Electromagnetic Induction and Faradays Law

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 • Faradays Law of Induction • Lenz’s Law • What happens to a conductor moving through a uniform magnetic field

3. General Information/Questions • What we know about electricity and magnetism: • An electric current produces a magnetic field • 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?

4. Induced Electromotive Force • Faraday’s Experiment • Battery • Switch • Iron Ring • Wire • 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. • He did, however, notice that a current was induced on the other side whenever he opened or closed the switch. • CONCLUSION: • A constant magnetic field does not 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

6. Faraday’s Law of Induction • 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 • Other things the induced emf in a loop of wire depends upon • The loop’s area • The loop’s angle to the magnetic field

7. Faraday’s Law of Induction • Magnetic Flux: • The measure of quantity of magnetism, taking into account the strength and extent of the magnetic field • Units: Weber [Wb] = [T·m2] Tesla·meter2 • 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

8. Faraday’s Law of Induction Faraday’s Law of Induction: • ΔΦ–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) • N – number of loops • ΔΦ–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: 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! • The changing magnetic field or flux that induces the current • The magnetic field produced by the induced current • 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. • For our purposes, you must assume: • The conductor has some length • The conductor has some velocity • The magnetic field is uniform • 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: • B – magnetic field • l – length of conductor • v – velocity of conductor