Download Goal: To understand what Electric Fields are

Goal: To understand Magnetic
Induction
Objectives:
1) Understanding how to Induce a Current
2) Understanding how to Induce a magnetic
field
3) To Learn more about transformers
4) To Understand how to Generate an AC
current
Inducing a current
• There are two ways to have fun with magnetic
fields.
• The first involves a changing magnetic field (in
terms of sign or magnitude).
• Any time a magnetic field changes all electrons
that are around it that are moved.
• This creates a current in any metal object – such
as a wire.
• So, the changing magnetic field creates a
current.
• This is called an induced current.
Induced currents
• So, a magnet traveling through a looped
wire will induce a current.
• The size of the current will depend on only
two things, the rate of change of the
magnetic field (so how quickly the
magnetic field changes) and the area of
the loop.
• The bigger the loop the bigger the current
and the more quickly the magnetic field
changes the bigger the current.
Induced magnetic fields
• Turns out it works both ways.
• A current can induce a magnetic field!
• Yes, that means that the current that the changing
magnetic field creates will create a magnetic field of
its own!
• In fact this induced magnetic field will be the
opposite in sign to the change in the magnetic field.
• So, if the magnetic field that induces the current
increases, then the magnetic field induced from the
induced current will be negative (positive if the
original field decreases).
• THEREFORE, I clearly cannot choose the glass in
front of you!
No, I have not even started yet!
• Wait, I have started, but have not finished
yet…
• Each loop will have its own current.
• If you have 10 loops, you get 10 currents.
• If they are all the same size then you get
the same current in each, and each
current is the same in that loop as it would
be if there was only 1 loop.
If all else is equal
• If all loops have the same amount of
resistance, since V = IR, then every loop
has the same amount of VOLTAGE!
• Note: When you build something that is
near either a changing magnetic field or
something with a current, you need to
watch out for induced currents, and be
sure you have resistance - otherwise….
Transformers revisited
• For a transformer, you have two sides with
different amounts of loops.
• Each loop has the same amount of
voltage!
• So, the more loops, the more voltage you
have on that side.
• That is why the ratio of voltages was the
ratio of the # of loops!
AC only!
• However, since transformers work
because of induced currents, therefore
transformers ONLY work using AC current.
• That is a big part of why we use AC
current.
Generating an AC current
• So, how do you generate an alternating current?
• Way #1 is move a magnet back and forth.
• As it goes in you will increase the magnetic field
and generate a current.
• When it comes back you will decrease a
magnetic field and generate a current which
flows in the opposite direction.
• This gives you a current which alternates from
positive to negative.
2nd way
• The 2nd way is to leave the magnetic but rotate
the loop.
• As you rotate the loop, the area the magnet sees
changes (drops to 0 when it is flat) and then will
be opposite when the loops is upside down.
• This is the most common way to generate
electricity – all you need is a way to turn the
loop.
• Ways to do this are to use steam or flowing
liquid water.
Conclusion
• We have learned about induction both
from induced magnetic fields and induced
magnetic currents.
• We have seen why transformers work.
• We have learned how to create an AC
circuit.