Electromagnetic
Induction
Any current or moving charge is surrounded
by a magnetic field. Likewise any moving
magnet or changing magnetic field creates a
current in a closed circuit.
Which is not an example of
induction
a. Walking past a magnet at a store with a
stolen good which has a metal tag inside of it
b. swiping a credit card
c. using your flash (jump) drive
d. touching a tablet screen
vocab
Induced= created
Emf= created voltage
Eddy current= created current
Electromotive force = emf
Solenoid = loop of wires
Flux = magnetic field per area
IF moving charges (current)in a wire produces
Induction
magnetism
around it question
What is created by moving a magnet past a bare
wire?
A. changing magnetic field
B. Voltage
C. An force on the charges in the wire
D. All of above
E. Only A and B
Flux : F = BAcosq
Analogy: B = rain drops
A = size of bucket
q = bucket tilt from vertical
Faraday’s Law
A voltage can be created in anything which
is exposed to a changing magnetic flux. If
that thing is conductive, a current will also
be induced.
The changing magnetic flux can be
changing in either the magnetic field (B)
or the area of magnetic field intersected.
How can you change the magnetic
field near a wire?
A. move a magnet closer or away from the wire
B. Rotate the magnet or the wire
C. Suddenly turn on or off the magnet if it’s an
electromagnet
D. Use an ac current near the wire
E. All of the above Shirlock
Lenz’s Law
Energy is still conserved so
a. it takes work to create new current
b. New current will oppose the change
(doesn’t always mean opposite directions)
Induction Matters!
Cool: make current w/o a battery!
* generators/flashlights/wireless power
* transformers to modify voltage
* resistive force w/o physical contact
Not cool: new current will oppose
whatever started it
* voltage = applied – induced
* random currents if change things
Faraday: math equation
• Emf induced in each coil of the conductor
is proportional to rate of change of
magnetic flux
• Emf = - DF/Dt
-D(NBA/t)
• N = # of wire loops
B = magnetic field
A = area of loop
t= seconds changed
How to change B flux
and capture it in a wire
1. Move the magnet or wire loop (change B lines)
2. Rotate the magnet or loop (change area)
3. Use two coils and change current of one
two versions of emf
1. solenoid containing “N” loops of wire:
catches a changing magnetic field or area
emf = -NA (DB/t) or -NB D A/ D t
2. Wire of length “L” moves distance Dx in Dt
So………..area = L*Dx, v = Dx/t,
emf= -B LDx /Dt = -BLv
Lenz’s Law
The induced emf resulting
from a changing magnetic
flux has a polarity that leads
to an induced current whose
direction is such that the
induced B – field opposes the
original flux change.
Lenz’s Law example
drop north magnet into coil on page
Coil sees increase in flux into page
New current must offset flux increase
by creating a B field out of page
hence current is counter clockwise
Induction by pulsing a current
• Left wire connect to a battery
Right side is just a wire
If current stays on or off, no new magnet lines going into right side so no
new current
But if suddenly change current, new magnet lines appear or disappear,
and current is induced in the right side!
Transformers: ac current induces new ac
current of different voltage
flux in = flux out
Vp /Np = Vs /Ns
Power in = power out
Ip*Vp = Is*Vs
Transformer questions
primary coil: 200V, I=1A, N=10 loops
Secondary: ? V, I = ??, N = 5 loops
Summary
1. Moving charge (current) produces magnetic field
- direct current produces a constant B
-alternating creates an alternating B field,
which then induces a counter current
2. Voltage or emf is generated in objects sitting in a changing
magnetic field
a. magnet or wire is moving
b. dc electromagnet is pulsed
c. use an ac electromagnet