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Magnetism > Portfolio Part A > Flashcards

Portfolio Part A Flashcards

1
Q

The Function of a magnetic shield is to

A

Divert the magnetic lines of force around the shielded object

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2
Q

If an electromagnet had 200 turns on the coil with a resistance of 5 Ohms, calculate the Magneto-motive force it would produce if connected to a 25v supply?

A

1000 ampere turns

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3
Q

Name the unit for magnetising force

A

(H) Ampere turns per meter

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4
Q

Describe the difference between magnetic leakage and magnetic fringing

A

Leakage: strays from the desired path

Fringing: when the area of the flux path is greater in the air gap than in the material of the magnetic circuit

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5
Q

Lenz’s law states that?

A

The direction of induced current will always oppose the changing magnetic field that produced it

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6
Q

List 3 practical applications for electromagnets

A

Motors, magnabend, headphones

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7
Q

Name 3 factors that affect the strength of a solenoid

A

Number of turns in the coil, length of core, cross sectional area

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8
Q

In what direction is the magnetic field around a conductor when the current flow Is away from the observer

A

Clockwise

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9
Q

State faraday’s law of electromagnetic induction

A

Placing a conductor in a changing magnetic field produces an EMF in that conductor

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10
Q

Which of the following devices are examples of electro magnetic induction

A: Electromagnets
B: Contactor Coil
C: Relay Coils
D: All of the above

A

D: All of the above

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11
Q

The current through an inductor is reduced from 5A to 1A in 0.75 seconds. If the inductance of the coil is 1.5 H, determine the induced voltage

A

8V

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12
Q

Magnetic sensors can be used to detect changes in magnetic fields

True or False

A

True

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13
Q

An electro magnet has 600 turns and the total reluctance of the core is 800 IN/Wb. Calculate the flux produced when 10A flows through the coil

A

7.5Wb

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14
Q

A coil of 800 turns has a flux of 90 micro Wb passing through it. If the flux is reduced to 40 micro Wb in 20ms, find the average induced voltage.

A

2V

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15
Q

When using the right hand rule to determine the direction of the magnetic field around a current carrying conductor, why are our fingers placed at a right angle around the conductor

A

Your fingers point at the direction of the magnetic field

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16
Q

A conductor placed at a right angle to a magnetic field with a density of 0.66T over a length of 0.1m. if a current of 25A is passed through the conductor, calculate the force exerted on the conductor

A

1.65 newtons

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17
Q

Define the term self-Inductance

A

Inductance occurs when a conductor had a voltage induced in it by its own magnetic field when in a changing field

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18
Q

If two conductors carry current in the same direction, the electromagnetic force will cause the conductors to repel each other

True or false

A

True

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19
Q

Which of the following is an example of mutual inductance

A: Multi-Coil transformer
B: Fluorescent Ballast
C: Solenoid Coil
D: All of the above

A

A

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20
Q

Which of the following is assigned the permeability of unity or 1

A: Actual Permeability
B: Absolute Permeability
C: Relative Permeability of air
D: Permeability of iron

A

C

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21
Q

If the rate at which a conductor cuts the magnetic field is reduced then

A: The magnitude of the voltage increases
B: The hotter the conductors become
C: The stronger the magnetic field around it
D: The magnitude of the induced EMF decreases

A

D

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22
Q

Which four components are used in the construction of a Electromagnetic relay

A: Overload, Stator, Rotor, Fuses
B: Fuses, Overload, Stator, Permanent Magnet
C: Coils, Contacts, Overloads, Permanent Magnets
D: fixed iron stator, Moving iron armature, Coil, Contacts

A

D

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23
Q

When applying Flemings right hand rule for conductors carrying current, your thumb points to

A: Direction of rotation
B: Direction of magnetic flux
C: Direction of current flow
D: Points of resistance

A

B

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24
Q

Which two factors determine iron losses in magnetic materials

A: Voltage and Eddy current losses
B: Iron and Hysteresis losses
C: Hysteresis and Eddy current losses
D: Copper and Brass losses

A

C

25
Q

Which two Solutions help reduce Hysteresis and Eddy current losses?

A: Reduce the turns on the coil and change the voltage
B: Create an air gap in the core and change the voltage
C: Increase the turns on the coil and Use a solid core
D: Increase the permeability of the core material and laminate the core

A

D

26
Q

Select the correct answer that defines Permeability.

A: The opposition to the magnetising force.
B: The lines of magnetism that extend from north pole to south pole
C: The ease at which a flux can be created in a material.
D: The number of lines of force per m²

A

C

27
Q

Select the correct answer that defines magnetic flux.

A: the number of lines of force per m²
B: The ease at which flux can be created in a material
C: The opposition to magnetising force
D: The lines of magnetism that extend from North pole to South

A

D

28
Q

Select the correct answer that defines flux density

A: The ease at which flux can be created in a material
B: The lines of magnetism that extend from North pole to South pole
C: The number of lines of force per m²
D: The ease at which flux can be created in a material

A

C

29
Q

Calculate the flux density of an air-cored former that has a cross sectional area of 200mm² and a total magnetic flux of 0.0000025 Webers. (Note: 1mm² = 1 x 10-⁶ m²)

A

0.00125 Wb/m²

30
Q

Name the common magnetic circuit types.

A: paramagnetic and diamagnetic
B: permanent magnet and electromagnetic
C: permanent and paramagnetic
D: Diamagnetic and electromagnetic

A

B

31
Q

What is the common name for the opposition to the establishment of magnetic flux

A

Reluctance

32
Q

A conductor 0.2m in length moving at right angles to a magnetic field of flux density 0.005T has a velocity of 20 m/s. Calculate the maximum induced voltage.

A

0.02V

33
Q

A bifilar winding inductor is constructed by winding 2 conductors in parallel

True of false

A

True

34
Q

Calculate the force exerted on two parallel conductors each carrying 50 amps, if the distance between them is 50mm.

A

0.01 newtons per metre

35
Q

What four factors determine the value of inductance in a coil

A: size of conductors and number of turns in the coil, length of the core, cross sectional area, Permeability of core material
B: Permeability of core, resistance of the coil, Flemings right hand rule, length of the core
C: right hand rule for conductors, number of turns, material core made from, resistance of coil.
D: capacitance in coil, amount of flux produced, length of core, type of material coil made from

A

A

36
Q

Three common types of inductor cores are.

A: air, iron, ferrite
B: Copper, iron, ferrite
C: Iron, air, brass
D: Iron, Aluminium, Ferrite

A

A

37
Q

Name three applications for inductors in electro-technology field…

A: RF applications, Anodes, Toroidal coils
B: RF applications, Diodes, DC connections
C: RF Applications, Kettering ignition coils, AC coils
D: RF application, Capacitors, Anodes

A

C

38
Q

Mutual induction occurs between coils when

A: One energized coil induces an EMF into a second coil
B: The coil is saturated
C: The second coil is at maximum resistance
D: The second coil has no load connected

A

A

39
Q

Snubbing circuits are fitted with components such as diodes, resistors, inductors and capacitors across the coil. These components are used to short out high self-induced voltages that try to keep the current flowing

True or False

A

True

40
Q

Define a time constant for an inductor connected to D.C

A

A time constant is the length of time during which the charge or discharge undergoes a 63.2% change

41
Q

The series field windings of a 120V DC generator have an ohmic resistance of 60 ohms and an inductance of 10H determine:

A: the time constant
B: the time it will take for the current to attain it’s full value
C: the final value of currant

A

A: 0.17s
B: 0.85s
C: 2 amps

42
Q

Name three possible causes of damage to electrical instruments

A

Overload, wrong connections, DC meter to AC

43
Q

When using an ohm meter name two safety precautions that should be taken

A

Make sure the circuit is off

*

44
Q

When choosing a meter to use 6000V which category of meter would be safest to use

A: Cat I
B: Cat II
C: Cat III
D: Cat IV

A

D

45
Q

Which material is used to make the core of a Toroidal Coil

A

Ceramic materials

46
Q

What is the effect of an air gap in a magnetic circuit

A: decrease the reluctance of that magnetic circuit
B: Increase the flux density of that magnetic circuit
C: Increase the reluctance of that magnetic circuit
D: Increase Magneto-motive force

A

C

47
Q

Like poles attract each other

True or false

A

False

48
Q

Name the three classifications of magnetic substances

A

Ferromagnetic, paramagnetic, diamagnetic

49
Q

Give two situations where reed switches are used

A

Security installations and domestic goods

50
Q

List 8 practical applications for permanent magnets

A

Magnetic shield, speaker, credit card, fridge door, alarms, microphones, small generators, electrical motors

51
Q

Calculate the values of a shunt resistor for a current range of 2A for a digital module that has a full scale input voltage of 200mV

A

0.1 ohms

52
Q

Name one method of extinguishing an arc across the open contacts of a D.C relay

A

Diode connected across the coil

53
Q

What is the difference in the construction of a D.C contactors and relays compared to A.C

A

DC has solid magnetic Core and A.C has a laminated iron

54
Q

There are 3 ways hall effect devices monitor motor rotation and speed

A

Small light emiting diodes, magnetic sensors, hall effect sum conductors

55
Q

In 1 time constant the current will increase to 63% of the value of the maximum current in an LR circuit. How many time constants will take the current to full load?

A

5 constants

56
Q

What is a dynamo meter used for and what is the practical name for it

A

Used to measure power consumed and it practical name is a watt-meter

57
Q

Give one practical application for each of the following meters

Moving coil meter

Moving iron meter

Dynometer

A

Moving coil meter - DC current test
Moving iron meter - DC and AC testing resistance
Dynamo meter - testing power consumption

58
Q

When selecting a meter for use in the electrical trade name three things that must be considered

A

Accuracy of the instrument needed

Sensitivity of the instrument needed

Cat rating for safety

59
Q

Name the practical applications where magneto-striction material is used

A

Stepper motors, ultrasonic cleaner, stress and pressure

Magnetism (2 decks)

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