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Chapter 22Magnetism
AP Physics BLecture Notes
22-01 Magnets
22-02 Earth’s Magnetic Field
22-03 Magnetic Fields
22-04 Magnetic Force on a Current-Carrying Conductor
22-06 Motion of a Charged Particle in a Magnetic Field
22-07 Magnetic Field of a Long Straight Wire
22-08 Magnetic Force Between Two Parallel Conductors
Magnetism
Sections
22-09 Magnetic Fields of Current Loops and Solenoids
Magnets have two ends – poles – called north and south.
Like poles repel; unlike poles attract.
Magnets
However, if you cut a magnet in half, you don’t get a north pole and a south pole – you get two smaller magnets.
Magnets
Chapter 22: Magnetism
The diagram represents the magnetic lines of force around a bar magnet. At which point is the magnitude of the magnetic field strength of the bar magnet the greatest?
(A) A (B) B (C) C (D) D
Earth’s Magnetic Field
S
N
Chapter 22: Magnetism
The south pole of a magnet points toward the Earth's
(A) South Pole.
(B) North Pole.
(C) center.
(D) middle latitudes.
Magnetic fields can be visualized using magnetic field lines, which are always closed loops.
Magnetic Fields
A uniform magnetic field is constant in magnitude and direction.
The field between these two wide poles is nearly uniform.
Magnetic Fields
The force on a moving charge in a magnetic field is related related to its charge and velocity.
θsinqvBF q
v
B
The direction is given by a right-hand rule.
The force is directedout of the screen.
Magnetic Fields
Determine the magnitude and direction of the force on an electron traveling horizontally to the east in a vertically upward magnetic field of strength 0.75 T.
east
north
Magnetic Fields
F4 )D( 2F
)C( F2 )B( F )A(
Chapter 22: Magnetism
The diagram shows a proton moving with velocity v entering a uniform magnetic field directed into the page. As the proton moves in the magnetic field, the magnitude of the magnetic force on the proton is F.If the proton were replaced by an alpha particle under the same conditions, the magnitude of the magnetic force on the alpha particle would be
A magnet exerts a force on a current-carrying wire. The direction of the force is given by a right-hand rule.
Magnetic Force on a Current-Carrying Conductor
MF MF
Magnetic Force on a Current-Carrying Conductor
The force on the wire depends on the current, the length of the wire, the magnetic field, and its orientation.
This equation defines the magnetic field B.
θsinILBF
Unit of B: the tesla, T.
B
I
LThe force is directedinto the screen.
mAN
1T 1
Magnetic Force on a Current-Carrying Conductor
The force on the wire depends on the current, the length of the wire, the magnetic field, and its orientation.
θsinILBF
L
B
I
ILBF
When the current is perpendicular to themagnetic field.
F
Magnetic Force on a Current-Carrying Conductor
A 1.5-m length of wire carrying 4.5 A of current is oriented horizontally. At that point on the Earth’s surface, the dip angle of the Earth’s magnetic field makes an angle of 38° to the wire. Estimate the magnitude of the magnetic force on the wire due to the Earth’s magnetic field of at this point.
Be
I
Magnetic Force on a Current-Carrying Conductor
Suppose a straight 1.00-mm-diameter copper wire could just “float” horizontally in air because of the force due to the Earth’s magnetic field which is horizontal, perpendicular to the wire, and of magnitude What current would the wire carry?
Be
I
Magnetic Force on a Current-Carrying Conductor
Chapter 22: Magnetism
The force on a current-carrying wire in a magnetic field is equal to zero when
(A) the current is parallel to the field lines.
(B) the current is at a 30° angle with respect to the field lines.
(C) the current is at a 60° angle with respect to the field lines.
(D) the current is perpendicular to the field lines.
Chapter 22: Magnetism
A vertical wire carries a current straight up in a region where the magnetic field vector points due north. What is the direction of the resulting force on this current?
(A) west
(B) north
(C) east
(D) down
If a charged particle is moving perpendicular to a uniform magnetic field, its path will be a circle.
F
v
q
BqvBF
rmv
qvB2
mvqrB Momentum
Motion of a Charged Particle in a Magnetic Field
What is the velocity of a beam of electrons that go undeflected when passing through perpendicular electric and magnetic fields of
magnitude 8.8 x 103 V/m and 3.5 x 103 T,
v
respectively? What is the radius of the electron orbit if the electric field is turned off?
Motion of a Charged Particle in a Magnetic Field
Alpha particles of charge +2e and mass of 6.6 x 1027 kg are
emitted from a radioactive source at a speed of 6.6 x 1027 m/s. What magnetic field strength would be required to bend them into a circular path of radius of 0.25 m?
rB
v
Motion of a Charged Particle in a Magnetic Field
Chapter 22: Magnetism
The diagram below shows an electron, e, located in a magnetic field. There is no magnetic force on the electron when it moves
(A) toward the right side of the screen
(B) toward the top of the screen
(C) into the screen
(D) out of the screen
Chapter 22: Magnetism
A charged particle is observed traveling in a circular path in a uniform magnetic field. If the particle had been traveling twice as fast, the radius of the circular path would be
(A) twice the original radius.
(B) four times the original radius.
(C) one-half the original radius.
(D) one-fourth the original radius.
Experiment shows that an electric current produces a magnetic field.
Magnetic Field of a Long Straight Wire
The field is inversely proportional to the distance from the wire:
The constant μ0 is called the permeability of free space, and has the value:
rπ2
IμB o
m/AT 10x 4πμ 7o
I
p
r
Magnetic Field of a Long Straight Wire
A power line carries a current of 95 A along the tops of 8.5 m-high poles. What is the magnitude of the magnetic field produced by this wire at the ground?
r
I
Magnetic Field of a Long Straight Wire
Chapter 22: Magnetism
A vertical wire carries a current straight down. To the east of this wire, the magnetic field points
(A) north.
(B) east.
(C) south.
(D) down.
A jumper cable used to start a stalled vehicle carries a 65-A current. How strong is the magnetic field 6.0 cm away from it?
I
p
r
Magnetic Field of a Long Straight Wire
The magnetic field produced at the position of wire 2 due to the current in wire 1 is:
The force this field exerts on a length L of wire 2 is:
d
II
π2
μ
LF 21o
d
I
π2
μB 1o
1
12LBIF
d
I
π2
μLIF 1o
1
Magnetic Force Between Two Parallel Conductors
Parallel currents attract; anti-parallel currents repel.
1I 2I
F F
1I 2I
F F
Magnetic Force Between Two Parallel Conductors
Determine the magnitude and direction of the force between two parallel wires 35 m long and 6.0 cm apart, each carrying 25 A in the same direction.
I
I
L
d
Magnetic Force Between Two Parallel Conductors
Chapter 22: Magnetism
Two long parallel wires are placed side-by-side on a horizontal table. If the wires carry current in the same direction,
(A) one wire is lifted slightly as the other wire is forced against the table's surface.
(B) both wires are lifted slightly.
(C) the wires attract each other.
(D) the wires repel each other.
The direction of the field is given by a right-hand rule.
Magnetic Fields of Current Loops and Solenoids
A solenoid is a long coil of wire. If it is tightly wrapped, the magnetic field in its interior is almost uniform:
L
INμB o
L
I I
N
B
Magnetic Fields of Current Loops and Solenoids
A thin 12-cm-long solenoid has a total of 420 turns of wire and carries a current of 2.0 A. Calculate the field inside near the center.
Magnetic Fields of Current Loops and Solenoids
A 30.0-cm-long solenoid 1.25 cm in diameter is to produce a field of 0.385 T at its center. How much current should the solenoid carry if it has 975 turns of the wire?
Magnetic Fields of Current Loops and Solenoids
Chapter 22: Magnetism
The diagram below shows an electromagnet made from a nail, a coil of insulated wire, and a battery. The south pole of the electromagnet is located closest to point
(A) A (B) B (C) C (D) D
Chapter 22: Magnetism
A current carrying loop of wire lies flat on a table top. When viewed from above, the current moves around the loop in a counterclockwise sense. What is the direction of the magnetic field caused by this current, outside the loop? The magnetic field
(A) circles the loop in a clockwise direction.
(B) circles the loop in a counterclockwise direction.
(C) points straight up.
(D) points straight down.
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