Electromagnetic Induction

Any current or moving charge is surrounded by a magnetic field. Likewise any moving magnet or changing magnetic field creates a current in a closed circuit.

Which is not an example of induction

a. Walking past a magnet at a store with a stolen good which has a metal tag inside of it

b. swiping a credit card

c. using your flash (jump) drive

d. touching a tablet screen

vocab

Induced= createdEmf= created voltageEddy current= created currentElectromotive force = emfSolenoid = loop of wiresFlux = magnetic field per area

Induction questionIF moving charges (current)in a wire

produces magnetism around it

What is created by moving a magnet past a bare wire?

A. changing magnetic field

B. Voltage

C. An force on the charges in the wire

D. All of above

E. Only A and B

Flux : F = BAcosqAnalogy: B = rain drops A = size of bucket q = bucket tilt from vertical

Faraday’s LawA voltage can be created in anything which is exposed to a changing magnetic flux. If that thing is conductive, a current will also be induced.

The changing magnetic flux can be changing in either the magnetic field (B) or the area of magnetic field intersected.

How can you change the magnetic field near a wire?

A. move a magnet closer or away from the wire

B. Rotate the magnet or the wire

C. Suddenly turn on or off the magnet if it’s an electromagnet

D. Use an ac current near the wire

E. All of the above Shirlock

Lenz’s Law

Energy is still conserved so

a. it takes work to create new current

b. New current will oppose the change

(doesn’t always mean opposite directions)

Induction Matters!Cool: make current w/o a battery! * generators/flashlights/wireless power * transformers to modify voltage * resistive force w/o physical contact

Not cool: new current will oppose whatever started it

* voltage = applied – induced * random currents if change things

Faraday: math equation

• Emf induced in each coil of the conductor is proportional to rate of change of magnetic flux

• Emf = - /DF Dt - ( /D NBA t)

• N = # of wire loops A = area of loopB = magnetic field t= seconds changed

How to change B fluxand capture it in a wire1. Move the magnet or wire loop

(change B lines)

2. Rotate the magnet or loop (change area)

3. Use two coils and change current of one

two versions of emf

1. solenoid containing “N” loops of wire: catches a changing magnetic field or area emf = -NA (DB/t) or -NB D A/ D t

2. Wire of length “L” moves distance Dx in Dt So………..area = L*Dx, v = Dx/t, emf= -B LDx /Dt = -BLv

Lenz’s Law

The induced emf resulting from a changing magnetic flux has a polarity that leads to an induced current whose direction is such that the induced B – field opposes the original flux change.

Lenz’s Law example

drop north magnet into coil on page

Coil sees increase in flux into page

New current must offset flux increase by creating a B field out of pagehence current is counter clockwise

Induction by pulsing a current• Left wire connect to a battery

Right side is just a wire If current stays on or off, no new magnet lines going into right side so

no new current

But if suddenly change current, new magnet lines appear or disappear, and current is induced in the right side!

Transformers: ac current induces new ac current of different voltage

flux in = flux out Power in = power out

Vp /Np = Vs /Ns Ip*Vp = Is*Vs

Transformer questions

primary coil: 200V, I=1A, N=10 loops

Secondary: ? V, I = ??, N = 5 loops

Summary

1. Moving charge (current) produces magnetic field

- direct current produces a constant B

-alternating creates an alternating B field, which then induces a counter current

2. Voltage or emf is generated in objects sitting in a changing magnetic field

a. magnet or wire is moving b. dc electromagnet is pulsed c. use an ac electromagnet