Magnetic Fields and Induced EMFs

1. Magnetic Fields and Induced EMFs

2. Electromagnetic Induction • Just as a magnetic field can be formed by a current in a circuit, a current can be produced using an external magnetic field. • The process of inducing a current in a circuit with a magnetic field is called electromagnetic induction

3. But How??? • Remember, a moving charge in a magnetic field experiences a force. • Well… a charge that is still will also experience a force if the magnetic field is moving around it. • The moving magnetic field causes the force on charges which will cause them to go around the circuit with a current.

4. Electromagnetic Induction • A current can only be produced if the magnetic field is changing, or what physicist call, a magnetic flux • There are three ways that a magnetic flux can be produced…

5. Induced Current method #1 • The circuit is moved into or out of a magnetic field. • This is relative, so the magnetic field could be moving, the circuit could be moving, or both could be moving.

6. Induced Current method #2 • The circuit is rotated in the magnetic field. • The strength of a magnetic field is dependent on the angle to the circuit. • By rotating the circuit, the strength of the magnetic field is changing and therefore a current can be induced.

7. Induced current #3 • The intensity of the magnetic field is varied. • This change in the strength of the magnetic field will cause a current to be induced.

8. Faraday’s Law • The emf produced is proportional to the change in the magnetic flux per unit of time and the number of loops in a circuit. Or

9. Magnetic Flux • Φ is the magnetic flux, or how much magnetic field is going through an area. • Picture a hula-hoop in the rain…

10. Hula-hooping in the rain • The harder the rain, the more water that will go through the hoop. • The bigger the hula-hoop, the more water that will go through the hoop. • Magnetic flux is the same as this. It is a factor of how much magnetic field there is and how much room does it have to flow through.

11. What if you turn the hoop? • Let’s say you are hula-hooping around your arm. Can any rain get through? • The angle also determines the amount of magnetic field that can get through.

12. Magnetic Flux • Therefore… Where, A is the area of the circuit loop, and B is the magnetic field strength

13. Faraday’s Law again • In total, Faraday’s Law is…

14. Summary • The things that effect electromagnetic induction according to Farday’s law are: • 1) The angle between the magnetic field and the circuit • 2) The strength of the magnetic field • 3) The area of the loop of the circuit • 4) The number of loops in the circuit • 5) How quickly the magnetic field changes

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16. Lenz’s Law • What direction is the induced current? • Lenz’s Law states: • “The magnetic field of the induced current opposes the change in the applied magnetic field”.

17. Right-hand rule #1 Again • Remember, a current can induce a magnetic field of it’s own. • The current that it induces will oppose the direction of the magnetic field flux.

18. Example • A circuit of wire is pulled through a magnetic field pointed out of the smart board. What is the direction of the induced current?

19. Solution • The current induced will be such that it will create a magnetic field to go back into the smart board. • Therefore, the direction of the current will be clockwise

20. Door Bells • Ever wonder why there is a small light behind the button of a door bell??

21. Door bell continued • Pushing the door bell disrupts the current in the left circuit. • This disruption causes there to be a decrease in the magnetic field in the left circuit. • According to Farday’s law, this decrease in the left circuit, causes a current to be induced in the right circuit.

22. Door bell continued • The induced current causes a magnetic field in the right circuit directed through the middle to the coil. • The iron bar is pushed outwards and hits the chime.

23. Door Bell