Applied Electricity

Reed relay

• A small current in one circuit is used to switch on a much larger current in another circuit.

• When the small current is switched on the electromagnet A activates and the iron lever is attracted to it. This causes the arm to move which closes the contact B in the other circuit.

• A The bell push is closed which allows a current to flow.

• B The electromagnet becomes magnetized and the armature is pulled to the magnet this makes the hammer hits the gong.

• As this happens the contact is broken so it not a complete circuit so the electromagnet is de-magnetized.

• C The armature is no longer attracted so it jumps back.

• This closes the contact, so electricity flows again and the armature is once more attracted to the electromagnet.The whole cycle is repeated until you stop pressing the bell push.

5-18. Electromagnetic Induction

The effect of producing an induced current is known as electromagnetic induction. The direction of the induced current can be reversed by reversing the motion of the wire or reversing the field direction. The strength of the current depends on the strength of the magnetic field and the speed of the wire's motion.

Electric Motors

An electric motor uses the sideways push of a magnetic field to turn a current-carrying wire loop. Electric motors use a commutator to change the direction of the current in the loop. Alternating current electric motors do not use commutators.

Michael Faraday (1791-1867) built the first electric

motor and discovered magnetic induction.

Loudspeaker – Moving Coil

A moving coil is moved by a changing input current.

The paper cone compresses and rarefies the air.

Input

Microphone

A moving coil activated by voice vibrations is used as a microphone.

The coil induces a current in the magnet that can be amplified or recorded.

Alternating and Direct CurrentAlternating current (ac) is current that flows in a back-and-forth manner; household current changes direction 120 times each second (60 Hz). Direct current (dc) flows in one direction.

The ac generator (or alternator) produces an ac current and can be modified to produce dc current by…..

Use of a rectifier which permits current to pass through it in only one direction.

AC Generator

• As the coil turns the direction and size of the induced current is varied

5-19. Transformers

A transformer is a device composed of two unconnected coils, usually wrapped around a soft iron core, that can increase or decrease the voltage of ac current.

TransformersThis how A.C. changes voltage up or down

V In

V Out

Turns 2

Turns 1=

Better Transformers

• Thick wire to reduce heat loss• Laminate core to reduce EDDY CURRENTS• Soft iron reduce the Hysteresis losses as the

core is magnetised and demagnetised• Figure 8 shape to cut flux leakage

Galvanometer

Device used in the construction of ammeters and voltmeters.

Based on principle that a current in a magnetic field experiences a force

Radial MagneticField

Current loop or coil

Spring

Scale

Galvanometer

Magnet forms a radial field

The springs balance the moment of the force or torque from the current in the magnetic field

Radial MagneticField

Current loop or coil

Spring

Scale

Galvanometer used as Ammeter• Typical galvanometer have an internal resistance of the

order of 60 Ω - that could significantly disturb (reduce) a current measurement.

• Built to have full scale for small current ~ 1 mA or less. • Must therefore be mounted in parallel with a small resistor

or shunt resistor.

Galvanometer 60 W

Rp

Galvanometer 60 W

Rp

• Let’s convert a 60 Ω, 1 mA full scale galvanometer to an ammeter that can measure up to 2 A current.

• Rp must be selected such that when 2 A passes through the ammeter, only 0.001 A goes through the galvanometer.

0.001 60 1.999

0.03002

p

p

A A R

R

• Rp is rather small!• The equivalent resistance of the circuit is also small!

Galvanometer used as Voltmeter• Must mount a large resistor in series to limit the current

going though the voltmeter to 1 mA.• Must also have a large resistance to avoid disturbing

circuit when measured in parallel.

Galvanometer 60 WRs

Galvanometer 60 WRs

Maximum voltage across galvanometer:

max 0.001 60 0.06V A V

Suppose one wish to have a voltmeter that can measure voltage difference up to 100 V:

100 0.001 60

99940

p

p

V A R

R

Large resistance

Photodiode

A photodiode is a PN junction in reverse bias. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a mobile electron and a positively charged electron hole.

LED

• While all diodes release light, most don't do it very effectively. In an ordinary diode, the semiconductor material itself ends up absorbing a lot of the light energy. LEDs are specially constructed to release a large number of photons outwards.

Bipolar Transistor

• The small current between the base and emitter controls the large current from collector to emitter

Transistor as amplifier

• Input signal controls the larger voltage output

• Looks exactly like the input but bigger amplitude

Transistor as switch

• Small change in base voltage turns on larger lamp

• Warnings, alarms and computer circuits

Transistor as switch in ECU

Voltage Inverter

• Input is opposite of the output

• Standard set up for an amplifier as well

Boolean Algebra to Logic Gates

• Logic circuits are built from components called logic gates.

• The logic gates correspond to Boolean operations +, *, ’.

OR+

AND*

NOT’

AND

A

B

A ∩ B

Logic Gate:

Series Circuit:

A B

A B A ∩ B

0 0 0

0 1 0

1 0 0

1 1 1

Truth Table:A∩B

A

B

AUB

Logic Gate:

Parallel Circuit:

A

B

A B AUB

0 0 0

0 1 1

1 0 1

1 1 1

Truth Table:AUB

OR

NOT

A

A’ or Ā

Logic Gate:(also called an inverter)

Single-throwDouble-poleSwitch:

A

A Ā

0 1

1 0

Truth Table:A’ or Ā

Computing Inputs

1101100101

01001101111101110111

10001111

0011110000001100

110001 001110

A bit of binary humor

Full Wave (Bridge) Rectifier

Full Wave Rectifier

Smoothing

• Add capacitor

Faraday's Law

• Any change in the magnetic environment (flux) of a coil of wire will cause a voltage (emf) to be "induced" in the coil.

Faraday's Law

Lenz’s Law

• http://courses.science.fau.edu/~rjordan/rev_notes/movies/MFA07AN1.MOV

• gives the direction of the induced electromotive force (emf) and current resulting from electromagnetic induction. The law provides a physical interpretation of the choice of sign in Faraday's law of induction, indicating that the induced emf and the change in flux have opposite signs.

Induction Motor

• No brushes to wear out• Induction motors use shorted

wire loops on a rotating armature and obtain their torque from currents induced in these loops by the changing magnetic field produced in the stator (stationary) coils

Bars are

really loops

Practical Induction Motor

• This implies that there is no current supplied to the rotating coils. These coils are closed loops which have large currents induced in them because of their low resistance.

Bars are

really loops

Induction Coil

• Father Nicholas Joseph Callan (1799 – 1864) was a Irish priest and scientist from Darver, Co. Louth, Ireland. He was Professor of Natural Philosophy in Maynooth College from 1834, and is best known for his work on the induction coil.

Induction Coil

Induction CoilIt is a type of electrical transformer used to produce high-voltage pulses from a low-voltage DC supply.

To create the flux changes necessary to induce voltage in the secondary, the DC current in the primary is repeatedly interrupted by a vibrating mechanical contact called an interrupter.

As in the relay the coil becomes an electro-magnet

This time it disconnects the supply.

• This turning off and on creates a large current in the greater windings of the secondary