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# Chapter 20 Magnetic Flux Faraday s Law - PowerPoint PPT Presentation

Lecture 31. Chapter 20 Magnetic Flux Faraday’s Law. 12 April 1999 Monday. Physics 112. Chapter 20. Induced Voltages and Inductance.

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Chapter 20

Magnetic Flux

12 April 1999

Monday

Physics 112

Induced Voltages and Inductance

We saw in Chapter 19 that moving charges (currents) create magnetic fields.Nature often reveals a great deal of symmetry. So, scientists wondered, “Can magnetic fields can create currents?” You might guess that the answer is, “Yes.” In this chapter, we’ll explore this physical process known as induction.

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Shortly after Oersted’s discovery that currents

create magnetic fields, Michael Faraday (England)

and Joseph Henry (US) conducted experiments

to determine if magnetic fields created electrical

currents...

Soft magnetic

material

primary coil

secondary coil

Secondary Circuit

Primary Circuit

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+

With theswitch open, no readingon the ammeter.

When theswitch is closed, theammeter needle deflects momentarilytoward the right,then returns to 0.

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+

With a constant current flowing through the

primary circuit, the ammeter shows no reading.

When the switch is opened, the ammeter needle

deflects to the left momentarily, then returns to 0.

_

+

Steady currentsin the primary produce constant magnetic fields, which result in no current in the secondary circuit.

Changing currents in the primary circuit result in changing magnetic fields, which result in induced currents in the secondary circuit.

S

N

If we move a bar magnet toward a loop of

wire, the ammeter deflects to the left.

When we stop moving the magnet, the

When we pull the magnet away from the loop,

the ammeter deflects to the right.

S

N

As the magnet moves toward the loop, the

magnetic field near the loop increases.

As the magnet moves away from the loop, the

magnetic field near the loop decreases.

Such results are consistent with Faraday’s

to induced currents.

x x x x x

x x x x x

x x x x x

side view

A New Quantity

Magnetic

Flux

How much magnetic

field is passing through

the loop?

Huh?

One way to think about this is to simply count the number of magnetic field lines passing through a loop.

x x x x x

x x x x x

x x x x x

30o

60o

side view

top view

Magnetic flux is the amount of magnetic field

perpendicular to the loop.

If you separate the magnetic field into

components parallel and perpendicular

to the loop, you see that...

Btot

q

30o

60o

This is also the direction of the normal to the loop!

top view

So, the Weber is the unit of magnetic flux.

B Flux

What is the magnetic flux through a square loop of side length 10 cm whose normal is at an angle of 60o to the direction of a uniform magnetic field of strength 1 T?

Potential

Differences

What causes currents to flow?

Having defined the magnetic flux, we can

now quantify the induced EMF we have

seen in the secondary circuit.

N is the number of turns