Magnetism 11.2From Forces to Induction

Occurrences• Today

– A Military Application of Magnetism– Introduction to Inductors– Begin next unit

• Friday– Quiz– Continue unit.

• Monday – More of the same old thing.

Remember Force on a Wire

F=Bil

Check this out …

Wire

The Real Deal

The figure shows a uniform magnetic field that is normal to the plane of a conducting loop with resistance R. Which one of the following changes will cause an induced current to flow through the resistor?

A) decreasing the area of the loopB) decreasing the magnitude of the magnetic fieldC) increasing the magnitude of the magnetic fieldD) rotating the loop through 90° about an axis in the plane of the paperE) all of the above

A conducting loop of wire is placed in a magnetic field that is normal to the plane of the loop. Which one of the following actions will not result in an induced current in the loop?

A) Rotate the loop about an axis that is parallel to the field and passes through the center of the loop.

B) Increase the strength of the magnetic field.

C) Decrease the area of the loop

D) Decrease the strength of the magnetic field.

E) Rotate the loop about an axis that is perpendicular to the field and passes through the center of the loop.

A conducting bar moves to the left at a constant speed v on two conducting rails joined at the left as shown. As a result of the bar moving through a constant magnetic field, a current I is induced in the indicated direction. Which one of the following directions is that of the magnetic field?

A) toward the rightB) toward the leftC) parallel to the long axis of the barD) into the pageE) out of the page

LET’S TALK ABOUT MIKE FARADAY

Important Definition From Last Time – Magnetic Flux

AREA

Magnetic Field

FLUX

What did LENTZ say of the FLUX

changes??

The Magnetic Flux Going Through The Loop:

iii

i AB )cos(

Add up all of these piecesthat are INSIDE the loop.

WAIT A SECOND …….

• You said that there is a conducting loop.• You said that there is therefore a VOLTAGE or

emf around the loop if the flux through the loop changes.

• But the beginning and end point of the loop are the same so how can there be a voltage difference around the loop?

• ‘tis a puzzlement!

REMEMBER when I said E Fields start and end on CHARGES???

DID I LIE??

The truth

• Electric fields that are created by static charges must start on a (+) charge and end on a (–) charge as I said previously.

• Electric Fields created by changing magnetic fields can actually be shaped in loops.

Why do you STILL think I am a liar?

Because you said that an emf is a voltage so if I put a voltmeter from one point on the loop around to the same point, I will get ZERO

volts, won’t I

The POTENTIAL between two points

• Is the WORK that an external agent has to do to move a unit charge from one point to another.

• But we also have (neglecting the sign):

sEV

s

So, consider the following:

E

Conductor

x x x x x x xx x x x x x xx x x x x x xx x x x x x xx x x x x x xx x x x x x x

zeroREemf

sEsEemf

2

Throughthe loopemft

Michael Faraday (1791-1867)

A: The way that you don’t want it to point! (Lenz’s Law)

Technically:

Q: WHICH WAY DOES E POINT?

emft

A rectangular circuit containing a resistor is perpendicular to a uniform magnetic field that starts out at 2.65 T and steadily decreases at 0.25 T/s. While this field is changing, what does the ammeter read?

Start Working on Unit 14

Friday: QUIZ + Do the Experiment