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