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Second Semester 2014
Magnetic Fields
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4.2.1 define magnetic field B or magnetic flux density B (and
its unit the tesla) in terms of the force on a current
carrying conductor;
4.2.2 state and apply Fleming’s left-hand rule to determine the
direction of force on a current carrying conductor placed
at right angles to a magnetic field;
4.2.3 select and use the equations BIL F= and θ sin BIL F= ;
4.2.4 select and use the equations for the motion of a charged
particle:
4.2.5 analyse the motion of charged particles in both electric
and magnetic fields;
4.2.6 sketch the magnetic field lines for a long straight currentcarrying wire, a long solenoid, a bar magnet and the
Earth.
2
Fleming’s LHR
Aims:
•To know that there is a force on a charged particle when
it moves in a magnetic field as long as its motion is not
parallel to the field.
•To recall that a force is exerted on a current-carrying
wire in a magnetic field, and, how this effect is applied in
loudspeakers.
•To predict the direction of the resulting force when a
wire carries a current perpendicular to a magnetic field
(Flemming’s LHR).
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Magnetic force
•When a current
passes through a
piece of foil a
magnetic field is
created around the
strip.
•The Permanent
horseshoe magnet
repels the foil
upwards.
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Reverse the current
•If the direction of
the electric current
is reversed the
magnetic field acts
in the opposite
direction.
•The permanent
horseshoe magnet
attracts the foil
downwards.
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Flemming’s apparatus
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Flemming’s apparatus
•The d.c. electric current passes through one
iron bar, across the moveable axle and back
through the other iron bar.
•The electric current passing through the axle
creates a magnetic field.
•The permanent magnet will either attract of
repel the axle causing it to move.
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Fleming’s Left Hand Rule
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Get your hands ready
•We are now going to look at the interaction of the
current in a wire with a magnetic field.
•We know that the two will either attract or repel when
a current is present.
•Fleming’s Left Hand Rule (LHR) will let us work out
the direction of force on a wire near a magnet.
•This will allow us to understand how a motor and
electric speaker work.
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LHR
•Grab a pen and get ready to write on your hand.
•The second finger of your left hand represents the
direction of the electric current, draw a ‘C’ on this finger.
•The first finger of your left hand represents the direction
of the magnetic field, draw a ‘F’ on this finger.
•Your left thumb represents the direction of thrust or force
on the wire, draw a ‘Th’ on your thumb.
•Now point these three so that they are at right angles to
each other!
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To use Fleming’s left-hand rule, hold the thumb and the first
two fingers of your left hand at right angles to each other.
direction of
force
(thumb)
direction of
magnetic field
(first finger)
direction of
current
(second finger)
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We can use the ‘Left Hand Rule’ to see
which way the wire moves
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LHR example
Which way will the wire move?
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Direction of the Force on a Wire
=?
Force on a Wire
Moving particles
•Remember that an electric current represents a flow of
positive particles from positive to negative.
•If a proton was moving through a magnetic field you
could use Flemming’s left hand rule to work out how
its motion would be changed.
•For negative particles, such as an electron, you can
still use the left hand rule but the direction of
force/thrust will need to be reversed.
•Any particle moving along a magnetic field line will
not be affected by the field.
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How to increase the turning effect
on the coil?
• Increase the current
• Increase a stronger magnet
• Increasing the number of turns on the coil
• Increasing the area of the coil in the field.
Practical motors
• Several sets of coils are used, each set at a
different angle and with its own pair of
commutator.
• Each coils contain hundreds of turns of wire
and are wound on a core call an armature.
• The pole pieces are curved to create a radial
magnetic field.
Summary – Flemming’s LHR
•When a proton, electron or current carrying wire are in
a magnetic field they will experience a force.
•Flemming’s left hand rule can be used to find the
direction of the force. Thumb = Force/thrust. First
finger = direction of magnetic field. Second finger =
direction of electric current.
•In a speaker the magnetic field from the current in the
wire either attracts or repels the permanent magnets, the
speaker moves in and out creating a sound wave.
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