Faraday’s Law of Induction
extNt
ΔΦ= −
ΔE
number of loops
Emf that drives the induced current
How quickly you change the number of external field lines through one loop.
Opposes the changein magnetic flux
(1791 - 1867)
EM Induction: Lenz’s LawThe induced EMF produces a a current that produces a magnetic field
that opposes the original changing magnetic field.
To find the direction of the induced current:1. Determine the flux change: Increasing? Decreasing?2. The induced B field will be opposite. Find that.3. Find the current with the RHR to produce the induced B field.
EM Inductance:TransformersThe AC current is changing in Coil 1, producing a changing Magnetic Flux in Coil 2, which induces a voltage and current. Since Coil 2 has fewer wraps than Coil 1 it will have a smaller Voltage than Coil 1.
Primary Voltage Secondary VoltageNumber of Primary Turns Number of Secondary Turns
=
EM Inductance:TransformersEnergy is Conserved so the Power is the same in both Coils.
If the voltage is less in Coil 2 than in Coil 1, then the current must be greater in Coil 2 in order to keep P = IV constant.
primary secondary(VI) = (VI)
S S
P P
V NV N
=
Voltage and Current Change but Energy & Power DO NOT!
//
S S
P P
P I NP I N
=
P S S S P SP P I V I V= = =
SP
S P
NII N
=
EM Inductance:Transformers
SS P
P
NV VN
=
PS P
S
NI IN
=
Step UP
TransformersSS P
P
NV VN
=
Step UP
Step Down
PS P
S
NI IN
=
Decrease VoltageIncrease Current
Increase VoltageDecrease Current
Power TransmissionStep up Voltage Step Down Current to reduce Power loss.
SS P
P
NV VN
= PS P
S
NI IN
=Transformers
Transformers & Sound Reproduction
Transformers and Auto Ignition
Converting 12V to 40,000V