7.2 Magnetic Field Strength
Calculating Magnetic Field Strength
A moving charged particle that enters a magnetic field at any direction other than parallel
will experience a force, FB, that depends on the charge, Q, and speed, v, of the particle,
the strength of the magnetic field, B.
Unit for magnetic Field
FB
strength is the Tesla (T):
= constant = B
Qv
1N
1N
= 1T
=
FB = QvB
C•m/s
A•m
If the charged particle’s motion is not at right angles to
the magnetic field then the equation above must be
modified:
FB = QvB sin Ɵ
p. 274
7.2 Magnetic Field Strength
The Magnetic Field Strength of a Solenoid (part 1)
Andrè Ampere investigated the magnetic fields associated with solenoids. He
discovered how the magnetic field forms around these solenoids
The strength, B, of the magnetic field inside a solenoid depends on:
1) The number of a turns, N, per unit length, L, of solenoid
2) The current in the solenoid I.
NI
B α
L
p. 276 - 277
7.2 Magnetic Field Strength
The Magnetic Field Strength of a Solenoid (part 2)
Inserting a constant changes
the proportionality into an
equation.
B = constant x
NI
L
The constant of proportionality is given the name of permeability of free space.
permeability of free space:
B = µo
µo = 4π x 10-7 Tm/A
NI
L
N
B = µonI
Where: n =
L
(n = number of turns
per unit length)
p. 277
7.2 Magnetic Field Strength
Magnetic Force on a Current in a Wire (part 1)
A current carrying wire has a magnetic field. When this current carrying wire is
placed in another external magnetic field, the wire will experience a force due to the
interactions of the two magnetic fields.
The right hand rule is also applied here to determine
the force that exists on the current carrying wire.
Point your right thumb in the direction of conventional current. Point your
fingers straight out in the direction of the magnetic field. The palm of your right
hand will now be pushing in the direction of the force on the wire segment
carrying current in the magnetic field.
p. 278
7.2 Magnetic Field Strength
Magnetic Force on a Current in a Wire (part 2)
Remember: FB = QvB
FB = BQ(l/t)
And: v = d/t = l/t
And: I = Q/t So: Q = It
FB = BIt(l/t)
Therefore:
FB = BIl
If the wire is not perpendicular to the magnetic
field then the component of the magnetic field
perpendicular to the wire segment must be used.
FB = BIl sin Ɵ
p. 279
7.2 Magnetic Field Strength
Key Questions
In this section, you should understand how to solve the following key questions.
Page 276 – Quick Check #3
Page 277 – Quick Check #2 & 3
Page 282 – 283 – Review 7.2 #1,3,7,8, & 9