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TRANSFORMERS

K Warne

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Magnetic Effect A magnetic field is generated around any conductor

when an electric current flows through it.

Electric current

Wire with current coming towards you Wire with current

going away from you

x

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Mutual Induction

• MUTUAL INDUCTION is defined as the changing electric …………………… producing a changing …………………….. field which can produce a changing ………………….. in another conductor.

• This is the basic principle of the ………………………

~

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Transformers• Coils of insulated conducting wire are wound

around a ring of iron constructed of thin isolated laminations or sheets.

• The laminations minimize eddy currents in the iron.

• Eddy currents are circulatory currents induced in the metal by the changing magnetic field. These currents produce an undesirable by-product—heat in the iron.

• Energy loss in a transformer can be reduced by using thinner laminations, very “soft” (low-carbon) iron and wire with a larger cross section, or by winding the primary and secondary circuits with conductors that have very low resistance.

• Transformers used to transmit and distribute power are commonly 98 to 99 percent efficient.

• While eddy currents are a problem in transformers, they are useful for heating objects in a vacuum.

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SOLENOID A coil generates a very concentrated (strong)

magnetic field in its center.Electric current

x

N

S

x

x

Increasing the number of coils strengthens the magnetic field.

The Right Hand Rule can also be applied to a solenoid!

INSIDE THE COIL THE FIELD GOES FROM SOUTH TO NORTH!!!

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Transformers• Ideally – all magnetic flux from primary coil links with secondary coil.• The number of turns of wire is directly proportional to the voltage in the coil.

Np Vp

Ns Vs

— = —

How many turns of wire would you need in a transformer that has 1000 turns in the primary and needs to step down the voltage from 220V to 9V?

........... Voltage

A. C in.

............. Voltage A. C

OUT.

STEP ................TRANSFORMER

..........

turns.........

. turnsVp = Vs

Np Ns

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TransformersFrom Faraday’s Law:“The size of the induced current is directly proportional to the rate of change of

the magnetic flux linkage.”• Therefore: work done by primary = energy gained by secondary

Since W = QV and Q = It .: W = VIt

Wp = Ws

......... = ...........

........... = .............

......... = ...........

Calculate the current in the secondary coil of the 220V/9V transformer if the current in the primary was 1.5A

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Hi -

This is a SAMPLE presentation only.

My FULL presentations, which contain loads more slides and other resources, are freely available on my resource sharing website:

www.sciencecafe.org.za

(paste into your browser if link above does not work)Have a look and enjoy!

Keith Warne