Lecture Outline Chapter 19 College Physics, 7 th Edition Wilson / Buffa / Lou © 2010 Pearson Education, Inc

Lecture Outline

Chapter 19

College Physics, 7th Edition

Wilson / Buffa / Lou

© 2010 Pearson Education, Inc.

How did Magnets Come to be…

• Thought to be supernatural

• Original magnets were called “lodestone”

• Found in Magnesia, Greece

19.1 Magnets, Magnetic Poles, and Magnetic Field Direction

Magnets have two distinct types of poles; we refer to them as north and south.



Pole- Force Law (Law of Poles): Like magnetic poles repel, and unlike poles attract.

[This is so much like another law we have recently seen]



• How does a compass work? – The north pole of a

compass needle was defined as North Pole Seeking. (That is the end will point towards North on Earth.)

– Confusing result of history! North pole of compass is…

MIND BLOWN!!!!


Two magnetic poles of opposite kind form a magnetic dipole.

All known magnets are dipoles (or higher poles)

If you break a magnet in half then…

Magnetic monopoles could exist but have never been observed.



• A magnet creates a magnetic field:

• The direction of a magnetic field (B) at any location is the direction that the north pole of a compass would point if placed at that location.

• (This is very similar to the electric field.)


North magnetic poles are attracted by south magnetic poles, so the magnetic field points from north poles to south poles.

The magnetic field may be represented by magnetic field lines.The closer together (that is, the denser) the B field lines, the stronger the magnetic field. At any location, the direction of the magnetic field is tangent to the field line, or equivalently, the way the north end of a compass points.



• Let’s look at some examples and labs!

19.2 Magnetic Field Strength and Magnetic Force

A magnetic field can exert a force on a moving charged particle.

This force depends on the charge of the particle, which leads a discussion on electromagnetism.

Suppose a positive particle enters a uniform magnetic field…what happens?



The magnitude of the force is proportional to the charge and to the speed:

SI unit of magnetic field: the tesla, T

Of course we can rearrange this equation to solve for force instead of magnetic field.



In general, if the particle is moving at an angle to the field,

The force is perpendicular to both the velocity and to the field.



A right-hand rule gives the direction of the force.

Since force is a _________ we must know direction!



• Let’s watch a little video on the right hand rule.

• Now let’s do some practice with this.

• Right Hand Rule Practice

http://buphy.bu.edu/~duffy/semester2/d12_RHR_practice.html

http://buphy.bu.edu/~duffy/semester2/d12_RHR_practice.html


• Conceptual Example:– In a linear accelerator, a beam of protons

travels horizontally northward. To deflect the protons eastward with a uniform magnetic field, which direction should the field point:

• A) Vertically downward• B) West• C) Vertically upward• D) South


• A particle with a charge of -5.0 x 10-4C and a mass of 2.0 x 10-

9 kg moves at 1.0 x 103 m/s in the positive x direction. It enters a uniform magnetic field of 0.20 T that points in the +y direction.

• A) Which way will the particle deflect as it enters the field?

• B) What is the magnitude of the force on the particle when it is in the field?

• C) What is the radius the particle will travel?

19.4 Magnetic Forces on Current-Carrying Wires

The magnetic force on a current-carrying wire is a consequence of the forces on the charges. All moving charges in this length of wire are acted upon by a magnetic force in the same direction.

θ is the angle between I and B.



The direction of the force is given by a right-hand rule:When the fingers of the right hand are pointed in the direction of the conventional current I and then curled toward the vector B, the extended thumb points in the direction of the magnetic force on the wire.



• Because a current-carrying wire is acted on by a magnetic force, it would seem possible to suspend such a wire at rest above the ground using Earth’s magnetic field. – A) Assuming this could be done,

consider long, straight wire located at the equator. What would the current direction have to be? U/D/E/W

– B) Calculate the current required to suspend the wire, assuming Earth’s magnetic field is 0.40 Gauss at the equator and the wire is 1.0 m long with a mass of 30 g.

19.5 Applications: Current-Carrying Wires in Magnetic Fields

A galvanometer has a coil in a magnetic field. When current flows in the coil, the deflection is proportional to the current.

Let’s make one of these….as well as a DC motor!


19.6 Electromagnetism: The Source of Magnetic Fields

Experimentally, we observe that a current-carrying wire creates a magnetic field.



The magnitude of the magnetic field near a long, straight, current-carrying wire is given by:

The constant μ0 is called the permeability of free space.



The field lines form circles around the wire; the direction is given by a right-hand rule.



• The maximum household current in a wire is about 15 A. Assume that this current exists in a long, straight, horizontal wire in a west-to-east direction. What are the magnitude and direction of the magnetic field the current produces 1.0 cm directly below the wire?


The magnetic field at the center of a circular current wire loop:



A solenoid is a wire coiled into a long cylinder. The magnetic field inside is given by:



• What your test will most likely do is ask you for the formula in a multiple choice question. Which means you must memorize the formula because I will test you on this with multiple choice questions!


• A solenoid is 0.30 m long with 300 turns and carries a current of 15.0 A. – A) What is the magnitude of the magnetic field

near the center of this solenoid? – B) Compare this with the result from the last

example with a long straight wire.


• Two long, parallel wires carry currents in the same direction. – A) Is the magnetic force between these

wires attractive or repulsive? – B) Wire 1 carries a current of 5.0 A and

the current in wire 2 is 10 A. Both have a length of 50 cm, and they are separated by 3.0 mm. Determine the magnitude of the magnetic field created by each wire.

– C) Determine the magnetic force that each wire exerts on the other.

19.7 Magnetic Materials

Atomic electrons have a property called “spin” that gives them a small magnetic moment. In multielectron atoms, the electrons are usually paired with an electron of the opposite spin, leaving no net magnetic moment.

However, this is not always the case, and some atoms do have a permanent magnetic moment. They will experience a torque in a magnetic field, and will tend to align with it.



In ferromagnetic materials, the forces between neighboring atoms are strong enough that they tend to align in clusters called domains. These domains are macroscopic in size.



When a ferromagnet is placed in a magnetic field, the domains tend to align with it.



When the external magnetic field is removed, the domains tend to stay aligned, creating a permanent magnet.

The most common ferromagnetic materials are iron, nickel, and cobalt. Some rare earth alloys are also ferromagnetic.


19.8 Geomagnetism: The Earth’s Magnetic Field

The Earth’s magnetic field is similar to that of a bar magnet, although its origin must be in the currents of molten rock at its core.

Its magnitude is approximately 10–5 to 10–4 T.


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Lecture Outline Chapter 19 College Physics, 7 th Edition Wilson / Buffa / Lou © 2010 Pearson Education, Inc