Download Motion Along a Straight Line at Constant

Book Reference : Pages 106-107
1.
To recap the nature of the magnetic field
around a bar magnet & the Earth
2.
To understand the nature of the magnetic
field around a current carrying wire
3.
To be able to calculate the magnitude &
direction of the force on a current carrying
wire in a magnetic field
Definition : A magnetic field is a force field which
surrounds either a magnet or a wire carrying an electric
current and will act upon, without contact, another
magnet or current carrying wire
Plotting Compass
Like the other fields we have studied we represent
magnetic fields diagrammatically using field lines or
lines of magnetic flux
We name the ends of a magnet “the poles”. (North and
& South). More correctly they should be referred to as
the “North seeking pole” and “South seeking pole”
Like poles repel
each other
Unlike poles
attract each
other
The arrows on a magnetic field line represent the path
which a “tiny free north pole” would take
The Earth has a
magnetic field just
like a giant
magnet.
The geographic
North pole has a
South magnetic
pole associated
with it (Since a
north seeking pole
of a magnet will
point towards it)
Earth Dipole
Watch terminology!!!
At the geographic
North pole there is a
magnetic pole which
we can refer to as
“Magnetic North”.
However it is a South
pole!
Watch the field lines!
Wires carrying a magnetic current produce a magnetic
field
“Maxwell’s corkscrew rule” can be used to establish the
direction of this field. Note the current direction is the
direction of “conventional current” positive to negative
A current carrying wire, with its associated magnetic
field will experience a “motor effect” if placed (at a nonzero angle) in a magnetic field
The force is perpendicular to both
the current & the magnetic field
Experimentally, it can be shown that the size of the force
due to the motor effect is related to the following :
1.
2.
3.
4.
The strength of the current
The strength of the magnetic field
The length of the wire
The angle between the field lines & current
In terms of angles, the force is greatest when the current
is perpendicular to the field and zero when parallel to
the field
The relationship between field, current and force can
best be remembered using “Fleming's left hand rule”
First finger (Field), seCond Finger (Current), Thumb
(moTion)
Expressing the observations as proportionalities :
Force  Current (F  I)
Force  length of wire (F  l) (that is a lower case L)
F  Il
For a given magnetic field we can turn this into an
equation where
F = BIl
(assuming current is perpendicular to field)
Where B is the magnetic flux density and is the force per unit
length, per unit current (Nm-1A-1) but given the unit of Tesla (T)
(Note we can introduce a sin  term to the above equation to consider angle
but it is beyond our spec
A straight horizontal wire of length 5m is in a uniform
magnetic field which has a magnetic flux density of
120mT. The wire is perpendicular to the field lines which
act due North. When the wire conducts a current of 14A
from East to West calculate the magnitude and direction
of the force on the wire
Using F = BIl
F = 120 x 10-3 x 14 x 5
F = 8.4N
Field due
North
Vertically downwards
Current East
to West
a) 2.4x10-2N West, b) 4.5A east to west, c) 0.2T vertically down, d)
8.0x10-3 N due south