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Chapter 20 Magnetic Flux Faraday’s Law

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 Faraday’s Law

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  1. Lecture 31 Chapter 20 Magnetic Flux Faraday’s Law 12 April 1999 Monday Physics 112

  2. Chapter 20 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.

  3. A _ + 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

  4. A _ + Faraday’s Observations: With theswitch open, no readingon the ammeter. When theswitch is closed, theammeter needle deflects momentarilytoward the right,then returns to 0.

  5. A _ + Faraday’s Observations: 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.

  6. A _ + Faraday’s Conclusions: 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.

  7. A 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 ammeter reads 0. When we pull the magnet away from the loop, the ammeter deflects to the right.

  8. A 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 experiment: Changing magnetic fields lead to induced currents.

  9. x x x x x 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.

  10. x x x x x 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...

  11. Btot q 30o 60o This is also the direction of the normal to the loop! top view

  12. Units! So, the Weber is the unit of magnetic flux.

  13. Concept Quiz! B Flux

  14. ? 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?

  15. EMF! 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 Faraday's Law

  16. Concept Quiz! Solenoids & Light Bulbs

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