1. Chapter 20 Magnetic Flux Faraday’s Law

2. 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. Induced Voltages and Inductance

3. 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... Primary Circuit primary coilsecondary coil Secondary Circuit + _ A Soft magnetic material

4. + _ A Faraday’s Observations: With the switch open, no reading on the ammeter. When the switch is closed, the ammeter needle deflects momentarily toward the right, then returns to 0.

5. + _ A Faraday’s Observations: When the switch is opened, the ammeter needle deflects to the left momentarily, then returns to 0. With a constant current flowing through the primary circuit, the ammeter shows no reading.

6. + _ A Faraday’s Conclusions: Steady currents in 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. 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. A S N

9. x x x x x side view Magnetic Flux One way to think about this is to simply count the number of magnetic field lines passing through a loop. How much magnetic field is passing through the loop?

10. x x x x x side view 30 o 60 o 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. 30 o 60 o top view B tot  This is also the direction of the normal to the loop!

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

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

15. Having defined the magnetic flux, we can now quantify the induced EMF we have seen in the secondary circuit. EMF! N is the number of turns What causes currents to flow? Potential Differences