Induced EMF

EMF

• EMF - It is potential difference and is measured in volts (V ).

Induced EMF

• Electromagnetic Induction - the process of inducing an electric current by suing a changing magnetic field

How is an induced magnetic field related to a magnetic field?

• - The magnitude of induced EMF is proportional to the rate of change of the magnetic field.

• The more rapidly the magnetic field changes, the greater the induced EMF.

Induced EMF is related to magnetic flux

• Flux –means “flow”

• Magnetic flux – measure of the number of magnetic filed lines that pass through a given area.

Calculating magnetic flux – Φ (phi)

• Magnetic filed – B crosses a surface area (A) , at right angle

• magnetic flux Φ = magnitude of magnetic field times area

• Φ = BA

What factors affect magnetic flux , Φ ?

• Magnetic flux depends on the magnitude of the magnetic field, B, its orientation with respect to a surface, θ, and the area of the surface, A

• - if magnetic flux is parallel to the surface then

• Φ = 0

If magnetic flux crosses the surface at an angle θ the formula is :

• Φ = BA cos θ

Faraday’s law relates magnetic flux and EMF

• Any change in magnetic flux Φ will induces voltage (emf) to be "induced" in the coil

• Change in magnetic flux Φ could be produced by:

• 1-changing the magnetic field strength,

• 2-moving a magnet toward or away from the coil,

• 3- moving the coil into or out of the magnetic field,

• 4- rotating the coil relative to the magnet, etc.

• EMF = −N (ΔBA/ Δt)

• N number of loops in the coil• Δt – time

• ΔBA = Φ magnetic flux– B is magnetic field– A is area

• emf is directly proportional to the change in flux

• emf is greatest when the change in time• Δt is smallest—that is, emf is inversely proportional to

Δt

• A coil has N turns, an emf will be produced that is N times greater than for a single coil, so that emf is directly proportional to N ( number of loops)

• EMF = −N (ΔBA/ Δt)

Lenz's law

• Lenz’s law states that an induced current always flow in a direction that opposes the change that cused it.

• The reason for the negative sign in Faraday’s law: EMF = − N (ΔBA/ Δt)

• ( - ) Indicates that the induced current opposes the change in magnetic flux.

Bar magnet being moved in a coil the strength of the magnetic field increases in the coil. The current induced in the coil creates another field, in the

opposite direction of the bar magnet’s to oppose the increase induction opposes any change in flux

a) Magnet moves into the coil –Current moves counter-clockwiseb) Magnet out of coil – current moves clockwise C) Magnet moves out of coil - Which way does the current move?

Faraday's Law