PHEP 15 Flashcards by Anita Dickson

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The strength of the magnetic field depends largely on?

The material involved and extent to which the material has been magnetized

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What produces the strongest artificial magnets?

Induced Magnetism (Electromagnets)

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Magnetism is a _________.

Force

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What are the three important facts about force lines?

-No lines cross - All lines are complete -All lines leave the magnet at right angles to the magnet

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What can be done to build a stonger magnetic field?

Add one or more turns of wire

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What happens when a conductor is moving through a magnetic field?

A voltage is induced in the conductor.

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The direction of current flow depends on?

The direction of flux cutting and the direction of the magnetic field.

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State Lenzs Law

In all cases of Electromagnetic induction, all induced EMF’s counter the motion producing EMF. Or more simply, for every force there is a counter force set up which tends to oppose the first force

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Reluctance in a __________ circuit corresponds to ____________in an electric circuit.

Magnetic & Resistance

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The amount of flux is measured by the number of lines, called____________.

Maxwells

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How many magnetic fields are involved when discussing induced magnetism?

2, - magnetic field and conductor field

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What direction are the fields traveling in when discussing induced magnetim?

The magnetic field travels in a straight line from North to South poles. The conductor field is in a circular field surrounding the conductor

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Permeability corresponds to ________ in an electric circuit.

Conductivity

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Permeability is a measurement of?

The relative ability of a material to carry flux

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Permeability is inversely proportional to?

Flux Density

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Describe why iron flux density can no longer increase after as the amp turns are continually increased?

Because the permeablility of the Iron has decreased due to saturation

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What can be done to increase the flux density of a metial after its become saturated?

nothing

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When turns of a conductor are wounds about an Iron core, an ___________ results.

Electro-Magnet

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Induction is the process of generating an?

Electrical voltage in a conductor.

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Mutual Induction is the transfer of electrical energy from one _________ to __________.

Coil or circuit to another coil or circuit

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What is inductance?

The ability of a coil or circuit to produce a counter-voltage within itself by self induction.

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What determines a coil’s inductance?

-the length -the number of turns - the diameter -the permeability of the core

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Inductance is measured in ___________.

Henries

Induction is a ___________, inductance ia an ____________, and inductive reactance is a ______________.

Process, Ability, Quantity

The driving force causing a magnetic field is known as?

Magneto-Motive Force

________ is a process by which a circuit induces an EMF in itself by its own magnetic field

Self Induction

Inductance of a coil is determined by four factors:

-Length of the coil -Number of turns on the coil -diameter of the coil -permeability of the coil

________ is when the current in a coil varies, the expanding and collapsing field around the coil cuts the turns of the coil & induces a voltage counter to the impressed voltage

Self Induction

What is the Effective Voltage Formula?

Eeff=0.707 x Epeak or RMS 0.707 times the peak value (RMS)

What is magnetism?

A measurable characteristic of a moving electrical charge.

There are three classifications of magnetic materials; what are these in order of magnetic ability?

1. Ferromagnetic materials (most magnetic - Iron) 2. Paramagnetic materials (sometimes magnetic - Aluminum) 3. Diamagnetic materials (never magnetic - Carbon)

What is the difference between a permanent and temporary magnet?

A temporary magnet loses it's magnetism quickly after it's magnetising force is lost. A permanent magnet will lose it's magnetism after a much longer time.

What is magnetic flux?

The lines of force of a magnetic field are collectively known as flux. Symbol is Φ.

What is flux density?

Flux density is the quantity of flux lines through a cross sectional area. Flux density determines the strength of the magnetic field. Symbol is ß.

What is permeability?

The ease at which flux magnetizes a material. Symbol is µ.

What are the seven characteristics of magnetic flux?

Lines: · Form closed loops · Are directional · Do not intersect · Try to shorten themselves · Repel each other · Seek the path of least reluctance · Can enter any material.

What is reluctance?

The opposition of a material to accepting a magnetic path through itself. Symbol is Rm.

What is residual magnetism?

Leftover aligned magnetic domains after a magnetising force is removed.

What is retentivity?

A material's ability to retain magnetism.

What is saturation?

When all magnetic domains in a material are aligned. Any further application of a magnetizing force will only produce waste.

Which material is more reluctant, air or soft iron?

Air

Magnetic flux flows from "_____", outside of a magnet.

North pole to south pole.

How would one use a left hand rule for coils?

· Thumb points to north pole. · Fingers point in direction of electron flow.

How would one use a left hand rule for conductors?

· Thumb points in the direction of electron flow. · Fingers curl in the same way as the magnetic field produced (north to south).

What are three factors affecting the strength of an electromagnet?

· The amount of current in the coil. · The number of turns in the coil. · The type of core used.

What are the two most common types of Iron losses? How are they reduced?

· Hysteresis losses: The flip flopping of magnetic domains in the core due to ac current, lost as friction heat. o Reduced with a silicon steel alloy core. · Eddy current losses: Heat losses due to induced circulating current in the core. o Reduced by lamination of the core.

What is Lenz's law of induction?

Induced voltage will cause a current to flow and it's magnetic effect will oppose the change which produced it.

What is inductance symbolized by, and what is it measured in?

Symbolized by L. Measured in Henrys (H).

What is Flemming's left hand rule?

· Thumb points in the direction of conductor motion. · First finger points in the direction of magnetic field from north to south. · Middle finger points in the direction of electron flow. (always perpendicular to forefinger)

What is Faraday's law of induction?

-A voltage will be produced across A conductor which cuts across magnetic flux. -This voltage is proportional to the rate of cutting flux. -1 Volt is produced by cutting across 100 million lines of force in 1 second.

What is the most noticeable effect of inductance?

Current takes longer to establish and dissipate.

What is mutual induction?

When current through one coil magnetically induces a voltage on a coil mounted on the same core but not electrically connected.

When both electrons in a pair of an atom are spinning in opposite directions, the net value of magnetism is...?

0 (zero)

Which materials are added to high carbon steel to make permanent magnets?

· Cobalt · Molybdenum · Chrome-tungsten · Alnico-5 (most common)

Magnetic flux in a magnetic circuit has a relation similar to what expression appearing in Ohm’s law? A) B) C) D)

Current

The total flux in a circuit divided by the cross-sectional area of the circuit path equals A) B) C) D)

Flux Density Flux density (B) =total flux (Ø)/cross-sectional area (A) B=Ø/A (it’s the amount of flux per unit area)

The electron theory of magnetism assumes that magnetic action is due to: A) B) C) D)

The electron movement and arrangement of the electron orbits Electron Theory of Magnetism Electrons spin on their axes as they orbit around the nucleus of the atom. This spinning motion causes each electron to become a tiny permanent magnet. Not all electrons spin in the same direction however. Electrons that spin in opposite directions tend to form pairs.

The strength of an electromagnetic field is NOT affected by the? A) B) C) D)

type of insulation covering the wire forming the solenoid Factors that affect the strength of electromagnets are: - the nature of the core material, - strength of the current passing through the core, - the number of turns of wire on the core and the shape and -size of the core

Permeability is the opposite of? A) B) C) D)

Magnetic reluctance

When a magnetic substance has reached the saturation point, it’s magnetic field: A) B) C) D)

Can not be increased The iron has become so saturated that it cannot hold any more flux regardless of how many ampere turns are used. (page 13)

The polarity of an electromagnet will depend upon the? A) B) C) D)

The direction of the current flow through the coil The polarity of the electromagnet is determined by the direction the current. The north pole of the electromagnet is determined by using your right hand.

To induce a voltage across a solenoid, there must be? A) B) C) D)

A change in the field cutting the coil. Then we can see that the faster the movement of the magnetic field the greater will be the induced emf or voltage in the coil, so for Faraday's law to hold true there must be “relative motion” or movement between the coil and the magnetic field and either the magnetic field, the coil or both can move

An induced voltage will not be created when? A) B) C) D)

A solenoid remains stationary within a fixed field Its not moving (The voltage is induced by motion)

The induced voltage that opposes the original EMF applied to the circuit is called the? A) B) C) D)

Counter EMF

Inductance is that property which aids any change in the current flow or in the magnetic field of an electric circuit. This definition is? A) B) C) D)

correct if the word aids is changed to opposes

Where the effects of inductance are noted within the same circuit where the original current exists it is called? A) B) C) D)

Self inductance

Hysteresis loss is most nearly associated with: A) B) C) D)

the heat produced by repeatedly magnetizing and demagnetizing an AC magnetic core

Inductive reactance is present in? A) B) C) D)

AC circuits if a coil forms part of the circuit

What is the magnitude of the effective current flowing through a 20 Henry choke coil of a negligible resistance which is connected to the 120 volt 60 cycle ac power line (I=E/XL) A) B) C) D)

about 0.016 amps XL=6.28 x Frequency x Inductance XL=6.28 x 60Hz x 20h XL=7,536 I=E/XL I=120v / 7,536

PHEP 15 Flashcards

(71 cards)