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Magnetism and Induction

Explore how electromagnetic fields, induction, and Faraday's and Lenz's laws shape current flow and voltage in circuits. Learn about generators and magnetic flux to grasp the principles behind induction phenomena.

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Magnetism and Induction

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

  2. Electromagnetism • Fields are formed around current carrying wires

  3. Solenoids • Fields are formed in solenoids or coils.

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

  5. 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.

  6. 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)

  7. 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

  8. 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

  9. 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)

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

  11. 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

  12. 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!!

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

  14. 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

  15. 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

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

  17. 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.

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

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

  20. 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

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

  22. 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!!

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

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

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