2/18/2011 Objectives Apply the laws of magnetism and induced emf.

2/18/2011 Faraday’s Law and Flux The magnitude of induced emf (voltage) is proportional to the rate of change of the magnetic flux passing through the loop of area A. So what is flux…..  Magnetic flux equals product of magnetic field perpendicular to coil’s face and area of coil  Flux is Measured in T-m 2 or webers where 1Wb = 1 T-m 2.

2/18/2011 Magnetic flux – when B is perpendicular to the face of the coil. The change in magnetic flux is equal to the magnetic field, B and the area, A. If either of these changes, the flux changes.

2/18/2011 Faraday’s Law Factors affecting magnitude of emf (voltage):  The rate of change of B or rate of change of magnetic flux.  Time; more rapidly magnetic field changes, greater induced emf  Proportional to number of wire loops. So what does this look like mathematically?

2/18/2011 Faraday’s Law of Induction – emf, ε, is equal to the negative of the number of loops of wire, N, multiplied by the change in magnetic flux, ΔΦ divided by the change in time Δt. The negative tells us the direction the emf acts and is Lenz’s Law…..

2/18/2011 Lenz’s Law Direction of induced emf gives rise to a current that opposes the original change in flux.  In real words… a changing magnetic flux induces an emf and that emf then creates a current that flows in such a way that it’s magnetic field is in opposition to the original changing flux.  So if the flux increases the induced current will be in a direction such that it’s magnetic field will be opposite in direction to the increasing flux.  So if the flux decreases the induced current will be in a direction such that it’s magnetic field will be in the same direction to the decreasing flux.  This is natures way of trying to keep the flux constant. Review Example 21-3

2/18/2011 Practice problem What is the direction of the induced current as the coil is reduced as shown? 1.Magnetic field is the dots coming out of page like the tips of arrows. 2.Red circle is the coil of wire.

2/18/2011 Practice problem answer What is the direction of the induced current as the coil is reduced as shown? 1. Is the flux increasing or decreasing? Decreasing because A of loop decreasing. 2. If flux is decreasing then induced current will move in a direction to create a magnetic field in the same direction as the decreasing flux. (out of the page) 3. To get a magnetic field coming out of the page the current must be counterclockwise in the red wire.

2/18/2011 Motional EMF Another way to induce an emf.  Produced when flux from external magnetic field flows through loop of changing cross- sectional area.  Induced current obeys Lenz’s law.  See Equation 21-3 on page 627. Motional EMF = -NB perp vl Review example 21-4.

2/18/2011 Flux and Electric Field A changing magnetic flux produces an Electric field. (we will talk about these in detail in Ch16- 17) E =F/q (definition on electric field rom Ch16) F = qvB (from Ch20 and don’t forget right hand rule

2/18/2011 Transformers How does an electrical transformer work? AC only due to need for changing flux.

2/18/2011 Motors and Generators Research motors, generators, and transformers. What do these things have to do with magnetism and electricity?  Generators transform mechanical energy into Electric energy.  Motors transform electric energy into mechanical energy.