AP Physics Day 38 – Electromagnetic Induction
Date 1/8/2004
Overview:
Finish Magnetism Test Revisions
Notes & Practice – Electromagnetic Induction
Existence of Induction
Lenz’s Law
Faraday’s Law of Induction
HW:
Ch 20:
Papers:
None
Materials:
Demos:
Magnet through copper tube
Magnet & Identical steel pellet
two long thin copper pipes
Push & Pull aluminum loop with a magnet vs. an incomplete loop
complete aluminum loop hanging from a string
incomplete aluminum loop hanging from a string
magnet
Jumping Ring Device
Magnets & Two Coins
Rails, Rods & Magnet Device?
Induction Coil and Telegraph Key?
Series of Coils and Magnet?
Labs:
none
Notes:
Drop Magnet & steel pellet through copper tube
Why does one fall slower? It’s a magnet! So? Copper isn’t a magnetic material!
Unless it has a electric current flowing through it!
But how does it get an electric current flowing through it?
Electromagnetic Induction! Changing magnetic fields create electric fields!
“Push & Pull” aluminum loop with a magnet (vs. an incomplete loop)
As magnet goes in, loop is pushed away – like poles together
As magnet comes away, loop is pulled toward – opposite poles together
Lenz’s Law – changing magnetic field creates loop of current, which tries to resist change in magnetic
field.
Jumping Ring Demo
Explain with Lenz’s law
(look back at copper pipe demo?)
Pick up coins with magnet (peso (steel) vs yen (aluminum))
How does it produce current? By creating a potential difference or emf around the loop – same as a
battery does
How much emf/voltage? Depends on how quickly magnetic field changes
Oscilloscope Demo
Hook Battery to oscilloscope – demonstrate that battery moves light
Hook solenoid to oscilloscope – demonstrate light moves as magnet moves or as loop moves
Series of Coils & Magnet Demo
Since magnetic field does not have to be constant inside loop, we have to look at total amount of field
inside loop: defined as magnetic flux (can think of this as counting up magnetic field lines)
Depends on how strong magnetic field is (stronger means lines closer together – more lines inside loop)
Depends on how big loop is (bigger loop catches more lines through it)
Depends on how loop is oriented (if area directly faces field you get more lines)
= BA cos
Sample Problems
Calculate flux in different situations

E = -N t
Sample Problems
Include current in loop
Rail, Rods & Magnets Demo