Download Lecture 14 - Iowa State University

Summary
Lecture 14
The number off students: 169.
Average score for this exam: 71 points (B) out of 100 points.
Score:
Maximum score: 98 pts (3 students)
Minimum score: 21 pts
Magnetism
Magnetic Field and its Sources
Effect of Magnetic Field on Electric Charge
Below 50 pts: 21 students (D and F)
”Difficult”: #8 (50%), #7 (54%), #25 (18%), #21, #22 (30%).
Problems:
“Simple”: #2, #3 (~86%), #13 (92%), #20 (~90%).
What can expect someone who received “low” grade?
What we can expect if someone for the first exam received “F”?
Future:
Just work more and you can expect high final grade.
Physics 112, Spring 2010, Feb 15, Lecture 14
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Physics 112, Spring 2010, Feb 15, Lecture 14
Magnetism
Two Sources of Magnetic Field
1. Magnets.
1. A magnet (permanent magnet).
2. Earth’s magnetic field.
2. The electric current.
The electric current flowing through
the coil made of conductive wire
produces a magnetic field.
3. Magnetic field of electric current.
4. Magnetic force on an electric current.
5. Magnetic force on moving electric charge.
A magnet is made of special
materials based on iron, cobalt,
nickel, and some other elements.
6. Magnetic field due to a long straight wire.
Permanent magnet produces a
magnetic field also is due to
microscopic currents in atoms.
7. Forces between two parallel wires.
8. Solenoids and electromagnets.
Bar magnet.
9. Torque on a current loop.
10. Magnetism of solids.
11. Ferromagnetism: domains and hysteresis.
12 V
12. Application.
Physics 112, Spring 2010, Feb 15, Lecture 14
U-shape magnet.
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Physics 112, Spring 2010, Feb 15, Lecture 14
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Magnets as Magnetic Dipole
Magnet Field Lines and Earth Magnetic Field
Any magnet is a magnetic dipole.
r
Fm
r
Fm
r
Fm
r
Fm
If we cut a magnet in half, we don’t
get a single north pole and a south
pole; we get two smaller magnets.
Magnetic field lines
continue through
the material; they do
not start or end on a
point.
1 magnetic dipole
r r
Fm Fm
Magnetic field lines leave north pole
and enter south pole.
2 magnetic dipoles
Like poles repel.
Unlike poles attract.
South magnetic pole is located at
north geographic pole of the Earth.
4 magnetic dipoles
Electric charge can be positive or negative while magnetic monopole (north
or south) was not found.
Physics 112, Spring 2010, Feb 15, Lecture 14
North magnetic pole is located at
south geographic pole of the Earth.
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Electric Current Produces Magnetic Field
Physics 112, Spring 2010, Feb 15, Lecture 14
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Magnetic Force on Electric Current in Wire
A magnet exerts a force on a current-carrying wire; the direction of the
force is given by the right-hand rule #3 (RHR-3).
The direction of the field is given by the right-hand rules.
Right-hand rule #1:
magnetic field of wire
with current.
The magnetic field of the Earth is generated by the
rotation of molten iron and nickel in the Earth’s core.
Magnetic field can
be visualized using
magnetic field lines.
All magnets have two poles: “north” and “south”.
Right-hand rule #2:
magnetic field of
loop with current.
Right-hand rule #3:
magnetic force on wire
with current or moving
positive charge.
Physics 112, Spring 2010, Feb 15, Lecture 14
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Force is always perpendicular
to I and B.
Physics 112, Spring 2010, Feb 15, Lecture 14
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Magnetic Force on Moving Charge
Magnitude of Magnetic Field
A magnetic field exerts a force only on moving electric charge.
r
B
r
v
+
z
x
Magnetic field is a vector quantity (has the magnitude and direction).
The electric charge moving perpendicular to a magnetic field, experience a
magnetic force.
B=
Fm = q v⊥ B
r
Fm
y
v - the index finger.
B - the middle finger.
Fm - the thumb.
The right-hand rule #3 gives us the
direction of the magnetic force acting
on moving positive electric charge.
Right-hand rule #3:
magnetic force on wire
with current or moving
positive charge.
If the charge is negative, change the
direction of the magnetic force.
Magnetic Force on Wire with Current
Fm is magnetic force;
v is velocity perpendicular to a magnetic field.
The units for magnetic field: tesla (T)
1T =
1N
1 C ⋅1 m / s
1 T = 1 tesla
- a magnetic field of Earth is 10-4 T.
- a maximal constant magnetic field is ~45 T (NLHMF, Florida State University).
Physics 112, Spring 2010, Feb 15, Lecture 14
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1. An electron moving perpendicular to a magnetic field with a velocity of
5 m/s experiences a magnetic force of 10-17 N. Find the magnetic field.
Fm = IlB sin θ
B=
θ = angle between I and B
Fm
10−17 N
=
= 12.5 T
q v⊥ (1.6 ×10−19 C )(5 m / s)
2. An electron is moving perpendicular to a magnetic field of 20 T with a
velocity of 10 m/s. What magnetic force is acting on the electron?
Right-hand rule #3:
magnetic force on wire
with current or moving
positive charge.
Fm = q v ⊥ B = (1.6 × 10 −19 C )(10 m / s )(20 T ) = 3.2 × 10 −17 N
Magnetic Force on Moving Charge
3. An electron is moving parallel to a magnetic field of 10 T with a velocity of
10 m/s. What magnetic force is acting on the electron?
1) into the page
Magnetic force is zero!
2) out of the page
Physics 112, Spring 2010, Feb 15, Lecture 14
q is electric charge;
Magnetic Field and Magnetic Force
The magnitude of the force on the wire depends on the current,
the length of the wire in the B field, the magnetic field, and its
orientation.
In which direction does the force act on the wire?
Fm = q vB sin θ
B is magnetic field;
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Physics 112, Spring 2010, Feb 15, Lecture 14
Fm
q v⊥
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Physics 112, Spring 2010, Feb 15, Lecture 14
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Magnetic Force
An electron moves on a circle at 2.0 x 107 m/s in a plane perpendicular to a
uniform magnetic field of 0.01 T .
Right-hand rule #3:
magnetic force on wire
with current or moving
positive charge.
1. Force on the electron:
Fm = q v⊥ B
2. Newton’s law for radial (centripetal) force:
Fradial = ma =
mv 2
r
mv⊥2
= q v⊥ B
r
Fradial = Fm
r=
mv⊥ (9.1×10−31 kg )(2 ×107 m / s )
=
= 0.011 m
qB
(1.6 ×10−19 C )(0.01 T )
Physics 112, Spring 2010, Feb 15, Lecture 14
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