Magnetism and Induction

Magnetism and Induction NCEA AS 3.6 Text Chapters: 15,16,17

Electromagnetism • Fields are formed around current carrying wires

Solenoids • Fields are formed in solenoids or coils.

Induction • If a wire is moved through a magnetic field then a voltage can be induced across the ends the wire.

Induction • If the wire is connected to a circuit then current will flow. The direction of induced current is determined by a right hand rule.

Right Hand Slap Rule • v=direction of wire movement • B=direction of magnetic field lines • F= force on a positive charge (ie direction of current flow)

Induction • The size of this induced voltage is given by: • V=BvL • (B=mag field strength, • v=velocity of movement, • L=length of wire in field) This is known as Faraday’s Law

Induced current I Opposing Force F=BIL Direction of movement Induction • The direction of the induced current is such that it creates an opposing force on the motion that is causing it. • This is known as Lenz’s Law

Induction • Induced voltage/current can be made larger if: • The mag field is stronger • The wire is longer • The movement is faster • (Solenoid has an iron core)

Induction • Induction can also occur if it is the magnetic field that is moved, rather than the wire.

Magnetic Flux • The magnetic field in a circuit is measured as magnetic flux Φ • Φ= BxA • B = mag field strength • A = area perpendicular to field • The unit for flux is the Weber Wb

Magnetic Flux • A useful analogy is using a net to catch whitebait in a stream…. • If you don’t hold the net straight up and down, you don’t catch many whitebait!!

Speed v L Faraday’s Law • According to Faraday’s Law, V=BvL • The area of the loop in the field is zero…..

v A L Faraday’s Law • Some time t later, A has changed by • ΔA = (vt x L) • ΔΦ = B x ΔA • ΔΦ = B x v x L x t • But V=BvL • So ΔΦ = V x t

Faraday’s Law (again) • Another way to look at Faraday’s Law is that the induced voltage in a circuit is determined by the rate of change of flux The negative sign is a reminder of Lenz’s Law

Generators • Rather than sliding a loop through a field, it is easier to spin it. • This is how a generator works

Generators • When the coil is horizontal, the induced current is maximum, as the coil is cutting across the field lines at right angles as it moves.

Generators • When the coil is vertical, the induced current is zero, as the coil is moving parallel to the magnetic field lines

B A N S Generators • If we start timing from when the coil is vertical, then at t=0, Φ= BxA

B θ N S A Generators • If the coil rotates with speed ω, then after time t the coil will have turned through angle θ = ωt • The flux will now be Φ= Bcosωt x A

Generators • Faraday’s Law says: • For a coil of N turns:

Generators • The formula for alternating generator voltage is often written as: • Where Vmax=BANω • This produces a voltage-time graph that looks like a sine curve • NB. Similarities to SHM!!

Generators • To generate A.C, slip rings are used…

Generators • To generate D.C, split rings are used.

Magnetism and Induction