Electromagnetic induction And you

When you move a magnet in a solenoid, what happens? Solenoid/magnet demo here Look closely, and describe what you see when the magnet moves into the solenoid What happens when the magnet stops? What happens when the magnet is pulled out of the solenoid?

So does the presence of the magnet (alone) induce a current in the wire? No – a changing magnetic field induces a current!!!!!

So if you move a solenoid (or loop) through a magnetic field… That will also induce a current in the wire!!!

Now I know what you are all thinking (Besides “Wozniak has way too much time on his hands”) WHY IS THIS HAPPENING????

Well – we will start with a simple example and define a few things I have a B field inside a partial loop of wire. I complete the circuit with a bar, and move the bar. I induce a separation of charges in the bar (a EMF) ! This results in a current in the circuit (conventional current). Apply the 2 nd right hand rule here

Thumb = motion of charges in bar when you push the bar (to the right) Fingers = magnetic field (into page) Palm = force on + charges in the bar (up)

Therefore the current Moves in a counter clockwise direction here!

Variables that influence the induced EMF The strength of the B field The area traversed The time interval of that traverse B field x area = magnetic flux = ϕ EMF = Δ ϕ Δ t This relationship is known as Faraday’s law

Power generating stations Use a similar process to generate electricity They use a large coil of wire that is made (with mechanical force) to rotate in a magnetic field. Just the opposite process of the motor you constructed.

One last related bit of information Lenz’s law: If a loop of wire experiences a changing magnetic field (B 1 ), then the wire induces a opposing magnetic field (B 2 ) Recall that the changing magnetic field induces a current in the wire – and that current has it’s own magnetic field. Perhaps some examples would best help explain this relationship.

Copper tube and magnet demo here Diagram of Lenses law on board