Goal: To understand how induction helps power our everyday lives Objectives: 1)To learn how to produce a Motional EMF 2)To be able to calculate Magnetic Flux 3)To learn about Faraday’s Law

Motional Electro-Magnetic Force (EMF) When a charge moves there is a force on it. If used properly you can use this to generate a current (energy). A lot of electricity is generated by generators. To do this you move metal through a magnetic field. The charges will move perpendicular to that. This creates a current.

Magnetic Flux Magnetic Flux is just a measure of the amount of magnetic field contained within some area. Magnetic Flux is in units of Webers (Wb) 1 Wb = 1 T * m 2 If the plane of some wire or object is tilted at an angle of θ towards or away from the plane of magnetic field lines the the Magnetic Flux is just: Φ = B * A * cos(θ) A is area and A * cos(θ) is the effective area inside the field.

You try one: You have a magnetic field of 20 T. You put a circular wire perpendicular to the field. If the wire is a circle with a diameter of 0.2 meters then what is the magnetic flux through the wire loop?

You try one: You have a magnetic field of 20 T. You put a circular wire perpendicular to the field. If the wire is a circle with a diameter of 0.2 meters then what is the magnetic flux through the wire loop? Φ = B * A * cos(θ) = BA = done on board

Another A wire loop with radius of 0.3 m is inserted at a 45 degree angle to a 5 T magnetic field. What is the magnetic flux through the loop?

Another A wire loop with radius of 0.3 m is inserted at a 45 degree angle to a 5 T magnetic field. What is the magnetic flux through the loop? Φ = B * A * cos(θ) = (done on board)

Faraday’s law When you change the flux then you produce a current. The induced emf is the rate at which you change the magnetic flux! So, V = - change in flux / change in time (- because the induced emf attempts to oppose that change in magnetic flux) V = - Δ Φ / Δt And Δ Φ = Φf - Φi

Faraday’s law – you try V = - Δ Φ / Δt And Δ Φ = Φf – Φi You put a 0.2 m in radius wire loop perpendicular to a 3 T magnetic field. Then something goes horribly wrong! To save the plant you work in you pull the loop from the field as quickly as you can. If you pull the loop free in 1.2 sec then what is the induced emf on that loop during that time?

Faraday’s law – you try V = - Δ Φ / Δt And Δ Φ = Φf – Φi You put a 0.2 m in radius wire loop perpendicular to a 3 T magnetic field. Then something goes horribly wrong! To save the plant you work in you pull the loop from the field as quickly as you can. If you pull the loop free in 1.2 sec then what is the induced emf on that loop during that time? Φf = BA = 0 because you have pulled it out of the magnetic field. Φi = BA = done on board Δ Φ = Φf – Φi = done on board V = - Δ Φ / Δt = done on board

Faraday’s law – you try #2 V = - Δ Φ / Δt And Δ Φ = Φf – Φi You put a 0.2 m in radius wire loop perpendicular to a 3 T magnetic field. Then in 1.2 sec you rotate it to 60 degrees. What is the induced emf?

Faraday’s law – you try #2 V = - Δ Φ / Δt And Δ Φ = Φf – Φi You put a 0.2 m in radius wire loop perpendicular to a 3 T magnetic field. Then in 1.2 sec you rotate it 60 degrees. What is the induced emf? Φi = BA = on board Φf = BAcos(60 degrees) = on board Δ Φ = Φf – Φi = on board V = - Δ Φ / Δt = on board

Coils If you have a coil you do the same thing except that you have more area effectively. N loops gives you N times more area. So: V = - N Δ Φ / Δt

Conclusion We have seen that motions of charges in a magnetic field produce a current and an induced EMF. Magnetic Flux is the amount of magnetic field filled by an area. The induced EMF is just the time rate of change of the Magnetic Flux. Tomorrow: direction of current, transformers, and LR circuits