W7D2 MagnetismOctober 5, 2011

Monday◦ Finish Capacitor

Activity◦ Dielectric in

Capacitors◦ Magnetism, Intro.

Wednesday◦ Magnetism

Friday◦ Quiz◦ Magnetism

Monday◦ More Magnetism

Wednesday◦ EXAMINATION #2

Potential Magnetism – as far as

we get Some old stuff might be

necessary.

◦ After Exam – continue with new material

This week

DONE

From the textbook…..

If you want some insight into how this comes about,check the website.

Try not to hurt yourself when these vectors are in difficult orientations!

The Right Hand Rule

Can you do this?A vector of magnitude 5 lies along the x axis, and a vector of magnitude 10 lies in the xy plane, 30 degrees from the x axis (see the figure). What is the magnitude and direction of the cross product C x D?

Made of mysterious materials (lodestone???)

Each Magnet has a North and South pole.

Break a magnet between the N and S pole and you wind up with two magnets, each with a N and S pole.

Magnets produce magnetic fields. (More on that later)

Moving charges also create magnetic fields. This is the next topic that we will discuss and explore now that you have done the experiment on this.

Magnets

Remember Coulomb?

Unit Vector

Source Charge

Inverse Square Law

Vector

Biot-Savart Law

02

270

4

Tesla-m1 10 (Exactly)

4 /

unitqv rB

r

Coulomb m s

Magnetic Permeability of

Space

Magnetic Field

IntensityTesla

SourceCurrentElement

Inverse Squareq

unit vector

B is a VECTOR (Unit – Weber)

B Activity

DISCUSSION◦ Does this agree

with your last experiment? Which kind of current???

◦ Does this agree with B-S?

Magnetic Field Around a Wire

Water current flows down hill.

Gravitational Field is pulling it down.

The flow is from high to low gravitational potential.

It is hard to swim against the current.

Electric Current flows in a similar way – from high to low electric potential (Voltage).

Let’s Discuss Current

02

270

4

Tesla-m1 10 (Exactly)

4 /

unitqv rB

r

Coulomb m s

CCCCCCCCCCCCCC

This is NOT the

norm

One Charge q is NOT thenorm .. A CURRENT is!

The amount of CHARGE that crosses a cross-section of a wire such as this in ONE SECOND is defined as a current of one Ampere.

Electric Current-1

The electron current flows against the Electric Field. The electric field exists because a battery (or something

else) creates the electric field. The conventional current flow is in the direction of the

electric field. Only the negative electrons actually move. The positive

(+) charges remain stationary.

Electric Current-2

The electrons “drift” at a drift velocity v. In time Dt they drift a distance vDt. The volume of electrons that flow in that time is AvDt. Let n be the number of electrons that can move per

unit volume.

Electric Current-3

A

Electric Current-4

A

(number of charges in the volume) x (charge/carrier)current=

(Amps)

timenA v e t

i nA v et

IMPORTANT !

Electric Current

Electron Current

ConventionalCurrent

Electron Current = Number of electrons crossing the area/second

Conventional Current = Positive charge crossing area/second

Which current is this equation for?

(number of charges in the volume) x (charge/carrier)current=

(Amps)

timenA v e t

i nA v et

IMPORTANT !

NO BOOKS – Just material from this lecture!!

Break a long wire carrying a current Iinto a bunch of small segments. Expressthe magnetic field DB at point A in terms of the current element Dl. Replace the equation above with this expression.