Download Physics     Week 5(Sem. 2) Magnetism

Physics Week 5(Sem. 2) Name____________________________ Chapter Summary Magnetism Magnetic Fields Permanent magnets have long been used in navigational compasses. The needle in a compass is supported to allow it to freely rotate in a plane. The needle will rotate allowing one side to point north and the other end to point south. Like electrical charges and their fields, magnetic fields exert forces on one another. For magnets, like poles repel and opposite poles attract. Unlike electric charges, north and south can not be isolated from one another. So no one has found a south pole, without a north pole. Even when a larger magnet is made smaller the smaller pieces each have a north and south pole. Magnetic fields have both magnitude and direction. The direction of the magnetic field at any point in space is the direction indicated by the north pole of a small compass needle placed at that point. Magnetic field lines can be drawn around a magnet to show direction. The magnetic field at any point is tangent to the magnetic field line at that point. And the strength of the magnetic field is proportional to the number of lines per unit area that passes through a surface oriented perpendicular to the lines. Magnetic field lines are closer together at the ends of the magnet where the field is stronger, this occurs at the poles. They always originate in the north pole and end in the south pole. Force of a Magnetic field on a Charge When a charge is placed in an electric field, the charge experiences an electric force and a magnetic force so long as: Geomagnetism The geographic north pole varies from the magnetic north pole slightly. This point also varies slightly with time so the angle the compass deviates from the geographic north pole is called the angle of declination. Currently, NYC is about 12o west, meaning the compass points 12o west of the geographic north pole. Ms. N. May 1. The charge must be moving, no magnetic charges work on stationary charges. 2. The velocity of the moving charge must have a component that is perpendicular to the direction of the magnetic field. Look at figure 21.6 for a visual. The test charge (qo) is moving with a velocity of v through a magnetic field. The vector of the magnetic field is labeled B, it is assumed to be constant in magnitude and direction. If the charge moves parallel or antiparallel then the charge experiences no magnetic field. If, the charge moves perpendicular to the field the charge experiences maximum possible magnetic force (Fmax). If it moves with some angle θ only the velocities perpendicular component factors into the magnetic force (less than Fmax). The force (F) is perpendicular to the plane that B and v are in, as seen in the figure. The Right Hand Rule (#1) can be used. The RHR 1 is to extend your right hand so the fingers point along the direction of the magnetic field B and the thumb points along the velocity of the charge v. The palm of the hand then faces in the direction of the magnetic force F that acts on the positive charge. If the charge is negative instead of positive then the force is in the opposite direction. The magnetic force can contribute to the net force on an object and thus cause the object to accelerate. Definition of the Magnetic Field The magnitude of a magnetic force (through a magnetic field) has been found to be directly proportional to the charge and to the component of the velocity that is perpendicular to the magnetic field. sin
The unit of the magnetic field strength is Ns/Cm, this unit is called the tesla (T). Often the tesla can be written as 1 N/Am. The strength of the magnetic field near the earth’s surface is approximately 1x10‐4 T. Thus the unit gauss (not SI) is often used, where 1 gauss = 1x10‐4 T. The Circular Trajectory Considering a special case, of a charged particle with a velocity that is perpendicular to a uniform magnetic field, to fully understand that the magnetic field does not do work. The magnetic force moves the charged particle in a circular path. Meaning when the positively changed particle is at a point 1, the magnetic force F is perpendicular to the velocity v and points directly upwards (as seen in the picture). This force causes the trajectory to bend upward. When the particle reaches point 2, the magnetic force still remains perpendicular to the velocity, but is not directed to the left in the drawing. The magnetic force always remains perpendicular to the velocity and is directed toward the center of the circular path. Recalling Centripetal Force F=mυ2/r, so qυB= mυ2/r. Thus r is Motion in a Magnetic Field For a charged particle in an electric field the particle will move parallel to the electric field or sideways in the figure (as seen in 21.10). For a charged particle in a magnetic field (as pictured), the particle is deflected Ms. N. May upwards. Applying the RHR 1 the velocity enters from the left, the field points into the page thus the magnetic field is up. For magnetism, the force is always perpendicular to the field. But for electricity, the force is always parallel (or antiparallel) to the field. Therefore, the work done by each field is different. Because the electric field exerts a force in the direction of the displacement, the electric field does work on the charged particle. Since the electric field can do work, it can change the kinetic energy of the particle. However, since the magnetic force is always perpendicular to the motion of the charged particle it never has displacement in the same direction. Thus the magnetic force cannot do work and cannot change the kinetic energy of the charged particle. The magnetic force can still alter the direction of the motion of the charged particle. This result shows that the radius of the circle is inversely proportional to the magnitude of the magnetic field, with stronger fields producing tighter circular paths. Ms. N. May Ms. N. May Ms. N. May Ms. N. May 4121 - 1 - Page 1
Name: ____________________________________________
1)
The diagram below shows a bar magnet.
Which arrow best represents the direction of the needle of a
compass placed at point A?
2)
A)
C)
B)
D)
A student sprinkled iron filings around a bar magnet and observed that the filings formed the pattern shown below.
The magnetic field is strongest at point
A) A
B) B
C) C
D) D
4121 - 1 - Page 2
3)
Which diagram below best represents the magnetic field
near a bar magnet?
5)
The diagram below shows electron e about to enter the
region between the poles of two magnets.
A)
B)
Upon entering the region between the poles, the moving
electron will experience a magnetic force directed
C)
A)
B)
C)
D)
D)
6)
4)
toward the south pole
out of the page
into the page
toward the north pole
The diagram below shows the lines of magnetic force
between two north magnetic poles.
The diagram below represents the magnetic lines of force
around a bar magnet.
At which point is the magnetic field strength greatest?
A) A
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
7)
B) B
C) C
D) D
The diagram below shows a proton moving with velocity v
about to enter 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, what would be the magnitude of the
magnetic force on the alpha particle?
A) 4F
C) 2F
B)
D) F
4121 - 1 - Page 3
8)
Which diagram best represents magnetic flux lines around a
bar magnet?
11)
Which diagram best represents the magnetic field near the
poles of a horseshoe magnet?
A)
A)
C)
B)
D)
B)
C)
D)
9)
12)
In the diagram below, a wire carrying an electron current
into the page, as denoted by X, is placed in a magnetic field.
The diagram below represents a conductor carrying a
current in which the electron flow is from left to right. The
conductor is located in a magnetic field which is directed
into the page.
The magnetic field exerts a force on the wire toward point
A) A
10)
B) B
C) C
D) D
The diagram below represents the magnetic field near
point P.
The direction of the magnetic force on the conductor will be
A)
B)
C)
D)
If a compass is placed at point P in the same plane as the
magnetic field, which arrow represents the direction the
north end of the compass needle will point?
A)
C)
B)
D)
toward the bottom of the page
toward the top of the page
into the page
out of the page
4121 - 1 - Page 4
13)
The diagram below represents an electron beam in a vacuum. The beam is emitted by cathode C, accelerated by anode A, and
passes through electric and magnetic fields.
In which direction will the force of the magnetic field act on the electron beam?
A) toward the bottom of the page
B) toward the top of the page
14)
C) out of the page
D) into the page
A magnetic field will be produced by
A) stationary ions
B) stationary protons
C) moving electrons
D) moving neutrons
Questions 44 through 48 refer to the following:
The diagram below represents a helium ion with a charge of +2 elementary charges moving toward point A with a constant speed (v) of
2.0 meters per second perpendicular to a uniform magnetic field between the poles of a magnet. The strength of the magnetic field is
0.10 weber per square meter.
15)
The helium ion is replaced by an electron moving at the
same speed. Compared to the magnitude of the force on the
helium ion, the magnitude of the force on the electron is
A) the same
B) greater
C) less
16)
If the strength of the magnetic field and the speed of the
helium ion are both doubled, the force on the helium ion will
be
C) quadrupled
D) halved
A) B
B) D
C) A
D) C
C) 6.4 x 10-20 N
D) 0.10 N
If the polarity of the magnet is reversed, the magnitude of
the magnetic force on the helium ion will
A) increase
B) decrease
C) remain the same
20)
The direction of the magnetic force on the helium ion is
toward point
The magnitude of the magnetic force exerted on the helium
ion is
A) 3.2 x 10-20 N
B) 0.20 N
19)
A) the same
B) doubled
17)
18)
The field around a permanent magnet is caused by the
motions of
A) neutrons
B) electrons
C) protons
D) nucleons
4121 - 1 - Page 5
21)
In the diagram below, what is the direction of the magnetic
field at point A?
Questions 17 through 21 refer to the following:
The diagram below represents an electron moving at 2.0 x 106
meters per second into a magnetic field which is directed into the
paper. The magnetic field has a strength of 2.0 newtons per
ampere-meter.
A)
B)
C)
D)
22)
toward the bottom of the page
toward the top of the page
to the left
to the right
Which diagram best represents the magnetic field around a
material of high permeability placed between unlike
magnetic poles?
A)
B)
24)
Which vector best indicates the direction of the force on the
electron?
A)
C)
B)
D)
C)
D)
23)
25)
If the strength of the magnetic field were increased, the
force on the electron would
A) increase
B) remain the same
C) decrease
In which diagram below is the magnetic flux density at
point P greatest?
26)
A)
B)
The electron is replaced with a proton moving with the same
velocity. Compared to the magnitude of the force on the
electron, the magnitude of the force on the proton would be
A) greater
B) less
C) the same
C)
27)
What is the magnitude of the force on the electron?
A) 6.4 x 10-13 newton
D)
B)
28)
8.0 x 106 newtons
C) 4.0 x 106 newtons
D) 6.4 x 106 newtons
If the velocity of the electron were increased, the force on
the electron would
A) increase
B) remain the same
C) decrease
29)
If a charged particle moving through a magnetic field
experiences a magnetic force, the angle between the
magnetic field and the force exerted on the particle is
A) 180D
B) 45D
C) 0D
D) 90D
4121 - 1 - Page 6
30)
In the diagram below, A, B, C, and D are points in the
magnetic field near a current-carrying loop. At which points
is the direction of the magnetic field into the page?
32)
An electron traveling at a speed (v) in the plane of this
paper enters a uniform magnetic field. Which diagram best
represents the condition under which the electron will
experience the greatest magnetic force as it enters the
magnetic field?
A)
B)
A) A and D
B) C and D
31)
C) B and C
D) A and B
Which diagram below best represents a magnetic field?
C)
A)
D)
B)
33)
C)
Which diagram best represents a magnetic field between
two magnetic poles?
A)
D)
B)
C)
D)
4121 - 1 - Page 7
34)
A wire 0.10 meter long is pushed through a magnetic field of
strength 4.0 newtons per ampere-meter in a direction
perpendicular to the field. If the speed of the wire is 2.0
meters per second, what is the magnitude of the induced
voltage across the ends of the wire?
A) 0.50 volt
B) 2.0 volts
35)
C) 0.20 volt
D) 0.80 volt
Each diagram below represents a cross section of a long,
straight, current-carrying wire with the electron flow into the
page. Which diagram best represents the magnetic field near
the wire?
A)
As two parallel conductors with currents in the same
direction are moved apart, their force of
A)
B)
C)
D)
36)
38)
repulsion decreases
repulsion increases
attraction decreases
attraction increases
B)
Which diagram best represents the magnetic field between
two magnetic north poles?
A)
C)
B)
C)
D)
D)
37)
A magnetic force is experienced by an electron moving
through a magnetic field. If the electron were replaced by a
proton traveling at the same velocity, the magnitude of the
magnetic force experienced by the proton would be
A) twice as great
B) zero
C) the same
D) half as great
39)
Which diagram best represents the lines of magnetic flux
between the ends of two bar magnets?
A)
B)
C)
D)
4121 - 1 - Page 8
40)
A particle with a charge of 2 x 10-6 coulomb crosses a
uniform magnetic field perpendicularly. The particle
experiences a force of 1 x 10-3 newton. If the particle has a
speed of 1 x 106 meters per second, the magnitude of the
field strength is
43)
Which arrow in the diagram below represents the direction
of the flux inside the bar magnet?
A) 2 x 103 weber/meter2
B) 5 x 10-4 weber/meter2
C) 5 x 10-1 weber/meter2
D) 2 x 10-3 weber/meter2
41)
The diagram below represents a conductor carrying an
electron current in magnetic field B. The direction of the
magnetic force on the conductor is
A) D
44)
A)
B)
C)
D)
42)
out of the page
into the page
toward the bottom of the page
toward the top of the page
The diagrams below show cross sections of conductors
with electrons flowing into or out of the page. In which
diagram below will the magnetic flux density at point A be
greater than the magnetic flux density at point B?
B) C
C) B
D) A
The diagram below shows an end view of a current-carrying
wire between the poles of a magnet. The wire is
perpendicular to the magnetic field.
If the direction of the electron flow is out of the page, which
arrow correctly shows the direction of the magnetic force F
acting on the wire?
A)
C)
B)
D)
A)
B)
C)
D)
45)
The existence of a magnetic field around a current-carrying
conductor can be demonstrated by placing the conductor
near
A) a battery
B) a compass needle
C) an electroscope
D) a pith ball
4121 - 1 - Page 9
46)
The diagram below shows a loop of wire between the poles
of a magnet. The plane of the loop is parallel to the magnetic
field. If an electron flow is established in the direction
shown in the loop, in which direction will a magnetic force
be exerted on segment AB?
47)
A volt is to electric potential as a tesla is to
A)
B)
C)
D)
48)
charge density
magnetic flux density
electric field intensity
electrical energy
The wire loop shown below has a clockwise electron
current.
What is the direction of the magnetic field at point P?
A)
B)
C)
D)
toward the top of the page
out of the page
into the page
toward the bottom of the page
A) out of the page
B) into the page
C) to the right
D) to the left
1. Which combination of units can be used to
express magnetic field strength?
(1) kg
s•C
(2) N
C
(3) N•m
C
(4) kg•m
C
(5) kg•m
s•C
5. A particle with a charge of +2 × 10–7 C and a
mass of
4 × 10–4 kg experiences an acceleration of 3.2
m/s2 due a magnetic field of 8 T. The velocity of
the particle may be
(1) 800 m/s parallel to the magnetic field.
(2) 400 m/s perpendicular to the magnetic field.
(3) 800 m/s perpendicular to the magnetic field.
(4) 800 m/s at an angle of 30º to the magnetic
field.
(5) 400 m/s at an angle of 30º to the magnetic
field.
2. Which of the following explains why a magnetic
field does no work on a moving charged
particle?
(1) The magnetic force depends on the direction
of motion of the particle.
(2) The magnetic force is conservative.
(3) The magnetic force depends on the speed of
the particle.
(4) The magnetic force is always perpendicular
to the direction of motion.
(5) There is always an electric field that cancels
the work done by the magnetic field.
6.
3. A particle with a charge of +3 µC and mass 2
×10–8 kg enters a 0.01 T magnetic field with a
velocity of 5 × 105 m/s perpendicular to the
field. The acceleration of this particle due to the
magnetic force is
(4) 2.5 × 105 m/s2
(1) 9.5 × 105 m/s2
(2) 1.5 × 105 m/s2
(5) 7.5 × 105 m/s2
(3) 4.5 × 105 m/s2
4. A particle with a mass of 1.2 × 10–5 kg
experiences an acceleration of 2 × 103 m/s2 as it
enters a magnetic field of 3 T with a velocity of
4 × 105 m/s at an angle of 30º with the magnetic
field. The magnitude of the charge on the
particle is
(1) 0.04 µC
(4) 0.4 µC
(2) 80 µC
(5) 4 µC
(3) 40 µC
In the figure above, what is the direction of the
magnetic force vector?
(1) To the right
(2) Out of the page
(3) To the left
(4) Into the page
(5) Upward in the plane of the page
7.
In the figure above, what is the direction of the
particle's velocity?
(1) Into the page
(2) Out of the plane of the page
(3) Upward, in the plane of the page
(4) To the left
(5) To the right
Base your answers to questions 8 and 9 on the
following.
Traveling at an initial speed of 1.2 × 106 m/s, a
particle (mass = 6 × 10–10 kg, charge = 1.0 × 10–10 C)
enters a region of uniform magnetic field with a
strength of 300 T at an angle of 30º to the field.
8. What is the magnitude of the acceleration of the
particle?
(4) 7.2 × 107 m/s2
(1) 3.0 × 1017 m/s2
(2) 3.0 × 107 m/s2
(5) 9.0 × 10–3 m/s2
(3) 1.8 × 10–2 m/s2
9. What is the speed of the particle after 1 s?
(4) 5.4 × 106 m/s
(1) 3.0 × 106 m/s
(5) 4.2 × 106 m/s
(2) 6.0 × 105 m/s
(3) 1.2 × 106 m/s
10. In order for a magnetic field to exert a force on
an object, which of the follow can be true?
I. The object is charged.
II. The object is moving parallel to the field.
III. The object is moving perpendicular to the
field.
(1) I and II only
(4) I only
(2) II only
(5) I and III only
(3) III only