1. Electromagnetic Induction and Faradays Law Ripon High School AP Physics 2012-2013

2. Stuff we ’ ll discuss in this unit: How to induce and emf using a changing magnetic field How to induce and emf using a changing magnetic field Faradays Law of Induction Faradays Law of Induction Lenz’s Law Lenz’s Law What happens to a conductor moving through a uniform magnetic field What happens to a conductor moving through a uniform magnetic field

3. General Information/Questions What we know about electricity and magnetism: What we know about electricity and magnetism: An electric current produces a magnetic field An electric current produces a magnetic field A magnetic field exerts a force on an electric current or moving electric charge A magnetic field exerts a force on an electric current or moving electric charge QUESTION: If an electric current produces a magnetic field, is it possible that a magnetic field can produce an electric current ? QUESTION: If an electric current produces a magnetic field, is it possible that a magnetic field can produce an electric current ?

4. Induced Electromotive Force Faraday ’ s Experiment Faraday ’ s Experiment Battery Battery Switch Switch Iron Ring Iron Ring Wire Wire Galvanometer Galvanometer

5. Induced Electromotive Force Faraday discovered through his experiment that a constant current could not induce a current on the other side of the iron ring. Faraday discovered through his experiment that a constant current could not induce a current on the other side of the iron ring. He did, however, notice that a current was induced on the other side whenever he opened or closed the switch. He did, however, notice that a current was induced on the other side whenever he opened or closed the switch. CONCLUSION: CONCLUSION: A constant magnetic field does not produce a current in a conductor A constant magnetic field does not produce a current in a conductor A changing magnetic field does produce a current in a conductor A changing magnetic field does produce a current in a conductor Induced Current: a current that is induced in a conductor due to a changing magnetic field Induced Current: a current that is induced in a conductor due to a changing magnetic field

6. Faraday ’ s Law of Induction Further experiments concerning the magnitude of the induced emf led to the following conclusions: Further experiments concerning the magnitude of the induced emf led to the following conclusions: The more rapidly the magnetic field changes, the greater the induced emf in a loop of wire The more rapidly the magnetic field changes, the greater the induced emf in a loop of wire Other things the induced emf in a loop of wire depends upon Other things the induced emf in a loop of wire depends upon The loop ’ s area The loop ’ s area The loop ’ s angle to the magnetic field The loop ’ s angle to the magnetic field

7. Faraday ’ s Law of Induction Magnetic Flux: Magnetic Flux: The measure of quantity of magnetism, taking into account the strength and extent of the magnetic field The measure of quantity of magnetism, taking into account the strength and extent of the magnetic field Units: Weber [Wb] = [T·m 2 ] Tesla·meter 2 Units: Weber [Wb] = [T·m 2 ] Tesla·meter 2 Represented by the Greek letter Phi: Φ Represented by the Greek letter Phi: Φ Think of magnetic flux as proportional to the amount of magnetic field lines passing through a certain area enclosed by the loop Think of magnetic flux as proportional to the amount of magnetic field lines passing through a certain area enclosed by the loop

8. Faraday ’ s Law of Induction Faraday ’ s Law of Induction: Δ Φ – change in flux Δt – change in time

9. Faraday ’ s Law of Induction If the circuit contains a number of loops closely wrapped together, the emfs induced in each wire simply add together (think of a bunch of batteries in series) If the circuit contains a number of loops closely wrapped together, the emfs induced in each wire simply add together (think of a bunch of batteries in series) N – number of loops Δ Φ – change in flux Δt – change in time

10. Lenz ’ s Law The minus sign prevalent in Faraday ’ s Law reminds us the direction the induced emfs act, summed up by Lenz ’ s Law: The minus sign prevalent in Faraday ’ s Law reminds us the direction the induced emfs act, summed up by Lenz ’ s Law: A current produced by an induced emf moves in a direction so that its magnetic field opposes the original change in flux

11. Lenz ’ s Law TWO MAGNETIC FIELDS EXIST! TWO MAGNETIC FIELDS EXIST! The changing magnetic field or flux that induces the current The changing magnetic field or flux that induces the current The magnetic field produced by the induced current The magnetic field produced by the induced current Important Rule: The second magnetic field opposes the change in the first Important Rule: The second magnetic field opposes the change in the first

12. Induced emf in a Moving Conductor Another way to induce an emf in a conductor is to move it through a magnetic field. Another way to induce an emf in a conductor is to move it through a magnetic field. For our purposes, you must assume: For our purposes, you must assume: The conductor has some length The conductor has some length The conductor has some velocity The conductor has some velocity The magnetic field is uniform The magnetic field is uniform All three are perpendicular to one another All three are perpendicular to one another

13. emf Induced in a Moving Conductor In order to calculate the induced emf in a conductor, the following equation applies: In order to calculate the induced emf in a conductor, the following equation applies: B – magnetic field l – length of conductor v – velocity of conductor