Do Now (1/23/14):

Do not touch the materials on your desk until instructed!

1. What is a magnet?

2. What are some properties of magnets?

3. How do you use magnets in your life?

4. Are some magnets stronger than others?

Investigate:

• Work with your group to complete the activity. You have ten minutes!!!

Like repels Like repels like…like…

Opposites attract!Opposites attract!

Magnetism Resources:

• http://coe.kean.edu/~afonarev/Physics/Magnetism/Magnetic%20Fields%20and%20Forces-eL.htm

•Magnets have been known for Magnets have been known for centuries.centuries.•The ancient Greeks used a stone The ancient Greeks used a stone substance called “magnetite.” They substance called “magnetite.” They discovered that the stone always discovered that the stone always pointed in the same direction. pointed in the same direction. •Later, stones of magnetite called Later, stones of magnetite called “lodestones” were used in navigation.“lodestones” were used in navigation.

What is Magnetism?What is Magnetism?

force of attraction or repulsion of a magnet due to the arrangement of its atoms, particularly its electrons.

•Magnetic effect is strongest at the poles (ends) •Each magnet has 2 poles – 1 north, 1 south.

Poles of a magnet always come in pairs!

If you cut a magnet in If you cut a magnet in half,half,

you get 2 magnets!you get 2 magnets!

Magnetic FieldsMagnetic Fields

The region where the The region where the magnetic forcesmagnetic forces

act is called the “magnetic act is called the “magnetic field”field”

Atoms themselves have magnetic properties due to the spin of the atom’s electrons.

These areas of atoms are called “domains”

Groups of atoms join so that their magnetic fields are all going in the same direction

When an unmagnetized substance is When an unmagnetized substance is placed in a magneticplaced in a magneticfield, the substance can become field, the substance can become magnetized.magnetized.This happens when the spinning electrons This happens when the spinning electrons line up in theline up in thesame direction.same direction.

An unmagnetized An unmagnetized substance looks likesubstance looks likethis…this…

While a magnetized While a magnetized substance lookssubstance lookslike this…like this…

How to break a magnet:

1. Drop it

2. Heat itThis causes the domains to become random again!

The Earth is a magnet:The Earth is a magnet:

Magnetic South Pole

Magnetic North Pole

surrounded by a magnetic field that is strongest near the North and South magnetic poles

Geographic North Pole

Geographic South Pole

SometimeSometimes, the s, the Earth’s Earth’s magnetic magnetic poles flip. poles flip. This This happens happens every every half-half-million million years or years or so.so.

Magnetic North Pole

Magnetic South Pole

Use the Earth’s magnetic field to find direction.

The needle of a compass always points toward the magnetic south pole.

We call this direction “North” (remember, opposites attract)

Do Now (1/24/14):(Do not touch the materials on

your desk until instructed!)

Write down three things you learned yesterday about magnets

Magnetic Field

1) Review:

a) Natural permanent magnets– Like poles repel, unlike attract– come in pairs (no monopoles)– Interact with earth;

define N (or north-seeking) pole as pole attracted to North pole of earth

b) Magnetic field direction:

- direction of force on N pole

B

Review

• What is an electric dipole?

Review:

• (in your notes) Draw the electric field of an electric dipole

Hypothesize (2 min):

• What do you think the magnetic field of a bar magnet would look like? Draw it in your notes.

• Discuss with your elbow partner

Investigation

• Work with your table to complete the Investigation. You have twenty minutes to complete the activity.

Results:

• Go to the board and draw the field lines that you discovered. If yours looks like another group’s, put a check mark next to it.

Field of a magnetic dipole

d) Magnetostatics for poles

(identical to electrostatics for charges)– 2 types: N, S vs +,-– Unlike attract, like repel– Inverse square law– Force along joining line– Magnetic Field:

B

F

qM

Why study magnetism?– No monopoles (yet)– Poles (dipoles) produced by moving charges– Charges affected by magnetic fieldThe fundamental unit is still charge– magnetic fields can be created by charge– The force on a charge can be due to magnetic field

Magnetic fields do not interact with stationary charges!!!!!

Magnetic Field• Represented by B

• Units: Tesla

• Brainstorm: how big do you think Earth’s magnetic field is?

10-5 T!!!!!

Force on a moving charge in a magnetic field

F qvBsin(Alternative definition of B)

• Proportional to component of v perp to B

• Perpendicular to B

• Perpendicular to v

Force on a moving chargewhat would result in a force of zero?

• charge at rest

• charge traveling parallel to B

F qvBsin

Force on a moving chargewhat would result in a maximum force?

• charge traveling @ 90° to B

F qvBsin

Example:

What is the force on an electron moving at 43 m/s in a magnetic field of 0.5 T?

Practice:

• Work on the Magnetic Force Worksheet

Do Now (1/27/14):

(On your Do Now sheet from last week)

• How fast is a proton moving in a magnetic field of 0.8 T if the magnetic force exerted on it is 0.9 N?

Finding Directions of B-Fields• Consider an arrow

OUT OF THE PAGE

INTO THE PAGE

The Right Hand Rule!

• Follow along on your paper

• Three different methods – find the one that works for you!

#1 (ON YOUR PAPER)

• Direction of v:

• Direction of B:

• Direction of F:

To the right

Out of the page

DOWN(TOWARDS THE BOTTOM OF THE PAGE)

Practice:

• Complete the Right Hand Rule Worksheet by the end of class.

• If you finish early, please continue working on your homework (Magnetic Force)

Do Now (1/28/14):1. Draw the following on your paper:

a. A magnetic field pointing to the right

b. A proton traveling towards the top of the page

c. What is the direction of the force exerted on the charge?

2. If the proton travels at 3000 m/s and the magnetic field has a magnitude of 4 T, what is the force exerted on the proton?

3. What is the proton’s acceleration?

RHR: Electron vs. Proton

• What if an electron travels through the field instead of a proton?

2) Magnetic field due to current (direction)

• Oersted (1820)

B I

r

Right Hand Rule #2:

3) Magnetic force on current

Direction from RHR1: B fingers, I thumb, F palm

F

IB

Force per unit length

defines B

a) Orthogonal case

Practice:

• Use the rest of class to work on your HW (Magnetic Force and/or Force on a Current-Carrying Wire)

Do Now (1/29/14):

• What is the force on a 15 cm wire carrying a 10 A current surrounded by a 0.2 T?

Force on a current carrying wire

• Look on your homework paper.

Example:

• A current in the +x direction and a magnetic field in the –y direction

Investigate!

• Work on ONE of the investigations for full credit.

• Work on both for extra credit!

Do Now (1/30/14):

• Come in quietly, pass in your Do Now’s and Homework, then wait for further instructions.

Bearth .5 gauss 5 10 5 T

Bfridge magnet .01T

Bsuper conducing 1 10 T

B F

I

N

Amtesla (T)Units:

b) General case

F

LIBsin

Force per unit length

4) Force between parallel wires

B I1

d;

F

I2B

F

k

I1I2

d

Attraction or repulsion?

Does it depend on reference frame?

+

+

FE

FE

FB

FB

v

v

v

v

+

+

-

-

+

+

-

-

+

+

-

-

• Define Ampere as the quantity of current that produces a force per unit length of 2 x 10-7 N/m for separation of 1 m

F

k

I1I2

d

k 1

40

8.988 109Nm2/C2

k (2 10 7 N/m)(1m)

(1A2)2 10 7N/A2• Then

• This defines C and gives

• Permeability of free space

0 2 k

F

0

2I1I2

dThen

4 10 7N/A2

k 0

2

5) Field due to long straight wire (magnitude)

B I

r

B 0

2I

r

6) Force on a moving charge

• Zero at rest

• Zero parallel to B

• Max perpendicular to B

F qvBsin(Alternative definition of B)

• Proportional to component of v perp to B

• Perpendicular to B

• Perpendicular to v

7) Motion of a charge in a magnetic field

a) Constant force

motion is parabolic

electric or gravitational field

not everywhere perp to velocity

not magnetic field

Mass spectrometer:

• Diagram:

b) Constant magnitude perpendicular to motion

radial field (circular motion)

mass on a string

magnetic field produces circular motion

(initial vel. perp. to B)

motion is circular

F Fc mv 2

r

Force due to the field:

F qvB

For circular motion:

So,

mv 2

rqvB

r mv

qBr depends on v, B

v

r

qB

m

angular freq. independent of speed, radius

Tracks in a bubble chamber

• electron-positron creation

• 1, 3 positive

• 2 negative

• energy: 3 > 2 > 1

• energy decreases by collisions

Example: Find speed and radius for proton

B = 0.10 T

V = 2100 V

c) Work done by magnetic field

W Fx cosF

displacement, x

Work by a force F

For a magnetic field,

0

Work = 0

d) Velocity selector

FE qE

Force due to E (down):

Force due to B (up):

FB qvB

For zero deflection, FE = FB :

qE qvB

v E

B

e) Mass SpectrometerIon energy:

KE 12 mv 2 qV

v 2qV

m

Radius of motion:

r mv

qB

m

qB

2qV

m

r m

q

2V

B

m

q

r2B2

2V

Additional Info about Magnets

William Gilbert, an English physician, predicted in 1600 that the Earth would be found to have magnetic poles.

The sun has a magnetic The sun has a magnetic field, too. It extends far field, too. It extends far above the sun’s surface.above the sun’s surface.

Other planets in the Other planets in the solar system also have solar system also have these magnetic fieldsthese magnetic fields

When a charged When a charged particle enters a particle enters a magnetic field, magnetic field, an electric force an electric force is exerted on it. is exerted on it. If a charged If a charged particle moves at particle moves at an angle to a an angle to a magnetic field, magnetic field, the magnetic the magnetic force acting on it force acting on it will cause it to will cause it to move in a spiral move in a spiral around the around the magnetic field magnetic field lineslines..

The solar wind is constantly bombarding the Earth’s magnetic field. Sometimes these charged particles penetrate that field. These particles are found in two large regions known as the Van Allen Belts.

The Earth’s magnetic field extends far into space. It is called the “magnetosphere.”

When the magnetic particles from the sun, called “solar wind”, strike this magnetosphere, we see a phenomenon called…

The Aurora Borealis in the Northern HemisphereThe Aurora Borealis in the Northern Hemisphere

And the Aurora Australis in the Southern HemisphereAnd the Aurora Australis in the Southern Hemisphere