MagnetismChapter 21 (pg 765)

Lets Review• Force equals mass times acceleration

– This is always true, even with magnetic force

• Centripetal force is caused by something– In the next couple of chapters the force will be

electromagnetism

• Torque equals force times lever arm• Electric fields are created by charged

particles– Electrons & protons

• Current equals amount of charge per unit time

The History of Magnetism• In the 4th century a Chinese

writing “Book of the Devil Valley Master” makes the first mention of magnetism

• “The lodestone makes iron come or it attracts it.“

• Around 201 used spoon shaped lodestones to show direction

• About 700 AD the Chinese magnetized a needle after noticing the lodestone could tranfer it’s magnetism to other materials

More History• In 1819 Danish physicist Hans Christian

Orsted (1777-1851) discovered by mere accident that a current carrying wire changed the deflection of a near by compass (watch for your self)

• This leads British physicist Michael Faraday (1791-1867) to demonstrate the process of induction in 1831 by producing a current in a wire by rotating a disc through a magnetic field

• This invention is called the Faraday Disc, which lead to today's generators

Magnetic Principles• Like “poles” repel like

– Magnetic poles are labeled as “North” and “South”

– This originates from the earths magnetic poles

– The Magnetic north pole of the earth is on Antarctica and the Magnetic South pole is in the northern part of Canada

– Since opposite poles attract the “North pole” of the compass points to the magnetic south pole – which is geographically North

– Confused?

Angle of declination• This is the difference

between true north and magnetic north.

• It changes through out the years, but still remains relatively close to an average value

• In New York the present declination angle is about 12o west, meaning the compass needle points 12o west of geographic north

It’s all about the pOles• Unlike electric field

lines that can start or end at infinity, magnetic field lines, up to this point in history, always form loops.

• There are always a North and South poles no matter how small you cut the magnet!

• Even cut all the way to a single electron.

So why aren’t all things magnets• All materials are

magnetic, but not all materials are magnets

• Inside materials exist magnetic domains

• When the domains align in the same direction the magnetic fields add together

• When the domains are random, some add and some subtract, causing a net filed on zero

Click here for a video of a magnetized frog.Click here to learn about the types of magnetism.

The latest data• This picture shows the

direction of the earths magnetic field in 2006

• This shows the Earth s magnetic field will eventually flip, so compasses will point South, not North

• It turns out this flip occurs about every million years

Click here for the link to article

Those lines?• The lines shown on the last slide are

magnetic lines of force, also know as Magnetic Field lines

• These are essentially the same as gravitational or electric field lines but show the direction a magnetized particle would move

• So place a piece of magnetized iron on a field line and it will or orient itself with its North pole facing the Blue line, or the magnetic South pole

Take a look

Take a look

Describing Direction• For electric flied we

mostly focused on 2-D representations

• Magnetic fields are perpendicular to electric fields, often we need to look at a three dimensional problem

• Look at table 21-1 on page 767 or the chart to the right

In the plane of the page

Out of the page (Up)

Into the page (Down)

Models of B – Fields

Magnetic force• Remember, all force

can be expressed by Newton second law

• Charge must be moving to experience a magnetic force

• The velocity of the moving charge must be perpendicular to the B(magnetic) – field

• The direction of the force is given by the “right-hand rule”

Right hand rule (cross product)• This is and easy way to find the direction in

space for either the B- Field, Force, or velocity direction.

• The magnetic force on a particle is a vector equal to:

• q is charge, v is velocity, and B is magnetic field strength

• However, as v and B become more parallel v x B goes to zero

BvqF

Consider the following• Watch what happens as goes to zero

v B

F

Parallel vectors• If v and B are parallel, the force on a

charged particle is zero• Keep in mind, the right hand rule is for a

positive (conventional) charge carrier• Since the force goes to zero when goes to

zero it’s said v x B equals vBsin • Rearranging to solve for B the result is:

sinvq

FB

o

Example 21-1

Magnetic force does no work• In this case the magnetic force simply

changes the direction of the particle• It does not change the particle energy• This means the speed remains constant• Does a changing direction with constant

speed sound familiar?

r

mvF

2

Example 21-3

Cathode Ray Tube• By changing the current in the steering coils,

the electron beam turns to strike different parts of the screen

• Click here to learn more about America’s favorite past time

Works Cited• Reference

– http://en.wikipedia.org/wiki/Magnetism– http://concise.britannica.com– http://inventors.about.com/od/

cstartinventions/a/Compass.htm

• Images– http://www.physics.ubc.ca/~outreach/

phys420/p420_05/dean/lodestone.jpg