Physics 102-002 Announcements

• WebAssign – – Chapter 24 due next

Wednesday

• Exam #3 is graded

– Corrections due Wed, Apr 25

Picture: The Earth's Magnetic Field: Gary A. Glatzmaier (UCSC) Pictured, a computer simulation shows the resulting magnetic field lines out to two Earth radii, with blue lines directed inward and yellow lines directed outward.

Average = 61.5349

Std Dev = 16.4568

Max Score = 94

Min Score = 30

Median Score = 58

Class Schedule4/9 Midterm Exam #3

4/11 Chapter 24 Magnetism, (Pg 458-470)

4/16 Chapter 26 Properties of Light

4/18 Chapter 28 Reflection and Refraction, Part 1 (Pg 530-540)

4/23 Chapter 28 Reflection and Refraction, Part 2 (Pg 540-551)

4/25 Chapter 29 Light Waves (Pg 558-568)

4/30 Chapter 30 Light Emission (Pg 558-568) Midterm Exam #4

5/2 Review

5/7 Final Exam

Note the change!!!

Chapter 24Magnetism

• Magnetic Forces• Magnetic Poles• Magnetic Fields• Magnetic Domains• Electric Currents and Magnetic Fields• Magnetic Force on Moving Charged Particles• Magnetic Force on Current-Carrying Wires• Earth’s Magnetic Field

Magnetic ForcesIn “Electrostatics” we learned about the force between charged particles that depends on the net charge on each and on their separation distance … this is the “Coulomb force”.

If 2 charges are in motion with respect to each other, there is an additional force between them, the Magnetic Force, that depends on their motion.

… the Electrostatic Force and the Magnetic Force are thus related to each other.

Physics Place video - Oersted.

We’ll see that there’s a reciprocity:

Moving charges (Currents) create magnetic fields …..

And …. Magnetic fields exert forces on moving charges.

Magnetic PolesMagnetic Poles exert a magnetic force on each other

There are 2 types of magnetic Poles:South (S) PolesNorth (N) Poles

Like poles repelOpposite poles attract

The strength of the repulsion or attraction obeys an inverse square law(The closer the poles are to each other, the stronger the force)

(similar to electric charges)

You can’t find a magnetic NORTH pole without finding a SOUTH pole partnered with it.

There is no such thing as a “magnetic monopole” – scientists have spent a lot of energy looking!

The North pole points “north” in the earth’s magnetic field.The South pole points “south”

A “horseshoe magnet” is just a bar magnetBent in a U shape. It has 2 poles too.

Cut a bar magnet in 2 pieces, you get 2 bar magnets:

N S N SS N+

Magnetic FieldsAlso similar to electrostatics … the space between 2 magnetic poles is filled by a MAGNETIC FIELD.

Physics Place figure

You can visualize the magnetic field by sprinkling iron filings around a magnet.

The direction of the field is from the North pole to the South both inside and outside of the magnet. Outside the magnet, the field starts on the North pole and circles around to the South Pole.

The earth has a naturally occuring magnetic field (caused by motion of the electrons making up the earth’s subterranean structure). The needle of a compass will line up with the earth’s magnetic field. If the needle starts out not lined up, the magnetic field exerts a torque on the needle and forces it to “get In line”,

Here is the magnetic field of a pair of bar magnets. The field is caused by the motion of the electrons in the Iron atoms making up the magnet.

Electrons are in motion 2 ways:- They’re “spinning”- They’re “orbiting” the nucleus

of the atom

Most common magnets are made from Iron, cobalt, and nickel

Question 1

Compared to the huge force that attracts an iron tack to a strong magnet, the force that the tack exerts on the magnet is

1. relatively small.2. equally huge.

Question 1 Answer

1. relatively small.2. equally huge.

Compared to the huge force that attracts an iron tack to a strong magnet, the force that the tack exerts on the magnet is

Magnetic DomainsIron becomes magnetized when large clusters of its atoms become lined up in the same direction. Such a cluster of atoms when aligned is called a Magnetic Domain.

There are millions of atoms in a domain, and there are many domains in a single crystal. Here is a cartoon depiction of magnetic domains.

Domains can also be aligned (or anti-aligned)with each other.

The magnetic strip on the back of your LOBO card is a large collection of magnetic domains used to record your personal information as a series of binary digits.

To make a magnet (or to “magnetize” a piece of metal) we have to get a significant number of the domains within it to line up. We can do that by subjecting it to an external magnetic field from another magnet, or by beating on it (depending on the softness of the metal).

Physics Place figure

Electric Currents and Magnetic FieldsWe said a moving charge creates a magnetic field.

This means that every current-carrying wire is surrounded by a magnetic field!!

There are several ways to demonstrate this is true … some of them will be shown in class. One is by surrounding a wire with compasses and then passing a current through the wire … the compasses line up with the magnetic field which circles around the wire.

If the wire is bent into a loop, the field can be built up so that it circles through the middle of the loop. If you loop the wire many times, you can build up a strong magnetic field that exists inside the coil of wire. This is called a Solenoid.

An Electromagnet is built by winding the wire around an iron core. The magnetic field inside the windings forces the iron’s domains into alignment and turns it into a very strong magnet.

Magnetic Force on Moving Charged Particles

A charged particle that is not moving will not be affected by a magnetic field. But if the charged particle begins moving in a magnetic field, the magnetic field will exert a force on the particle, deflecting it from its “normal” path. This is the principle behind devices such as the Mass Spectrometer.

If that path is inside a wire, the magnetic field will deflect the electrons moving through the wire, and will make the wire move.

Because of the nature of the field, though, the field exerts no force on the particle if the particle is moving parallel to the magnetic field lines. The force on the particle is maximum if it moves perpendicular to the field lines.

The odd thing is that the direction of the deflecting force is perpendicular to BOTH the field’s direction and the direction of motion of the particle.

If the particle moves continuously in the magnetic field (and doesn’t exit the field for some reason), the particle can be made to move in a circle! This is the concept behind the cyclotron.

Physics Place figure

Magnetic Force on Current Carrying WiresIf charges are moving through a wire when they enter a magnetic field, the wire will be deflected to one side.

Physics Place figure

The direction that the wire gets deflected is determined by the direction the current is flowing and the direction of the magnetic field, and the wire is deflected perpendicular to both of those directions.

So electric currents deflect magnets, and magnets deflect electric currents.

This is the fundamental relationship that lets us build electric motors (and electric meters)!!

Electric meter(a galvanometer)

Really just a compass inside a coil of wires that lets us measure current, voltage, and many other things.

Electric Motors

A motor is really just a galvanometer in which the “compass” is allowed to turn all the way around. The forces between the moving current and the magnetic field provide the push to keep the motor turning.

Question 2

When the current flows in the wire that is placed in the magnetic field as shown, the wire is forced upward. I f the wire is made to form a loop as shown, the loop will tend to

1. rotate clockwise.2. rotate counterclockwise.3. remain at rest.

Question 2 Answer

1. rotate clockwise.2. rotate counterclockwise.3. remain at rest.

When the current flows in the wire that is placed in the magnetic field as shown, the wire is forced upward. I f the wire is made to form a loop as shown, the loop will tend to