physics2 lecture

8/2/2019 Physics2 Lecture

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Table of Contents

I. Lesson 1 : Fundamentals of Electricity

i. What is matter and atom?

ii. What are electrons, protons, neutrons?

iii. What are classifications of electrical materials?

iv. What are resistance, current, and voltage?


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Table of Contents

II. Lesson 2: Current

i. What is Electric Charge?

ii. What is Current and Current Flow?

iii. What is Voltage Source?


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What is Matter?

Anything that occupies space and has weight.

It maybe found in any one of three states: solid, liquidorgas.

Example : the air we breath, water we drink, the clothing wewear and even ourselves

Matter may be either an elementor a compound


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What is Atom?

An ATOM is the smallest particle of an element thatretains the characteristics of the element.

Is made up of protons, neutrons, and electrons.

The protons and neutrons group together to form the

center of the atom called nucleus.

The electrons orbit the nucleus in shells located atvarious distances from the nucleus.


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What is Element?

The basic building block of nature.

It is a substance that cannot be reduced toa simpler substance by chemical means.

There are now more than a 100 known elements(Periodic Table).

Example : gold, silver, copper, and oxygen.


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What is Compound?

chemical combination of two or more elements.can be separated by chemical but not by physicalmeans.

example : water which consist of hydrogen and oxygen, andsalt, which consists of sodium and chlorine.


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What is Molecule?

The smallest partof the compound that still retainsthe propertiesof the compound.

The chemical combination of two or more atoms.


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Atom1

Atom1 & 2

Atom

1

Atom

1 & 2

Atom2

Atom

2

Molecule

Element 2Element 1

Compound

Figure 1-1 An illustration of a chemical combination


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What is Mixture?

The physicalcombinationof elements andcompound.

Example: air which is made of oxygen , nitrogen, carbon

dioxide, and other gases and salt water, which consists of

salt and water.


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Exercise Questions:

1. In what formscan matter be found?

2. What is a substancecalled that cannot be reducedto a simpler substance by chemical means?

3. What is the smallest possible particle that retains

the characteristics of a compound?

4. What is the smallest possible particle that retainsthe characteristics of an element?


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A Closer Look @ ATOMS

an atom is the smallestparticleof an element.

Atoms of different elements differ from each other.

If there are over 100 known elements, then there are

over 100 known atoms.


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The Nucleusis located at the center of the atom.It contains positively charged called Protonsanduncharged particles Neutrons.

Negatively charged particles called Electronsorbit

around the nucleus.

The number of protons in the nucleus of the atomcalled the elements Atomic Number.


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The Atomic Weight is the mass of the atom.

It is determined by the totalnumber of protons andneutrons in the nucleus.

The electrons orbit in concentric circlesabout the

nucleus.

Each orbit is called a Shell.

These shells are filled in sequence; K is filled first, then

L, M, N, and so on (figure 1-3).


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The outer shell is called the Valence Shell and the number

of electrons it contain is the Valence.

The farther the valence shell is from the nucleus, the lessattraction the nucleus has on each valence electron.

Conductivity of an atom depends on its valence band.The greater the number of electrons in the valence shell is theless conductive than an atom having three electrons in thevalence shell.


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Shell

DesignationTotal Numberof Electrons

K 2

L 8

M 18

N 32

O 18

P 12

Q 2

Number of electrons each shell can accommodate


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Conductor

Materials that contain alarge number of free

electrons.

Material Conductance

Silver High

Copper

Gold

Aluminum

Tungsten

Iron

Nichrome Low


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Insulator

The opposite of conductors prevent the flow ofelectricity.

Insulators are stabilized by absorbing valenceelectrons from other atoms to fill their valence shells,

thus eliminatingfree electrons.

Mica is the best insulator because it has the fewestfree electrons in its valence shell.


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Insulator MaterialInsulation

Properties

Mica High

Glass

Teflon

Paper (Paraffin)

Rubber

Bakelite

Oils

Porcelain

Air Low


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Semiconductor

Halfway between conductors and insulators.

Neither good conductors nor good insulators but areimportant because they can be altered to function asconductor or insulator.

Silicon and Germanium are two semiconductormaterials


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Semiconductor

An atom that has the same number of electrons and protonsis said to be electrically balanced.

A balance atom that receives one or more electrons is nolonger balanced. It is said to be negatively charged and iscalled a Negative Ion.

A balance atom that loses one or more electrons is said tobe positively charged and is called a Positive Ion.

The process of gaining or losing electrons is called

Ionization.


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Exercise Questions:

1. What atomic particle has a positive chargeand a large

mass?

2. What atomic particle has no chargeat all?

3. What atomic particle has a negative chargeand a small

mass?4. What does the numberof electronsin the outermost

shelldetermine?

5. What is the term for describing the gainingand losingof

electrons?


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Current

Given an appropriate external force, the movementofelectrons is fromnegatively chargedto positivelychargedatoms.

The symbol (I), is used to represent current.

The amount of current is the sum of the chargesof themovingelectrons past a given point.


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Current

An electron has a very small charge, so the charge of6.24x 1018 electrons is added together and called aCoulomb (C).

When one coulomb of charge moves past a singlepoint in one second it is called an Ampere (A).

The ampere is named for a French physicist, AndreMarie Ampere(1775-1836).

Current is measured in amperes.


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Voltage

When there is an excess of electrons(negative charge)at one end of a conductor and a deficiency of electrons(positive charge) at the opposite end, a current flowsbetween the two ends.

A current flows through the conductor as long as thiscondition persists.


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Voltage

The source that creates this excess of electrons at oneend and the deficiency at the other end represents thePotential.

The potential is the ability of the sourceto perform

electrical work.

The actual work accomplished in a circuit is a result ofthe difference of potential available at the two ends of aconductor.


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Voltage

It is this Difference of Potential that causes electrons to move

or flow in a circuit. (fig.1-8)

The difference of potential is referred to as ElectromotiveForce (emf) or Voltage.

Voltage is the force that moves the electrons in the circuit.

The symbol E is used in electronics to represent voltage.

The unit for measuring voltage is the volt (V) named forCount Alessandro Volta (1745-1827), inventor of the first cellto produce electricity.


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LOAD

Difference

of Potential

Electrons flow in a circuit because

of the difference of potential


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Resistance

Opposition to the flow of electrons (the current).

Resistance is measure in Ohms, a unit named for theGerman physicist George Simon Ohm (1787-1854). Thesymbol for the ohm is the Greek letter Omega ().

Every material offers some resistance or opposition tocurrent flow.

The degree of resistance of a material depends on its size,shape, and temperature.


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Resistance

Materials with low resistanceare called Conductors.

Conductors have many free electrons and offer littleresistance to current flow.

Materials with high resistanceare called Insulators.

Insulators have few free electrons and offer highresistance to current flow.


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Exercise Questions:

1. What is the device called that supply a voltage?

2. What symbol is used to represent voltage?

3. What is the term used to describe oppositionto currentflow?

4. What is the main difference between conductors andinsulators?

5. What is the symbol used to representresistance?


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Electric Charge

Two electrons together or two protons together

represent likecharges.

Like charges resist being brought together and insteadmove away from each other. This movement is calledRepelling.

this is the first law of electrostatic charges: likecharges repel each other.

According to the second law of electrostatic chargesunlike charges Attracteach other.


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Electric Charge

The amount of attracting or repelling force that acts between

two electrically charged bodies depends on two factors: theirchargeand the distancebetween them.

Single electrons have a charge too small for practical use.The unit adopted for measuring charges is the Coulomb (C) ,named for Charles Coulomb.

The electrical charge (Q) carried by6,240,000,000,000,000,000 electrons(six quintillion, twohundred eighty quadrillion or 6.24x1018) represents onecoulomb.


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+ +

1 C = 6.24 x 1018 electrons

+

Unlike Charges Attract Each Other

Basic Laws of Electrostatic Charge


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Current

Electrical charges are created by the displacement ofelectrons.

When there is an excessof electrons at one point and adeficiencyof electrons at another point, a difference of

potential exists between two charged bodies connected by aconductor, electrons will flow along the conductor.


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Current Flow

An electric current consist of the drift of electronsfroman area of negative charge to an area of positivecharge.

The measure of current flow is the Ampere (A).

An ampere represents the amount of currentin aconductor when one coulomb of charge moves past apoint in one second.


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Current Flow

The relationship between amperes and coulombs persecond can be expressed as:

I = Q / t

Where: I - current measured in ampereQ - quantity of electrical charge in coulombst - time in seconds


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Examples

Example 1:

What is the current in amperes if 9 coulombs of charge flowpast a point in an electrical circuit in 3 sec.? Answer is 3A

(amperes)

Example 2:

A circuit has a current of 5 amperes. How long does it takefor one coulomb to pass a given point in the circuit?

Answer is 0.2 s


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Current Flow

Engineers and scientist once thought that current flowedin a direction opposite to electron flow.

Later work revealed that the movement of an electronfrom one atom to the next created the appearance of a

positive charge called Hole, moving in the oppositedirection


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Cu

Cu

Cu

-

+

-

+

Electron

Copper

Atom

Hole

As electrons move from one atom to another, they

create the appearance of positive charge , called a Hole.

N ti P iti


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+ -

+

+

+

+

-

--

-

--

++

+ -

NegativePotential

Conductor PositivePotential

Hole FlowElectron Flow

Current Flow

Electron movement occurs in the opposite direction to hole movement


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Current Flow

These new free electrons travel toward the positive endof the conductor and collidewith other atoms.

The electrons drift from the negative to the positive end ofthe conductor because like charges repel.

In, addition, the positive end of the conductor whichrepresents a deficiency in electrons, attracts the freeelectrons because unlike charges attract.


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For example, visualize a long hollow tube filled with Ping-pongballs.

As a ball is added to one end of the tube, a ball is forced outthe other end of the tube.

Although an individual ball takes time to travel down the tube,the speed of its impact can be far greater.

Ping-Pong

Balls

Hollow

Tube


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Electrons in a conductor react like Ping-Pong balls

in a hollow tube.


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Voltage Source

The devicethat supplies electronsfrom one end of a conductor (thenegative terminal) and removesthem from the other end of the

conductor (the positive terminal.

It can be thought of asa kind of pump


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Exercise Questions:

1. What are the two lawsof electrostatic charges?

2. What does an electrical chargerepresent?

3. Define coulomb?


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Table of Contents

I. Lesson 1 : Voltagei. What is voltage?

ii. Common Voltage Sources

iii. Cells and Batteries

iv. Series, Parallel, Series-Parallel Connections

v. Voltage Rise & Voltage Drop


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Table of Contents

II. Lesson 2: Resistancei. What is resistance?

ii. Things that Affects Resistance

iii. Resistors

iv. Major Resistor Categories

v. Resistor Bands

vi. Connecting Resistors


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Table of Contents

III. Lesson 3: Ohms Lawi. What is Ohms Law?

ii. Ohms Law Formula

iii. Application of Ohms Law

iv. Kirchhoffs Current Law

v. Kirchhoffs Voltage Law


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Table of Contents

IV. Lesson 4 : Poweri. What is Power?

ii. Power Formula

iii. Power Application


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What is Voltage?

Voltage or difference of potential, is the forcethat causes the electrons to move in acommon direction.


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Common VoltageSources

1. Friction- is the oldest known method of producing

electricity.


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2. Magnetism

- is the most common method of producingelectrical energy today.

The Generator is based from this principle.


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3. Chemical / Chemical Cell

- the second most common method of

producing electricity.


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4. Light Energy

- can be converted directly to electrical energy bylight striking a photosensitive substance in a

photovoltaic cell.



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5. Heat Energy

- can be converted directly to

electricity with a device called athermocouple.


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6. Pressure

- is applied to a certain crystalline material suchas quartz, tourmaline, or barium titanate, a small voltage

is produced.


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Cells and Batteries

A Battery is a combination

of two or more cells.

Cells that cannot be

recharged are calledPrimary Cells.

Cells that can be rechargedare call Secondary Cells.


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Connecting Cellsand Batteries

cells or batteries can be connected in either series-aiding or series-opposing configurations.


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Series-Aiding

In a series-aiding configuration the same current

flows through all the cells or batteries. This can beexpressed as:

The Total Voltage is the sum of the individual cellvoltages and can be expressed as:

IT = I1 = I2 = I3

ET = E1 + E2 + E3


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Series-Opposing

All the positive terminals are connected together andall the negative terminals are connected together.

The total current available is the sum of the individualcurrents of each cell or battery. This can be

expressed as:

IT = I1 + I2 + I3


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Voltage Rise & Voltage Drop

VOLTAGE RISE

Potential Energy introducesinto a circuit

VOLTAGE DROP

Potential Energy used up in acircuit.


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What is Resistance?

Resistance is opposition to the flow of current. Everymaterial offers some resistance or opposition to the

flow of the current.


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Things that AffectsResistance

1. Size & Type of Wires- are chosen to keep the electrical resistance as

low a possible. In an electric circuit, the larger thediameter of the wire, the lower the electrical

resistance to current flow.


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Things that AffectsResistance

2. Temperature- affects the resistance of an electrical conductor.

In most conductors, resistance increases withtemperature.


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Resistors

Resistors are components manufactured to possessa specific value of resistance to the flow of current.

A resistor is the most commonly used component inan electronic circuit.


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Resistors Tolerance

A Resistors Tolerance is the amount that the resistor

may vary and still be acceptable.

Resistors are available with tolerances of +20%,+10%,+5%, +2% and +1%.

Precision resistors are available with even smallertolerance.

In most electronic circuits, resistors of 10% toleranceare satisfactory.


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Major Resistor Categories

Molded Carbon Composition ResistorMost commonly used resistor in electroniccircuits.

It is inexpensive and is manufacturedin the standard resistorvalues


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Wirewound Resistor

Constructed of a nickel-chromium alloy wirewound on a ceramic form.

These kinds of resistors are used for high-

current circuits where precision is necessary.


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Film Resistors

Film resistors are becoming increasingly popular.

They offer the small size of the compositionresistor with the accuracy of the wirewound

resistor.


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Resistor Identification

Color-coded strip system is used to display theresistor value.

The strips can be seen and read in any position thatthe resistor is placed.


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STANDARD RESISTOR COLOR CODE

Two significant-Figures Color Code


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Reading a Resistors Value

The First Band, closest to the end of the resistor,represents the first digit of the resistor value.

Note: The 1stband cant have the color Black, Silverand Gold.

The Second Band represents the second digit of theresistor value.


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Reading a Resistors Value

The Third Band represents the number of zeros to beadded to the first two digits.

The Fourth Band represents the tolerance of theresistor.


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Connecting Resistors

A Series Circuit contains two or more resistor and

provides one path for current to flow.

The current flows from the negative side of thevoltage source through each resistor to the positive

side of the voltage source.


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Connecting Resistors

A Parallel Circuit contains two or more resistors and

provides two or more paths for current to flow.

Each current path in a parallel circuit is called branch.

The current flows from the negative side of the voltage

source, through each branch of the parallel circuit, tothe positive side of the voltage source.


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What is Ohms Law?

Ohms law defines the relationship among threefundamental quantities: current, voltage, andresistance.

It states that current is directly proportionalto voltage

and inversely proportionalto resistance.


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Ohms Law Formula

RVE

AIcetanresis

voltage

current

Note :

I current (amperes, A)

E voltage (volts, V)

R resistance (ohms, )


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Different Formulas Derived from Ohms Law


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Application of Ohms Law

In a series circuit the same current flows throughout the

circuitIT = IR1 = IR2 = IR3. . . . . .= IRn

The total voltage in a series circuit is equal to the voltagedrop across the individual loads (resistance) in the circuit.

VT = VR1 + VR2 + VR3 . . . . . .+ VRn


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The total resistance in a series circuit is equal to the

sum of the individual resistances in the circuit.

RT = R1 + R2 + R3 +. . .+ Rn


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In parallel circuit, the same voltage is applied to each

branch in the circuit.

VT = VR1 = VR2 = VR3 = VRn

The total current in a parallel circuit is equal to the

sum of the individual branch currents in the circuit.

IT = IR1 + IR2 + IR3 . . . + IRn


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The reciprocal of the total resistance is equal to the

sum of the reciprocal of the individual branchresistances in a parallel circuit

1/RT = 1/R1 + 1/R2 + 1/R3 +. . .+ 1/Rn


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Example(Series)

What is the total current flow in the circuit shown in

figure below.R1 = 560

R2 = 680

R3 = 1 k

ET = 12 V

IT = ?


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Answer toExample IT = ?

R3 = 1 k

R2 = 680

R1 = 560

ET

= 12 V

First solve for the total resistance

of the given circuit:

RT = R1 + R2 + R3 (series connection)

RT = 560 + 680 + 1000 = 2240 or 2.24 k

Now solve for the total current flow:

IT = ET / RT

IT = 12 / 2240 = 0.0054 A or 5.4 mA


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Example(Parallel)

What is the value of R2in the circuit shown in figurebelow?

R3= 5.6 k

R1 = 1 k

IT = 200 mA

ET = 120 V

R2 = ?


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Answer toExample

IT = 200 mA

R1

= 1 k R2 = ?

R3 = 5.6 k

ET

= 120 V

First solve for thecurrent that flows through R1 and R3, voltage are the

same in each branch of a parallel circuit:

ET = VR1 = VR2 = VR3 = 120 V (parallel connection)IR1 = VR1 / R1 = 120 / 1000 = 0.12 A or 120 mAIR3 = VR3 / R3 = 120 / 5600 = 0.021 A or 21 mA

IT = IR1 + IR2 + IR3 ; 0.2 = 0.12 + IR2 + 0.021 ; IR2 = 0.059 A

Now solve for the resistor R2flow:

R2 = VR2 / IR2 = 120 / 0.059 = 2033.9


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Example(Series-Parallel)

What is the current through R3 in the circuit shown in

the figure below?

R1 = 1 k

ET = 120 V R2 = 2 k

R3 = 5.6 k

R4 = 4.7k

R6 = 3.3kR5 = 1.5k


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Example(Series-Parallel)

R1 = 1 k

ET = 120 VR2 = 2 k

R3 = 5.6 k

R4 = 4.7k

R6 = 3.3kR5 = 1.5k

First determine the equivalent resistance

(RA) for resistor R1 and R2

1/RA = 1/R1 + 1/R2 = 1/1000 + 1/2000RA = 666.67

RB = (1/4700 + 1/(1500+3300))-1

= 2375.73

RT = RA + RB + R3 = 666.67 + 2375.73 + 5600= 8642.4

IT = VT/RT = 120 / 8642.4 = 13.885 mA = IR3


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Kirchhoffs Current Law

The algebraic sum of all the currentsentering andleaving a junction is equal to zero.

or

The algebraic sum of the current entering ajunction is equal to the algebraic sum of currentleaving of that same junction.


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Kirchhoffs Voltage Law

The algebraic sum of all voltages around a closedcircuit equals zero.

or

The sum of all the voltage drops in a closed circuitwill equal the voltage source.


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Introduction to Meters

Meters are the means by which the invisible action ofelectrons can be detected and measured.

Meters are indispensable in examining the operationof a circuit.

Two types of meters are available.


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Introduction to Meters

Analog Meter

- uses a graduated scale with a pointer. Analogmeters provide a better graphic display of rapid changes incurrent or voltage.

Digital Meter

- is provided a reading in numbers. Digital meters areeasier to read and provide a more accurate reading thananalog meters.


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Digital Meter Analog Meter


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Types of Meter

Voltmeter is used to measure the voltage (differenceof potential) between two points in a circuit.

Resistance is measured with an ohmmeter.

An ammeter is used to measure current in a circuit.


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Types of Meter


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Tips on Using a Meter

On both analog and digitalmeters, the red terminal ispositive and the blackterminal is negative.

Ohmmeters are read from

right to left and havenonlinear scales.

The number of divisions onthe meter scale depends onthe meter is designed tomeasure.

Ammeters and voltmeters

are read from left to rightand have a linear-scale.


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What is Power?

Power is the rate at which work done.

Power is expended every time a circuit is energized.

Power is directly proportional to both current andvoltage.


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Power Formula

As mentioned, the basic unit of Power is the watt.

A watt (W) is equal to the voltage across a circuit multipliedby the current through the circuit.

It represents the rate at any given instant in which work isbeing done, moving electrons through the circuit.

The symbol P represents electrical power.


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Power Application

To determine the power dissipated by a component,multiply the voltage drop across the component by currentflowing through the component.

P = IE

The total power dissipates in a series or parallel circuit is

equal to the sumof the power dissipatedby the individualcomponents. This can be expressed as:

PT = PR1 + PR2 + PR3 + . . . + PRn


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Examples

What voltage is required to deliver 2 amperes ofcurrent at 200 watts?

Given: Solution:

P = 200 watts P = IV

I = 2 amps 200 = 2(V)

V = ? 100 volts = V


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Table of Contents

I. Lesson 1 : Magnetism

i. What is magnetism?

ii. Magnetic Field

iii. Magnetic Induction

iv. Electromagnetism


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Table of Contents

II. Lesson 2: Inductance

i. What is an inductor?

ii. Inductors in series / parallel

iii. L / R Time Constants


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Table of Contents

III. Lesson 3: Capacitance

i. What is a capacitor?

ii. Capacitors in Series / Parallel

iii. RC time constants


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What is Magnetism?

A phenomenon associated with magnets and

magnetic materials or properties.MAGNET any substance or material which can attractmagnetic materials (bits or pieces of iron).

It came from the word magnesia, an ancient name of

Turkey where this phenomenon was first discovered.MAGNETIC MATERIALS anything which results withmagnet (attracted or repelled by)

NONMAGNETIC MATERIALSmaterials that dont reactwith magnet (wood).


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Types of MagneticMaterials

1. Ferromagnetic materials

- strongly attracted by magnet (Fe, Ni, Co)

2. Paramagnetic materials

- slightly attracted by magnet (Al, Pt)

3. Diamagnetic materials- weakly repelled by magnet (Zn, Au, Bi)


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Parts of a Magnet

MAGNETIC POLE

- the end region of a magnet where magnetism

(power to attract) is greatest.

NORTH POLE (N)

- the pole of the magnet which points to the northdirection

SOUTH POLE (S)

- the pole of the magnet which points to the southdirection

N SNP


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Parts of a Magnet

NEUTRAL POLE (NP)

- the point of the magnet where there is no polarity

POLE STRENGTH (P)

- a measure of the ability of the magnetic pole to attractor repel magnetic materials.

N SNP

N S1 N S2P1 = 50 A-m P2 = 100 A-m

Unit: ampere-meter (A-m)

P2 > P1


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Two Kinds ofMagnet1. Natural Magnet

- a magnet which has natural magnetic property.

Example : lodestone

2. Artificial / Temporary Magnet

- a magnet which acquires its magnetic property

through the passage of electric current in a wireor a conductor.

Example : Electromagnet


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a) every magnetic substance consists of tiny magnetscalled dipoles.

No further cutting of magnet can separate north pole fromsouth pole.

b) in a non-magnetized material, dipoles arehaphazardly, or disorderly arranged

Atomic Theory of Magnet

N S


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c) In a magnetized material, dipoles are aligned and thatthe like poles point in one direction.

The greater the alignment, the stronger is the magnet.

ISOLATED POLE - an imaginary pole, considering that N pole canbe separated from S pole and vice versa.

Atomic Theory of Magnet

N S

N S

N S

N S

N S

N S

N S

N S


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Method of Magnetizing aSubstance

To magnetize a substance a magnetic forcewhich

causes its dipoles to align is applied:a) passing of electric current thru it.

b) heating a material and cooling it beside a permanentmagnet.

c) hammering it against a permanent magnet found innature.


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Law of Magnetism

a) Like poles repel each other

N S

N SN

S

NS


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Law of Magnetism

b) Unlike poles attract each other

N S N S

NS

NS


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Magnetic Force(Coulombs Law of Magnetism)

Consider two isolated poles:

N S

r

P1 P2


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The magnetic force between the magnetic poles isdirectly proportional to the product of the two pole

strengths but inversely proportional to the squareof the distance between them.

N S

r

P1 P2

2

21

r

PPF

2

21

r

PPkF


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Attractive magnetic forcethe force between two

unlike poles

Repulsive magnetic forcethe force between two

like poles

N S

r

P1 P2

2

21

r

PP

kF

Where:

Fforce of attraction or repulsion betweentwo magnetic poles (N)

rdistance between two magnetic poles(m)

Ppole strength of the magnetic pole(Ampere-meter or A-m)

kconstant of magnetism N-m2/ (A-m)2


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In air :

NS

r

P1 P2

221

rPPkF

22

7

mA

mN10x1k

N m2 / (A m)2

OR web / A - m


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In other medium:

N S

r

P1 P2

221

rPPkF

Where is the permeability ofthe medium, a measure ofthe ability of a medium to

concentrate magnetic fieldwithin its vicinity.

4

k for vacuum: 710x4

410x4k

7

mA/web10x1k7

Note: magnetic force is a vector


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Sample Problems

1. What is the force between two magnetic poles of strength 40

A-m and 50 A-m having a distance of 10 cm in the air?2. Consider the system, determine the net force on the S-

pole.

3. Given a system, determine the net force on isolated N-pole.

N1 N2S

10 A-m 15 A-m 20 A-m

40 cm20 cm

N S N

P = 100 A-m P = 200 A-m

10 cm10 cm


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Magnetic Fields

A region where an independent N-pole placed in it, will experience a

force.

MAGNETIC LINES OF FORCE thelines used to describe a magneticfield.

The actual path taken by the motionof an independent unit N-pole in a

magnetic field emerging from N-poleof a magnet and terminating to its S-

pole.

N

S


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Permeability Explained

If an iron bar is placed in between the poles of a magnet,the lines of force are concentrated in the iron bar becauseit has higher permeability than air.

N S N S

Iron bar

Iron ring


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RESIDUAL MAGNETISM - a

weak magnetic field when themagnet and iron bar areseparated.

Magnetic Induction

MAGNETIC INDUCTION

is the effect a magnet hason an object withoutphysical contact.

IRON BAR

in passing through the iron bar, themagnetic lines of force cause the

domains in the iron bar to align inone direction, the iron bar is now amagnet


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Magnetic Induction

RETENTIVITYthe ability of a material to retain its magnetic field after themagnetizing force is removed.

Soft iron has low retentivity while Alnico, an alloy made of aluminum, nickel,and cobalt, has high retentivity.

ELECTROMAGNETIC INDUCTION

is the principle behind the generation of electricity:

for the electromagnetic induction to occur, either the conductor must moveor the magnetic field must move.

The voltage produced in the conductor is calledINDUCED VOLTAGE.


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Magnetic Induction

The stronger the magnetic field, the greater the inducedvoltage.

The faster the conductor moves through the field, thegreater the induced voltage.

the maximum voltage is induced when the conductor moves at rightangles to the field.

angles less than 90 degrees induce less voltage.

if a conductor is moved parallel to the flux lines, no voltage is induced.

the longer the conductor, the greater the induced voltage.


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Magnetic Induction(Magnetic Flux Density - B)

2r

PpkF

The strength of a magnetic field

the magnetic force exerted by thefield on a unit N-pole placed in a

region

P

FB

Where:

B magnetic induction (web/m2)

P pole strength (A-m)

r distance (m)

K magnetism constant (web/A-m)

N N

P P

p22

2

r

kP

P

1

r

kPp

P

rPp

kB


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Due to isolated pole:

2rkPB N

S

Ar

B is also a vector, therefore themagnitude and direction is

determined by vector

analysis.

B

B A

r

B is always away from N-pole &

B is always towardS-pole


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Due to several magnetic poles:

N1

S2

A

N2

S1

BS2rS2

BS1rS1

BN2rN1

BN1rN2

At point A, Bnet = B

Bnet = B = BN1 + BN2 + BS1 + BS2

where:

BN1 = kPN1 / rN12

BN2

= kPN2

/ rN2

2

BS1 = kPS1 / rS12

BS2 = kPS2 / rS22


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Faradays Law

The basic law of magnetism states:

The induced voltage in a conductor is directly proportional tothe rate at which the conductor cuts the magnetic lines of

force.


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physics2 lecture

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