Chapter 29 - Magnetic Fields
1. The force on a charge moving through a magnetic field is F = qv × B.
(a) A charge traveling parallel to the magnetic field does not experience a force and
continues unaffected.
(b) A charge passing perpendicular to a uniform magnetic field experiences a force
and travels in a circle of radius
r=
mv
.
qB
(c) A charge passing at an other angle through a uniform magnetic field travels in
a helix.
2. The Hall effect can differentiate the signs of the charge carriers that comprise the
current. The sign of the Hall voltage is the signature of the signs of the charge
carriers.
× B.
3. The force on a current carrying wire is F = iL
4. A current carrying loop acts like a permanent magnet and has a magnetic dipole
The direction of µ is given by the right-hand rule.
moment µ = NiA.
(a) When the magnetic moment is placed in an uniform magnetic field, it experiences
and the moment attempts to line up with the field.
a torque given by τ = µ × B
(b) The potential energy of the magnetic moment is given by U = −µ · B.
5. When we compare electric and magnetic forces we find a lot of differences.
(a) Electric forces are parallel to the electric fields. Magnetic forces are perpendicular
to the magnetic fields.
(b) Electric forces can do work and, thereby, change the kinetic energy of a charge.
Magnetic forces never do any work. The kinetic energy is constant.
(c) A charge passing through a uniform electric field travels in a parabolic trajectory.
In a uniform magnetic field a charge travels in a helical path which simplifies to
a circle when the velocity is perpendicular to the magnetic field.
Chapter 30 - Magnetic Fields due to Currents
1. Magnetic fields are created by currents. Fields can be calculated using the Biot-Savart
law:
= µ0 i ds × r .
dB
4π r 3
Often a a complicated problem can be solved by using the superposition of simpler
elements and adding the magnetic fields due to these elements.
2. Ampere’s law is
· ds = µ0 i
B
enclosed .
It is necessary to determine the signs of the enclosed currents. To do this curl the
fingers of your right hand around the Amperian loop, with your fingers pointing in the
direction of integration. A current passing through the loop in the general direction
of your outstretched thumb is assigned a plus sign, and a current moving generally in
the opposite direction is assigned a minus sign.
3. There are different magnetic fields for different geometries.
(a) A long straight wire
B=
µ0 i
.
2πr
B=
µ0 iφ
.
4πR
(b) At the center of circular arc φ
(c) Along the axis of a circular loop
B=
µ0 IR2
.
2(z 2 + R2 )3/2
(d) A solenoid
B = µ0 ni.
(e) A toroid
B=
µ0 Ni
.
2πr
(f) A magnetic dipole
= µ0 µ .
B
2π z 3
4. Parallel wires carrying currents in the same direction attract each other, whereas
parallel wires carrying currents in opposite directions repel each other. The magnitude
of the force on a length L of either wire is
Fab =
µ0 Lia ib
.
2πd
Chapter 31 - Induction and Inductance
· dA.
1. The magnetic flux is defined ΦB = B
2. Faraday’s law states the magnitude of the emf induced in a conducting loop is equal
to the rate at which the magnetic flux ΦB changes with time
E=
dΦB
.
dt
3. Lenz’s law says that an induced current has a direction such that the magnetic field
due to the induced current opposes the change in the magnetic flux that induces the
current
dΦB
E =−
.
dt
4. A changing magnetic field produces an electric field
· ds = − dΦB .
E
dt
5. Electric potential has meaning only for electric fields that are produced by static
charges; it has no meaning for electric fields that are produced by induction.
6. The self-inductance of an inductor is defined as
L=
The inductance of a solenoid is
NΦ
.
i
L = µ0 n2 Al.
7. An induced emf appears in any coil in which the current is changing:
EL = −L
di
.
dt
(a) The emf in the inductor is in the same direction as the current if the current is
decreasing.
(b) The emf in the inductor is in the opposite direction as the current if the current
is increasing.
8. RL circuits have similar behavior to RC circuits.
(a) The circuit has a characteristic time τL = L/R. Initially, an inductor acts to
oppose changes in the current through it. A long time later, it acts like ordinary
connecting wire.
(b) When the battery is connected the current increases according to the equation
i=
E
(1 − e−t/τL ).
R
(c) When the battery is disconnected, the current decays according to
i=
E −t/τL
.
e
R
9. Analogous to a capacitor, energy is stored in an inductor. This time the energy is
stored in the magnetic field.
(a) The magnetic energy is UB = 12 Li2 .
(b) The energy density is uB = B 2 /2µ0 .
10. If two coils are close together, the magnetic field from one could link the coils of the
other. It can be shown that the mutual inductance is the same regardless of which
coil we consider to be the primary (source) or secondary (receiver). The induced emf
in one coil because of the change of current in the other coil is
E2 = −M
di1
.
dt