Electromagnetic induction Lenz’s law Faraday’s law The nature of electromagnetic waves

1. Electromagnetic Induction and Electromagnetic Waves • Electromagnetic induction • Lenz’s law • Faraday’s law • The nature of electromagnetic waves • The spectrum of electromagnetic waves Topics: Sample question: The ultraviolet view of the flowers on the right shows markings that cannot be seen in the visible region of the spectrum. Whose eyes are these markings intended for? Slide 25-1

2. Electromagnetic Induction Slide 25-8

3. Motional emf Slide 25-9

4. Magnetic Flux Slide 25-10

5. Checking Understanding A loop of wire of area A is tipped at an angle to a uniform magnetic field B. The maximum flux occurs for an angle . What angle will give a flux that is ½ of this maximum value? A. B. C. D. Slide 25-11

6. Answer A loop of wire of area A is tipped at an angle to a uniform magnetic field B. The maximum flux occurs for an angle . What angle will give a flux that is ½ of this maximum value? C. Slide 25-12

7. Faraday’s Law Slide 25-15

8. 21.5 Electric Generators A generator is the opposite of a motor – it transforms mechanical energy into electrical energy. This is an ac generator: The axle is rotated by an external force such as falling water or steam. The brushes are in constant electrical contact with the slip rings.

9. 21.5 Electric Generators A dc generator is similar, except that it has a split-ring commutator instead of slip rings.

10. 21.5 Electric Generators A sinusoidal emf is induced in the rotating loop (N is the number of turns, and A the area of the loop): (21-5)

11. Checking Understanding A long conductor carrying a current runs next to a loop of wire. The current in the wire varies as in the graph. Which segment of the graph corresponds to the largest induced current in the loop? Slide 25-16

12. Answer A long conductor carrying a current runs next to a loop of wire. The current in the wire varies as in the graph. Which segment of the graph corresponds to the largest induced current in the loop? Slide 25-17

13. Lenz’s Law Lenz’s law There is an induced current in a closed, conducting loop if and only if the magnetic flux through the loop is changing. The direction of the induced current is such that the induced magnetic field opposes the change in the flux. Slide 25-13

14. Using Lenz’s Law Slide 25-14

15. Saul notes on using Lenz’s Law Lenz’s law says that the B-field from the induced current will oppose the change in magnetic flux from the source B-field. • Find direction of B-field vector in loop • Determine if magnetic flux from source is increasing or decreasing • If magnetic flux from source is decreasing, direction of Binduced is same as Bsource • If magnetic flux from source is increasing, direction of Binduced is opposite of Bsource • Use RHR 3 to find direction of induced current Slide 25-23

16. Right Hand Rules (RHR) for magnetic forces & fields Right-hand rule 1 (RHR 1) => for finding magnetic forceFB= q*v_vector x B_vector (Cross-Product Rule) Point right hand in the direction the charges are moving (current or velocity) Rotate your right hand until you can point your fingers in the direction of the magnetic Field Thumb points in direction of force for + chargeForce is in opposite direction for - charges(Text references pp 804-806) Right-hand rule 2 (RHR 2) => Finding direction of B from I Point thumb of right hand in direction of current I, B-field lines curl in direction of fingers (Text references pp 795 & 797) Right-hand rule 3 (RHR 3) => Finding direction of current in a loop from direction of B-field Point thumb of right hand in direction of B-field Fingers of right hand curl in direction of current Slide 25-23