1 Introduction to Electricity John R. Ebden Seneca College.

1

Introduction to ElectricityIntroduction to Electricity

John R. EbdenJohn R. Ebden

Seneca College

Introduction to Electricity 2

What is electricity?What is electricity?

Static electricityStatic electricity

CurrentCurrent

VoltageVoltage

ResistanceResistance

ACAC

DCDC

?


Atoms and electronsAtoms and electrons

Electronics is based on controlling the Electronics is based on controlling the movement of electrons movement of electrons


Static electricity Static electricity

Rubbing objects can cause electrons to be Rubbing objects can cause electrons to be added or removed from the object. added or removed from the object.

The object then becomes charged The object then becomes charged because it no longer has the same number because it no longer has the same number of electrons as protons.of electrons as protons.

+ - + +Unlike charges attract Like charges repel



When a plexiglass rod is rubbed with a silk When a plexiglass rod is rubbed with a silk cloth, electrons move from the rod to the cloth, electrons move from the rod to the cloth, leaving the rod positively charged, and cloth, leaving the rod positively charged, and the cloth negatively charged. the cloth negatively charged.



If the now positively charged rod is placed If the now positively charged rod is placed near your hair, it will cause the electrons in near your hair, it will cause the electrons in your hair to move towards the end of the hair. your hair to move towards the end of the hair.

Your hair will become attracted to the positive Your hair will become attracted to the positive charged rod making it stand up.charged rod making it stand up.



Static electricity occurs when electric charges Static electricity occurs when electric charges build up on an object, but the electric charges build up on an object, but the electric charges cannot move around. cannot move around.

He is charged up !! He just lost his charge !!



Static electricity is a big problem for Static electricity is a big problem for companies that deal with non-conducting companies that deal with non-conducting materials like plastics. materials like plastics.

The high voltages can destroy sensitive The high voltages can destroy sensitive electronic componentselectronic components

At companies like Celestica personnel must At companies like Celestica personnel must wear ESD coats, work on ESD surfaced wear ESD coats, work on ESD surfaced tables, and wear grounded wrist strapstables, and wear grounded wrist straps


Current electricityCurrent electricity

There is another type of electricity, current There is another type of electricity, current electricity, which consists of a continuous flow electricity, which consists of a continuous flow of electrons. of electrons.

It requires a source of electrons and a pathway It requires a source of electrons and a pathway to carry or conduct them. to carry or conduct them.

The source of electrons might be a battery or a The source of electrons might be a battery or a generator.generator.

G



Current electricity must flow along a Current electricity must flow along a pathway, usually metal, called a conductor. pathway, usually metal, called a conductor.

Copper , aluminium , and gold are common Copper , aluminium , and gold are common conductors. conductors.

Some materials prevent the flow of electrons Some materials prevent the flow of electrons and are called insulators. and are called insulators.



A bulb or an electric appliance may be part A bulb or an electric appliance may be part of that pathway. Electrons are returned to of that pathway. Electrons are returned to the battery or generator along the pathway the battery or generator along the pathway completing the electric circuit. completing the electric circuit.


Voltage Voltage

Current electricity flows because there is a Current electricity flows because there is a difference in electrical pressure or potential difference in electrical pressure or potential between two points along a conductor. between two points along a conductor.

This potential difference is called voltage. This potential difference is called voltage.


Direct current (DC) and Direct current (DC) and alternating current (AC) alternating current (AC)

Electric circuits can be connected to Electric circuits can be connected to either a direct current (DC) or an either a direct current (DC) or an alternating current (AC) electricity supply.alternating current (AC) electricity supply.


Direct current (DC) Direct current (DC)

The stream of electrons flow in only one The stream of electrons flow in only one direction around the circuit, from the direction around the circuit, from the negative terminal to the positive terminalnegative terminal to the positive terminal


Direct current (DC) Direct current (DC)

In the early days, the concept of electron In the early days, the concept of electron flow was not fully understood so scientists flow was not fully understood so scientists randomly decided that current in a conductor randomly decided that current in a conductor flowed from the positive terminal and into the flowed from the positive terminal and into the negative terminal. negative terminal.

It is still convention today to show current It is still convention today to show current flowing in this direction (ie. opposite to flowing in this direction (ie. opposite to electron flow). electron flow).


Alternating current (AC) Alternating current (AC)

The electrons move backwards and forwards The electrons move backwards and forwards around an electric circuit in this casearound an electric circuit in this case


How electricity is madeHow electricity is made

AC (most common) is made using a AC (most common) is made using a generator.generator.

DC is made by:DC is made by: batteries (chemical reaction)batteries (chemical reaction) power supplies (rectification of AC)power supplies (rectification of AC)


DC Sources - Batteries DC Sources - Batteries


DC Sources – Power DC Sources – Power Supplies Supplies


Making ACMaking AC

AC Electricity can be made or generated by AC Electricity can be made or generated by moving a wire (conductor) through a moving a wire (conductor) through a magnetic field. magnetic field.


MagnetismMagnetism

A bar magnet has a north and south pole. It A bar magnet has a north and south pole. It is placed under a sheet of paper and iron is placed under a sheet of paper and iron filings are sprinkled over the top of the filings are sprinkled over the top of the paper.paper.

These lines of filings show the magnetic field These lines of filings show the magnetic field around the magnet.around the magnet.


Making ACMaking AC

If a coil of wire is moved within a magnetic If a coil of wire is moved within a magnetic field electricity is produced in the coil.field electricity is produced in the coil.

As loop is “cutting” more lines of magnetic As loop is “cutting” more lines of magnetic field when it is horizontal, the current is a field when it is horizontal, the current is a maximum.maximum.


Making ACMaking AC

When loop is vertical no magnetic field lines When loop is vertical no magnetic field lines are being “cut”, and current is zero.are being “cut”, and current is zero.


Making ACMaking AC

When loop has rotated 180When loop has rotated 180oo it is flat again. it is flat again. Current is now maximum but in the opposite Current is now maximum but in the opposite

direction.direction.


Making ACMaking AC

The current produced changes direction The current produced changes direction every half turn (180 degrees ). This is called every half turn (180 degrees ). This is called alternating current or AC. alternating current or AC.

The generators at large power stations The generators at large power stations produce nearly all the electricity we use in produce nearly all the electricity we use in this way. this way.


Making ACMaking AC

The voltage in the loop varies with angle as The voltage in the loop varies with angle as shown. shown.


Making ACMaking AC

1 rev = 1 cycle = 360 degrees1 rev = 1 cycle = 360 degrees

domestic supply = 60 cycles/sec = 60 Hzdomestic supply = 60 cycles/sec = 60 Hz

loop rotates at 60 rev/sec = 3600 rpmloop rotates at 60 rev/sec = 3600 rpm


Making ACMaking AC

In many parts of Europe the frequency In many parts of Europe the frequency employed is 50 Hertz (cycles/sec)employed is 50 Hertz (cycles/sec)


Making ACMaking AC

3 phase current is used to supply 3 phase current is used to supply commercial buildings as it is more efficient.commercial buildings as it is more efficient.

Produced by 3 loops rather 1Produced by 3 loops rather 1


CurrentCurrent

Can compare water flowing through a pipe Can compare water flowing through a pipe with current in a conductor “flow of electrons”with current in a conductor “flow of electrons”


CurrentCurrent

A flow meter measures the water flow in A flow meter measures the water flow in litres/seclitres/sec

An ammeter measures the current in AAn ammeter measures the current in A(1 A = 6.24 (1 A = 6.24 10 101818 electrons/sec) electrons/sec)


CurrentCurrent

1 Coulomb (C) = charge carried by 6.24 1 Coulomb (C) = charge carried by 6.24 10 101818 electronselectrons

1 Ampere = 1Coulomb/second (A = C.sec1 Ampere = 1Coulomb/second (A = C.sec-1-1))


CurrentCurrent

The movement of charge is called electric The movement of charge is called electric current.current.

The more electrons per second that pass The more electrons per second that pass through a circuit, the greater the current.through a circuit, the greater the current.

Current is the rate of flow of charge.Current is the rate of flow of charge.

I = Q/tI = Q/t


VoltageVoltage

A pump can be used to cause the water to A pump can be used to cause the water to flow in the pipe.flow in the pipe.

The force that makes the water flow is called The force that makes the water flow is called pressure (measured in Pa or psi)pressure (measured in Pa or psi)


VoltageVoltage

A generator is an electrical “pump”A generator is an electrical “pump” The electrical pressure is called electrical The electrical pressure is called electrical

potential or voltage (measured in V)potential or voltage (measured in V)


VoltageVoltage

Force between charges is:Force between charges is:

F = kQF = kQ11QQ22/r/r22

To separate unlike charges To separate unlike charges (or bring like charges (or bring like charges together) work is donetogether) work is done

W = Force x distance W = Force x distance

W = W = FdxFdx


VoltageVoltage

When two objects have a difference in When two objects have a difference in charges, we say they have a potential charges, we say they have a potential difference or voltage between them.difference or voltage between them.

The unit of voltage is the volt.The unit of voltage is the volt.

Thunderclouds have hundreds of millions of Thunderclouds have hundreds of millions of volts between them. volts between them.


VoltageVoltage


VoltageVoltage

A difference in potential energy is defined as A difference in potential energy is defined as voltage.voltage.

The voltage between two points is one volt if The voltage between two points is one volt if it requires one joule of energy to move one it requires one joule of energy to move one coulomb of charge from one point to another.coulomb of charge from one point to another.

VV = Work/Charge or V = W/Q = Work/Charge or V = W/Q Volt = Joule/CoulombVolt = Joule/Coulomb Voltage is defined between points.Voltage is defined between points.


Ammeters and voltmetersAmmeters and voltmeters

Ammeters are connected into the circuit. The Ammeters are connected into the circuit. The circuit must be “broken” to put one in.circuit must be “broken” to put one in.

Voltmeters are connected across Voltmeters are connected across components to measure voltage. No need to components to measure voltage. No need to break the circuitbreak the circuit

Note: voltage does not flow, current does


Measuring Measuring voltagevoltage

Voltmeter is placed Voltmeter is placed in parallelin parallel

No need to change No need to change circuitcircuit


Measuring Measuring currentcurrent

Ammeter is Ammeter is placed in placed in seriesseries

Will need to Will need to “break” the “break” the circuitcircuit



When water flows in a pipe, its flow is When water flows in a pipe, its flow is impeded by factors such as the size of the impeded by factors such as the size of the pipe, roughness of the pipe, etc.pipe, roughness of the pipe, etc.

Anything that impedes or resists current flow Anything that impedes or resists current flow in a circuit is called resistance. in a circuit is called resistance.



Electrical resistance is measured with an Electrical resistance is measured with an ohmmeter. Units are ohms (ohmmeter. Units are ohms ())

The power must be off before making any The power must be off before making any measurements.measurements.


Ohm’s LawOhm’s Law

Relates current, voltage and resistance:Relates current, voltage and resistance:

I = V/R or, V = IR or, R = V/I I = V/R or, V = IR or, R = V/I


Example - What is the Example - What is the current?current?

Lamp has resistance of 96 Lamp has resistance of 96 and battery is 12 V and battery is 12 V

I = 12V/96I = 12V/96 = 0.125A = 125x10 = 0.125A = 125x10-3-3A = 125mAA = 125mA


Meters - Analogue (VOM)Meters - Analogue (VOM)


Meters - Digital (DMM)Meters - Digital (DMM)


Meters - OscilloscopeMeters - Oscilloscope

Used for analysing waveforms

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1 Introduction to Electricity John R. Ebden Seneca College.

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