1. Lecture 9 Electromagnetic Induction Chapter 20.1  20.4 Outline Induced Emf Magnetic Flux Faraday’s Law of Induction

2. Electromagnetic Induction If an electric current produces a magnetic field, then a magnet should be able to generate an electric current. A current is produced in a wire when there is relative motion between the wire and a magnetic field.current Such a current is called an induced current (emf). The effect is called electromagnetic induction (discovered by Michael Faraday in the XIX century). The strength of the current depends on the magnetic field strength and the wire’s speed.

3. Magnetic Flux The emf is induced by the magnetic flux, rather than the magnetic field. Magnetic flux is a measure of the total number of magnetic lines passing through an area. Consider a loop of wire in the presence of a uniform magnetic field B. If the loop area is A, then the magnetic flux  B through it is defined as: magnetic flux  B = B  A = BA cos  B  is the component of B, perpendicular to the plane of the loop

4. Faraday’s Law of Induction The induced emf exists only when there is a relative motion between the magnet and the circuit. Faraday’s law states that the instantaneous induced emf equals the rate of change of magnetic flux through the circuit.law If a circuit has N loops and the flux through each loop changes by the amount of  B during interval of time  t, the average emf ℇ induced in the circuit during this time is: ℇ =  N  B /  t

5. Polarity of the Induced Emf The induced emf produces an induced current. The induced current produces its own magnetic field. The induced magnetic field opposes the change of the external magnetic field. The polarity (direction) of the induced emf is determined by Lenz’s law.law

6. Motional Emf Let us consider an application of Faraday’s law to a conductor of length l moving at a constant velocity v through a uniform magnetic field. Picture F = qvB qE = qvB E = vB  Magnetic force on electrons in the conductor E is the electric field induced in the conductor E is uniform  the potential difference across the conductor ends  V = E l

7. Summary The phenomenon of electromagnetic induction refers to appearance of an electric field under influence of a magnetic field. Faraday’s law connects the induced emf and time change in the magnetic flux. Lenz’s law defines the polarity of the induced emf Motional emf is an emf produced in a moving conductor under influence of a magnetic field.