Université de Strasbourg Électromagnétisme
Licence MPA, L2–S4 Année 2012–2013
TD 4
Champs magnétiques dans la matière
Exercice 4.1
Calculez le couple exercé sur la boucle carrée
de la Fig. 1 dû à la boucle circulaire (supposez
que ra,b).
6.1.
MAGNETIZATION
259
Problem 6.1 Calculate the torque exerted on the square loop shown in Fig. 6.6, due to the
circular loop (assume ris much larger than aor h).
If
the square loop
is
free to rotate, what
will its equilibrium orientation be?
r
Figure 6.6
Problem 6.2 Starting from the Lorentz force law, in the form
of
Eq. 5.16, show that the torque
on any steady current distribution (not just asquare loop)
in
auniform field B
is
mx
B.
Problem 6.3 Find the force
of
attraction between two magnetic dipoles,
mt
and
m2,
oriented
as
shown
in
Fig. 6.7, adistance rapart, (a) using Eq. 6.2, and (b) using Eg. 6.3.
z
I
E
r
Figure 6.7
x
E
Figure 6.8
y
Problem 6.4 Derive Eq. 6.3. [Here's one way to do
it:
Assume the dipole is
an
infinitesimal
square,
of
side E(if it's not, chop it up into squares, and apply the argument to each one).
Choose axes as shown
in
Fig. 6.8, and calculate F=IJ(dl xB) along each
of
the four sides.
Expand Bin aTaylor
series-on
the right side, for instance,
oBI
B=B(D.
E.
z) B(D.
D.
z) +
E-
.
oy
(O,O,z)
For amore sophisticated method, see Prob. 6.22.]
Fig. 1: c
D. J. Griffiths
Exercice 4.2
En partant de l’expression de la force de Lorentz F=RI(dl×B), montrez que le couple
exercé par n’importe quelle distribution de courant stationnaire (pas seulement une boucle
carrée comme dans le cours) dans un champ magnétique uniforme Bs’écrit N=m×B.
Exercice 4.3
Démontrez que la force exercée par un champ magnétique Bsur
un dipôle magnétique s’écrit F=∇(m·B). Pour ce faire, suppo-
sez que le dipôle est une boucle carrée infinitésimale, de côté e(cf.
Fig. 3), et calculez F=IRdl×Ben effectuant un développement
de Taylor de Bpour epetit.
6.1.
MAGNETIZATION
259
Problem 6.1 Calculate the torque exerted on the square loop shown in Fig. 6.6, due to the
circular loop (assume ris much larger than aor h).
If
the square loop
is
free to rotate, what
will its equilibrium orientation be?
r
Figure 6.6
Problem 6.2 Starting from the Lorentz force law, in the form
of
Eq. 5.16, show that the torque
on any steady current distribution (not just asquare loop)
in
auniform field B
is
mx
B.
Problem 6.3 Find the force
of
attraction between two magnetic dipoles,
mt
and
m2,
oriented
as
shown
in
Fig. 6.7, adistance rapart, (a) using Eq. 6.2, and (b) using Eg. 6.3.
z
I
E
r
Figure 6.7
x
E
Figure 6.8
y
Problem 6.4 Derive Eq. 6.3. [Here's one way to do
it:
Assume the dipole is
an
infinitesimal
square,
of
side E(if it's not, chop it up into squares, and apply the argument to each one).
Choose axes as shown
in
Fig. 6.8, and calculate F=IJ(dl xB) along each
of
the four sides.
Expand Bin aTaylor
series-on
the right side, for instance,
oBI
B=B(D.
E.
z) B(D.
D.
z) +
E-
.
oy
(O,O,z)
For amore sophisticated method, see Prob. 6.22.]
Fig. 2: c
D. J. Griffiths
Exercice 4.4
Déterminez la force d’attraction entre deux di-
pôles magnétiques m1et m2séparés de la dis-
tance ret orientés comme sur la Fig. 3.
6.1.
MAGNETIZATION
259
Problem 6.1 Calculate the torque exerted on the square loop shown in Fig. 6.6, due to the
circular loop (assume ris much larger than aor h).
If
the square loop
is
free to rotate, what
will its equilibrium orientation be?
r
Figure 6.6
Problem 6.2 Starting from the Lorentz force law, in the form
of
Eq. 5.16, show that the torque
on any steady current distribution (not just asquare loop)
in
auniform field B
is
mx
B.
Problem 6.3 Find the force
of
attraction between two magnetic dipoles,
mt
and
m2,
oriented
as
shown
in
Fig. 6.7, adistance rapart, (a) using Eq. 6.2, and (b) using Eg. 6.3.
z
I
E
r
Figure 6.7
x
E
Figure 6.8
y
Problem 6.4 Derive Eq. 6.3. [Here's one way to do
it:
Assume the dipole is
an
infinitesimal
square,
of
side E(if it's not, chop it up into squares, and apply the argument to each one).
Choose axes as shown
in
Fig. 6.8, and calculate F=IJ(dl xB) along each
of
the four sides.
Expand Bin aTaylor
series-on
the right side, for instance,
oBI
B=B(D.
E.
z) B(D.
D.
z) +
E-
.
oy
(O,O,z)
For amore sophisticated method, see Prob. 6.22.]
Fig. 3: c
D. J. Griffiths
Exercice 4.5
On considère une plaque épaisse perpendiculaire au plan yz et s’étendant de x=−aàx=a.
La plaque est parcourue par une densité de courant uniforme J=J0ˆz, et un dipôle magnétique
m=m0ˆ
xse situe à l’origine.
(a) Déterminez la force exercée sur le dipôle.
(b) Même question si m=m0ˆy.