Download Magnetic Field Lines

Magnetic Field Lines
 All pass from north to south
 Never cross each other
 Concentration of lines indicates relative strength of field
 Direction of field at a point is to the tangent of the lines.
Magnetic Field (strength): B = [T = tesla]
: looking in-to page from North to South (arrow tail - away),
: looking out-of page from South to North (arrow head - toward),
Magnetic Induction by a Current-Carrying Wire:
Induction: when a magnet causes magnetic properties in another
object.
 A magnetic field is induced by the electrons moving in a
current-carrying wire so that the field encircles the wire.
Right-Hand Grip Rule: Imagine that you grab the wire with your
right hand so that your thumb points in the direction of the current
(I), your fingers will then point in the direction of the magnetic
field lines (B) induced by the current.
N.B. I, is the conventional current, i.e., opposite to electron flow!
Solenoid: A coil of wire wound into a cylindrical shape.
Looking into one end of a solenoid; if a current
travels clockwise, field lines will go in-to the
solenoid (), making it the South Pole; if a
current travels anticlockwise, field lines go
out-of the solenoid (), making it the North Pole.
Magnetism and Electron Orbits: The orbits of electrons around
atoms can be considered to be currents inducing magnetic fields.
When many of these orbits are aligned, so are the micro-magnetic
fields, creating a larger overall magnetic field around the entire
substance. Some substances are temporary magnets because the
electron structure of its atoms is not protected from disturbances
and the orbits are eventually randomly oriented.
Magnetism and Charged Particles: Other charged particles (e.g.,
alpha and beta) are also deflected by a magnetic field. To
determine the direction of the deflection, the Right-Hand Grip rule
can be used, where the current (I) is in the direction of motion of
positive charges and opposite to the direction of negative charges.
Magnetic Force on an electric current:
F= nBIl(sin)
F: magnetic force on an electric current,
n: number of current-carrying wires,
B: magnetic field (strength) caused by magnets around the
wires,
I: current travelling through the wires,
l: length of the wires,
: angle between the current and the magnetic field,
N.B. sin(90o)= 1, sin(0o)= 0, .: F┴ = nBIl, F║= 0,
.: B = [T = Nm-1A-1]
i.e., Magnetic field (strength) is the force exerted by a magnet on a
current-carrying wire per meter of current-carrying wire.
Right-Hand Slap Rule: With your flat, open right hand, when your
fingers point in the direction of the magnetic field (B), and your
thumb points in the direction of the current (I), then the magnetic
force (F) is in the direction of you palm: ‘slap’.
Moving Electric Charge: A moving electron can be considered as
a current travelling in the opposite direction, so that the RightHand Slap rule applies to free electrons moving through a
magnetic field (your thumb should point in the direction opposite
to the of motion of the electron).
Electromagnetic Meters and Motors:
Magnetic force on a loop of current-carrying wire: A current
carrying loop in a magnetic field experiences a force in one
direction on one side of the loop and an equal force in the opposite
direction on the other side of the loop. These forces are in balance
at the centre of mass of the loop so no translational motion is
experienced by the loop. However, the forces are on opposite sides
of the loop so there is a net rotational force (torque). In this way,
electrical energy is transformed into rotational kinetic energy.
Commutator: In DC Motors, a device called a commutator is used
to flip the direction of the current through the loop every half turn
so that the rotational forces (torques) on the loop are always in the
same direction: relative to the magnets the current doesn’t change!

= nBIlsin), .: F┴ = nBIl = Fmax, F║= 0
= Fmin,