equipment compasses bar magnets yellow wire & power supply

Equipment

• Compasses• Bar Magnets• Yellow Wire & power supply

Forces and Fields (6)

In the most fundamental equations about the universe, we find fields.

Black holes, the Aurora Borealis, and microwave ovens all are understood in terms of fields.

Fields are abstract, but quite real.

Mr. KlapholzShaker Heights

High School

MAGNETIC FORCE & FIELD (B)

Magnet basics

• The poles of a magnet are the places where the field (B) is the greatest.

• All magnets have two poles. If you try to break a magnet, you end up with two small magnets (each with a North and a South pole)

Compass• Perform a preliminary exploration.• Keep the compass off of your desktop; free your

needle (it should shiver).• What does your compass do?• Slowly turn the compass’s container; what effect

does this have on the needle? Do the letters or the numbers affect the needle?

• The compass is measuring the Earth’s _ _ _ _ _ _ _ _ _ _ _ _ _ .

Compass• Perform a preliminary exploration.• Keep the compass off of your desktop.• What does your compass do?• Slowly turn the compass’s container; what effect


• The compass is measuring the Earth’s Magnetic Field.




• A compass is a _ _ _ _ _ _ _ _ _ _ _ _ _ detector.




• A compass is a Magnetic Field detector.

How could we use a compassto get home?

What exactly would we do?

Home

S

W E

N

StartHere

Bar Magnet

• Perform a preliminary exploration at your desk.

• Does the bar have a magnetic field? How do you know?

N S

ActivityThe most important pattern in magnetism:

Map the Field due to a Bar Magnet

A

What does the field look like? [A, B, C]

Yellow Wire• Does a wire make a magnetic field?• Does a current make a magnetic field?• Make the compass point toward the wire (!).• Is the field perpendicular or parallel to the wire?• Compare the field on one side of the current to

the field on the other side. • Inside a current loop, is the field especially weak

(due to subtraction / cancellation) or especially strong (due to addition / enhancement)?

• All magnetism comes from _ _ _ _ _ _ _.

Yellow Wire• Does a wire make a magnetic field?• Does a current make a magnetic field?• Make the compass point toward the wire (!).• Is the field perpendicular or parallel to the wire?• Compare the field on one side of the current to

the field on the other side. • Inside a current loop, is the field especially weak

(due to subtraction / cancellation) or especially strong (due to addition / enhancement)?

• All magnetism comes from CURRENT.

We have 3 big results:

• The magnetic field never points toward the current.

• The magnetic field is perpendicular to current.• The magnetic field on one side of a wire is

opposite in direction to the field on the other side.

How do we make sense of this? And…What does the field look like?

The Magnetic Field (B) due to a current (I)See how it agrees with our data.

Pennsylvania State Univ.Wikipedia

I

Here is a current. What does the magnetic field look like?

I

B

B

B

B

B

If you placed a compass next to one of the circles, is would align tangent to the circle.

Do you see that this accounts forthe three big results?

Now tilt that wire so it points at your eye. What would the magnetic field look like?

Here is the current. What would the magnetic field look like?

I

Which way does the field go?

I

Arrow Basics

What would you see if the arrow was aimed right at your eye?

What would you see if the arrow was away from your eye?

The Right-Hand Rule

1. Visualize the field loops around the current.2. Put your right thumb in the direction of the current.3. Your fingers will naturally curl in the direction of the

magnetic field.

Now go back and see how this agrees with the example that we did.

The current is moving away from you.

I

The current is moving away from you. Fill in the 8 compasses.

I

A

B

C

D

Now tilt this wire so the current is going toward the left. What is the field above and below the wire? [Compare with the compasses A and C]

A

C

Notice that in the plane of the paper, the field is well-defined

Same Wire. In what direction is the field between you and the wire?

Which compass is this [A, B, C, D]?

Same Wire. In what direction is the field BEYOND the wire? Is this compass A, B, C, or D?

Draw the field

When we made a loopwith the yellow wire,was the field strongor weak?

Solenoid

Magnetic Force

• The magnetic force acts only on moving charges.

• The force is perpendicular to the movement, and perpendicular to the field.

Motor Effect Examples• Washer / Dryer• Pencil Sharpener• Blender / Mixer / Coffee Grinder• Drip coffee maker• Clock / Watch• Windshield wipers• Car windows• Hard Drive (iPod)• CD spinner• Speakers• Fan (in computer, in AC, in heating a home)

A current in a magnetic field will experience a force.I & B F

To have a magnetic force, the field must come from some source other than the current itself.Example: Put a current in the Earth’s magnetic

field, and a tiny force will push the current.

First Idea / Second Idea

• Recall the Yellow wire and the compass.• An Electric Current makes a Magnetic Field.• I B

• Recall today’s demonstrations.• A current in a field will be forced.• I & B F• {Note: this B is not made by this I.}

Find the Direction of the Force.(The field below is made by a permanent magnet or the earth.)

BB

Find the Direction of the Force. (Now someone adds a current to the system. The current is not making the

field.)

I

I

BB

Find the Direction of the Force.

I

I

F B

The Right Hand Rule for Magnetic Force

1. Put your fingers in the direction of the current (there is more than one way to do this).

2. Fold your fingers in the direction of the magnetic field.

3. Your thumb points in the direction of the magnetic force.


I

B

I

B

F

Find the Direction of the Force

I

I

B B

The Force is out of the page.

I

I

B F B

Find the Direction of the Force (Actually, first, in which direction is the current?)

B+ -

Find the Direction of the Force

B

I I

B

I I


I B

There is no component of I that is perpendicular to B. There is no force.

I B

There is no component of I that is perpendicular to B. There is no force.

I B

Also, there is no force if the current is oppositein direction to the magnetic field.


B

I

The Force is Out of the page.Here’s why …

B

I

Only the part of the current that is perpendicular to the field makes a force.

B

I I

The Force is Out of the page.

F

Find the direction of the force on the electron beam.

e-

e-

BB

e-

e-

FB

A wire (9.4 meters) carries a current of 0.51 A. The wire is in a magnetic field of 0.022 Teslas. How

much force acts on the wire when the wire is: (a) parallel to the field, (b) perpendicular to the field.

A wire (9.4 meters) carries a current of 0.51 A. The wire is in a magnetic field of 22 mT. How much

force acts on the wire when the wire is: (a) parallel to the field, (b) perpendicular to the field.

(a) There is no force.(b) F = BIL = (22x10-6)(0.51)(9.4) = 1.1 x 10-6 N

Contrast between the 2 Right-Hand Rules in Magnetism

Rule 1 Rule 2

How we 1ST saw it

Cause

Effect

Rule

more

Rule 1 - How we first saw it.


A current in a yellow wire

affected a compass.

Rule 1 - The Cause

Rule 1 - The Cause

Current ( I )

Rule 1 - The Effect

Rule 1 - The Effect

Magnetic Field ( B)

Rule 1 - The Rule, in brief


Put your Right thumbin the direction of I.

Your fingers will curlin the direction of B.

Rule 1 - More Details


Before you use your right hand,visualize the shape of the field.


For example, if I is moving out of the page, in which direction is B at the bottom of the

page?

I


THIS IS THE VISUALIZATION STEP THAT MAKES ALL THE DIFFERENCE!

I


I

This will let you narrow the choices (for thedirection of B) down to just 2 choices.

Then, the rule is so helpful.


I

The answer istoward the Right.

If you skip thevisualization step,

it’s confusing.


A wire was placed near a

permanent magnet.When we put a current in the wire,

the wire had a force on it.

Rule 2 - The Cause

Rule 2 - The Causes

Current ( I)

AndMagnetic Field (B)

{ Compare to Rule 1 ! }

Note: the B is not made by the I.The B is made by some other source (perhaps it is made by

a permanent magnet or some other current.)

Rule 2 - The Effect

Rule 2 - The Effect

Force ( F)


Put your fingers

in the direction of I.

Fold them in the direction of B

Your thumb will pointin the direction of F.

Rule 2 - An Example:

B

I

B

I

In which direction is the force?

Rule 2 - An Example:

B

I

B

I

The force is out of the page.

Amount of magnetic force on a current

F = I L B

http://www.lvl39.com/forums/amateur-articles/1511-electromagnetic-current.html

equipment compasses bar magnets yellow wire & power supply

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