Transcript Electromagnetic Forces and Fields

Lesson 2 : RULES for Magnetic Fields.
 The
shape of the magnetic field produced by
the moving charges in a wire is circular. It
grows weaker as one moves away from the
wire.
 What is the direction of the field around the
current carrying wire?
 For
the direction of the B around a current
carrying wire we let the thumb of the left
hand indicate the direction of electron
flow. The finger tips of the hand that circle
the wire indicate the direction of the B
field.
 Left
hand is
used for
negative
charges
 Right
for
positive charges
 The
flow of positive charge is referred to as
conventional current. Some textbooks use this and
talk in terms of right hand rules rather than left
hand rules.
 When
current
flows through
each loop of
wire in a coil a
magnetic field is
set up in such a
way that it
makes the coil
act like a single
bar magnet.
- the magnetic field around a
straight wire is very weak
- to make the field stronger, we can
bend the wire into many loops
 the
current goes in the same direction for each wire
 the magnetic field of each wire adds up to create a
much stronger field
 Fingers
curl
around the coil in
the direction of
the electron flow.
 Outstretched
thumb points to
the end of the coil
that acts as the
north end of a
fixed magnet.

Is the direction of the
current shown, e¯
flow or conventional
current?
 Need
to use right
hand to B field
direction as
shown:
 ∴ is conventional
current.
 Determine
position of each compass
A
solenoid can be made to be a much
stronger magnet by placing a ferromagnetic
material (usually steel) inside the coil.
 Electromagnets have many uses.
 Magnetizing ferromagnetic material
 Bells
 Relay switches
 Changes
current to another circuit
 Ampere
investigated the interaction
between two current-carrying wires parallel
to each other.
What is the effect on the wires?
 The
direction of the fields is different
therefore the fields attract each other. If the
field directions are the same they would
repel.
 Current
is the rate at which e¯ flow through a
wire.
 Units: C/s or ampere (A)
 Symbol: I
q
I
t
 The
B field around a  F = magnetic force
current carrying wire (N)
depends in part on  I = current (A) or
the current in the
(C/s)
wire.
 l = length
(perpendicular to B)
 B = magnetic field
m

strength in tesla (T)
F  Il B
 If
two wires, each
one meter long and
one meter apart,
carry current so that
the force between
the two wires is 2.00
x 10-7 N, the current
in each wire is one
ampere.
A
30 cm long wire is placed in a 0.50 T B field
that is directed out of the page. The e¯ flow is
to the right, with a current of 1.2 A. Determine
the force acting on the wire.
Fm  Il B
Fm  1.2 A 0.30m 0.50T
Fm  0.18 N
 What
is the
direction of
the force?