Hammer
goes back
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Diaphragm
inwards
Compression
Resistance
decreases
Diaphragm
inwards
Current increases
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I
When the current
is larger than a value,
the electromagnet can
attract the iron arm
t
f = 50 Hz
Diode allows the current flowing
in one direction only
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S
N
S
S
N
N
Reed
switch
Reeds touch one another
when magnetised
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Reed relay
Bar magnet is replaced
by an electromagnet
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Switch is closed
Current flows
through the
solenoid
Main circuit is
completed
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A large
current
flows
A small
current flows
Relay
A small current
is used to control
a large current
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Current flows
Attract the contact
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Reed switch is on when
the door is opened
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Reed switch
is off when
the door is
closed
Neutral
point
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Induced North pole is induced.
N
Iron bar is attracted downwards
S
N
M : South pole
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Iron bar returns to the original
position
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Induced South pole is induced.
S
Iron bar is attracted downwards
N
S
M : North pole
2 dots within 1 cycle
100 dots in 1 second
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The soft iron bar is magnetized to
attract the arm and hit the bell
The arm will be attracted
permanently.
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Increase no. of turns per unit length
Increase e.m.f. of cell
The soft iron is
magnetized to attract the
steel ball for a while
To avoid the sparks when
contacts touch each other
Current flows through
the circuit, magnetic
field is produced and
induced poles are
produced on the
contacts. They attract
each other and
conduct electricity.
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7a) Attract downwards
7b) No.
7c) Same current ->
Greater attractive force
Increase no. of turns
per unit length
Weaker spring to have
greater extensions
under same amount of
force
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Yes
Yes
No
Reduce R ->increase I
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N
S